CHAPTER 15 ROAD DRAINAGE AND THE WATER
ENVIRONMENT
Cross Tay Link Road
Revision Date Status Author Technical Checker Approver Number Reviewer P01.1 10.07.19 WORK IN E. REID, J J. PRESTON, R. McLEAN C. CARDNO PROGRESS WALKER J. MOORE P01 15.11.19 S4 FOR E. REID, J J. MOORE R McLEAN D. RITCHIE STAGE WALKER APPROVAL BIM Reference: 119046-SWECO-EWE-000-RP-EN-20016
This document has been prepared on behalf of Perth and Kinross Council by Sweco for the proposed Cross Tay Link Road Project. It is issued for the party which commissioned it and for specific purposes connected with the above-captioned project only. It should not be relied upon by any other party or used for any other purpose. Sweco accepts no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties.
Prepared for: Prepared by: Perth and Kinross Council Sweco Pullar House Suite 4.2, City Park 35 Kinnoull Street 368 Alexandra Parade Perth Glasgow PH1 5GD G31 3AU
CONTENTS
15 ROAD DRAINAGE AND THE WATER ENVIRONMENT ...... 1 15.1 Executive Summary ...... 1 15.2 Introduction ...... 1 15.3 Scope of Assessment ...... 2 15.4 Methodology ...... 7 15.5 Baseline Conditions ...... 17 15.6 Potential Effects ...... 30 15.7 Mitigation and Enhancement ...... 51 15.8 Residual Effects ...... 60 15.9 Cumulative Effects ...... 61 15.10 Summary of Effects ...... 62 15.11 Statement of Significance ...... 76
FIGURES
Figure 15.1: Water Environment and Mitigation ...... 19 Figure 15.2: 200 year Flood Extents (Baseline) ...... 44 Figure 15.3: 200 year Flood Extents (with Scheme) ...... 57
TABLES
Table 15.1: Summary of Consultations and Actions Taken ...... 3 Table 15.2: Criteria for assessing Baseline Sensitivity ...... 9 Table 15.3: Criteria for assessing Impact Magnitude ...... 11 Table 15.4: Criteria for assessing Impact Significance* ...... 13 Table 15.5: Overview of Watercourses ...... 20 Table 15.6: Summary of Watercourse Sensitivity ...... 22 Table 15.7: Summary of Construction Activities in/near each Watercourse ...... 30 Table 15.8: Proposed Drainage Outfall Locations ...... 47 Table 15.9: Summary of Routine Runoff Assessment (soluble pollutants) ...... 48 Table 15.10; Summary of Routine Runoff Assessment (sediment pollutants) ...... 50 Table 15.11: Summary of Spillage Risk Assessment ...... 51 Table 15.12: Indicative Pollutant Reduction Factors used for HAWRAT assessment ...... 58 Table 15.13: Summary of the Simple Index Approach (CTLR carriageway) ...... 59 Table 15.14: Summary of Effects on RDWE sub-topics ...... 64
APPENDICES
Appendix 15.1: Baseline Water Environment Appendix 15.2: Flood Risk Assessment Appendix 15.3: DMRB Routine Runoff / Spillage Risk Assessment Appendix 15.4: Engineering Activities
CHAPTER 15 CROSS TAY LINK ROAD ROAD DRAINAGE AND THE WATER EIA REPORT (VOLUME 2) ENVIRONMENT
15 ROAD DRAINAGE AND THE WATER ENVIRONMENT
15.1 EXECUTIVE SUMMARY
This chapter provides an assessment of the effects of the proposed Cross Tay Link Road (CTLR) Project on the surface water environment, during both construction and operational phases. The assessment also considers road drainage provision of the operational scheme and potential effects to receiving watercourses. The main watercourse in the study area is the River Tay, which is a designated Special Area of Conservation (SAC) for its populations of Atlantic salmon, lamprey species, freshwater pearl mussel and otter, as well as being a sensitive freshwater habitat. As such it is of very high sensitivity.
The baseline conditions and potential effects were assessed by means of desk-based assessment, a river reconnaissance survey and consultations, as well as hydraulic modelling and drainage assessments following DMRB procedures.
One of the greatest risks to the water environment and aquatic ecology is of silt-laden and contaminated runoff entering the River Tay SAC directly from works on the banks, as well as the cumulative effects of in-channel works in nearby tributaries during the construction phase. However, these construction works will be subject to a number of control and management measures, and in particular construction site licensing under the Controlled Activities Regulations.
Following implementation of best practice and site-specific mitigation measures during construction and operation detailed in this chapter, all residual effects are predicted to reduce to no more than minor magnitude and Slight significance, and therefore not significant in the context of the EIA Regulations.
15.2 INTRODUCTION
This chapter provides an assessment of the effects of the proposed CTLR Project on various sub-topics of the surface water environment; namely hydrology and flood risk, fluvial geomorphology, water quality and drainage during both construction and operational phases. These sub-topics can be defined as:
• Hydrology and Flood Risk – the flow of water on or near the surface. Flooding has many sources including coastal, river (fluvial), surface water (pluvial), sewer and groundwater. • Fluvial Geomorphology – landforms associated with river channels (channel morphology) and the fluvial and sediment transport processes which form them (fluvial processes and sediment regime). • Water Quality – various attributes of watercourses and water bodies including water quality and supply, biodiversity, dilution and removal of waste products and recreation. • Drainage – a system of collecting overland or underground waters, often by channels, drains and sewers to a point of discharge or treatment.
These sub-topics are closely linked and are also linked to ecological receptors (Chapter 9: Biodiversity) and groundwater and contaminated land (Chapter 10: Hydrogeology and Soils). Baseline characteristics and potential effects of the proposed CTLR Project, as well as any associated mitigation, is often applicable to these interrelated topics and cross-referencing has been added throughout this chapter where relevant.
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This chapter is supported by the following figures and appendices:
• Figure 15.1: Water Environment and Mitigation; • Figure 15.2: 200 year Flood Extents (Baseline); • Figure 15.3: 200 year Flood Extents (with Scheme); • Appendix 15.1: Water Environment Baseline Conditions. Provides the baseline of the water environment for the proposed CTLR Project • Appendix 15.2: CTLR Flood Risk Assessment Report. A report detailing the flood risk and associated modelling as a result of the proposed CTLR Project • Appendix 15.3: Water Quality Calculations. Provides calculations undertaken with the HAWRAT model to determine the impact of the proposed CTLR Project on the water quality • Appendix 15.4: Engineering in the Water Environment. Provides information on the watercourse crossings (bridges and culverts) that are to be constructed or modified as part of the proposed CTLR Project, including justification for each engineering option and reasons for rejection of alternatives
15.3 SCOPE OF ASSESSMENT
15.3.1 Study Area
The general study area for the water quality and geomorphology assessments was up to 0.5km from the footprint of the proposed CTLR Project, as shown on Figure 15.1. The flood risk assessment (FRA) considered various sources of flood risk (including fluvial, tidal, groundwater and pluvial sources) within a wider study area as per best practice guidance.
15.3.2 Site Visits
A river reconnaissance walkover survey was undertaken in May 2018 by Sweco geomorphologists to better understand the existing morphological features and sediment/fluvial processes within the watercourses in the study area. This informed the understanding of the baseline environment, and the resulting potential changes and vulnerability for channel change as a result of the proposed CTLR Project.
Specific surveys for the hydraulic modelling work, including topographic/bathymetric surveys and a culvert inspection survey, are detailed in the Hydrology and Flood Risk Methodology section below.
15.3.3 Key Consultations
Table 15.1 outlines the key consultations and meetings undertaken with Statutory and Non-Statutory consultees to inform the approach and key issues raised for the Road Drainage and Water Environment (RDWE) assessment. This includes a summary of how key issues were considered in the EIA.
In addition, there were various meetings and correspondence with SEPA to discuss and agree the hydraulic modelling approach, methods and results, which has not been included here.
Refer to Chapter 5: Consultation and Scoping for more detailed information on consultations, including the EIA Scoping Opinions received from SEPA, the Council and Scottish Water and how these issues were dealt with in the EIA.
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Table 15.1: Summary of Consultations and Actions Taken
Consultee Purpose (Date) Summary of Key Issues Action Taken Through optioneering at DMRB Stage 3, the chosen SEPA’s preference for crossings with no in-river River Tay Crossing Bridge includes no in-channel structures. Dredging works unacceptable to SEPA and piers, with the piers slightly set-back from the bank bed disturbance on minor watercourses to be avoided edge to minimise adverse effects on the River Tay where possible. SAC. Due to the requirement to cross watercourses, • A minimum of two levels of treatment (SuDS) is some bed disturbance was unavoidable in minor required for drainage outfalls in accordance with watercourses. Justification for proposed engineering the CIRIA SUDS Manual (and as agreed at works is included in Appendix 15.4. DMRB Stage 2). The type of SuDS would be Meeting to discuss assessed on a case-by-case basis and SEPA The drainage design and SuDS measures are SEPA’s requirements for would accept a shut-off system on the SuDS designed in line with SEPA and CIRIA best practice. the proposed drainage, outlet to deal with a spillage or pollution incident. The assessment in this chapter demonstrates that flooding and • For any SuDS ponds/basins constructed in the the proposed SuDS measures are sufficient to SEPA environmental aspects floodplain, the feature should be sized to protect receiving watercourses from pollution. of the scheme, including attenuate, and store flows up to the 1 in 200 the River Tay Crossing year event and be protected from inundation up Hydraulic modelling was undertaken in line with Bridge structure to the 1 in 30 year event. SEPA’s Technical Flood Risk Guidance for (September 2017) • The road infrastructure would need to be at a Stakeholders and a climate change uplift of 20% level above the 1 in 200 year return period flood was added to the 200 year peak flow estimate. event with an appropriate freeboard and climate change allowance. Detailed hydraulic modelling was undertaken for the • Fluvial modelling should utilise the most relevant and River Tay and some of its tributaries including the up-to-date data where required, e.g. previous flood Cramock Burn. For the River Tay, a previous 1D models for the River Tay and River Almond, hydraulic model was updated, which included a 4km construction of Phase 1 of the Scheme, and stretch of the River Almond (refer to Section 15.4.3 requirements for modelling of Cramock Burn. for details of the flood modelling methodology).
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Consultee Purpose (Date) Summary of Key Issues Action Taken
• The RDWE chapter of the EIAR will include DMRB Joint meeting with SNH routine runoff and accidental spillage risk calculations A summary of the DMRB routine runoff and and SEPA to provide an to demonstrate that the drainage design/SuDS accidental spillage risk assessment, as well as update on flood provision is adequate to protect receiving CIRIA’s Simple Index Approach, is provided in modelling, as well as watercourses (i.e. minimum two levels of SuDS). Section 15.6.4 and Section 15.7.2 of this chapter. The methodologies and detailed inputs/outputs is discussion on drainage, • Drainage outfalls and pollution prevention measures provided in Appendix 15.3. water quality and will be designed and installed in accordance with
Controlled Activities SEPA and CIRIA best practice guidance, and any Best practice and site specific measures for pollution Regulations (CAR) site-specific measures will be included as necessary. prevention and drainage outfalls is provided in (June 2018) Locations of outfalls will be provided to SNH/SEPA for Section 15.7. comment as the drainage design develops.
The likely activities requiring CAR authorisation as part A schedule of activities requiring CAR authorisation of the proposed CTLR Project including culverts and was issued and discussed with SEPA. It is Meeting to discuss latest channel realignments, works in the floodplain and road anticipated that further CAR Pre-Application drainage, flood drainage discharges were discussed with SEPA. discussions will be undertaken with SEPA and CAR modelling and CAR Freeboard requirements for the culverts were discussed licenses will be sought during the tender process licensing (December with SEPA. As was the flood modelling approach for and transferred to the successful tenderer upon 2018) Bertha Loch Burn and compensatory storage contract award. Refer to Section 15.4.1 of this requirements. chapter for more details.
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Consultee Purpose (Date) Summary of Key Issues Action Taken SEPA confirmed that the provision of compensatory floodplain storage for the River Tay Crossing Bridge Hydraulic modelling predicted that 4m3 of existing River was not required for the minimal loss of existing Tay floodplain was displaced due to the eastern pier of floodplain in this location. the River Tay Crossing Bridge, which is very minimal
compared to the much larger River Tay catchment. SEPA confirmed that the proposed mitigation at
Bertha Loch Burn to protect receptors to the south Bertha Loch Burn flood mitigation (in the form of a 160m Meeting to discuss the was appropriate. long flood embankment on south bank) was required to latest Scheme flood risk protect Bertha Park Lodge and access road from assessment (FRA) and flooding. The crest of the embankment was designed to • SEPA confirmed that the design of the River CAR licensing contain the 1 in 200 year flood event with a 35% climate Almond crossing to convey the 1 in 200 year requirements (July change allowance to ensure future resilience. flood event (without an allowance for climate 2019) change) was appropriate for a temporary Requirements for compensatory floodplain storage of bridge structure. approximately 750m3 for the temporary River Almond • SEPA agreed that consideration of the 0.5% bridge crossing to mitigate for the loss of existing AEP flood event was overly conservative for floodplain up to the 1 in 200 year flood event was a temporary structure and confirmed that discussed.1 compensatory floodplain storage was not required in this location.
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Consultee Purpose (Date) Summary of Key Issues Action Taken Through optioneering at DMRB Stage 3, the chosen River Tay Crossing Bridge structure included no in- channel piers (to minimise adverse effects on the • The main potential issues would include disruption to River Tay SAC and disruption to fishing rights) and fishing rights during construction, potential in-channel aimed to minimise adverse visual impacts (an works and visual intrusion from a new crossing alternative ‘statement structure’ was discounted on Meeting to discuss structure. this basis). TDSFB preferences for • The main fishing season is from July to mid-October the River Tay Crossing and the main spawning season is November / Piling works on the banks of the River Tay will avoid Bridge and in-channel December. the sensitive spawning period for fish. Refer to works (May 2017) • Recreational activity including rafting / canoeing is Chapter 9: Biodiversity for further information. prevalent in the River Tay upstream of the proposed Tay District CTLR Project (e.g. Stanley / Aberfeldy), rather than in Information on the watercourses including water- Salmon the smoother downstream reaches. dependent recreational activities on the River Tay, is Fisheries included in the baseline (Section 15.5) and was one Board metric used to assign baseline sensitivity. (TDSFB) Construction of the east and west piers of the River Tay Crossing Bridge will take place within a • It is likely that the piers for the River Tay Crossing cofferdam to minimise risk of pollution. The Meeting to discuss the Bridge will be constructed within a cofferdam to appointed Contractor’s programme will take into latest River Tay and ensure a dry working environment and also minimise account sensitive seasonal restrictions including the River Almond structure risk of pollution to the river. Installation and main fishing season. designs and TDSFB construction should be programmed out with the main preferences for these fishing season (i.e. July to mid-October). The proposed temporary crossing of the River crossings (September • Any crossings (permanent and/or temporary) of the Almond for construction vehicles to access the west 2018) River Almond should take account of existing flooding pier of the River Tay Crossing Bridge was subject to and may be subject to an FRA. an FRA. Further details are provided in Appendix 15.2.
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15.4 METHODOLOGY
Details of the assessment topic methodologies are provided in the following section, including legislation and guidance applicable to RDWE and any assumptions/limitations to the assessment.
15.4.1 Legislation and Guidance
The assessment was carried out in accordance with the following legislation, policy and guidance.
Flood Risk Management (Scotland) Act 2009 (FRMS Act)
The FRMS Act sets in place a statutory framework in Scotland for delivering a sustainable and risk- based approach to managing flooding. It places a duty on Scottish Ministers, SEPA, local authorities, Scottish Water and other responsible authorities to manage and reduce flood risk and to promote sustainable flood risk management. Under Section 42 of the FRMS Act, planning authorities (in this case the Council) require applicants to provide an assessment of flood risk where a development is likely to result in a material increase in the number of buildings at risk of being damaged by flooding.
Water Framework Directive 2000/60/EC (WFD)
The WFD, transposed into Scottish law by the Water Environment and Water Services Act 2003 (WEWS Act), sets targets for restoring and improving the status of water bodies and preventing deterioration. The Scottish Government prepared and implemented a River Basin Management Plan (RBMP) for the Scotland river basin district for 2015-2027 and supplementary Area Management Plans outlining how the water environment will be managed and improved to meet WFD objectives over time (i.e. for all waterbodies to achieve or maintain an overall status of ‘good’ by 2027 or over agreed timescales).
To do this, SEPA introduced a risk-based classification system in 2009, whereby natural and heavily modified/artificial water bodies are classified into one of five quality classes, comprising of ecological, hydromorphological, chemical and physical elements. SEPA identify improvement measures in order for failing water bodies to meet WFD objectives, where cost effective to do so.
The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended) (CAR)
The CAR is a primary tool for achieving the WFD objectives in Scotland. This legislation controls engineering works within inland water bodies, as well as point source discharges, abstractions and impoundments. There are three different levels of authorisation under CAR: General Binding Rules (GBR), Registration and Licence (either Simple or Complex); the level of regulation increasing with higher risk activities. For the proposed CTLR Project specific activities will require CAR authorisation, which must be obtained from SEPA prior to the start of construction to ensure adequate design and site measures are in place to protect water bodies. A ‘schedule’ of CAR activities was agreed with SEPA in December 2018 based on the design presented and included:
• engineering activities (e.g. temporary and permanent works associated with the River Tay Crossing Bridge, channel culverting and realignment, and other in-river works such as drainage outfalls and scour protection); • outfalls draining more than 1km of carriageway length; • road cuttings intercepting groundwater; and • a construction site licence (for a road greater than 5km in length or 4ha in area).
Although CAR consent is separate to EIA and obtaining planning permission, much of the information collated as part of the CTLR Specimen Design and EIA will be used to support the CAR licence applications, including reasons for selection of engineering options and rejection of alternatives (e.g. choice of river crossing structures). Further information is provided in Appendix 15.4 (Engineering in the Water Environment).
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Scottish Planning Policy (SPP)
SPP2 requires planning authorities to consider all sources of flooding and their associated risks when preparing development plans and reviewing planning applications. The predicted effects of climate change also need to be taken into account. The general position is to prevent development which would have a significant probability of being affected by flooding and/or would increase the probability of flooding elsewhere.
Due to the nature and scale of the proposed CTLR Project, crossing areas of flood risk cannot be avoided and SEPA specify that ‘essential transport infrastructure’ can be located in a flood risk area for operational reasons. The SPP risk framework for flood risk categorises some of the CTLR study area as ‘medium to high risk’ (annual probability of coastal or fluvial flooding is greater than 0.5% (1:200 years). SPP states that development in ‘medium to high’ risk areas may be suitable for:
“essential infrastructure designed and constructed to remain operational during floods and not impede water flow”.
The following technical guidance and resources were also utilised:
• DMRB Volume 11, Section 3, Part 10 (HD45/09): Road Drainage and the Water Environment (Highways Agency et al., 2009)3; • Technical Flood Risk Guidance for Stakeholders v10 (SEPA, 2018)4; • Flood Modelling Guidance for Responsible Authorities, v1.1 (SEPA, undated)5; • The SUDS Manual C753 (CIRIA, 2015); • SEPA’s Regulatory Method for SuDS (2017)6; • The Guidebook of Applied Fluvial Geomorphology (Sear et al., 2003)7; • Review of Impact Assessment Tools and Post Project Monitoring Guidance (Skinner and Thorne, 2005)8; • The Fluvial Design Guide (Environment Agency, 2010)9; and • CAR Practical Guide v8.3 (SEPA, 2019)10.
15.4.2 Approach to Assessment
Baseline information was collected and reviewed for each sub-topic (hydrology and flood risk, fluvial geomorphology and water quality) and was used to determine the sensitivity (value/importance) of each receptor (watercourse) (Table 15.2). Impact magnitude is influenced by the timing, scale, location, duration and likelihood of a potential impact occurring and potential change to the baseline conditions (which can be either adverse or beneficial) (Table 15.3).
2 Scottish Government (2014) Scottish Planning Policy. Accesed 01/03/2019 [https://www.gov.scot/publications/scottish-planning- policy/] 3 Highways Agency et al. (2009) HD 45/09: Design Manual for Roads and Bridges (DMRB), Volume 11, Section 3, Part 10, Road Drainage and the Water Environment, 2009. The Highways Agency, Scottish Executive Development Department, The National Assembly for Wales and The Department of Regional Development Northern Ireland. Accessed 15/04/2019 [http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol11/section3/hd4509.pdf] 4 SEPA (2018) Technical Flood Risk Guidance for Stakeholders – SEPA requirements for undertaking a Flood Risk Assessment. Version 10, July 2018. Accessed 01/03/2019 [https://www.sepa.org.uk/media/162602/ss-nfr-p-002-technical-flood-risk-guidance- for-stakeholders.pdf] 5 SEPA Flood Modelling Guidance for Responsible Authorities, Version 1.1. Accessed 01/03/2019 [https://www.sepa.org.uk/media/219653/flood_model_guidance_v2.pdf] 6 SEPA (2019) Regulatory Method (WAT-RM-08) – Sustainable Urban Drainage Systems (SUDS or SUD Systems), Version 6.3, March 2019. Accessed 01/03/2019 [https://www.sepa.org.uk/media/219048/wat-rm-08-regulation-of-sustainable-urban- drainage-systems-suds.pdf] 7 Sear, D.A. Newson, M.D. and Thorne, C.R. (2003) Guidebook of Applied Fluvial Geomorphology. R&D Technical Report FD1914. DEFRA/ Environment Agency. Flood and Coastal Defence R&D Programme. 8 Skinner, K. and Thorne, C.R. (2005) Review of Impact Assessment Tools and Post Project Monitoring Guidance. Report prepared for SEPA. Accessed 15/04/2019 [https://www.sepa.org.uk/media/152207/wat_sg_30.pdf] 9 Environment Agency (2010) The Fluvial Design Guide. Accessed 15/04/2019 [http://evidence.environment- agency.gov.uk/FCERM/en/FluvialDesignGuide/Fluvial_Design_Guide_Overview.aspx] 10 SEPA (2019) The Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended) - A Practical Guide. Version 8.3 February 2019. Accessed 01/03/2019 [https://www.sepa.org.uk/media/34761/car_a_practical_guide.pdf]
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Impact significance is a function of the sensitivity of a receptor and the magnitude of impact (before and after mitigation) (Table 15.4). These tables are based on DMRB HD45/09 guidance or other industry accepted guidance, and a sensitivity rating have been assigned to each receptor based on professional judgement. Where two alternatives are provided for the significance of effects, a single significance rating was chosen based on professional judgement.
Impact Assessment Criteria
Table 15.2: Criteria for assessing Baseline Sensitivity Sensitivity Criteria/Examples (value/importance) Hydrology and Flood Risk:
• Floodplain containing, or flood defence protecting, more than 100 residential properties from flooding. Floodplain containing nationally important infrastructure (e.g. hospitals, schools, care homes, emergency service stations), ecosystems or other uses of very high value.
Channel Morphology:
• The watercourse is in a natural state with no artificial modifications or morphological pressures. • The watercourse exhibits a wide range of features (e.g. riffles, pools, bar forms and a variety of natural bank profiles). • Rare stream types (e.g. active braided rivers). Very High Fluvial Processes and Sediment Regime:
Receptor has a high • Watercourse is in a state of equilibrium, with the sediment regime quality and rarity on reflecting the nature of the natural catchment and fluvial system. regional or national • Predominantly natural watercourse which displays a wide range of fluvial scale processes (e.g. erosion, deposition, varied flow types). • WFD morphology status of ‘High’.
Water Quality:
• WFD physico-chemical status of ‘High’, specific pollutants status of ‘Pass’. No identified pollutant pressures. • Water chemistry complies with published Environmental Quality Standards (EQS) for freshwaters. • Habitats and/or species protected under EU legislation (e.g. SAC, SPA, Ramsar site). • Designated salmonid waters under WFD. • Regionally important potable water supply. • Watercourse widely used for recreation, directly related to watercourse quality (e.g. salmon fishery, water sports)
Hydrology and Flood Risk:
• Floodplain containing, or flood defence protecting, between 11 and 100 High residential properties or commercial/industrial premises from flooding or
containing locally important infrastructure (e.g. electrical sub-stations, Receptor has a high major roads and railway lines), ecosystems or other uses of high value. quality and rarity on local scale Channel Morphology:
• The watercourse appears to be in a generally natural state with limited
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Sensitivity Criteria/Examples (value/importance) artificial modifications or morphological pressures. • The watercourse exhibits a range of geomorphological features (e.g. riffles, pools, bar forms, and a variety of natural bank profiles). • Where modifications have occurred, there is significant evidence of the river returning to its natural form. Fluvial Processes and Sediment Regime: • Watercourse has a sediment regime reflecting the nature of the natural catchment and fluvial system. • Watercourse displays a range of fluvial processes. • WFD morphology status of at least ‘Good’.
Water Quality:
• WFD physico-chemical status of at least ‘Good’, specific pollutants status of ‘Pass’. None or very limited pressures identified. • Water chemistry generally complies with EQS for freshwaters. • Habitats and/or species protected under EU or UK legislation, including SSSIs. • Designated salmonid/cyprinid waters under WFD. • Watercourse used for recreation and locally important potable water source.
Hydrology and Flood Risk:
• Floodplain or defence protecting 10 or fewer commercial/industrial properties from flooding, or with limited hydrological linkage to important ecosystems or other uses of high value.
Channel Morphology:
• The watercourse displays some geomorphic features (e.g. riffles, pools, bar forms). • The channel cross-section has been modified in places with obvious signs of changes to the channel morphology. Some natural recovery to the channel form may be evident (e.g. depositional features, bank Medium erosion).
Fluvial Processes and Sediment Regime: Receptor has a medium quality and rarity on • Watercourse has had significant modifications which have caused local scale notable alterations to the sediment transport pathways, sources and deposition areas. • Watercourse displays some natural fluvial processes, including varied flow types, however anthropogenic modifications have had an obvious impact on natural flow regime. • WFD morphology status of at least ‘Moderate’ or not classified by SEPA.
Water Quality:
• WFD physico-chemical status of at least ‘Moderate’, specific pollutants status of ‘Pass’ or not classified by SEPA. • Water quality likely to be affected by pollutant inputs or other pressures. • Could support a limited number of protected habitats or species. • Watercourse not generally used for recreation or water supply.
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Sensitivity Criteria/Examples (value/importance) Hydrology and Flood Risk:
• Watercourse or floodplain with limited constraints and a low probability of flooding of residential and commercial/industrial properties and containing no ecosystems or civil infrastructure.
Channel Morphology:
• The watercourse has been modified to an extent where a uniform, featureless channel has been created. • Channel displays very limited morphological diversity, with uniform banks and absence of bars. Low Fluvial Processes and Sediment Regime:
Receptor has a low • Highly modified sediment regime with little to no capacity for natural quality or rarity on local recovery. scale • Watercourse has a uniform flow type with minimal secondary currents and/or displays an unnatural flow regime. • Limited evidence of active fluvial processes. • WFD morphology status of ‘Poor’ or ‘Bad’ or not classified by SEPA.
Water Quality:
• WFD physico-chemical status of at least ‘Poor’ or ‘Bad’, specific pollutants status of ‘Fail’ or not classified by SEPA. • Water quality highly likely to be affected by pollutant pressures. • Supports no protected habitats or species. • Likely to be heavily modified or an artificial waterbody, including short road and field drains. • Likely to exhibit no flow during dry periods and not used for water supply.
Table 15.3: Criteria for assessing Impact Magnitude
Impact Magnitude Criteria/Examples
Hydrology and Flood Risk:
• Increase in peak flood level (0.5% annual probability) >100mm and/or significant increase in the extent of ‘medium to high risk’ areas, resulting in an increased risk of flooding to more than 100 residential/commercial properties. Major Adverse • Significant changes to the existing flow regime as a result of extensive changes to catchment and/or construction footprint. Results in the loss of attribute and/or quality Channel Morphology: and integrity of the attribute • Significant and extensive changes to the planform and/or cross-section of the channel, including modification of bank profiles and replacement of the natural bed within a reach or multiple reaches.
Fluvial Processes and Sediment Regime: • Significant shift from baseline conditions with potential to alter processes at the catchment scale or over multiple reaches.
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Impact Magnitude Criteria/Examples
• Significant impacts to the watercourse bed, banks and vegetated riparian corridor resulting in significant changes to sediment characteristics, transport processes, sediment load and turbidity. • Changes are likely to be irreversible and impacts are at the waterbody scale.
Water Quality:
• Major shift away from baseline conditions that may be long-term or temporary. Serious pollution risks from multiple in-channel works resulting in substantial/irreversible deterioration of the quality of existing water and effect on aquatic ecology. • Failure of both soluble and sediment-bound pollutants in HAWRAT and compliance failure with EQS values. • Calculated risk of pollution from a spillage >2% annually.
Hydrology and Flood Risk:
• An increase in peak flood level (0.5% annual probability) >50mm resulting in an increased risk of flooding to 11-100 residential/commercial properties. • Moderate changes to the existing flow regime as a result of changes to catchment and/or construction footprint.
Channel Morphology:
• Moderate changes to channel planform and/or cross section of a reach (e.g. modification to bed and bank profiles, new embankments, etc.)
Moderate Adverse Fluvial Processes and Sediment Regime: • A shift from baseline conditions with the potential to significantly alter Results in effect on processes over a reach or moderate change over multiple reaches. integrity of attribute, or • Moderate changes and impacts to watercourse bed, banks and loss of part of attribute vegetated riparian corridor resulting in some changes to sediment characteristics, transport processes, sediment load and turbidity.
Water Quality:
• Moderate shift from baseline conditions that may be long-term or temporary. Pollution risks from in-channel works or works in close proximity to bank resulting in partial deterioration in the quality of existing water and effect on aquatic ecology. • Failure of both soluble and sediment-bound pollutants in HAWRAT but compliance with EQS values. • Calculated risk of pollution from a spillage >1% annually and <2% annually.
Hydrology and Flood Risk: Minor Adverse • Slight changes to the flow regime and/or increase in the extent of Results in some ‘medium to high risk’ areas. Increase in peak flood level (0.5% annual measurable changes in probability) >10mm, resulting in an increased risk of flooding to less than attributes quality or 10 residential/commercial properties. vulnerability Channel Morphology:
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Impact Magnitude Criteria/Examples
• Localised modifications to channel planform and/cross section (e.g. installation of outfalls).
Fluvial Processes and Sediment Regime: • Minimal shift from baseline conditions with impacts localised to the reach scale. • Limited changes and impacts to watercourse bed, banks and vegetated riparian corridor resulting in limited (but notable) changes to sediment characteristics, transport processes, sediment load and turbidity.
Water Quality:
• Minor shift away from baseline conditions, generally temporary in nature. Measurable deterioration in the quality of the water resulting from in- channel or bankside works but of limited duration and extent. • Failure of either soluble or sediment-bound pollutants in HAWRAT. • Calculated risk of pollution from a spillage >0.5% annually and <1% annually.
Hydrology and Flood Risk:
• Negligible change in peak flood level (0.5% annual probability) <+/- 10mm. • Negligible change in the extent of “medium to high risk” areas (SPP) and negligible changes to existing flow regime. Negligible Channel Morphology, Fluvial Processes and Sediment Regime: Results in effect on • Minimal or no measurable change from baseline conditions which is attribute, but of barely distinguishable. insufficient magnitude • Any changes are highly localised and have no effect on a reach scale. to affect the use or integrity Water Quality:
• Imperceptible change to water quality or aquatic ecology. • No risk identified by HAWRAT (Pass both soluble and sediment-bound pollutants). • Risk of pollution from a spillage <0.5%.
Table 15.4: Criteria for assessing Impact Significance* Magnitude of Impact Sensitivity Major Moderate Minor Negligible Large / Very Sight / Moderate / Very High Large Neutral Large Large High Large Moderate / Large Slight / Moderate Neutral
Medium Moderate / Large Moderate Slight Neutral
Low Moderate Slight / Moderate Neutral Neutral *Shaded cells represent significant effect
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15.4.3 Specific Methodologies
Detailed baseline and impact assessment methodologies for hydrology and flood risk, fluvial geomorphology and water quality are outlined below.
Hydrology and Flood Risk
An FRA has been undertaken in accordance with DMRB HD45/09 Methods E and F (Assessing Flood Impacts), as well as SEPA’s Technical Flood Risk Guidance for Stakeholders and Scottish Planning Policy. Refer to Appendix 15.2: CTLR Flood Risk Assessment Report.
The FRA considered hydrology and flood risk from a variety of sources within 1km of the footprint of the CTLR Project, including river, surface water, groundwater and reservoir flooding. This was informed by an initial desk-study of existing data including the SEPA Flood Maps 11 . These provide indicative mapping of the baseline flood risk from river, surface water and coastal flooding and at a range of likelihoods 12 (low, medium and high), and was used to review the existing flood risk in the area. Consideration was also given to minor watercourses (with catchment areas less than 3km2; the threshold used for SEPA online mapping).
Flood risk from surface water (pluvial), groundwater and reservoirs were also assessed based on available mapping, historic flood records and by qualitative assessment. The British Geological Society (BGS) “Susceptibility to Groundwater Flooding Maps” were purchased for the study area to inform this.
Detailed hydraulic modelling was undertaken for the River Tay, the River Almond, as well as three minor watercourses: the Cramock Burn, Bertha Loch Burn and Broxy Kennels Drain. These were identified as potential sources of flood risk based on a review of the SEPA Flood Maps. They will also be directly intercepted by the proposed CTLR Project and will require the installation of hydraulic structures, including the new River Tay Crossing Bridge and culverts. The models were used to refine the flood risk information and assess the existing baseline flood conditions for the 0.5% Annual Exceedance Probability (AEP) (1:200 year return period) design flood event. Post-development modelling was then undertaken to inform the design of hydraulic features and to quantify the impact of the CTLR Project on flood risk and identify the need for any mitigation.
SEPA’s Technical Flood Risk Guidance for Stakeholders recommends that an allowance/uplift of 20% should be added to the 200-year peak flow estimate to account for climate change. This has been included in this assessment, hence all references to the 0.5% AEP (1:200) design scenario flood events in this chapter incorporate this allowance.
For the River Tay, a previous 1D hydraulic model was reviewed and updated using ISIS modelling software. The model extends for approximately 53km along the River Tay, from the vicinity of Ballathie Hotel to Dundee Harbour, as well as a 4km stretch of the River Almond and 23km of the River Earn upstream of their confluences. The cross-section data on which the model was based was outdated, therefore bathymetric and topographic surveying was commissioned and built into the model to improve the representation and accuracy of the river reach close to the proposed River Tay Crossing Bridge. Several cross- sections were extended using LIDAR data (Scotland Phase 1 (2011-2012) data) where the current extent was considered insufficient. Furthermore, the tidal downstream boundary of the model was also updated to incorporate climate change, and more detailed ‘roughness’ (friction) values were applied.
Three 1D-2D flood models were also constructed for reaches of the Cramock Burn, Bertha Loch Burn and Broxy Kennels Drain and their associated floodplains, using Infoworks ICM (v9.0) modelling software. These were constructed using topographic survey data, supplemented using LiDAR where necessary. Existing hydraulic structures and sediment depths were represented in the 1D channel
11 SEPA Flood Risk Management Maps. Accessed 15.04/2019 [http://map.sepa.org.uk/floodmap/map.htm] 12 The chance of a flood occurring: High likelihood (1:10 year or more frequent); Medium likelihood (between 1:10 and 1:200 year) and Low likelihood (200 year or less frequent)
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where these could impact flows (either high or low). Dimensions of hydraulic structures were informed by a commissioned culvert inspection survey.
These watercourses are all ungauged and therefore a variety of techniques to estimate inflows were used (including FEH, ReFH2 and IOH124). These were in line with SEPA’s Technical Flood Risk Guidance, and sensitivity testing of parameters such as roughness and inflows was undertaken to ensure that predictions were suitably robust. Simulation of the 0.5% AEP (1:200 year return period) flood event for both the baseline and post-development scenarios provided information on:
• in-channel flows; • flow velocities; • flood inundation extents; and • changes in water depths and levels downstream of the CTLR Project.
Flood maps are presented within the FRA Report (Volume 2, Appendix 15.2) along with full details of the methodology used to obtain the estimate of existing peak water levels (within the river channel) at the locations of interest and the maximum predicted flood extent for the design event (1:200 year including climate change).
Fluvial Geomorphology
Since the DMRB does not provide a methodology for assessing impacts on fluvial geomorphology, impact assessment criteria have been developed using industry-accepted methods including Sear et al. (2003), Skinner and Thorne (2005) and Environment Agency (2010).
The baseline assessment for fluvial geomorphology was informed by both a desk-top assessment and a site visit. Baseline information related to fluvial geomorphology was collected from a range of sources including:
• bedrock and superficial geology mapping; • historic OS mapping; • SEPA WFD monitoring data; and • other publicly available reports related to the watercourses in the study area.
The site visit in May 2018 used standard river reconnaissance methods and forms, as detailed in Skinner and Thorne (2005), although the forms were digitised in an ArcGIS survey app (Esri Collector) to aid data collection and processing. The survey provided a better understanding of the existing morphological features and sediment/fluvial processes within the watercourses in the study area. This informed the understanding of the baseline environment, and the resulting potential changes and vulnerability for channel change as a result of the CTLR Project.
Potential construction and operational impacts were assessed qualitatively based on the indicative in- channel works and activities known at this stage and expert judgement used to determine vulnerability to morphological change.
Water Quality
The water quality baseline assessment was informed by WFD data obtained from SEPA’s Water Environment Hub Interactive Map13. SEPA also supplied monthly monitoring results covering several years up to May 2018 for a range of chemical parameters at two monitoring locations:
• River Tay at Queens Bridge (downstream of study area east of Perth; NGR NO 1215 2322); and • River Almond at Almond Bridge (within study area; NGR NO 0941 2655).
13 SEPA (2018) Water Environment Hub (2018) Accessed 15/04/2019 [https://www.sepa.org.uk/data-visualisation/water-environment-hub/]
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Where no SEPA monitoring data was available for the smaller watercourses, a review of current and historic surrounding land-uses was undertaken to infer water quality and any potential sources of pollution that could influence existing water quality.
Surface water sampling data was collected on one occasion from upstream and downstream locations on the watercourses within the study area during the ground investigation works in 2018. The concentrations were compared against Environmental Quality Standard (EQS) screening values for freshwaters which provided in-situ chemistry data to augment the WFD information on SEPA’s online database (or provided information to infer water quality where no SEPA monitoring data were available).
Potential construction impacts on water quality were assessed qualitatively based on expert judgement. The assessment also considered potential for likely significant effects on the qualifying features of the River Tay SAC (refer to Chapter 9: Biodiversity).
To assess potential operational impacts on water quality as a result of road traffic and the requirement for level/types of SuDS, the following assessment methods were undertaken as prescribed in CIRIA C753 and DMRB HD45/09 guidance (and agreed with SEPA):
• A9 trunk road (realigned A9 carriageway and grade-separated junction) – as per CIRIA guidance, trunk roads and motorways have a high pollution hazard level and therefore the risk assessment process set out in DMRB HD45/09 should be followed. This includes Methods A, B and D (refer to Volume 2, Appendix 15.3: Water Quality Calculations for further information):
− Methods A and B: Effects of Routine Runoff on Surface Waters - estimate the magnitude of potential short term and longer-term impacts to water quality associated with discharge of operational road drainage. Calculated concentrations of specific elements are compared against freshwater pollutant thresholds and EQS values to assess compliance with the WFD. This was undertaken using the Highways Agency Water Risk Assessment Tool (HAWRAT). − Method D: Pollution Impacts from Accidental Spillages - estimate the probability of an accidental spillage from a heavy goods vehicle (HGV) leading to a serious pollution incident.
• Remainder of the CTLR carriageway from the River Tay Crossing Bridge to the tie-in at the A94 junction – this type of carriageway has a medium pollution hazard level and therefore it is considered appropriate to use CIRIA’s Simple Index Approach. For this approach, the combined pollutant mitigation index for the SuDS components must exceed the pollution hazard index for the type of land-use (in this case a non-trunk road with medium hazard), for the proposed SuDS mitigation to be considered acceptable.
The above methods informed the number and type of SuDS measures required to drain the new carriageway to protect receiving watercourses and has been discussed with SEPA.
15.4.4 Assumptions and Limitations
The minor watercourses within the study area are ungauged and historic flood data was sparse for the study area, and therefore it was not possible to calibrate or validate these models. However, a conservative approach was adopted, and industry-accepted methods applied to each of the modelled watercourses to allow for comparison of flow estimates, as recommended in SEPA’s guidance. Similarly, a sensitivity analysis was undertaken on a range of key parameters to provide confidence in the accuracy of the modelling outputs.
The assessment relied on assumptions about the methods of working for in-channel and bankside works. More specific details on in-channel works and construction techniques will be provided by the appointed Contractor, as well as locations of construction compounds and laydown areas. However, indicative locations and sizes of these features have been included in the site layout plan to allow for a robust assessment of the potential impacts from construction in the EIA.
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Chemistry sampling of watercourses only provided a snapshot of water quality conditions rather than a longer-term trend. This sampling data was only used to augment the longer-term classification data available on SEPA’s online database or other desk-based analysis and is therefore not considered a limitation to the assessment.
15.5 BASELINE CONDITIONS
There are eleven watercourses within the RDWE study area ranging from large rivers such as the River Tay to minor watercourses and small field/forestry drains (see Figure 15.1).
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Legend Proposed CTLR Project ± ! Outfall Location SuDS Mitigation 500m Study Area Watercourse and Flow Direction Waterbody Water Framework Directive (WFD) Status WFD (2017)
Outfalls into existing Paddocks Culvert River Tay (R Isla to R Earn Confluences; SEPA ID:6498) Overall status: Good Morphology status: Good Physico-chemical status: High Specific Pollutants status: Pass
! R ed go ! rt D on ra in Outfall into existing Outfall into existing Outfalls into Broxy Culvert Broxy Railway Culvert Whiggle Burn
n 0 0.5 1 le Bur Whigg Km ! ! Broxy Outfall into Kennels ! Sheriffton Wood Outfalls into Sheriffton existing ditch Drain ! Drain Burn realignment ! ! ! ! ! mock Burn Cra ! ! ! Outfall into Outfall into van Drain Cara Highfield ! Annaty Burn existing Cundy Plantation Drain urn Outfalls into ! mock B Bertha Loch ! Cra !! Cramock Burn Burn Outfalls to Caravan Drain ! n Outfall into r Contains OS data © Crown u
Outfall into existing ditch B Copyright and database right
y Bertha Culvert R t 2019 iv a er n n 04/11/2019 Ta P01 For Information JM DR !! y Outfalls into A existing drainage Rev. Rev. Date Drawing Suitability Drawn Appr'd d Almon along Highfield Track River Outfalls into River Almond Sweco, Suite 4/2, City Park, 368 Alexandra Parade Glasgow, G31 3AU, Tel: +44 (0)141 414 1700
Client
River Almond (R East Pow to R Tay Confluences; SEPA ID:6506) Project Overall status: Good Morphology status: Good Cross Tay Link Road Physico-chemical status: High Specific Pollutants status: Pass Drawing Title Figure 15.1 Water Environment and Mitigation
Scale @ A3 1:22,500 Project No. 119046 Status S2 BIM No. 119046-SWECO-EGN-000-DR-GS-20060
This drawing should not be relied on or used in circumstances other than those for which it was originally prepared and for which Sweco UK Limited was commissioned. Sweco UK Limited accepts no responsibility for this drawing to any party other than the person by whom it was commissioned. Any party which breaches the provisions of this disclaimer shall indemnify Sweco Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2019 OS 100016971. Use of this data is subject to terms and conditions. Contains public sector information licensed under the Open Government Licence v3.0. UK Limited for all loss or damage arising therefrom.
CHAPTER 15 CROSS TAY LINK ROAD ROAD DRAINAGE AND THE WATER EIA REPORT (VOLUME 2) ENVIRONMENT
An overview of the watercourses considered in this assessment is outlined in Table 15.5.
Table 15.5: Overview of Watercourses Major Watercourse Minor Watercourse Field/Forest Drain
• Bertha Loch Burn • Broxy Kennels Drain • Highfield Plantation • River Tay (SAC) • Redgorton Drain Drain (multiple) • River Almond • Cramock Burn • Sheriffton Wood Drain • Gelly/Whiggle Burn • Caravan Drain • Annaty Burn
The proposed CTLR Project does not directly cross the River Almond (with the exception of a temporary bridge if a southern haul road (Option 1) to access the west abutment of the River Tay Crossing Bridge is taken forward), Redgorton Drain, Gelly/Whiggle Burn, Annaty Burn or the minor Caravan Drain. However, these watercourses have been included in the assessment because they are:
• hydrologically-linked; • proposed to receive operational road runoff from the completed carriageway; and • tributaries of the River Tay which could result in cumulative effects.
Ecological designations (refer to Chapter 9: Biodiversity for more information) and other sensitivities associated within the study area include:
• River Tay and its key tributaries (including the River Almond) is designated as a SAC under the EU Habitats Directive for its populations of Atlantic salmon, lamprey species, freshwater pearl mussel (FWPM) and otter, as well as being a sensitive freshwater habitat. • River Tay (and key tributaries) are designated salmonid waters under the WFD. • The study area is located within a designated surface water Drinking Water Protected Area (DWPA) under the WFD and the River Tay is an important water supply source for Scottish Water.
There are a number of existing CAR licences on the watercourses in the study area, including consented discharges to, and abstractions from, the River Tay and River Almond, including:
• discharges from Almondbank Wastewater Treatment Works (WWTW) (NGR NO 0690 2560) and Scone WWTW (NGR NO 1190 2530), as well as private properties and landholdings; • abstractions for hydropower and irrigation for agriculture; and • a TDSFB abstraction for fish production (NGR NO 0555 2706).
Site investigation data has shown elevated concentrations of heavy metals have been recorded widely in the groundwater across the study area. Refer to Chapter 10: Hydrogeology and Soils for further information.
Table 15.6 summarises the sensitivity of each watercourse for the RDWE sub-topics within the study area, drawing on the sensitivity criteria outlined in Table 15.2. Baseline conditions are described in more detail in Volume 2, Appendix 15.1.
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Table 15.6: Summary of Watercourse Sensitivity Watercourse Baseline Description Sensitivity Hydrology and Flood Risk • Long history of flooding dating back to 1800s causing significant damage to property within Perth and disruption to transport routes. • Extensive flood defences, which protect residential/commercial properties within areas of Very High Perth. • Floodwaters predicted to be contained within the banks of the river in the location of the proposed River Tay Crossing Bridge for the 0.5% AEP (1 in 200-year return period) flood event. River Tay
Fluvial Geomorphology • Wide actively-meandering river sits on wide fluvial plain. • WFD morphology status: ’Good’ in 2017, minimal morphological pressures on the reach. High • Main process is sediment transportation. Evidence of moderate erosion on the left bank which has been remedied by the installation of a deflector vein. On the right bank, some erosion and undercutting of the bank toe were observed.
Water Quality • WFD overall status: ’Good’, Physico-Chemical status: ‘High’ and Specific Pollutants status: ’Pass’ (2017). • Water sampling in 2017-18 indicated concentrations of metals and polycyclic aromatic hydrocarbons (PAHs) generally all fell within EQS for freshwaters. • Designated SAC and salmonid waters; supports European protected species including Very High Atlantic salmon, lamprey species, otter and freshwater pearl mussel (FWPM). Good water quality for sensitive species. • Study area located within a designated drinking water protected area (DWPA) and close to a Scottish Water abstraction. • River is important for recreation, particularly salmon angling.
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Watercourse Baseline Description Sensitivity Hydrology and Flood Risk • Long history of flooding. • SEPA flood mapping shows flooding along the southern bank of the River Almond, resulting in extensive flooding (>100 Very High River Almond residential/commercial properties) within Perth to the west of the Highland Mainline Railway. • Extensive system of flood defences, which protect areas of Perth.
Fluvial Geomorphology • Moderate sized river in a narrow valley with pool-riffle typology and uniform/tranquil flow. • WFD morphology status: ’Good’ in 2017. High • Evidence of geomorphic processes such as erosion and deposition. There are several morphological pressures on the channel such as scour protection on banks and bed around bridges.
Water Quality • WFD overall status: ‘Good’, Physico-Chemical status: ‘High’ and Specific Pollutants status: ’Pass’ (2017). • Water sampling in 2017-18 indicated concentrations of metals and PAHs generally all fell within EQS for freshwaters. High • Designated SAC and salmonid waters (associated with the River Tay). Evidence of breeding salmon and lamprey and supports foraging otter.
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Watercourse Baseline Description Sensitivity Bertha Loch Burn Hydrology and Flood Risk • Not shown on SEPA flood mapping. • Baseline modelling predicts significant flooding along the southern bank upstream of the existing A9, resulting in Medium increased flooding onto the A9 road at the junction with Bertha Park access road for the 0.5% AEP (including climate change) flood event.
Fluvial Geomorphology • Narrow channel, which has been artificially straightened for Low agricultural drainage. • Uniform dimensions and displays few geomorphic features.
Water Quality • Not monitored by SEPA. Surrounding land-use mainly woodland and arable farmland and crossed by the A9 and Highland Mainline; may receive intermittent point source Medium pollutants and diffuse agricultural runoff. • No protected aquatic species recorded; however, considered to be suitable habitat to support salmonids and lamprey in the lower reaches.
Broxy Kennels Drain Hydrology and Flood Risk • Not shown on SEPA flood mapping. Low • Baseline modelling predicts that the extent of flooding upstream of the existing A9 culvert is limited, and no residential/commercial receptors are affected. Fluvial Geomorphology • Small, straightened, over-deepened channel with uniform Low dimensions throughout its length. • The drain displays few geomorphic features. Water Quality • Not monitored by SEPA. • Likely to flow intermittently and does not support any protected Low aquatic species. • Very low pollutant dilution and sediment dispersal capacity.
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Watercourse Baseline Description Sensitivity Hydrology and Flood Risk • Baseline modelling shows no flooding on the north bank (which is a refinement of the indicative SEPA flood mapping which identified flooding in this location, which was accepted by SEPA). • Flooding predicted to occur along the southern bank at three Medium Cramock Burn locations between the Stormontfield Road Bridge and upstream of the culvert adjacent to Scone Camping and Caravanning
Club. Floodwaters flow southwards along the eastern perimeter of Perth Racecourse affecting two commercial properties and a small path.
Fluvial Geomorphology • Burn has been straightened and over-deepened, and displays few geomorphic features along the northern perimeter of Perth Racecourse Low • Closer to River Tay confluence, burn appears more natural and displays some morphological diversity. • culverted in several locations, with some hard bank protection
Water Quality • Not monitored by SEPA. Surrounding land-use mainly arable farmland, crossed by A93 and Stormontfield Road; may receive intermittent point source pollutants and diffuse agricultural runoff. • Water sampling indicated slightly elevated concentrations of copper and ammonia at downstream sampling site, may be due Medium to agricultural pollution, untreated sewage and/or decay of plant and animal material, which can result in an increase in algal growth and is toxic to aquatic species. • Surveys downstream of Perth Racecourse showed potential presence of lamprey and evidence of otter, but it generally lacks suitable habitat for other fish species. • Low pollutant dilution and sediment dispersal capacity.
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Watercourse Baseline Description Sensitivity Hydrology and Flood Risk • Not shown on SEPA flood mapping. Caravan Drain • Represented within the Cramock Burn hydraulic model. Low • For the 0.5% AEP (plus climate change) flood event, the drain was not predicted to overtop and did not affect any commercial/residential receptors nearby.
Fluvial Geomorphology • Straightened drainage ditch with relatively uniform channel Low dimensions and limited geomorphic features throughout length.
Water Quality • Likely to flow intermittently and does not support any protected aquatic species. Low • Very low pollutant dilution and sediment dispersal capacity.
Sheriffton Wood Drain Hydrology and Flood Risk • Not shown on SEPA flood mapping. • No previous incidents of flooding identified and drain Low located in isolated area surrounded by agricultural land only.
Fluvial Geomorphology • Relatively straight ditch with uniform channel dimensions Low and likely to be ephemeral.
Water Quality • Drain is likely to flow intermittently and does not support any protected aquatic species. Low • Very low pollutant dilution and sediment dispersal capacity.
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Watercourse Baseline Description Sensitivity Redgorton Drain Hydrology and Flood Risk • Not shown on SEPA flood mapping. • No previously recorded incidents of flooding and the Low surrounding area is largely rural, with no sensitive residential/commercial receptors at risk.
Fluvial Geomorphology • Narrow and shallow drain with limited geomorphic features Low and few modifications or channel pressures.
Water Quality • Not monitored by SEPA; may receive intermittent road runoff pollutants. Low • Low pollutant dilution and sediment dispersal capacity and does not support any protected aquatic species. Hydrology and Flood Risk • SEPA flood mapping indicates the 0.5% AEP flood extent would be limited, generally to the bank edge. Whiggle/Gelly Burn • The culvert at Langedge Bridge was in good condition with Low low blockage risk, and presence of a high river terrace along the southern bank would contain any out-of-bank flooding.
Fluvial Geomorphology • Small gravel and cobble stream bed, with intermittent steep but short valley sides. Some bedrock exposed at the base of the channel banks. Low • Channel has sinuous planform with some alternating depositional gravel and cobble bars, and some evidence of bank erosion.
Water Quality • Not monitored by SEPA. The burn may receive nutrient- rich runoff from agriculture and woodland soils, as well as Medium intermittent road runoff inputs. • Low pollutant dilution and sediment dispersal capacity and not known to support any protected aquatic species.
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Watercourse Baseline Description Sensitivity
Hydrology and Flood Risk • SEPA flood mapping indicates the 0.5% AEP flood extent would be limited to the bank edge. • The burn is close to the A94 road, but this road remains outside of the 0.1% AEP (1:1000 year) SEPA Low flood extent. • Reported incidents of flooding within Scone in 2004 Annaty Burn affecting properties along Den Road, however this is over 2km downstream of the study area.
Fluvial Geomorphology • Small to moderate cobble and gravel bed stream. Medium Mostly natural with few modifications and has riffle glide morphology.
Water Quality • Not monitored by SEPA. May receive intermittent point source pollutants from roads and diffuse agricultural runoff. • Sampling showed slightly elevated concentrations of ammonia, copper and zinc against published EQS at Medium both sampling sites, which may be due to pollutants contained in upstream agricultural and road runoff. • Low pollutant dilution and sediment dispersal capacity. • Not known to support any protected aquatic species, but there is evidence of otter using the burn.
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Watercourse Baseline Description Sensitivity
Hydrology and Flood Risk • Not shown on SEPA flood mapping. Highfield Plantation Drain • No previous incidents of flooding identified and drain Low located in isolated area surrounded by woodland with no sensitive residential/commercial receptors nearby.
Fluvial Geomorphology • Several shallow, straight drainage channels which mainly contain leaf litter. Some of the channels Low contained standing water at the time of the site visit but no discernible flow.
Water Quality • Not monitored by SEPA. May receive intermittent nutrient-rich runoff from surrounding woodland. Low • Drains likely to be ephemeral and have a very low pollutant dilution and sediment dispersal capacity. • Do not support any protected aquatic species.
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15.5.1 Existing Drainage
The existing A9 within the study area is currently drained through kerbs and gullies which connect into filter drains within the verge. The filter drains then discharge into various outfalls including the River Almond, Bertha Loch Burn and Broxy Kennels Drain, which then ultimately flows into the River Tay. Due to the gullies being connected to the filter pipes, there is minimal treatment achieved of any contaminated runoff other than in the sump of the gully.
Drainage from other roads including the A93 and A94 is generally either over-the-edge (i.e. no treatment and infiltration into surrounding soil) or is conveyed to nearby watercourses via gullies.
15.6 POTENTIAL EFFECTS
This section describes the potential effects on the water environment in the absence of mitigation during the construction and operational phases of the proposed CTLR Project.
Generic construction impacts are described first that could affect all watercourses, followed by any key construction effects that could potentially occur on specific watercourses. This process is then followed for operational effects, including an assessment of the operational HAWRAT routine runoff and accidental spillage calculations in the water quality section.
A summary of the specific construction works/activities in or near each watercourse, that are known at this stage or can be reasonably assumed, is provided in Table 15.7. Refer to Chapter 2: Project Description for further information on the key engineering works/activities of the proposed CTLR Project. Further details and justification for the watercourse engineering works (including new and replacement culverts) is provided in Appendix 15.4 (Engineering in the Water Environment).
The EIA considered three potential temporary options for accessing the west abutment of the River Tay Crossing Bridge during construction (Option 1 – 3, as outlined in Chapter 2: Project Description). The southern access option (Option 1) will require a temporary bridge crossing of the River Almond as well as a minor crossing of Bertha Loch Burn. Access from the west (Option 2) requires a minor crossing of Bertha Loch Burn only, whilst access from the north requires no temporary watercourse crossings.
Table 15.7: Summary of Construction Activities in/near each Watercourse
Watercourse Proposed Construction Works • Extensive works close to banks including earthworks and road construction associated with the realigned A9(T), grade separated junction, River Tay Crossing Bridge piers/abutments and CTLR carriageway • Works in floodplain including east pier of River Tay Crossing Bridge • 2 main construction compounds – i) west of A9 interchange; ii) immediately east of the River Tay Crossing Bridge or adjacent to Stormontfield Road roundabout River Tay • River Tay Crossing Bridge spanning river (piers out with the normal channel but in the floodplain) - works including cofferdam installation, piling and pier construction on both banks, rip-rap scour protection to prevent undermining of pier foundations • Temporary haul route to River Tay Crossing Bridge west pier – two alternatives (Option 1 and Option 3) • No direct outfalls but cumulative effects from various construction works in tributaries (see below)
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Watercourse Proposed Construction Works • Temporary haul route to River Tay Crossing Bridge west pier (south access) requires a temporary bridge over River Almond if this access option is taken forward. North bridge abutment sheet-piled, south River Almond abutment reinforced earth, both temporary abutments in the floodplain. Scour protection boulders at abutments to prevent undermining (if needed) • 2 drainage outfalls
• 1 new box culvert under realigned A9 (approx. 70m long) • 1 replacement box culvert (approx. 10m long) (replacing twin 900mm pipes) under existing A9 Bertha Loch Burn • Temporary haul route to River Tay Crossing Bridge west pier (south and west access options) requires a temporary crossing of burn • 1 drainage outfall
• 1 new box culvert and channel realignment (under realigned A9 and 2 Broxy Kennels Drain slip roads) (approx. 200m long) • 2 drainage outfalls
• 1 replacement box culvert (Stormontfield Road culvert) (approx. 10m Cramock Burn long) - replacing masonry arch structure (including ‘Caravan • 6 direct/indirect drainage outfalls including: 2 no. from Stormontfield Drain’ and a minor drain Road upgrade, 3 no. to ‘Caravan Drain’ and a ditch at Balboughty at Balboughty Cottages before flowing shortly downstream to Cramock Burn Cottages) • A SuDS Wetland Area feature (refer to Section 15.7.2 and Chapter 2: Project Description for more details)
• New 900mm pipe culvert under CTLR carriageway (and 60m of Sheriffton Wood Drain realigned channel) • 2 drainage outfalls
Redgorton Drain • 2 drainage outfalls (outfall into existing Paddocks Culvert)
Whiggle Burn • 2 drainage outfalls
• 1 drainage outfall Annaty Burn • Potential satellite compound at A94 roundabout
• 2 drainage outfalls Highfield Plantation • 2 new pipe culverts under CTLR carriageway and several other drains Drain will be intercepted by cut-off ditches • Potential satellite compound in vicinity of drains
15.6.1 Generic Construction Impacts
The potential risk of pollution and siltation is highest during the construction phase when there is most activity on site, particularly in-channel works and works in close proximity to channel banks. The construction programme is anticipated to be around 2.5 years of which the major infrastructure works on the west and east banks of the River Tay, including construction of the River Tay Crossing Bridge, is assumed to take the full construction period.
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Hydrology and Flood Risk
• Soil compaction from works traffic may reduce soil infiltration capabilities and increase surface water runoff from temporary access routes, construction compounds and other temporary construction areas. This has the potential to increase localised ponding and/or lead to uncontrolled discharge to nearby watercourses. This potential impact is likely to be greatest for watercourses with smaller catchments and is therefore not predicted to result in a significant change to the flow regime of the River Tay and its larger tributaries. • Discharge of construction site drainage may have an impact on watercourse flows and the sediment regime of the receiving watercourse, and in turn affect channel capacities and flow conveyance downstream. • Temporary increase in fluvial flood risk as a result of construction activity occurring in the floodplain, particularly during large rainfall events, due to loss of floodplain storage. Temporary construction plant/materials may increase risk of flooding by causing a flow restriction/blockage if not properly secured during flood events. • Crossings of smaller watercourses will likely require diversions and/or temporary cofferdams with over-pumping to provide a dry working area for construction, which may result in temporary narrowing of the channel and constrictions in flow conveyance. This could result in short-term changes to existing flow patterns and velocities. • Inadequate or inappropriate temporary drainage provision may increase surface water (pluvial) flood risk.
Fluvial Geomorphology
• Construction activities in or in close proximity to watercourses, such as drainage outfalls and new culverts, have potential to result in the release of fine sediment into channels. This release of fine sediment can potentially lead to an increase in turbidity and siltation may occur. Siltation can cause a decrease in the morphological diversity of the river bed since features such as pools can become smothered by silt deposition. This will impact the ecology of gravel bed channels due to loss of habitat. • Loss of morphological diversity and increased fine sediment supply can occur due to vegetation/tree removal on the river banks. Clearance of vegetation from the river banks can lead to bank instability due to increased erosion of the exposed banks, and loss of vegetation roots which bind the bank together adding stability. This can lead to disturbance of the existing bank profiles and addition of fine sediment to the channel. Vegetation loss can also result in loss of morphological diversity due to loss of tree roots and a reduction in the addition of woody debris to the channel. Woody debris and root intrusion into the channel encourage the formation of geomorphic features such as riffles and pools. • Channel realignment has high potential to result in increases to the fine sediment supply in watercourses, particularly during the initial formation of the channel when it is free of vegetation, as there will be increased rates of channel scour. Rates of scour may increase further if the realigned channel is straightened (and therefore has an increased stream gradient). If the new channel dimensions and planform are not designed appropriately, channel instability may be triggered. Additionally, morphological diversity may be lost since bedforms which have developed over long time periods may be destroyed.
Water Quality
Construction activities have the potential to cause water pollution and have short-term as well as long- term chronic adverse effects on watercourses, groundwater and aquatic ecology.
• Site clearance and ground preparation works for temporary and permanent infrastructure including soil stripping, tree/vegetation removal, compound preparation, earthworks, excavations and stockpiling of soils can result in mobilisation and release of suspended solids to watercourses. Suspended solids found in construction site runoff can lower the chemical and ecological quality of watercourses. Severe weather events, resulting in flooding during the construction period have the potential to exacerbate the release of fine sediment into the watercourses and could result in collapse of stockpiles.
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• Previously unidentified contamination/contaminated soils can be disturbed due to construction works, and in particular deep excavations (e.g. large road cuttings and bridge foundations), or the mobilisation of contaminants due to groundwater pumping. Contamination can pollute watercourses (and groundwaters) and can be toxic to aquatic species. • Construction compounds and other works locations near watercourses may include potentially polluting activities such as concrete pouring, cement mixing, vehicle washing, re-fuelling and oil/fuels transport and storage, which can result in toxic pollutants entering watercourses. In- channel works or works at the bank edge present the greatest risk due to direct entry of pollutants and spillages, which can result in acute pollution incidents. Toxic elements can be transported downstream or build up on the river bed affecting water quality and aquatic habitats. • Spillages of concrete and unset cement into watercourses (and groundwaters) can increase alkalinity, presenting a high risk to aquatic species which can be sensitive to changes in water pH. • Organic waste inputs through damage to services or unsatisfactory disposal of sewage from site welfare facilities can pollute watercourses and groundwater leading to an increase in algae and oxygen deficits. • The capacity of a watercourse to dilute pollutants and disperse sediments (based on size and flow) can act to either lessen or worsen the potential effects of silt-laden or polluted runoff entering watercourses.
15.6.2 Specific Construction Impacts
An assessment of predicted construction effects on watercourses, based on the activities in Table 15.7 are summarised below, for each of the RDWE sub-topics.
For the purposes of this assessment, the proposed drainage outfalls to the ‘Caravan Drain’ and a minor drain at Balboughty Cottages are included in the assessment of the Cramock Burn, since the outfalls are positioned a short distance upstream of the Cramock Burn.
River Tay
Hydrology and Flood Risk
Temporary work/development on the banks and in the floodplain of the River Tay could result in minor changes to the existing flow regime. The main works in the 0.5% AEP (1:200 year) floodplain relates to construction of the east pier of the River Tay Crossing Bridge, which is very small compared to the catchment of the River Tay, and therefore is predicted to result in a negligible flood risk impact. Temporary potential effects on hydrology and flood risk of the River Tay (very high sensitivity) are predicted to be of negligible magnitude and Neutral significance.
Fluvial Geomorphology
The River Tay Crossing Bridge is proposed to span the full width of the river and so there will be no in- stream construction involved. However, there is likely to be construction, excavations, earthworks, temporary roads, plant and vehicle washing, and stripping of vegetation in close proximity to the watercourse. These activities have high potential to result in the release of fine sediment into the channel. Stripping of vegetation from the banks can also lead to bank instability due to increased erosion of the exposed banks. This can lead to disturbance of the existing bank profiles and addition of more fine sediment to the River Tay channel, particularly during high rainfall events.
Overall, construction works on the River Tay (high sensitivity) are predicted to have a potential impact of moderate magnitude and Moderate significance on geomorphology.
Water Quality
There is a high risk of silt-laden runoff and suspended sediment release to the River Tay from the extensive construction works on the west bank, as well as site works on the east bank. Large volumes of silt-laden and contaminated runoff from exposed earthworks, including the realigned A9 and grade- separated junction construction footprint, as well as temporary haul routes, could be carried into the
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river having a significant effect on water quality and sensitive aquatic ecology, including freshwater pearl mussels (FWPMs). FWPMs are particularly sensitive to increases in suspended sediment concentrations. Over an anticipated construction programme of 2.5 years, many heavy rainfall events could exacerbate this risk if sediment and silt runoff is not controlled.
Two main construction compounds are proposed in close proximity to the west and east banks of the River Tay. Pollutants carried in runoff from various construction activities in these compounds, such as concrete and plant/vehicle wash waters, could enter the River Tay causing pollution.
Installation of cofferdams for the piling and pier construction works of the River Tay Crossing Bridge on the bank edge could result in sediment release and oil/fuel/concrete leaks and spillages directly into the river.
Various construction works in and near other tributaries of the River Tay could result in elevated levels of suspended sediment and pollutants entering these smaller watercourses and being transported into the River Tay downstream. The cumulative effect of these works could be significant; however, the River Tay is a very large watercourse with an estimated low flow (Q95) of approximately 46m3/s in the vicinity of the proposed CTLR Project, and its ability to dilute and disperse pollutants will mitigate the effects somewhat.
Overall, potential effects on water quality of the River Tay (very high sensitivity) during construction are predicted to be of major magnitude and Large significance.
River Almond
Hydrology and Flood Risk
Construction of a temporary bridge crossing for a potential southern haul route and installation of new drainage outfalls may result in constriction of flow and temporary increase in flood risk. The deck of the temporary bridge crossing over the River Almond (south haul route) is designed to have a freeboard greater than 600mm above the 0.5% AEP water level (without climate change), however both the northern and southern abutments, as well as the northern embankment, were predicted to be within the 0.5% AEP floodplain. The crossing is temporary and is predicted to have a negligible impact on water levels along the River Almond (<5mm increase). Therefore, the impact on flood risk is low. It should be noted that due to the temporary nature of the crossing structure, no allowance was provided for climate change, as agreed with SEPA (refer to Table 15.1).
An increase in impervious surfaces due to the construction site and the temporary haulage route could result in increasing temporary flows (volume and intensity) to the river, resulting in increased temporary flood risk, although the potential impact is likely to be small compared to the size of the River Almond catchment.
This is predicted to result in an impact of minor magnitude and Moderate significance on hydrology and flood risk of the River Almond (very high sensitivity).
Fluvial Geomorphology
A potential temporary bridge and construction near the banks would likely involve excavation and earthworks close to the River Almond and stripping of vegetation on the river banks. There is high potential for siltation, and consequently loss of morphological diversity on the river bed as a result. There is also potential for silt deposition on the exposed gravel bar deposits on the River Almond. When silt deposition occurs on gravel bars, they can support vegetation growth. This leads to the loss of exposed gravel bar features, which provide important fish habitat.
Similar to the River Tay, vegetation clearance on the banks can lead to bank instability, which can result in disturbance of the existing bank profiles and addition of more fine sediment to the channel. Installation of two drainage outfalls/headwalls could result in disturbance of the existing river banks and release of minor, localised quantities of fine sediment into the river.
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Overall, potential geomorphological impacts on the River Almond (high sensitivity) are predicted to have an impact of minor magnitude and Moderate significance.
Water Quality
Construction of two drainage outfalls/headwalls in the river bank could result in release of sediment into the downstream reach of the River Almond, impacting on water quality and aquatic ecology.
Construction works associated with a potential temporary haul route and bridge over the River Almond, including piling works on the north bank, could result in sediment release and oil/fuel/concrete leaks and spillages directly into the river. Over an anticipated construction programme of 2.5 years, many heavy rainfall events could exacerbate this risk if sediment and silt runoff is not controlled.
The River Almond is a medium-sized watercourse, and with an estimated low flow (Q95) of approximately 1.0m3/s in the vicinity of the proposed CTLR Project, is considered to have a moderate pollutant dilution and dispersal capacity. Overall, potential effects on water quality of the River Almond (high sensitivity) are predicted to be of moderate magnitude and Large significance.
Bertha Loch Burn
Hydrology and Flood Risk
Increased impervious surfaces due to the construction site and a potential southern/western haul route could result in increasing temporary flows (volume and intensity) to the burn, resulting in a temporary increase in flood risk, although the potential impact is likely to be relatively small compared to the size of the Bertha Loch Burn catchment.
Construction of the realigned A9 over the watercourse, including construction of a new and replacement culvert and new drainage outfall, is likely to require temporary flow diversion/over-pumping, resulting in narrowing of the channel, changes to flow conveyance and temporary increase in flood risk.
This is predicted to result in an impact of moderate magnitude and Moderate significance on hydrology and flood risk of Bertha Loch Burn (medium sensitivity).
Fluvial Geomorphology
Construction of a new and replacement culvert is likely to cause disturbance to the existing bed and banks and result in the release of fine sediment into Bertha Loch Burn. A new drainage outfall could also result in minor and localised fine sediment release.
Overall, construction works on Bertha Loch Burn (low sensitivity) are predicted to have an impact of moderate magnitude and Slight significance on geomorphology.
Water Quality
Construction works including installation of a new drainage outfall/headwall, a new box culvert and a replacement box culvert, could result in release of sediment into the downstream reach of Bertha Loch Burn. Construction of a temporary crossing for a potential southern/western haul route to the west pier of the River Tay Crossing Bridge could contribute to elevated silt concentrations, and these activities could result in pollutant leaks and spillages to the burn from construction plant and in-channel working methods. Due to multiple in-channel and near-channel works, as well as having a low pollutant dilution capacity, the impact on Bertha Loch Burn (medium sensitivity) is predicted to be of major magnitude and Large significance for water quality.
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Broxy Kennels Drain
Hydrology and Flood Risk
Construction of a new box culvert and channel realignment under the A9 junction is likely to require temporary flow diversion/over-pumping, with potential effects on flow conveyance and temporary increase in flood risk. Installation of two new drainage outfalls could also result in channel constriction and exacerbate temporary flood risk.
This is predicted to result in an impact of moderate magnitude and Moderate significance on hydrology and flood risk of Broxy Kennels Drain (low sensitivity).
Fluvial Geomorphology
Open sections of Broxy Kennels Drain will be culverted upstream of the realigned A9, resulting in loss of all open sections and any existing morphological diversity in the drain. Construction of a long channel realignment and drainage outfalls is also likely to result in release of fine sediment, which will be transported downstream to the River Tay since there is no vegetation or natural stream bed on the Broxy Kennels Drain to trap sediment.
Overall, potential effects on Broxy Kennels Drain (low sensitivity) associated with a long culvert and channel realignment is predicted to be of major magnitude and Moderate significance on geomorphology.
Water Quality
Construction of a long box culvert and channel realignment (approximately 200m in length) could result in increased sediment supply and pollutant leaks and spillages to the burn from construction plant and in-channel working methods. Installation of two drainage outfalls/headwalls in the bank could also increase the risk of increased sediment and pollution.
Due to extensive in-channel works, and combined with a very low pollutant dilution capacity, the impact on Broxy Kennels Drain (low sensitivity) is predicted to be of major magnitude and Moderate significance for water quality.
Cramock Burn
Hydrology and Flood Risk
There will be an increase in impervious surfaces in the catchment due to the construction site and any temporary access routes, potentially increasing temporary flows to the burn resulting in a temporary increase in flood risk. However, the potential impact is likely to be relatively small compared to the size of the Cramock Burn catchment. Removal and replacement of a culvert and installation of new drainage outfalls is likely to require temporary flow diversion/over-pumping, which could temporarily narrow the channel resulting in changes to flow conveyance and temporary increase in flood risk.
This is predicted to result in an impact of moderate magnitude and Moderate significance on the hydrology and flood risk of Cramock Burn (medium sensitivity).
Fluvial Geomorphology
Replacement of Stormontfield Road culvert is likely to result in fine sediment release into the watercourse. In addition, construction will occur along Stormontfield Road in close proximity to the Cramock Burn, which is likely to involve excavation, earthworks and stripping of vegetation close to the burn. There is high potential for fine sediment release, and consequently loss of morphological diversity on the river bed.
Installation of new drainage outfalls has potential to result in the release of small localised quantities of fine sediment into the Cramock Burn.
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Overall, potential construction impacts on the Cramock Burn (low sensitivity) are predicted to be of moderate magnitude and Slight significance on geomorphology.
Water Quality
Removal and installation of a replacement box culvert under Stormontfield Road could result in release of sediment, as well as pollutant leaks and spillages to the Cramock Burn from construction plant and in-channel working methods. Works to install drainage outfalls/headwalls in the channel banks could result in increased sediment supply to the Cramock Burn.
Overall, the potential impact on the water quality of Cramock Burn (medium sensitivity), considering the low dilution capacity, is predicted to be of major magnitude and Moderate significance.
Sheriffton Wood Drain
Hydrology and Flood Risk
Construction of a new pipe culvert and associated channel realignment under the CTLR road embankment is likely to require temporary flow diversion/over-pumping (if water is present in the channel), with potential effects on flow conveyance and temporary increase in flood risk. Installation of two new drainage outfalls could also result in channel constriction although this is considered to be very localised and will only affect surrounding agricultural land if any flooding occurs (considered unlikely).
Overall, an impact of moderate magnitude and Slight significance is predicted on the hydrology and flood risk of Sheriffton Wood Drain (low sensitivity).
Fluvial Geomorphology
Realignment of the channel and installation of a pipe culvert has high potential to result in fine sediment release, particularly while the new channel is initially bare and free of vegetation. This fine sediment is likely to have minimal impact on the drain as it would be trapped in the grassy channel bed downstream. However, there is a risk that sediment could be transported further downstream and deposited in the River Tay during high rainfall events.
Overall, construction works on the Sheriffton Wood Drain (low sensitivity) are predicted to have an impact of moderate magnitude and Slight significance on geomorphology.
Water Quality
Construction of a new pipe culvert and associated channel realignment (approximately 60m in length) could result in high risk of increased sediment supply and pollutant leaks and spillages to the burn from construction plant and in-channel working methods. Installation of two drainage outfalls/headwalls could also increase the risk of increased sediment and pollution to this small ditch a short distance upstream of the River Tay.
Due to the nature of the in-channel works, and combined with a very low pollutant dilution capacity, the impact on Sheriffton Wood Drain is predicted to be of moderate magnitude and Slight significance for water quality.
Redgorton Drain
Hydrology and Flood Risk
Installation of two new drainage outfalls may require temporary flow diversion/over-pumping, with potential effects on flow conveyance and temporary increase in flood risk, although this is considered to be very localised. This is predicted to result in an impact of minor magnitude and Neutral significance on the hydrology and flood risk of Redgorton Drain (low sensitivity).
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Fluvial Geomorphology
Installation of two new drainage outfalls into Redgorton Drain has potential to result in the release of small localised quantities of fine sediment into the drain and then the River Tay a short distance downstream. This is predicted to result in an impact of minor magnitude and Neutral significance on the geomorphology of Redgorton Drain (low sensitivity).
Water Quality
Construction of two drainage outfalls/headwalls in the bank of Redgorton Drain, a short distance upstream of the River Tay, could result in release of sediment. The impact on Redgorton Drain (low sensitivity) is predicted to be of minor magnitude and Neutral significance for water quality.
Whiggle Burn
Hydrology and Flood Risk
There could be a potential temporary increase in flows as existing drainage is to be re-routed from the Cramock Burn to the Whiggle Burn, which could cause a localised temporary increase in flood risk. Installation of two new drainage outfalls may require temporary flow diversion/over-pumping, with potential effects on flow conveyance and temporary increase in flood risk.
As the channel is steep-sided and the floodplain confined in this location, construction impacts are predicted to be of minor magnitude and Neutral significance on the hydrology and flood risk of the Whiggle Burn (low sensitivity).
Fluvial Geomorphology
Installation of two new drainage outfalls has potential to result in the release of minor localised volumes of fine sediment into the watercourse. This is predicted to have an impact of minor magnitude and Neutral significance on the geomorphology of the Whiggle Burn (low sensitivity).
Water Quality
Construction of two drainage outfalls/headwalls in the bank could result in release of sediment to the Whiggle Burn. Overall considering the low dilution capacity, the potential impact on water quality of the Whiggle Burn (medium sensitivity) is predicted to be of minor magnitude and Slight significance.
Annaty Burn
Hydrology and Flood Risk
An increase in impervious surfaces due to construction works in the catchment and construction of a new drainage outfall may result in constriction of flow and temporary increase in flood risk. Potential effects on hydrology and flood risk of Annaty Burn (low sensitivity) are predicted to be of moderate magnitude and Slight significance.
Fluvial Geomorphology
Installation of a new drainage outfall has potential to result in the release of minor localised volumes of fine sediment into the watercourse. This is predicted to have an impact of minor magnitude and Slight significance on the geomorphology of Annaty Burn (medium sensitivity).
Water Quality
Construction of a drainage outfall/headwall in the bank of Annaty Burn could result in release of sediment to the burn. The location of a potential satellite construction compound at the A94 roundabout could be within 300m of the Annaty Burn, however any pollutants carried in contaminated runoff from the compound is unlikely to reach the burn. As such, the potential impact on the water quality of Annaty Burn (medium sensitivity) is predicted to be of minor magnitude and Slight significance.
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Highfield Plantation Drains
Hydrology and Flood Risk
There could be localised changes to the hydrology of the area as several drains will be re-routed and intercepted by cut-off ditches during construction and installation of two new pipe culverts may require temporary flow diversion/over-pumping (if water is present in the channels). Installation of two new drainage outfalls could also result in channel constriction although this is very localised and will only affect surrounding woodland if any flooding occurs (considered unlikely).
Potential effects on hydrology and flood risk of the Highfield Plantation Drains (low sensitivity) are predicted to be of moderate magnitude and Slight significance.
Fluvial Geomorphology
Two of the Highfield drainage channels are proposed to be realigned and piped under the new road alignment. In addition, there may be a requirement to pump water out of the drains during construction. These drains have uniform, man-made channels with little morphological diversity. Therefore, the construction is predicted to have an impact of minor magnitude and Neutral significance on the geomorphology of the Highfield Plantation Drains (low sensitivity).
Water Quality
Construction of new pipe culverts under the CTLR carriageway and two new drainage outfalls could result in increased sediment supply and pollutant leaks and spillages to these forest drains from construction plant and in-channel working methods. There could also be a minor risk of mobilisation of contaminants if any groundwater pumping is required from the proposed road cutting in this location.
The location of a potential satellite construction compound could be within close proximity of the Highfield Drains. Pollutants carried in runoff from various construction activities in this compound, such as concrete and plant/vehicle wash waters, oils and fuels could run off into the drains causing pollution.
However, due to the relative low importance of the drains from a water quality perspective, and remote distance of any sensitive watercourses downstream, the impact on the Highfield Plantation Drains (low sensitivity) is predicted to be of minor magnitude and Neutral significance for water quality.
15.6.3 Generic Operational Impacts
The completed proposed CTLR Project includes new and upgraded road carriageways, embankments and cuttings, a grade-separate junction at the A9 interchange, the River Tay Crossing Bridge structure as well as a permanent road drainage system and SuDS features.
Hydrology and Flood Risk
• Increased permanent hardstanding/impermeable areas within the catchment resulting in an increase in surface water runoff (volume and intensity) to watercourses, and subsequent increase in flood risk. • Existing floodplain storage capacity can be reduced with new development in a floodplain, increasing localised flood risk and/or flooding elsewhere in the catchment. • Road embankments can intersect catchments of watercourses and create a barrier to, or alter the patterns of, runoff pathways in the area. Unmitigated, these flows can accumulate behind embankments and present a source of surface water flooding or alter the inflows to other waterbodies. However, the proposed drainage system of the proposed CTLR Project has been designed to ensure that connectivity between watercourses and their catchments is maintained. These have generally been designed to work with the existing topography to ensure that catchments remain connected to their natural watercourses. • Hydraulic structures (bridges/culverts) and channel realignments can affect the flow behaviour of a channel. Undersized structures can constrict flows increasing flood levels upstream, whereas removing or replacing an undersized culvert may increase forward conveyance
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thereby worsening flooding downstream. Channel realignment resulting in shorter channels can result in steepening of the channel gradient and therefore result in faster flows downstream. • Increased flows to watercourses receiving operational road drainage. This can alter the catchment response to storm events, which may become 'flashier' thereby increasing flood risk and stream power downstream if there is no suitably designed SuDS to attenuate runoff. The impact may be greater if the proposed discharge to an outfall is outside the original catchment resulting in a net gain in the contributing area of one catchment but a net loss in another. • A lack of maintenance of the drainage system and SuDS features can increase the risk of blockage and surface water flooding to surrounding areas.
Fluvial Geomorphology
• Increased runoff due to an increase in new impermeable road areas/paved surfaces may result in an increase in fine sediment supply to watercourses. This release of fine sediment can potentially lead to an increase in turbidity and siltation. Siltation can cause a decrease in the morphological diversity of the river bed, since features such as pools can become smothered by silt deposition. This could impact the ecology of gravel bed channels due to loss of habitat. • Scour around new drainage outfalls can lead to an increase in the supply of fine sediment to watercourses. If outfalls are poorly positioned they can also induce basal scour, which can impact channel morphology and headwalls can reduce bank morphological diversity. • Realigned sections of channel may result in channel instability and changes in existing flow and sediment regimes if the stream gradient and dimensions of the new channel do not closely replicate that of the existing channel. Under- or over-sized culverts and channels may be subject to increased rates of erosion or deposition.
Water Quality
• A future increase in traffic volumes can lead to an increase in the volume of contaminated road runoff entering the drainage system and downstream watercourses. There are a wide range of pollutants found in road runoff which may have an effect on the receiving waters and associated ecology, including suspended solids and contaminants bound to them (such as metals, oils and de-icing salt). • However, a new road project (such as the proposed CTLR Project) would result in beneficial impacts on road drainage due to the higher standard of highway design and improved drainage design compared to the existing road, which provides little or no treatment (refer to Section 5.5.1). • Potential contamination of watercourses (and groundwater) by leachable contamination from imported fill materials or SuDS drainage. • New or extended culverts and channel realignments can potentially change the flow and sediment regime of a watercourse and this could have an associated effect on water quality by mobilising suspended solids and releasing previously ‘locked’ contaminants into the water column. Changes in water turbulence through long culverts can also locally affect atmospheric oxygenation of the water. • Long new and extended culverts can have an effect on water quality due to oxygen sags caused by the lack of light, which restricts aquatic plant photosynthesis and rapid microbiological degradation of biodegradable matter. Structures that are relatively wide and/or short in length would allow better light penetration and therefore have a lower effect on water quality, however this is considered to be a very localised effect.
15.6.4 Specific Operational Impacts
An assessment of predicted operational effects on watercourses, based on the activities in Table 15.7 is summarised below, for each of the RDWE sub-topics. Refer to Appendix 15.2 (Flood Risk Assessment) for more detailed information on potential flooding effects. A summary of the operational HAWRAT routine runoff and accidental spillage risk assessment is provided in the water quality section (more detailed information is provided in Appendix 15.3: Water Quality Calculations).
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River Tay
Hydrology and Flood Risk
Increased flood risk due to alterations in the hydraulic behaviour of the River Tay caused by in-channel elements associated with the River Tay Crossing Bridge including the piers. The bridge has been designed so that the underside of the deck has a freeboard of 600mm and the only element of the bridge within the 0.5% AEP (1 in 200 year) floodplain is the eastern pier. This causes a floodplain displacement of approximately 4m3, which compared to the overall River Tay catchment is extremely small. Hydraulic modelling indicates that this will have a minimal impact (<5mm) on water levels along this reach near the bridge crossing. Overall, potential effects on hydrology and flood risk of the River Tay (very high sensitivity) are predicted to be of negligible magnitude and Neutral significance.
Fluvial Geomorphology
The River Tay Crossing Bridge will clear span the river, with the piers slightly set back from the river banks. Therefore, there is predicted to be an impact of negligible magnitude and Neutral significance on the geomorphology of the River Tay (high sensitivity).
River Almond
Hydrology and Flood Risk
Given the relatively large catchment area, new road drainage inputs to the River Almond (very high sensitivity) is predicted to have a minimal impact on water levels downstream, resulting in an impact of negligible magnitude and Neutral significance on hydrology and flood risk.
Fluvial Geomorphology
The installation of drainage outfalls in the River Almond will increase the amount of artificial bank protection along the river. However, this is likely to be limited to short localised extents. If the outfalls are poorly positioned along the bank, there is a risk of increased basal scour which could impact the channel morphology. This is therefore predicted to have an impact of minor magnitude and Slight significance on the geomorphology of the River Almond (high sensitivity).
Bertha Loch Burn
Hydrology and Flood Risk
The realigned A9 cuts through the Bertha Loch Burn catchment; however, the drainage provision along the western side of the road will maintain hydrological connectivity. There is an increase in impermeable road surfaces in Bertha Loch Burn catchment, with drainage from the A9 grade-separated junction (currently within the Broxy Kennels Drain catchment) proposed to discharge to the Bertha Loch Burn at the drainage outfall. However, the change in area is small and change in flows and levels is likely to be minimal.
Hydraulic modelling of the Bertha Loch Burn indicates that the installation of the new culvert sized to DMRB standards for the realigned section of the A9 occupies part of the floodplain and causes a “squeezing” of floodwater. Flows are channelled southwards along the embankment of the realigned A9 towards the Bertha Park access road and increases flows onto the tie-in point between the realigned and existing A9 road, as well as to a lodge (residential property) within Bertha Park. Overall, potential effects on hydrology and flood risk of Bertha Loch Burn (medium sensitivity) are predicted to be of moderate magnitude and Moderate significance.
Fluvial Geomorphology
The installation of a new (approximately 70m long) and replacement (approximately 10m long) culvert will increase the amount of artificial bank and bed on the Bertha Loch Burn, resulting in a loss of morphological diversity. The new outfall installation is proposed in the existing Highland Mainline Railway culvert, and therefore this will have a minimal impact on existing bank morphology. Overall, November 2019 PAGE 41 OF CHAPTER 15
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predicted effects on the geomorphology of Bertha Loch Burn (low sensitivity) are predicted to be of minor magnitude and Neutral significance.
Broxy Kennels Drain
Hydrology and Flood Risk
Hydraulic modelling indicates that the new 1.2m diameter 186m long box culvert upstream of the A9 would have sufficient capacity to transfer the predicted 0.5% AEP (including 20% climate change) inflows with over 600mm freeboard retained. The new arrangement would also remove an existing undersized culvert upstream of the A9, which would remove the current localised flood risk. The reduction in freeboard in the Highland Mainline Railway culvert downstream was also found to be negligible (<5mm).
There will be an overall increase in impervious surfaces, although there will be an overall reduction in the catchment area as drainage from the A9 grade-separated junction will be re-routed and discharge to the Bertha Loch Burn. The realigned A9 cuts through the catchment; however, the proposed drainage provision will maintain hydrological connectivity. Potential effects on hydrology and flood risk of the Broxy Kennels Drain (low sensitivity) are therefore predicted to be of negligible magnitude and Neutral significance.
Fluvial Geomorphology
Open sections of the Broxy Kennels Drain will be culverted as part of the proposed CTLR Project resulting in a loss of morphological diversity and habitat. Since the existing drain has a uniform channel with limited morphological diversity, the potential impacts are predicted to be of moderate magnitude and Slight significance on the geomorphology of Broxy Kennels Drain (low sensitivity).
Cramock Burn
Hydrology and Flood Risk
The hydraulic baseline modelling identified that the area to the north of Cramock Burn does not flood for the 0.5% AEP (1:200 year) including climate change event (as shown on Figure 15.2), and therefore the CTLR carriageway will not cause a displacement of flood storage.
Hydraulic modelling indicates that new impermeable surfaces in the floodplain and new road drainage inputs are predicted to have a minimal impact on water levels in the Cramock Burn downstream. The modelled flood extents are largely insensitive to any increase in flows.
The replacement of the existing Stormontfield Road culvert is predicted to have a greater freeboard than the existing culvert, which surcharges in the baseline scenario. Hydraulic modelling predicts that this will have negligible impact (<5mm) on flood extents or water levels downstream, with the maximum flood extent and flooding along the southern bank of the Cramock Burn. Potential effects on hydrology and flood risk of Cramock Burn (medium sensitivity) are predicted to be of negligible magnitude and Neutral significance.
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Legend Proposed CTLR Project ± 200 year flood extents (river) Watercourse and Flow Direction
Broxy Kennels Drain
R ed go rt on D ra in
n le Bur Whigg
Broxy Kennels Drain Sheriffton Wood Drain
mock Burn Cra Caravan Drain 0 0.5 1 Highfield Km Plantation Drain Bertha Loch k Burn Cramoc Burn
R n i r v u e r B T a y y t
a
n
n
A d Almon River
Contains OS data © Crown Copyright and database right 2019
P01 04/11/2019 For Information JM DR
Rev. Rev. Date Drawing Suitability Drawn Appr'd
k Burn Cramoc Sweco, Suite 4/2, City Park, 368 Alexandra Parade Glasgow, G31 3AU, Tel: +44 (0)141 414 1700 Bertha Loch Client Burn
Project Cross Tay Link Road
Drawing Title Figure 15.2 200 year Flood Extents (Baseline)
Scale @ A3 1:22,500 Project No. 119046 Status S2 lmond R ver A i BIM No. 119046-SWECO-EGN-000-DR-GS-20058 Ri ve r Ta This drawing should not be relied on or used in circumstances other than those for which it was y originally prepared and for which Sweco UK Limited was commissioned. Sweco UK Limited Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2019 OS 100016971. Use of this accepts no responsibility for this drawing to any party other than the person by whom it was data is subject to terms and conditions. commissioned. Any party which breaches the provisions of this disclaimer shall indemnify Sweco Contains public sector information licensed under the Open Government Licence v3.0. UK Limited for all loss or damage arising therefrom.
CHAPTER 15 CROSS TAY LINK ROAD ROAD DRAINAGE AND THE WATER EIA REPORT (VOLUME 2) ENVIRONMENT
Fluvial Geomorphology
The installation of outfalls in the banks of Cramock Burn will increase the amount of artificial bank protection along these reaches. However, this is likely to be limited to short localised extents. If the outfalls are poorly positioned, there is a risk of increased basal scour which could impact the channel morphology. As the new culvert is replacing an existing structure under Stormontfield Road, there is likely to be a minimal change in the amount of artificial bank and bed on the Cramock Burn and minimal additional loss of morphological diversity.
Overall, this is predicted to have an impact of minor magnitude and Neutral significance on the geomorphology of Cramock Burn (low sensitivity).
Sheriffton Wood Drain
Hydrology and Flood Risk
The new CTLR carriageway embankment cuts through the catchment; however, the drainage provision to the north of the embankment will maintain hydrological connectivity.
There is potential for the new pipe culvert, channel realignment and new road drainage inputs to impact on flows and levels downstream, however there are no sensitive receptors downstream before discharging to the River Tay a short distance downstream. The new 900mm pipe culvert has been designed to accommodate flows for the 0.5% AEP event with adequate freeboard (300mm), as agreed with SEPA. Potential effects on hydrology and flood risk of Sheriffton Wood Drain (low sensitivity) are therefore predicted to be of minor magnitude and Neutral significance.
Fluvial Geomorphology
The proposed channel realignment, culverting and drainage outfalls on Sheriffton Wood Drain are considered to have relatively minimal effect on Sheriffton Wood Drain as it currently exhibits a straightened, uniform channel, which is often dry and lacks morphological diversity. There is therefore considered to be low chance of initiating channel instability by realigning the channel and other permanent works.
Overall, potential impacts are predicted to be of moderate magnitude and Slight significance on the geomorphology of Sheriffton Wood Drain (low sensitivity).
Redgorton Drain
Hydrology and Flood Risk
New road drainage inputs are predicted to have a minimal impact on water levels before entering the River Tay a short distance downstream, resulting in an impact on the hydrology and flood risk of Redgorton Drain (low sensitivity) of negligible magnitude and Neutral significance.
Fluvial Geomorphology
The new outfalls installation is proposed in the existing Paddocks Culvert, which will therefore have a minimal impact on existing bank morphology. This is predicted to have an impact of negligible magnitude and Neutral significance on the geomorphology of Redgorton Drain (low sensitivity).
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Whiggle Burn
Hydrology and Flood Risk
New road drainage from the CTLR carriageway (currently within the Cramock Burn catchment) is proposed to discharge to the Whiggle Burn. Given the catchment size of the watercourse and the contained extent of the floodplain the additional flows are predicted to have a minimal impact on water levels and flood patterns downstream. This is therefore predicted to result in a potential impact of minor magnitude and Neutral significance on the hydrology and flood risk of Whiggle Burn (low sensitivity).
Fluvial Geomorphology
Two new drainage outfalls on the Whiggle Burn will increase the amount of artificial bank protection along the watercourse, which will be limited to a short, localised extent. If the outfalls are poorly positioned there is a risk of increased basal scour which could impact channel morphology. This is predicted to have an impact of minor magnitude and Neutral significance on the geomorphology of Whiggle Burn (low sensitivity).
Annaty Burn
Hydrology and Flood Risk
Given the relatively large catchment area, new road drainage inputs to Annaty Burn (low sensitivity) is predicted to have a minimal impact on water levels downstream, resulting in an impact of negligible magnitude and Neutral significance on hydrology and flood risk.
Fluvial Geomorphology
The new drainage outfall on Annaty Burn will increase the amount of artificial bank protection along the watercourse, which will be limited to a short, localised extent. If the outfall is poorly positioned there is a risk of increased basal scour which could impact channel morphology. This is predicted to have an impact of minor magnitude and Slight significance on the geomorphology of Whiggle Burn (medium sensitivity).
Highfield Plantation Drains
Hydrology and Flood Risk
The new CTLR carriageway cuts through the catchment, which is in cutting in this location. The proposed drainage provision will maintain hydrological connectivity where required, whilst other minor drains will be re-routed. Due to the low sensitivity of these drains for hydrology and flood risk, potential effects are predicted to be of minor magnitude and Neutral significance.
Fluvial Geomorphology
Since the Highfield Plantation Drains are manmade, uniform drainage channels, the operation of the proposed CTLR Project is predicted to have an impact of negligible magnitude and Neutral significance on the geomorphology of the drains (low sensitivity).
Water Quality
In total, 16 new drainage outfalls are proposed to discharge to watercourses during the operational phase, as summarised in Table 15.8, from both the realigned A9 (and grade-separated junction and side roads) and the CTLR mainline carriageway. The indicative drainage outfall locations are shown on Figure 15.1. There is also a proposed outfall to an existing cundy (old stone drain) which receives drainage from a minor access road upgrade as shown on the figure. Although the cundy eventually enters the Broxy Kennels Drain, a proportion of the received runoff will have infiltrated into the ground through the base of the cundy by the time it reaches Broxy Kennels Drain, and therefore has not been included in the assessment below.
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In addition, there are five outfalls proposed to collect pre-earthwork drainage rather than proposed carriageway runoff. This includes an outfall to the Whiggle Burn, two outfalls to the Caravan Drain and two outfalls to existing ditches in the Highfield Plantations area, which are also shown on Figure 15.1. These additional outfalls are not included in the assessment below as they are not proposed to receive carriageway drainage.
As part of the drainage design, several networks were required to outfall into the same watercourse, such as two networks on the River Almond. In these instances, one outfall headwall is proposed rather than two to minimise the potential visual impact of multiple precast concrete units in the channel banks.
Table 15.8: Proposed Drainage Outfall Locations Approx. Outfall Road Drainage Impermeable Proposed Receiving Watercouse Location Length (m) Road Drainage treatment (NGR) Area (ha) NO 0936 Redgorton Drain 221m 0.278 Filter drains 2866
NO 0932 Redgorton Drain 294m 0.432 Filter drains 2875
NO 0925 Filter drains and Broxy Kennels Drain 893m 3.864 2786 detention basin
NO 0917 Broxy Kennels Drain * 196m 0.081 Filter drains 2764
NO 0956 Filter drains and Bertha Loch Burn * 1,636m 4.038 2710 detention basin
NO 0964 Filter drains and River Almond 842m 3.318 2663 detention basin
NO 0959 River Almond 269m 0.336 Filter drains 2661 Grass-topped NO 0979 Sheriffton Wood Drain 785m 1.341 filter drains and 2763 detention basin Grass-topped NO 0992 Sheriffton Wood Drain 423m 0.709 filter drains and 2764 detention basin Cramock Burn (at NO 1152 Kerbs & gullies / 216m 0.083 Stormontfield Road upgrade) 2711 filter drains
Cramock Burn (at NO 1150 Kerbs & gullies / 245m 0.196 Stormontfield Road upgrade) 2709 filter drains Grass-topped Caravan Drain (then NO 1092 2,863m 6.329 filter drains and Cramock Burn) 2748 detention basin Drain at Balboughty NO 1268 Grass-topped Cottages (then Cramock 295m 1.008 2759 filter drains Burn)
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Approx. Outfall Road Drainage Impermeable Proposed Receiving Watercouse Location Length (m) Road Drainage treatment (NGR) Area (ha) Grass-topped NO 1283 Whiggle Burn 1,185m 3.319 filter drains and 2804 treatment pond Grass-topped NO 1386 Highfield Plantation Drain 483m 1.308 filter drains and 2721 detention basin Grass-topped NO 1484 Annaty Burn 532m 1.114 filter drains and 2695 detention basin * Some of the networks that drain to Broxy Kennels Drain and Bertha Loch Burn drain a minor access road and a link road that connects to the A9 grade-separated junction and therefore are not subject to a detailed HAWRAT assessment
Tables 15.9 and 15.10 summarise the results of the HAWRAT routine runoff assessment for soluble and sediment pollutants for the watercourses proposed to receive operational road drainage, following the three-step approach in HAWRAT. The results of the spillage risk assessment are provided in Table 15.11. The routine runoff/spillage risk assessment methodologies and HAWRAT outputs are described in Appendix 15.3.
Individual outfall assessments are summarised first, followed by two in-combination (cumulative) assessments for two drainage outfalls draining to Redgorton Drain and the River Almond. In- combination assessments are undertaken where more than one outfall discharges in close proximity in the same reach of watercourse.
Table 15.9: Summary of Routine Runoff Assessment (soluble pollutants) Runoff Runoff Specific Specific Annual Pollutant PASS Watercourse Step Threshold Threshold Average EQS (ug/l) / FAIL Value Concentration 24hr 6hr (mean) Dissolved 23.22 21 42 - - FAIL Step 1: In copper Runoff Dissolved 68.11 60 120 - - FAIL zinc Dissolved 0.38 21 42 0.08 1 PASS Redgorton Step 2: In copper Drain River Dissolved 1.25 60 120 0.29 7.8 PASS zinc Dissolved Step 3: 0.23 21 42 0.05 1 PASS copper With Dissolved mitigation 0.75 60 120 0.17 7.8 PASS zinc Dissolved 23.22 21 42 - - FAIL Step 1: In Copper Runoff Dissolved 68.11 60 120 - - FAIL Zinc Redgorton Dissolved 0.57 21 42 0.12 1 PASS Drain Step 2: In Copper River Dissolved 1.85 60 120 0.44 7.8 PASS Zinc Step 3: Dissolved 0.34 21 42 0.07 1 PASS With Copper
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Runoff Runoff Specific Specific Annual Pollutant PASS Watercourse Step Threshold Threshold Average EQS (ug/l) / FAIL Value Concentration 24hr 6hr (mean) mitigation Dissolved 1.11 60 120 0.26 7.8 PASS Zinc Dissolved 23.22 21 42 - - FAIL Step 1: In Copper Runoff Dissolved 68.11 60 120 - - FAIL Zinc Dissolved 5.10 21 42 1.30 1 FAIL Step 2: In Copper Broxy Drain River Dissolved 15.90 60 120 4.46 7.8 FAIL Zinc Dissolved Step 3: 1.75 21 42 0.45 1 PASS Copper With Dissolved mitigation 5.43 60 120 1.59 7.8 PASS Zinc Dissolved 23.22 21 42 - - FAIL Step 1: In Copper Runoff Dissolved 68.11 60 120 - - FAIL Zinc Dissolved 2.70 21 42 0.68 1 PASS Bertha Loch Step 2: In Copper Burn River Dissolved 8.50 60 120 2.31 7.8 FAIL Zinc Dissolved Step 3: 0.86 21 42 0.24 1 PASS Copper With Dissolved mitigation 2.69 60 120 0.85 7.8 PASS Zinc Dissolved 23.22 21 42 - - FAIL Step 1: In Copper Runoff Dissolved 68.11 60 120 - - FAIL Zinc Dissolved 0.02 21 42 0.00 1 PASS Step 2: In Copper River Almond River Dissolved 0.06 60 120 0.01 7.8 PASS Zinc Dissolved Step 3: 0.01 21 42 0.00 1 PASS Copper With Dissolved mitigation 0.02 60 120 0.00 7.8 PASS Zinc Dissolved 23.22 21 42 - - FAIL Step 1: In Copper Runoff Dissolved 68.11 60 120 - - FAIL Zinc Dissolved 0.00 21 42 0.00 1 PASS Step 2: In Copper River Almond River Dissolved 0.01 60 120 0.00 7.8 PASS Zinc Dissolved Step 3: 0.00 21 42 0.00 1 PASS Copper With Dissolved mitigation 0.00 60 120 0.00 7.8 PASS Zinc Redgorton Step 1: In Dissolved 23.22 21 42 - - FAIL Drain Runoff Copper
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Runoff Runoff Specific Specific Annual Pollutant PASS Watercourse Step Threshold Threshold Average EQS (ug/l) / FAIL Value Concentration 24hr 6hr (mean) (combined Dissolved 68.11 60 120 - - FAIL assessment) Zinc Dissolved 0.88 21 42 0.19 1 PASS Step 2: In Copper River Dissolved 2.85 60 120 0.69 7.8 PASS Zinc Dissolved Step 3: 0.53 21 42 0.11 1 PASS Copper With Dissolved mitigation 1.71 60 120 0.42 7.8 PASS Zinc Dissolved 23.22 21 42 - - FAIL Step 1: In Copper Runoff Dissolved 68.11 60 120 - - FAIL Zinc Dissolved River Almond 0.02 21 42 0.00 1 PASS Step 2: In Copper (combined River Dissolved assessment) 0.07 60 120 0.01 7.8 PASS Zinc Dissolved Step 3: 0.01 21 42 0.00 1 PASS Copper With Dissolved mitigation 0.02 60 120 0.01 7.8 PASS Zinc
Table 15.10; Summary of Routine Runoff Assessment (sediment pollutants) Velocity Low Flow Deposition Deposition Deposition PASS Watercourse Step Velocity Index Threshold Index Value / FAIL (m/s) Threshold (m/s) Step 2 Redgorton Drain 0.11 0.1 0 100 PASS (Tier 1) Step 2 Redgorton Drain 0.11 0.1 0 100 PASS (Tier 1) Step 2 0.05 272 100 FAIL (Tier 1) Broxy Drain 0.1 Step 2 0.11 0 PASS (Tier 2) Step 2 0.07 134 100 FAIL Bertha Loch (Tier 1) 0.1 Burn Step 2 0.14 0 PASS (Tier 2) Step 2 River Almond 0.29 0.1 0 100 PASS (Tier 1) Step 2 River Almond 0.29 0.1 0 100 PASS (Tier 1) Redgorton Drain Step 2 (combined 0.11 0.1 0 100 PASS (Tier 1) assessment) River Almond Step 2 (combined 0.29 0.1 0 100 PASS (Tier 1) assessment)
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Table 15.11: Summary of Spillage Risk Assessment Threshold of Spillage Risk (1:X return period) Within Acceptability Acceptable Watercourse Without (1:X year return With mitigation Limits? period) mitigation Redgorton Drain 1:53,807 1:89,678 Yes
Redgorton Drain 1:43,153 1:71,922 Yes
Broxy Drain 1:6,874 1:19,639 Yes Bertha Loch 1:4,703 1:13,438 Yes Burn River Almond 1:200 1:5,623 1:16,067 Yes
River Almond 1:35,461 1:59,101 Yes Redgorton Drain (combined 1:11,919 1:19,865 Yes assessment) River Almond (combined 1:4,262 1:12,177 Yes assessment)
The results in Table 15.9 indicate that all drainage discharges return a ‘Fail’ result at Step 1, as this is for concentrated pollutants in road runoff before mixing (and dilution) in receiving watercourses (i.e. worse case). The results indicate that all soluble and sediment-bound pollutants (for both single and combined outfalls assessments) return a ‘Pass’ result at Step 2 (with in-river mixing prior to any mitigation measures in place), with a few exceptions:
• Broxy Kennels Drain – failure of both soluble pollutants (dissolved copper and zinc) against short-term Runoff Specific Thresholds (RSTs) and compliance failure of dissolved copper against published Environmental Quality Standards (EQS) at Step 2. • Bertha Loch Burn – failure of dissolved zinc against the RSTs at Step 2 (but compliance with EQS).
The spillage risk results (Table 15.11) indicate that risk is well within acceptable thresholds (1:200 year return period) when considering each outfall in isolation and the combined assessment.
For the watercourses proposed to receive road drainage, potential effects on water quality are predicted to be:
• Negligible magnitude and Neutral significance on Redgorton Drain (low sensitivity) and the River Almond (high sensitivity); • Major magnitude and Moderate significance on Broxy Kennels Drain (low sensitivity); and • Minor magnitude and Slight significance on Bertha Loch Burn (medium sensitivity).
SuDS are required for new developments under the Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended) (CAR), even for watercourse outfalls that achieve a ‘Pass’ result at Step 2 prior to any mitigation. SuDS proposals are described in Section 15.7.
15.7 MITIGATION AND ENHANCEMENT
The following mitigation measures are required in order to prevent, reduce or offset any significant potential effects described in Section 15.6 above, during both construction and operational phases.
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15.7.1 Construction
Due to the extent of temporary and permanent land required to be cleared and prepared for construction, particularly on the west bank of the River Tay, and over a relatively long construction programme, the risk of silt-laden runoff entering the River Tay (and its nearby tributaries), presents the greatest risk to water quality and protected species/habitats of the river. The mitigation presented below primarily addresses this issue, as well as any other significant effects identified on the RDWE sub- topics.
Controlling Runoff from Working Area
• Prior to construction, the appointed Contractor will prepare a Construction Environmental Management Plan (CEMP), building on the information laid out in the EIAR, describing construction methods/techniques and mitigation measures identified in the EIA to protect the water environment. The content of the CEMP will be discussed and agreed with the Planning Authority and key environmental bodies such as SEPA. • As part of the CAR regulatory regime, the appointed Contractor will apply to SEPA for a Construction Site Licence, which will include a Pollution Prevention Plan (PPP). The PPP will contain robust measures to deal with surface water runoff from the construction site, including any temporary SuDS, to the satisfaction of SEPA. • Early in the construction programme, the appointed Contractor will implement sediment/pollution control measures to minimise the risk of silt-laden and polluted runoff entering watercourses. This is likely to include temporary SuDS features, silt/sediment traps and fences, and cut-off ditches around site compounds to prevent sediment from leaving the site. Silt barriers/netting could be included on temporary bridges/river crossings carrying construction vehicles, such as on the River Almond and Bertha Loch Burn. • Temporary haul roads and bridges over or near watercourses will be regularly inspected for excessive dirt/mud deposits and maintained. • An Environmental Clerk of Works (EnvCoW), approved by the Planning Authority, will undertake daily/frequent site inspections to ensure working methods and temporary mitigation measures are effectively protecting watercourses around the site. Silt barriers/netting and other temporary sediment control measures will be replaced when required to ensure optimum effectiveness. • The appointed Contractor will be required to install a network of pre-earthworks/cut-off drains to keep runoff from the natural catchment separate from construction site runoff. ‘Clean’ runoff from the natural catchment will be directed towards watercourses, ensuring that the site treatment systems do not become overwhelmed by additional runoff waters and do not reduce the effectiveness of the site treatment facilities by diluting the pollutants. • During site clearance works, bankside trees/vegetation will be retained wherever possible to help bind the soil and minimise erosion. The extent and duration of bare/exposed surfaces will be limited as much as possible, and restoration works undertaken as soon as possible following construction, to minimise risk of silt-laden runoff entering watercourses. • The appointed Contractor will carefully consider the timing of works in his programme to ensure that areas of site clearance are appropriately managed to help minimise the risk of large volumes of silt-laden runoff entering watercourses and the downstream cumulative effect of this on the River Tay.
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Watercourse Engineering Works
• The appointed Contractor will prepare Construction Method Statements to plan and manage in- channel and near-channel works to be approved by SEPA prior to construction. This will include key activities such as i) the River Tay Crossing Bridge pier/foundation works and associated cofferdam installation; and ii) culvert construction and channel realignments, and any associated diversions/over-pumping required to create dry working conditions and minimise risk of sediment mobilisation and pollution events and minimise risk of temporary flooding. • Installation of new and replacement culverts will be undertaken during low flow conditions to minimise risk of pollution and sediment release, and the length of channel disturbed will be minimised as much as possible. • Channel realignments will be designed to have appropriate dimensions and stream gradients, to avoid initiating channel instability. The length of existing channel should be maintained as much as possible to avoid significant increases/decreases in gradient and subsequent effects on flow. • Drainage outfalls will be positioned in the river bank to limit the potential for scour around the outfall headwall, following SEPA’s guidance on outfalls14. • Piling works on the banks of the River Tay will avoid the sensitive fish spawning period (November to May inclusive). Refer to Chapter 9: Biodiversity for further information. The appointed Contractor’s programme will also take into account other seasonal constraints such as the River Tay fishing season.
Pollution Prevention
• The appointed Contractor will adhere to SEPA and CIRIA best practice guidance to manage and reduce the risk of water pollution and sediment release including SEPA's Pollution Prevention Guidelines (PPG)/Guidance for Pollution Prevention (GPP) series 15 , SEPA's Engineering in the Water Environment Good Practice Guides16 and CIRIA guidance17. • The CEMP will include a Pollution Incident Plan (or equivalent) to describe the measures to minimise the risk of a serious pollution incident and actions to take in the event of a spillage during construction, taking cognisance of SEPA's GPP 21 (Pollution incident response planning) and GPP22 (dealing with spills). • Emergency spill kits will be available on site to deal with accidental spillages and leaks, ideally in all construction vehicles, plant and high risk areas. All site staff will be trained in their use. • Potentially polluting activities such as concrete pouring, cement mixing, refuelling and all washout areas will be undertaken within site compounds or controlled areas a safe distance from watercourses and will be appropriately bunded/contained to prevent any uncontrolled runoff to watercourses. Works will not be undertaken within 50m of watercourses, but where this is not practical, then a minimum buffer of 10m will be applied. • Plant and machinery will be stationed on hardstanding surfaces with spillage/drip trays used where required. Construction plant will be regularly checked for leakages and will undergo regular maintenance. • Rather than 10m buffers from watercourses, we would recommend 50m buffers are applied to watercourses and 50m applied to spring, well or borehole. • Best practice measures associated with storage of oils and fuels will be followed in compliance with CAR and SEPA's GPP2 (Above ground oil storage tanks). Oils will be stored within a leak-
14 SEPA (2008) Engineering in the Water Environment Good Practice Guide: Intakes and outfalls. First edition, October 2008. Accessed 19.04.2019 [https://www.sepa.org.uk/media/150984/wat_sg_28.pdf] 15 PPGs are in the process of being replaced by the new GPP series. Relevant guidance includes: GPP2 (Above ground oil storage tanks); GPP4 (Treatment and disposal of wastewater); GPP5 (Works and maintenance in or near water); PPG6 (Working at construction and demolition sites); GPP8: Safe storage and disposal of used oils; GPP13 (Vehicle washing and cleaning); GPP21: Pollution incident response planning) and GPP22 (Dealing with spills). 16 SEPA (2008) WAT-SG-23 Bank Protection Rivers and Lochs, 1st edition, April 2008. Accessed 04/03/2019 [https://www.sepa.org.uk/media/150971/wat_sg_23.pdf]; SEPA (2009) WAT-SG-29 Temporary Construction Methods, 1st edition, March 2009. Accessed 04/03/2019 [https://www.sepa.org.uk/media/150997/wat_sg_29.pdf]. 17 CIRIA (2001) Control of water pollution from construction sites – guidance for consultants and contractors (C532); CIRIA (2006) Control of water pollution from linear construction projects: technical guidance (C648); CIRIA (2006) Control of water pollution from linear construction projects: site guide (C649); CIRIA (2016) Environmental good practice on site pocket book, 4th edition (C762); CIRIA (2015) The SUDS Manual (C753)
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proof container and be contained within a secondary containment system with a capacity of 110% or more of the containers storage capacity, in line with CAR General Binding Rule (GBR) 28. • If any unexpected contamination is identified during earthworks or construction (e.g. hydrocarbon impacted soils), work in such areas will be temporarily halted until a suitably qualified professional (SQP) has been consulted to assess the situation and provide advice. • Sewage from site welfare facilities will be disposed of appropriately either to the foul sewer with the permission of Scottish Water, or in accordance with GPP4 (Treatment and disposal of wastewater where there is no connection to the public foul sewer). • Water quality monitoring will be undertaken pre, during and post-construction, and requirements for this (i.e. parameters, frequency and sampling locations) will be agreed with SEPA and Scottish Water if required.
Flood Risk
• The period of exposure of bare areas and uncontrolled runoff from newly paved areas will be limited as far as practicable to reduce increase in runoff to watercourses. • Appropriate action will be taken in the event of predicted heavy rainfall to protect unsecured materials/plant and items located in site compounds or around site to prevent their movement or release. Plant and materials will be stored in safe areas out with the floodplain where practicable. • Temporary drainage systems will alleviate localised flood risk and prevent obstruction of surface runoff pathways.
15.7.2 Operation
Flood Risk
• The River Tay Crossing Bridge has been designed to have a soffit of 1600mm above the design (1:200 year including climate change) flood level, which is greater than the 600mm minimum freeboard allowance prescribed within the DMRB 18. The flood extents are predicted to be contained within the near bank area and the only design element within the design floodplain is the eastern pier. This is 1.5m in diameter with a depth of 2.44m (from the ground level to the design water level) and is predicted to result in approximately 4m3 of floodplain displacement. Following consultation with SEPA it was agreed that compensatory storage would not be required given this minimal displacement, as highlighted in Table 15.1. • Pre-earthwork ditches, cross-drainage and flood relief culverts have been implemented as part of the drainage design to ensure that the completed proposed CTLR Project does not impede existing drainage pathways or increase surface water flood risk. • SuDS features including treatment ponds and detention basins will be sized to store and attenuate the 0.5% AEP (1 in 200 year) plus climate change flood volume and have a restricted outflow at the 50% AEP (1 in 2 year) Greenfield runoff rate to receiving watercourses to prevent downstream flooding and scour. A hydro-brake or similar control device will be used to achieve this restricted discharge. • SuDS detention basins and treatment ponds will be located out with the 0.5% AEP (1 in 200 year) floodplain. • For the Bertha Loch Burn, the proposed CTLR Project is predicted to cause a ‘squeezing’ of the floodplain and channelling a greater volume of floodwater southwards. A 160m flood embankment is proposed along the southern bank of the burn upstream of the realigned A9 road to contain flows (refer to Figure 15.3). This has been sized for the 0.5% AEP event with a 35% climate change allowance to provide an additional level of resilience. The two culverts
18 Highways Agency et al. (2009) BA 59/94: Design Manual for Roads and Bridges (DMRB), Volume 1, Section 3, Part 6, The Design of Highway Bridges for Hydraulic Action, 2009. The Highways Agency, Scottish Executive Development Department, The National Assembly for Wales and The Department of Regional Development Northern Ireland. Accessed 15/04/2019 [http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol1/section3/ba5994.pdf]
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under the existing A9 carriageway will be replaced with a box culvert (2m wide and 1.6m high) to prevent any backing-up upstream and flooding between the realigned and existing A9 roads. • The new culvert along the Bertha Loch Burn has been designed to accommodate a 750mm freeboard to facilitate mammal passage, whilst the Broxy Kennels Drain culvert has 600mm freeboard, which meets DMRB requirements. • The replacement culverts for the Stormontfield Road culvert (on Cramock Burn) and the existing A9 culvert on Bertha Loch Burn have been designed with 300mm freeboard, following consultation with SEPA. This allowance was agreed with SEPA to be appropriate given the surrounding constraints, including proximity to nearby access roads and properties, and therefore a greater freeboard allowance in this location was not possible. These new culverts would still provide an overall net betterment compared to the existing culverts which are predicted to surcharge for the 0.5% AEP (1 in 200 year) flood event.
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Legend Proposed CTLR Project ± Proposed Flood Embankment 200 year flood extents (river) Watercourse and Flow Direction
R ed go rt on D ra in
n le Bur Whigg
Broxy Kennels Sheriffton Wood Drain Drain
mock Burn Cra
Caravan Drain Highfield Plantation Drain Bertha Loch k Burn Cramoc Burn
R n i r v u e r B T a y 0 0.5 1 y t a Km n
n d A Almon River
Contains OS data © Crown Copyright and database right 2019
P01 04/11/2019 For Information JM DR
Rev. Rev. Date Drawing Suitability Drawn Appr'd
k Burn Cramoc Sweco, Suite 4/2, City Park, 368 Alexandra Parade Glasgow, G31 3AU, Tel: +44 (0)141 414 1700
Client
Project
Bertha Loch Cross Tay Link Road Burn Drawing Title Figure 15.3 200 year Flood Extents (with scheme)
Scale @ A3 1:22,500 Project No. 119046 Status S2 R i BIM No. 119046-SWECO-EGN-000-DR-GS-20059 ve r Ta This drawing should not be relied on or used in circumstances other than those for which it was y originally prepared and for which Sweco UK Limited was commissioned. Sweco UK Limited Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2019 OS 100016971. Use of this accepts no responsibility for this drawing to any party other than the person by whom it was data is subject to terms and conditions. commissioned. Any party which breaches the provisions of this disclaimer shall indemnify Sweco Contains public sector information licensed under the Open Government Licence v3.0. UK Limited for all loss or damage arising therefrom.
CHAPTER 15 CROSS TAY LINK ROAD ROAD DRAINAGE AND THE WATER EIA REPORT (VOLUME 2) ENVIRONMENT
Drainage
The drainage system of the proposed CTLR Project has been designed in accordance with CIRIA’s The SUDS Manual, C753 (2015).
• Drainage outfalls and headwalls (where required) will be positioned to limit potential for scour and comply with SEPA's Good Practice Guide: Intakes and Outfalls (2008) and positioned above the 20% AEP (1 in 5 year) water level, where possible. • Where it can be accommodated, outfall ditches will be used rather than hard engineered pipes and headwalls, to remove the need for precast concrete headwalls in the river banks and require less maintenance. • SuDS features will be lined with an impermeable layer, where required depending on the underlying ground conditions, to prevent surface water infiltration and therefore directing all carriageway drainage to receiving watercourses. • The road drainage network and SuDS measures will be maintained and periodically inspected to avoid failure and reduce the risk of sub-optimal performance, blockage and flooding.
In addition, a SuDS Wetland Area has been included in the design, removing the requirement for three separate outfalls to the Caravan Drain in close proximity to each other (instead the wetland feature will attenuate and treat runoff from the CTLR carriageway and outfall to the Caravan Drain from a single outfall). As well as having attenuation and treatment benefits, the SuDS Wetland Area will also provide improved amenity and visual impact as well as creating wildlife habitat. Refer to Chapter 2: Project Description for more information.
A9 Carriageway
The number and type of SuDS has been discussed and agreed in consultation with SEPA and Transport Scotland during the design and EIA stages. One or two levels of SuDS features are provided to treat carriageway runoff from the A9 carriageway (filter drains and some with additional detention basins) (Table 15.8). Two levels of SuDS are provided prior to outfalling to Broxy Kennels Drain and Bertha Loch Burn, which resulted in failure of one or more of the thresholds in ‘Step 2’ of the HAWRAT assessment (refer to Table 15.9).
Various studies have been undertaken on the efficiencies of different SuDS measures, resulting in a range of published pollutant reduction factors (sources include DMRB HD45/09 and CIRIA’s SUDS Manual. Taking a precautionary approach, Table 15.12 summarises the SuDS pollutant removal factors used in HAWRAT for the CTLR drainage assessment.
Table 15.12: Indicative Pollutant Reduction Factors used for HAWRAT assessment Indicative Pollutant SuDS Measure Notes Reduction (%) Filter Drains 40% n/a
Detention Basin or Treatment 50% n/a Pond After the first level of treatment, subsequent SuDS components are assumed to have half the efficiency 65% Filter Drain and Basin/Pond in quoted. A factor of 0.5 is used to combination account for the reduced (40 + (50/2) = 65%) performance of secondary (or more) components associated with already reduced inflow concentrations.
With this SuDS mitigation in place, concentrations of dissolved copper and zinc are all within the RST and EQS thresholds for all receiving watercourses (Step 3 – with mitigation) (see Table 15.9), and spillage risk reduced even further (see Table 15.11).
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CTLR Carriageway
To ensure that the new CTLR carriageway has sufficient drainage provision before runoff enters receiving watercourses, CIRIA’s Simple Index Approach19 was followed. This involved two stages:
• assign pollution hazard indices based on the proposed land use; and • select SuDS features with a total pollution mitigation index that equals or exceeds the pollution hazard index.
This information is presented in Table 15.13. For this type of road (assigned a Medium hazard level), it was determined that two levels of SuDS are required to ensure adequate treatment prior to entering receiving watercourses. However, three levels of treatment are included in the drainage design for the majority of outfalls which drain this section of carriageway, comprising:
• grassed-top filter drains (two levels of treatment); and • either a detention basin or treatment pond (one level of treatment).
Grass-topped filter drains effectively provide two levels of treatment; the grass-top element provides sediment and hydrocarbon capture as the water dissipates through the soil, whilst the second level of treatment (filter drain) provides the filter media underneath the grass top. The piped network then discharges into either ponds or basins depending on the levels of the out-falling watercourse, providing the third level of treatment and attenuation.
Table 15.13: Summary of the Simple Index Approach (CTLR carriageway) Pollution Hazard Index Pollution Hazard Total Land-Use Level Suspended Metals Hydrocarbons Solids Medium 0.7 0.6 0.7 Road (excluding SuDS Features Pollution Mitigation Index20 low traffic roads, 1.Swale21 0.5 0.6 0.6 highly frequented 2.Filter Drain 0.4 0.4 0.4 lorry approaches 3.Detention Basin 0.5 0.5 0.6 to industrial estates, trunk 3.Treatment 0.7 0.7 0.5 roads/motorways) pond/wetland
Combined surface water pollution 0.95 or >0.95 >0.95 >0.95 mitigation index22
Table 15.13 indicates that the combined pollution mitigation of the SuDS exceeds the specified ‘medium’ pollution hazard index for this type of road. Regardless of whether the final SuDS component is a detention basin or treatment pond, the SuDS in combination is considered to be more than sufficient to protect the receiving watercourses, as well as downstream River Tay, from pollution (i.e. a factor of 0.95 or greater pollution reduction capability for total suspended solids, metals and hydrocarbons).
19 CIRIA (2015), The SUDS Manual, C753, Section 26.7.1 20 The SuDS pollution mitigation index for each SuDS component can only be assumed to deliver these pollutant reduction factors if the design follows the design guidance for hydraulics and treatment set out in CIRIA’s SuDS Manual 21 For purposes of this assessment, the grass-top element of the filter drains is represented by a swale 22 With SuDS in combination, after the first level of treatment subsequent SuDS components are assumed to have half the efficiency quoted. A factor of 0.5 is used to account for the reduced performance of secondary (or more) components associated with already reduced inflow concentrations
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There are three outfalls with different levels of treatment provision, which comprise:
• Two outfalls to Cramock Burn (at Stormontfield Road): kerbs & gullies and filter drains (1 level of treatment prior to each outfall) – a relatively low traffic volume is predicted on this section of Stormontfield Road after the proposed CTLR Project is in operation (<300 vehicles per day). Therefore, the pollution hazard index assigned to the CTLR carriageway in Table 15.13 is considered to be overly conservative for this section of road, which would have a much lower pollution risk. There is minimal treatment provision on the existing Stormontfield Road and therefore the inclusion of filter drains provides a betterment over the existing drainage arrangement, which is considered to be sufficient. • Drain at Balboughty Cottages (prior to outfall to the Cramock Burn): grassed-topped filter drains (2 levels of treatment) – this mitigation still meets or exceeds the required pollutant indices to protect the Cramock Burn in this location, as shown in Table 15.13. In addition, these SuDS only serve a short section of the A93 road to the A93 junction which has lower traffic levels (and therefore pollution risk) compared to the main CTLR carriageway and is therefore considered to be sufficient.
15.8 RESIDUAL EFFECTS
This section describes the residual effects on the water environment (following implementation of mitigation) during both the construction and operational phases of the proposed CTLR Project.
15.8.1 Construction
Hydrology and Flood Risk
Following implementation of best practice and site-specific mitigation during construction, temporary residual effects on hydrology and flood risk are predicted to reduce to no more than minor magnitude and Slight significance, and in most cases will be negligible magnitude and Neutral significance.
Fluvial Geomorphology
Following the implementation of mitigation, no significant residual effects on the geomorphology of watercourses in the study area during the construction phase of the proposed CTLR Project are expected to occur. Residual effects are considered to be reduced to Slight or Neutral significance for all watercourses.
Water Quality
Following implementation of best practice and site-specific mitigation during construction, there is a temporary residual risk of construction site runoff entering the River Tay directly from works on the banks as well as cumulative effects from works in nearby tributaries. However, these construction works will be subject to a number of control and management measures, as stated in Section 15.7.1, and in particular construction site licensing under CAR.
Overall, residual effects on the water quality of the River Tay and its tributaries are predicted to reduce to minor magnitude and Slight significance.
15.8.2 Operation
Hydrology and Flood Risk
There will be an overall net reduction in the catchment area of the Broxy Kennels Drain as the new graded junction will now discharge (via a SuDS) to the Bertha Loch Burn catchment. This was taken into account in the hydraulic modelling which indicated that this would result in a minimal change in peak flows downstream.
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A section of the new CTLR carriageway drainage, which is estimated to be within the Cramock Burn catchment, is proposed to discharge into the Whiggle Burn. This will therefore result in a small net increase in discharge to the Whiggle Burn of approximately 151l/s, which is minimal compared to the estimated 0.5% AEP inflow of 2.44m3/s. Given that there are no sensitive receptors downstream and the channel is confined, the residual effect on flood risk downstream is predicted to be of negligible magnitude and Neutral significance on the Whiggle Burn.
The installation of a flood embankment on the south bank of Bertha Loch Burn will result in a reduction in the freeboard within the Highland Mainline Railway culvert of 24mm. This amounts to a decrease of 4.7%-5.2% of the freeboard capacity, which is within model tolerance and is therefore predicted to be of negligible magnitude and Neutral significance on Bertha Loch Burn.
Permanent residual effects on the other watercourses are also predicted to be of negligible magnitude and Neutral significance for hydrology and flood risk.
Fluvial Geomorphology
Following implementation of best practice and embedded mitigation for watercourse crossings, channel realignments and drainage outfall structures, residual effects on geomorphology are predicted to be no more than of minor magnitude and Slight significance for all watercourses.
Water Quality
With implementation of SuDS measures and a planned inspection/monitoring regime of the drainage system and SuDS, residual effects on water quality are predicted to be of negligible magnitude and Neutral significance for all watercourses proposed to receive road drainage.
15.9 CUMULATIVE EFFECTS
Other development proposals in close proximity of the proposed CTLR Project considered in this cumulative assessment include:
• Luncarty South; • Bertha Park; • Expansion of North Muirton Industrial Estate; and • North Scone.
Please refer to Volume 3 (Cumulative Assessment) for further information. Potentially significant cumulative effects that could arise on the RDWE sub-topics include:
• Increase in silt-laden and polluted runoff to River Tay and its tributaries (construction); • Increase in flood risk due to development in the floodplain (operation); and • Increase in road drainage and polluted discharges to receiving watercourses (operation).
These development sites are located in proximity to the River Tay and a number of its tributaries including the River Almond, Bertha Loch Burn and the Cramock Burn. If one or all of these development proposals were to be constructed concurrently or immediately before or after the proposed CTLR Project, this could result in a potential significant risk of silt-laden runoff and pollutants entering the River Tay (and indirectly through its tributaries) from these construction sites affecting water quality and the qualifying species of the SAC designation, particularly FWPMs. FWPMs are particularly vulnerable to increases in suspended sediment concentrations. A number of large rainfall events could occur during the anticipated construction programme (2021-23) of the proposed CTLR Project, and the risk of large volumes of silt-laden runoff entering the River Tay could be exacerbated if construction of other development sites occurred simultaneously or immediately before or after (effectively extending the construction programme and period of exposed soils) if construction site runoff is not suitably controlled.
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To mitigate these potential significant effects on water quality and protected aquatic ecology, these other development proposals will require the same level of surface water management during construction as the mitigation specified for the proposed CTLR Project in Section 15.8, and in particular:
• Construction site works will be subject to CAR licensing; the appointed Contractor on each site will need to prepare a Construction Site Licence with an accompanying Pollution Prevention Plan to demonstrate to SEPA how surface water will be effectively managed during construction. This will likely include temporary SuDS and other silt/sediment control measures to collect, treat and attenuate site runoff before it is discharged to watercourses. • Preparation of Construction Method Statements detailing working practices/techniques and mitigation to protect surface waters from silt/sediment and pollution, particularly for high risk in- channel works, to be approved by SEPA prior to construction. • Construction programming to avoid in-channel working during sensitive spawning periods for fish.
During operation, additional development in the floodplain of the River Tay and its tributaries may increase risk of flooding to sensitive receptors. However, encroachment into the floodplain of the River Tay in the study area is predicted to be minor and therefore cumulative risk of flooding is not predicted to be significant in 2023 and beyond. In the event of encroachment into the 0.5% AEP (1:200 year) floodplain due to other development proposals, mitigation such as provision of compensatory floodplain storage will be required to offset this impact in agreement with the Council and SEPA. This should be located as close to the area of lost storage as possible to minimise risk of flooding elsewhere.
Runoff from residential/commercial/mixed use development, including road infrastructure, could drain directly to the River Tay, or indirectly through its tributaries. This could result in pollution events if large volumes of silt-laden and polluted runoff are discharged untreated. Other development proposals in the study area, including those listed above, will require a permanent SuDS system to treat and attenuate surface water before it is discharged to receiving watercourses, in line with CAR guidance and national/local planning policy.
With the mitigation measures specified above, these potential cumulative effects are not predicted to be significant.
15.10 SUMMARY OF EFFECTS
A summary of effects on the sub-topics of the water environment is provided in Table 15.14. Only potential effects (pre-mitigation) predicted to be significant (i.e. Moderate significance or greater) is provided.
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Table 15.14: Summary of Effects on RDWE sub-topics Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) Road Drainage and the Water Environment Construction Hydrology and Flood Risk Period of exposure of bare surfaces and uncontrolled runoff from construction areas will be limited as far as practicable Temporary bridge to minimise increase crossing, haul route in runoff. and works in floodplain of River Almond Temporary drainage resulting in increased systems will alleviate overland flows, 750m3 localised flood risk temporary loss of Temporary Moderate and prevent Y Slight existing floodplain obstruction of surface (0.5% AEP) and runoff pathways. increase in temporary flood risk (if southern Materials/plant access option is taken located around site forward). will be secured in the event of heavy rainfall to prevent their movement or release. Plant and materials will be stored in safe areas
23 I = International; E = European; UK = United Kingdom; R = Regional; L = Local
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) out with the floodplain where practicable.
SEPA advised compensatory floodplain storage was not required for temporary construction works in River Almond floodplain. Works in small As above. In addition: catchment, including construction of a new Contractor will and replacement prepare construction culvert and new method statement(s) drainage outfall likely for in-channel to require temporary working methods, Temporary Moderate Y Slight flow diversion/ over- such as flow pumping, resulting in diversion/over- narrowing of channel, pumping and changes to flow measures to conveyance and minimise temporary increased flood risk to flooding, to be Bertha Loch Burn. approved by SEPA. Works in small catchment, including construction of a new box culvert, long channel realignment Temporary Moderate As above Y Neutral and new drainage outfalls likely to require temporary flow diversion/over-
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) pumping, resulting in narrowing of the channel, changes to flow conveyance and increased flood risk to Broxy Kennels Drain. Increase in impermeable surfaces in catchment, installation of new drainage outfalls and removal/replacement of culvert, likely to require temporary flow Temporary Moderate As above Y Neutral diversion/ over- pumping, resulting in narrowing of the channel, changes to flow conveyance and increased flood risk to Cramock Burn. Fluvial Geomorphology Significant earthworks Contractor will and vegetation prepare construction removal on/near banks method statement(s) resulting in release of for in-channel and fine sediment to River near-channel working Tay and increased methods, and bank instability. Temporary Moderate measures to Y Slight Cumulative effects of minimise risk of works in tributaries sediment resulting in elevated mobilisation, to be levels of suspended approved by SEPA. sediment being transported During site clearance
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) downstream into River works, bankside Tay. trees/vegetation will be retained wherever possible to help bind the soil and minimise erosion.
Extent/duration of exposed surfaces will be limited as much as possible, and restoration works undertaken as soon as possible following construction, to minimise risk of silt- laden runoff. Construction works on banks of River Almond As above, also: including a temporary bridge, drainage Drainage outfalls will outfalls, earthworks be positioned in the and vegetation river bank following removal resulting in Temporary Moderate Y Slight SEPA best practice release of fine guidance to limit the sediment to river, as potential for scour well as bank around the outfall instability/loss of bank headwall. morphological diversity. Construction of culvert, As above, also: long channel realignment and Temporary Moderate Installation of new Y Neutral drainage outfalls culverts will be resulting in release of undertaken during
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) fine sediment to Broxy low flow conditions to Kennels Drain. minimise risk of sediment release, and the length of channel disturbance will be minimised as much as possible.
Channel realignment will be designed to replicate existing channel length/gradient as much as possible to avoid significant increase/decrease in flows and changes to sediment regime (increased rates of erosion/deposition). Water Quality High risk of silt-laden Contractor will apply and polluted runoff to to SEPA for a River Tay from Construction Site construction site, Licence, which will including earthworks include a Pollution and bridge Prevention Plan construction (e.g. (PPP), under the Temporary Large Y Slight cofferdam and pier Controlled Activities installation). Significant Regulations (CAR). effect on water quality The PPP will contain and sensitive aquatic robust measures to ecology, including deal with surface freshwater pearl water runoff from the mussels, which are construction site,
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) very sensitive to including any increases in temporary SuDS. suspended sediment concentrations. Contractor will Cumulative effects of implement works in tributaries sediment/pollution resulting in elevated control measures to levels of suspended minimise the risk of sediment and silt-laden and pollutants being polluted runoff transported leaving the downstream into River construction site. An Tay. EnvCoW will undertake daily/frequent site inspections to ensure temporary mitigation measures are working effectively, and silt barriers/netting will be replaced when required.
A network of pre- earthworks/cut-off drains will keep runoff from the natural catchment separate from construction site runoff.
Areas of site clearance will be
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) carefully managed around the construction site to reduce risk of large volumes of contaminated runoff entering the River Tay and tributaries.
Contractor will adhere to SEPA and CIRIA best practice guidance to manage risk of water pollution and sediment release during construction including SEPA's PPG/GPP series.
A Pollution Incident Plan (or equivalent) will be prepared to minimise the risk of a serious pollution incident and include actions to take in the event of a spillage during construction. Emergency spill kits will be available at key areas on site and all site staff will be trained in their use.
High risk polluting
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) activities will be undertaken in site compounds or controlled areas a safe distance from watercourses (minimum 10m) and will be appropriately bunded to prevent any uncontrolled runoff to watercourses.
Plant/machinery will be stationed on hardstanding surfaces with drip trays, and will be regularly checked for leaks. Oil/fuels will be stored in line with CAR and SEPA best practice.
Piling works on the banks of the River Tay will avoid the sensitive fish spawning period.
Water quality monitoring will be undertaken pre, during and post- construction, and
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) requirements for this (i.e. parameters, frequency and sampling locations) will be agreed with SEPA and Scottish Water if required.
As above (apart from specific mitigation requirements for River Tay). In addition:
Silt barriers/netting will be included on Construction works temporary bridges including temporary carrying construction haul route (if south vehicles, such as on access option is taken the River Almond and forward) and drainage Bertha Loch Burn. outfalls resulting in Temporary Large Y Slight An EnvCoW will silt-laden and polluted undertake runoff to River Almond, daily/frequent site impacting on water inspections to ensure quality and aquatic temporary mitigation ecology. measures are working effectively, and ensure temporary barriers/netting are replaced when required.
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) Construction works including new/replacement box culverts, drainage outfall and a temporary Temporary Large As above Y Slight haul route (south or west access options) resulting in silt-laden and polluted runoff to Bertha Loch Burn. Construction of a long box culvert and channel realignment, and drainage outfalls resulting in increased sediment supply and Temporary Moderate As above Y Neutral pollutant leaks/ spillages to Broxy Kennels Drain from construction plant and in-channel working methods.
Removal/installation of a replacement box culvert and drainage outfalls resulting in release of sediment and risk of pollutant Temporary Moderate As above Y Slight leaks/spillages to Cramock Burn from construction plant and in-channel working methods.
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) Operation Hydrology and Flood Risk An approximate 160m flood embankment along south bank of river, Installation of new immediately culvert within existing upstream of the floodplain predicted to realigned A9 road, ’squeeze’ floodwaters. will be created to Modelling indicates contain flows. This flows are channelled has been designed southwards along the for the 0.5% AEP embankment of the event with a 35% realigned A9 towards Permanent climate change Moderate Y Neutral Bertha Park access (intermittent) allowance to provide road and increases an additional level of flows onto the tie-in resilience. The point between the culvert under the realigned and existing existing A9 A9, as well as to a carriageway will be lodge (residential replaced and sized to property) within Bertha prevent any backing- Park. up upstream and flooding between the realigned and existing A9 roads.
Fluvial Geomorphology No significant effects predicted before mitigation Water Quality Road drainage Permanent Two levels of SuDS Moderate Y Neutral proposed to Broxy (intermittent) (filter drains and
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Nature of 23 Significance Mitigation / Geographical Importance Effect Potential Effect of Potential Enhancement Significance of Residual Effect (Permanent/ Effect Measures I E UK R L Temporary) Kennels Drain detention basin) to predicted to result in treat polluted runoff failure of both soluble prior to outfall. pollutants (dissolved These measures are copper and zinc) shown in HAWRAT against short-term to reduce pollutant Runoff Specific concentrations to Thresholds (RSTs) and within acceptable compliance failure of levels. dissolved copper against Environment Road drainage Quality Standards network and SuDS (EQS) at Step 2 components will be maintained and periodically inspected to avoid failure and reduce the risk of sub-optimal performance, blockage and flooding. Road drainage proposed to Bertha Loch Burn predicted to result in failure of one Permanent soluble pollutant Slight As above Y Neutral (intermittent) (dissolved zinc) against RSTs, but achieves compliance with EQS at Step 2.
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15.11 STATEMENT OF SIGNIFICANCE
One of the greatest risks to the water environment and aquatic ecology is of silt-laden and contaminated runoff entering the River Tay SAC directly from works on the banks, as well as the cumulative effects of in-channel and near-channel works in nearby tributaries during the construction phase. However, these construction works will be subject to a number of control and management measures, and in particular construction site licensing under CAR.
Following implementation of best practice and site-specific mitigation measures during construction and operation outlined in this chapter, all residual effects are predicted to reduce to no more than minor magnitude and Slight significance.
Overall, no adverse effects have been predicted from the proposed CTLR Project which are considered significant in the context of the EIA Regulations.
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CHAPTER 15 – ROAD DRAINAGE AND THE WATER ENVIRONMENT
Appendix 15.1 – Water Environment Baseline
Cross Tay Link Road
Revision Date Version Author Technical Checker Approver Number Reviewer P01 31.05.19 DRAFT E REID J MOORE R. McLEAN D. RITCHIE
P02 16.10.19 FINAL E REID J MOORE R. McLEAN D. RITCHIE
BIM Reference: 119046-SWECO-EWE-000-RP-EN-20036
This document has been prepared on behalf of Perth and Kinross Council by Sweco for the proposed Cross Tay Link Road Project. It is issued for the party which commissioned it and for specific purposes connected with the above-captioned project only. It should not be relied upon by any other party or used for any other purpose. Sweco accepts no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties.
This document contains confidential information and proprietary intellectual property. It should not be shown to other parties without consent from Perth and Kinross Council.
Prepared for: Prepared by: Perth and Kinross Council Sweco Pullar House Suite 4.2, City Park 35 Kinnoull Street 368 Alexandra Parade Perth Glasgow PH1 5GD G31 3AU
CHAPTER 15 APPENDIX 15.1 CROSS TAY LINK ROAD WATER ENVIRONMENT BASELINE EIA REPORT (VOLUME 2) CONDITIONS
1 WATER ENVIRONMENT BASELINE CONDITIONS 1.1 INTRODUCTION
This appendix provides detailed information on the baseline conditions of the watercourses within the 0.5km study area of the proposed CTLR Project, for each of the sub-topics of the Road Drainage and the Water Environment (RDWE) chapter (namely hydrology and flood risk, fluvial geomorphology and water quality). A summary of the key characteristics and baseline sensitivity of each watercourse is provided in Section 15.5 (Baseline Conditions) of Volume 2, Chapter 15: Road Drainage and the Water Environment. This appendix is supported by Figure 15.3: Water Environment and Mitigation.
1.2 WATERCOURSE BASELINE DESCRIPTIONS
There are eleven watercourses within the RDWE study area ranging from large rivers such as the River Tay to minor watercourses and small field/forestry drains (see Figure 15.3). An overview of the watercourses considered in the assessment is outlined in Table 1.1.
Table 1.1: Overview of Watercourses
Major Watercourse Minor Watercourse Field/Forest Drain • River Tay (SAC) • Bertha Loch Burn • Highfield Plantation Drain • River Almond • Broxy Kennels Drain (multiple) • Redgorton Drain • Sherrifton Wood Drain • Cramock Burn • Caravan Drain • Gelly / Whiggle Burn • Annaty Burn
Baseline information is provided for hydrology and flood risk, followed by a breakdown of each of the watercourses for fluvial geomorphology and water quality.
1.3 HYDROLOGY AND FLOOD RISK
1.3.1 Major Watercourses
The River Tay and the River Almond have a long history of flooding causing significant damage to properties within Perth and disruption to transport routes dating back to the 1800s. The updated hydraulic modelling of the River Tay supports the SEPA flood mapping with floodwaters predicted to be contained within the banks of the river in the location of the River Tay Crossing Bridge for the 0.5% AEP (1 in 200-year return period) flood event. Flood levels at the proposed crossing location were slightly higher than previous estimates following revisions made to the original model, with a peak 200-year water level of approximately 10.1m AOD predicted at the crossing. However, the SEPA mapping also predicts extensive flooding for the 0.5% AEP flood event along the eastern bank downstream of Perth Racecourse, which extends inland to Scone Palace, although this does not appear to intercept any residential/commercial receptors.
Flooding is predicted for the 0.5% AEP flood event along the southern bank of the River Almond shown on the SEPA flood mapping, resulting in extensive flooding within the City of Perth to the west of the Highland Mainline Railway. This appears to affect over 100 residential and commercial properties.
Both watercourses have an extensive system of flood defences, which include embankments, floodwalls, pumps and storage areas which protect areas of Perth. As a result of the existing floodplain and flood defences in place, the River Tay and River Almond have an overall Very High sensitivity for hydrology and flood risk.
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1.3.2 Minor Watercourses
Bertha Loch Burn and Broxy Kennels Drain are not shown on the SEPA flood mapping due to their small catchment size (<3km2), hence the level of flood risk from these sources is unclear. The SEPA 0.5% AEP (1:200 year) flood mapping appears to show an extensive area of flooding to the north of the Cramock Burn which affects Scone Caravan Park. There is also flooding predicted from the southern bank of the Cramock Burn which extends along the eastern boundary of Perth Racecourse and affects several buildings. However, the exact source and mechanisms of the flooding to the north is unclear and is thought to relate to issues with the representation of the Cramock Burn within the SEPA modelling.
The baseline 0.5% AEP (including climate change) flood extent shown in the Cramock Burn modelling differs from that shown in the SEPA flood mapping, with no flooding predicted to the north of the burn. Instead, flooding occurs along the southern bank of the Cramock Burn at three locations between the Stormontfield Road Bridge and upstream of the culvert adjacent to Scone Camping and Caravan Park. Floodwaters flow southwards along the eastern perimeter of Perth Racecourse affecting two commercial properties and a small path. Model sensitivity testing indicated that these results were robust with the area to the north remaining unaffected by flooding from the Cramock Burn. The Cramock Burn is considered to have an overall Medium sensitivity for hydrology and flood risk.
The baseline modelling for the Bertha Loch Burn predicts significant flooding along the southern bank upstream of the two culverts under the existing A9 road for the 0.5% AEP design event (including climate change). The floodwaters extend southwards and accumulate to the north of the public road to Bertha Park. A Culvert Inspection Report prepared by Hamilton Industrial Services (March 2018) reported significant sediment accumulation within one of the existing A9 culverts. Modelling found this sediment significantly worsened flooding from the southern bank upstream, increasing flows onto the A9 at the junction with the Bertha Park access road. The Bertha Loch Burn has therefore been classified as having a Medium sensitivity for hydrology and flood risk.
The baseline modelling for the Broxy Kennels Drain predicts backing up of water upstream of the existing A9 culvert due to an undersized (0.5m diameter) box culvert close to the inlet. This results in the northern bank of the channel immediately upstream of the A9 culvert overtopping slightly. However, the extent of the floodwaters is limited, and no residential/commercial receptors are affected. Therefore, the Broxy Kennels Drain has a Low sensitivity for hydrology and flood risk.
For the Gelly/Whiggle Burn and Annaty Burn, the SEPA flood mapping shows that the 0.5% AEP flood extent would be limited, generally to the bank edge of these watercourses. The main potential constraint along the Gelly/Whiggle Burn is the culvert at Langedge Bridge. A site visit undertaken indicated that this was in good condition with no blockages, and the presence of a high river terrace along the southern bank would contain any additional flooding.
The Annaty Burn is close to the A94 road, but this road remains outside of the 0.1% AEP (1:1000 year) SEPA flood extent. Whilst there were reported incidences of flooding from the Annaty Burn within Scone in 2004, which affected properties along Den Road, this is over 2km downstream. Within the study area, both watercourses are surrounded by agricultural land with a limited number of residential/commercial receptors nearby. Therefore, the Gelly/Whiggle Burn and Annaty Burn are both classified as having a Low sensitivity for hydrology and flood risk.
The catchment of Redgorton Drain is less than 3km2 and therefore there is information provided by SEPA flood mapping. The watercourse is heavily modified with a 1m diameter culvert under the A9. However, the culvert condition survey indicates that this is in a good condition with no blockages identified. Similarly, no previously recorded incidents of flooding could be found. Given that the surrounding area is largely rural, the Redgorton Drain is considered to have a Low sensitivity for hydrology and flood risk.
1.3.3 Land and Forestry Drains
Due to the drainage area of the Highfield Plantation Drains and the Sherrifton Wood Drain being less than 3km2 there is no information on SEPA’s flood mapping. No previously recorded incidents of
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CHAPTER 15 APPENDIX 15.1 CROSS TAY LINK ROAD WATER ENVIRONMENT BASELINE EIA REPORT (VOLUME 2) CONDITIONS flooding were identified and given that both watercourses are in isolated areas surrounded by agricultural land or woodland, both are classified as Low sensitivity for hydrology and flood risk.
The Caravan Drain connects into the Cramock Burn and was represented within the Cramock Burn hydraulic model discussed above. For the 0.5% AEP (including climate change) flood event, the drain was not predicted to overtop and did not affect any commercial/residential receptors nearby. The drain is classified as Low sensitivity for hydrology and flood risk.
1.3.4 Additional sources of flood risk
SEPA’s surface water (pluvial) flood mapping indicates there is a very low risk of surface water flooding within most of the study area, with some localised areas at high risk to the east of the River Tay. These localised patches of existing surface water flood risk do not pose a significant risk to the proposed CTLR Project.
BGS hazard mapping indicates that susceptibility to groundwater flooding is variable in the study area. There is the potential for groundwater flooding to occur at the surface during periods of extended intense rainfall at i) the grade-separated junction between the realigned A9 and the new link road (west of the River Tay); ii) from the eastern bank of the River Tay to the Stormontfield Road junction; and iii) around the junction with the A93. There is also moderate risk of groundwater flooding between the Stormontfield Road junction and the A93 junction, as well as at the connection with the A94. There are however, no known reported historical instances of groundwater flooding in the area and based on the BGS groundwater hazard mapping, the proposed road cutting locations are located in areas of limited potential for groundwater flooding.
SEPA’s Reservoir Inundation Map classifies the Bertha Loch as being at high risk of reservoir flooding, and mapping indicates that a breach would result in flooding along the southern bank of the Bertha Loch Burn, upstream of the A9 road. Floodwaters appear to flow southwards towards the River Almond affecting the existing A9. The River Tay is also predicted to be affected in the case of a breach of Loch Ericht and Loch An Daimh (both designated as high risk further upstream in the catchment) with extensive parts of north-eastern Perth and the area to the south of Perth Racecourse affected. However, floodwaters are shown to remain in bank upstream of Perth, near the proposed CTLR infrastructure. It should however be noted that these maps are designed to be used for emergency planning purposes and show the worst-case scenario in which all reservoirs simultaneously fail, which is considered extremely unlikely.
1.4 FLUVIAL GEOMORPHOLOGY AND WATER QUALITY
1.4.1 River Tay
The River Tay (Photo 1.1) is the longest river in Scotland (approximately 190km) and has a catchment area of approximately 5,000km2. The river flows easterly from its source in western Scotland through a number of lochs including Loch Dochart, Loch Lubhair and Loch Tay, before flowing south through Perth where it becomes tidal and discharges to the Firth of Tay.
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Photo 1.1: River Tay at the location of the River Tay Crossing Bridge (May 2018)
Fluvial Geomorphology
The River Tay is approximately 115m wide in the location of the River Tay Crossing Bridge and has a sinuous planform. The channel has a low gradient and sits on a wide fluvial plain. The channel typology is active meandering. The WFD morphology status of this reach of the River Tay (River Isla to River Earn Confluences) was classified as ‘Good’ in 2017.
The dominant process on the reach appears to be sediment transportation; the river displays fast uniform flows and lacks depositional features. Some evidence of erosion was observed on both banks. On the left bank there is evidence of moderate erosion having occurred previously across the whole bank but has been remedied by the installation of a deflector vein. On the right bank, some erosion and undercutting of the bank toe was observed.
There are few morphological pressures on this reach. A flow deflection vein composed of boulders is located on the left bank, which appears to have been in place for some time There are also sporadic short sections of bank protection (rip-rap) located further downstream on the left bank. Overall, the River Tay has been assigned a High sensitivity to modification for geomorphology.
Water Quality
The SEPA monitored reach of the River Tay in the study area (R Isla to R Earn Confluences, ID: 6498) had an overall status of ‘Good’ in 2017. It has been classified with a Physico-Chemical status of ‘High’ and Specific Pollutants status of ‘Pass’.
In accordance with this, chemistry sampling data collected from the river showed that concentrations of detected metals and polycyclic aromatic hydrocarbon (PAHs) generally all fell within EQS thresholds for freshwaters. The exceptions were slightly elevated concentrations of copper at both sampling sites, as well as an exceedance of zinc at the upstream sampling site. These dissolved heavy metals may
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CHAPTER 15 APPENDIX 15.1 CROSS TAY LINK ROAD WATER ENVIRONMENT BASELINE EIA REPORT (VOLUME 2) CONDITIONS have originated from various upstream sources as they are commonly used in industry and present in routine road runoff. SEPA monthly monitoring results taken at Queens Bridge in 2017 and 2018 indicate general compliance with dissolved copper and zinc concentrations against EQS for freshwaters.
There is an area of made ground associated with the former A9 trunk road, located to the south of the River Tay Crossing Bridge to the west of the River Tay, which could be a source of contamination (refer to Volume 2, Chapter 10: Hydrogeology and Soils for more information).
The River Tay is designated salmonid waters under the WFD. The River Tay supports European protected species including Atlantic salmon, lamprey species, otter and freshwater pearl mussel (FWPM). FWPM are particularly sensitive to elevated concentrations of suspended sediment and their presence is therefore a good indicator of water quality (refer to Chapter 9: Biodiversity for further information on aquatic species).
The River Tay is important for recreation and in particular salmon angling. Rafting/canoeing is known to be undertaken on the River Tay upstream of the study area, such as at Stanley and Aberfeldy, rather than in the smoother downstream reaches in the vicinity of the proposed CTLR Project. The watercourse is very large and is therefore considered to have a very high pollutant dilution and sediment dispersal capacity.
The study area is located within a designated drinking water protected area (DWPA), where a Scottish Water abstraction is located. The River Tay supplies Perth Gowans Terrace Water Treatment Works (WTW); the abstraction point is north-east of Perth. Overall, the River Tay has been assigned Very High sensitivity for water quality.
1.4.2 River Almond
The River Almond (Photo 1.2) is a tributary of the River Tay and is approximately 48km long. Its source is to the south-east of Loch Tay, flowing east through Almondbank before joining the Tay to the north of Perth (confluence at NGR NO 1010 2670).
Photo 1.2: River Almond (May 2018). (A) shows the old road bridge. (B) shows rock pavement protecting the right bank upstream of the A9 road bridge.
Fluvial Geomorphology
The River Almond is a tributary of the River Tay which sits in a relatively narrow valley composed of terraced fluvial deposits and has a pool-riffle typology within the study area. The channel has a slightly sinuous planform with a uniform/tranquil flow. There are 15m high banks on either side of the channel, both of which are straight and relatively steep. The WFD morphology status of this reach (River East Pow to River Tay Confluences) was classified as Good in 2017.
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Both erosion and deposition are evident within the reach. The bed is composed of gravel and cobbles, and two medial bars were observed between the old road bridge and the A9 bridge. Erosion is occurring on both banks from parallel flow. Some undercutting is occurring and has resulted in small shallow landslides. Along steeper sections of the banks some shallow slumping of bank material have occurred, leading to build up of bank debris at the toe of the slopes.
There are several morphological pressures on this reach. Three bridges cross the River Almond within the study area; the old road bridge, the A9 bridge and the Highland Mainline Railway bridge. The old bridge has a large in-channel pier and bank protection on both banks, comprising rip-rap and gabion baskets. There is a boulder revetment/pavement under the A9 road bridge, and stone wall and concrete abutments under the rail bridge. The river bed under the rail bridge is reinforced with a rock mattress.
Overall, the River Almond has been assigned a High sensitivity to modification for geomorphology.
Water Quality
The reach of the River Almond monitored by SEPA (R East Pow to R Tay Confluences, ID: 6506) in the study area achieved an overall status of ‘Good’ in 2017. This included a Physico-Chemical status of ‘High’ and Specific Pollutants status of ‘Pass’.
In accordance with this, chemistry sampling data collected from the River Almond showed that concentrations of detected metals and PAHs generally all fell within published EQS for freshwaters, with the exception of dissolved copper at both upstream and downstream sampling sites. Again, this may have originated from various upstream sources such as routine road runoff. SEPA monthly monitoring results taken at Almond Bridge in 2017 and 2018 show general compliance with dissolved copper concentrations against EQS for freshwaters.
The River Almond is also designated salmonid waters under the WFD (associated with the River Tay). There is evidence of breeding salmon and lamprey and it supports foraging otter (see Chapter 9: Biodiversity for further information). The watercourse is medium-sized and is considered to have a moderate pollutant dilution and sediment dispersal capacity.
The Inveralmond Brewery and Inveralmond Industrial Estate are situated near the confluence of the Almond and the Tay. Overall, the River Almond has been assigned High sensitivity for water quality.
1.4.3 Bertha Loch Burn
Bertha Loch Burn (Photo 1.3) flows east from Bertha Loch Burn for approximately 1.7km to the River Tay. The burn is joined by another outflow from the loch upstream of the confluence.
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Photo 1.3: Bertha Loch Burn facing upstream from near A9 embankment (May 2018)
Fluvial Geomorphology
Bertha Loch Burn is a tributary of the River Tay which crosses a fluvial plain, situated in a wider valley composed of hummocky raised marine deposits. The burn has a narrow channel, which appears to have been artificially straightened for agricultural drainage. The channel width is approximately 2.5m at top of bank and the flow is uniform/tranquil. The channel has uniform dimensions throughout its length from the farm trail to the A9 and displays few geomorphic features. Very limited erosion and deposition were observed in the channel. The section close to the confluence with the River Tay is more natural with riffles and stone line development.
Bertha Loch Burn does not have a morphology classification as it is not monitored by SEPA. It has therefore been assigned a Low sensitivity to modification for geomorphology.
Water Quality
Bertha Loch Burn is not monitored by SEPA. Surrounding land-use is predominantly woodland and arable farmland and it is crossed by the A9 and Highland Mainline Railway; therefore, the burn may receive intermittent point source pollutants from road and railway runoff and diffuse agricultural runoff, which could affect water quality.
Chemistry sampling data collected from the burn showed that concentrations of detected metals and PAHs generally all fell within published EQS for freshwaters, with the exception of dissolved copper which was slightly elevated at both sampling sites.
The watercourse is small and is considered to have a low pollutant dilution and sediment dispersal capacity. No protected aquatic species were recorded in the burn; however there is considered to be suitable habitat to support salmonids and lamprey in the lower reaches. Overall, Bertha Loch Burn has been assigned Medium sensitivity for water quality.
1.4.4 Broxy Kennels Drain
Broxy Kennels Drain (Photo 1.4) is a minor field ditch, sitting in a low point between hummocky raised marine deposits and is a tributary of the River Tay. Despite historic OS mapping showing the drain to
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CHAPTER 15 APPENDIX 15.1 CROSS TAY LINK ROAD WATER ENVIRONMENT BASELINE EIA REPORT (VOLUME 2) CONDITIONS be more extensive, it currently has less than 200m of open channel. It appears to have been culverted both in the upstream agricultural land and under the existing A9 and railway embankment downstream.
Photo 1.4: Broxy Kennels Drain facing downstream towards A9 embankment (May 2018)
Fluvial Geomorphology
Broxy Kennels Drain has a small straightened channel. The current channel is over-deepened and is approximately 2m wide and 2m deep, with uniform dimensions throughout its length. The drain displays few geomorphic features. Broxy Kennels Drain does not have a morphology classification as it is not monitored by SEPA. Overall, it has been assigned a Low sensitivity to modification for geomorphology.
Water Quality
Broxy Kennels Drain is not monitored by SEPA. Surrounding land-use is mainly farmland and grassland and it is culverted beneath the existing A9 carriageway and Highland Mainline Railway. The drain is likely to flow intermittently and does not support any protected species. It has a very low pollutant dilution and sediment dispersal capacity. Overall, Broxy Kennels Drain has been assigned Low sensitivity for water quality.
1.4.5 Cramock Burn
Cramock Burn (Photo 1.5) originates to the north of Scone and flows roughly west for 5km before discharging to the River Tay (NGR NO 0999 2701).
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Photo 1.5: (A) shows Cramock Burn close to the River Tay confluence. (B) shows Cramock Burn upstream on the northern perimeter of the racecourse where is has been straightened and over- deepened (May 2018)
Fluvial Geomorphology
Cramock Burn is currently crossed by Stormontfield Road. The burn is a tributary of the River Tay, crossing a wide fluvial plain in the section north of Perth Racecourse and hummocky raised marine deposits close to Stormontfield Road. Upstream of Stormontfield Road, Cramock Burn has a more natural sinuous planform. However, downstream of Stormontfield Road along the northern perimeter of the racecourse, the burn appears to have been straightened. The straightened section of Cramock Burn has an over-deepened channel with uniform dimensions and displays few geomorphic features. Closer to the River Tay confluence, Cramock Burn appears more natural and displays some morphological diversity and some bank erosion. Generally the burn has a sandy, gravel bed and cohesive banks.
Immediately downstream of Stormontfield Road culvert, the Cramock Burn has a somewhat irregular layout. It appears that some flood relief channels and surface water ponds have been created. The burn has been culverted in several locations, with hard bank protection on both sides of the channel upstream and downstream of some of these culverts.
Cramock Burn is not monitored by SEPA. Overall it is assigned a Low sensitivity to modification for geomorphology.
Water Quality
The burn is not monitored by SEPA; the surrounding land-use is mainly arable farmland, some woodland and is crossed by the A93 and Stormontfield Road and runs between Scone Camping and Caravan Park and Perth Racecourse. The burn may therefore receive intermittent point source pollutants from road runoff and diffuse agricultural runoff, which could impact water quality.
Chemistry sampling data showed that concentrations of detected metals and PAHs generally all fell within EQS for freshwaters, apart from slightly elevated concentrations of copper and ammonia at the downstream sampling site. An elevated concentration of ammonia may be attributable to agricultural pollution, untreated sewage and/or decay of plant and animal material, which can result in an increase in algal growth and is toxic to fish and aquatic species.
The watercourse is fairly small and is considered to have a low pollutant dilution and sediment dispersal capacity. Ecology surveys downstream of Perth Racecourse highlighted potential presence of lamprey and potential evidence of otter, but it generally lacks suitable habitat for any other fish species (see Chapter 9: Biodiversity for further information).
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Overall, Cramock Burn has been assigned Medium sensitivity for water quality.
Caravan Drain
The ‘Caravan Drain’ (Photo 1.6) runs on the northern perimeter of Scone Caravan Park, before flowing south on the western perimeter before joining the Cramock Burn.
Photo 1.6: Caravan Drain upstream of confluence with Cramock Burn (May 2018)
Fluvial Geomorphology
The Caravan Drain is a straightened drainage ditch with relatively uniform channel dimensions throughout its length. The channel is approximately 5m wide at top of bank and approximately 4m deep. The channel has a silty sand bed and banks and uniform/tranquil flow. The banks are steep and straight with occasional slumping of material off the banks. There are limited geomorphic features in this channel. The Caravan Drain is not monitored by SEPA. Overall, it has been assigned Low sensitivity to modification for geomorphology.
Water Quality
This drain is likely to flow intermittently and does not support any protected species. The burn has a very low pollutant dilution and sediment dispersal capacity. Overall, this drain has been assigned Low sensitivity for water quality.
1.4.6 Sheriffton Wood Drain
Sheriffton Wood Drain is a straightened field ditch (Photo 1.7), which flows from an unnamed pond near Sheriffton Wood to the River Tay (approximate NGR NO 0959 2760).
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Photo 1.7: Sheriffton Wood Drain facing upstream from near River Tay (May 2018)
Fluvial Geomorphology
This relatively straightened ditch has uniform channel dimensions, approximately 3m wide at top of bank and 1.5m deep. There was a small stream flowing in the channel at the time of the survey, but it is likely that this is an ephemeral ditch. It is not monitored by SEPA and has been assigned a Low sensitivity to modification for geomorphology.
Water Quality
This drain is likely to flow intermittently and does not support any protected species. The drain has a very low pollutant dilution and sediment dispersal capacity. Overall, this drain has been assigned Low sensitivity for water quality.
1.4.7 Redgorton Drain
Redgorton Drain (Photo 1.8) is a small tributary of the River Tay, originating at Redgorton and is culverted under the A9 carriageway and Highland Mainline Railway before discharging to the Tay (NGR NO 0970 2820).
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Photo 1.8: Redgorton Drain facing downstream towards River Tay confluence (May 2018)
Fluvial Geomorphology
Redgorton Drain has a narrow (approximately 0.75m width) and shallow (approximately 0.3m depth) channel with a cobble and gravel bed. Limited modifications and channel pressures were observed during the site visit. Redgorton Drain is not monitored by SEPA and has been assigned a Low sensitivity to modification for geomorphology.
Water Quality
Redgorton Drain is crossed by the B8063 road and runs close to a number of other minor roads and therefore may receive intermittent road runoff pollutants. An elevated concentration of dissolved copper was identified from the water sampling data, which may be attributable to road drainage inputs.
The watercourse has a low pollutant dilution and sediment dispersal capacity. It does not support any protected aquatic species. Overall, this drain has been assigned Low sensitivity for water quality.
1.4.8 Gelly/Whiggle Burn
The Whiggle Burn (Photo 1.9) flows from Muirward Wood in a westerly direction, before becoming the Gelly Burn and outfalling to the River Tay at approximate NGR NO 1040 2860.
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Photo 1.9: Whiggle Burn flowing through Muirward Wood (May 2018)
Fluvial Geomorphology
Whiggle Burn has a gravel and cobble stream bed, with intermittent steep but short valley sides. Some bedrock is exposed at the base of the channel banks. The channel has a sinuous planform with some alternating depositional gravel and cobble bars, and some evidence of bank erosion. The Whiggle Burn is not monitored by SEPA and has been assigned Low sensitivity to modification for geomorphology.
Water Quality
Whiggle Burn is not monitored by SEPA; in its upstream reach the surrounding land use is woodland and downstream the land-use is a mixture of woodland and arable farmland. The burn may therefore receive nutrient-rich runoff from agriculture and woodland soils, as well as road runoff inputs at the A93 and Stormontfield Road crossings. An elevated concentration of dissolved copper was identified from the water sampling data, which may be attributable to road drainage inputs.
There is an infilled quarry at Balboughty Farm, in the vicinity of the A93 roundabout, which could be a source of contamination (approximately 100m from Whiggle Burn).
The watercourse is small and is considered to have a low pollutant dilution and sediment dispersal capacity. It is not known to support any protected aquatic species or habitats. Overall, Whiggle Burn has been assigned Medium sensitivity for water quality.
1.4.9 Annaty Burn
Annaty Burn (Photo 1.10) is referred to as Mill Lade in its upper reaches. It flows from the east of Balbeggie, through the southern portion of Scone and outfalls to the Tay (NGR NO 1178 2525).
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Photo 1.10: Annaty Burn in the eastern portion of the study area (May 2018)
Fluvial Geomorphology
Annaty Burn is a tributary of the River Tay, which crosses undulating till deposits. It has an approximate width of 2m and a cobble and gravel stream bed. The channel appears to be mostly natural with few modifications and has riffle glide morphology. In some reaches the stream is unconfined, but close to the road it crosses through a more confined valley.
The Annaty Burn does not have a morphology classification as it is not monitored by SEPA. Overall it has been assigned a Medium sensitivity to modification for geomorphology.
Water Quality
Annaty Burn is not monitored by SEPA and flows through a largely rural agricultural catchment, as well as part of Scone and is crossed by the A93 and A94 roads, as well as a number of minor roads. The burn may therefore receive intermittent point source pollutants from roads and diffuse agricultural runoff, which could impact on water quality. Chemistry sampling data collected from the burn showed slightly elevated concentrations of ammonia, copper and zinc against published EQS at both sampling sites, which may be attributable to pollutants contained in agricultural and road runoff.
The watercourse is fairly small and is considered to have a low pollutant dilution and sediment dispersal capacity. It is not known to support any protected aquatic species or habitats, but there is some evidence of otter using the watercourse.
Overall, Annaty Burn has been assigned Medium sensitivity for water quality.
1.4.10 Highfield Plantations Drains
The Highfield Plantations (Photo 1.11) is a network of artificial forestry drains which drain into the Cramock Burn.
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Photo 1.11: Highfields Plantation Drain in vicinity of the proposed CTLR carriageway (May 2018)
Fluvial Geomorphology
The Highfield Plantation contains several shallow, relatively straight drainage channels which are cut into till. The channels have a straight planform and uniform dimensions, approximately 0.5m wide by 0.5m deep, and mainly contain leaf litter. Some of the channels contained standing water at the time of the site visit but no discernible flow.
These drains are not monitored by SEPA and have been assigned a Low sensitivity to modification for geomorphology.
Water Quality
The surrounding land-use is commercial forestry and therefore the Highfield Drains (and downstream Cramock Burn) may receive intermittent nutrient-rich runoff. The drains are likely to be ephemeral and have a very low pollutant dilution and sediment dispersal capacity and support no protected aquatic species. Overall, the drains have been assigned Low sensitivity for water quality.
November 2019 PAGE 15 OF APPENDIX 15.1
CHAPTER 15 APPENDIX 15.1 CROSS TAY LINK ROAD WATER ENVIRONMENT BASELINE EIA REPORT (VOLUME 2) CONDITIONS
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November 2019 PAGE 16 OF APPENDIX 15.1
AppendixNovember 120195. 2 – Flood Risk AssessmentPAGE 0 OF CHAPTER 6
Report
Sweco UK Limited FRA Report Sweco 2nd Floor Quay 2 Flood Risk Assessment Report 139 Fountainbridge Edinburgh, EH3 9QG +44 131 550 6300
22 October 2019 Project Reference: 119046 Document Reference: 119046-SWECO-EWE-000-RP-EN-20035 Revision: P02 Prepared For: Perth and Kinross Council
www.sweco.co.uk 1 of 112 Status / Revisions
Rev. Date Reason Prepared Reviewed Approved for issue
P01 02.05.19 S3 - For JJW 02.05.19 JP 02.05.19 CC 21/06/19 Review
P02 23/06//19 Updated JJW 22.10.19 JP 22.10.19 JPF 23/06/19 following review
© Sweco 2018. This document is a Sweco confidential document; it may not be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, photocopying, recording or otherwise disclosed in whole or in part to any third party without our express prior written consent. It should be used by you and the permitted disclosees for the purpose for which it has been submitted and for no other.
Reg. Office Address: Reg. No.: 2888385 Sweco UK Limited James Walker Sweco UK Limited Reg. Office: Leeds Sweco 2nd Floor Quay 2 Assistant Consultant Grove House 139 Fountainbridge +44 131 550 6462 Mansion Gate Drive www.sweco.co.uk Edinburgh, EH3 9QG Leeds, LS7 4DN +44 131 550 6300 [email protected] +44 113 262 0000 2 of 112 Executive Summary Sweco were commissioned by Perth and Kinross Council (PKC) to undertake a Flood Risk Assessment (FRA) in support of the planning application for the proposed Cross Tay Link Road (CTLR) project. The aim of the FRA was to demonstrate that the development proposal, inclusive of any mitigation elements, is compliant with Scottish Planning Policy (SPP, 2014)1 and the Flood Risk Management (Scotland) Act (2009). The CTLR proposal involves the construction of a new 6-kilometre link road running west to east connecting the existing A9 road with Stormonfield Road, the A93 and A94. The design includes the installation of: · a new bridge over the River Tay and associated infrastructure; · a new culvert crossing along the Bertha Loch Burn; · a new culvert along the Broxy Kennels Drain; and · replacement of the Stormontfield Road bridge culvert along the Cramock Burn. A review of the SEPA flood mapping indicated that the 1:200-year fluvial flood extent is contained within the banks of the River Tay in the vicinity of the bridge crossing. Most of the route of the link road is at very low fluvial flood risk. However, the area along the eastern bank of the River Tay, near to the proposed bridge, is predicted to be inundated for the 1 in 1000-year event - classified as low risk. The Bertha Loch Burn and Broxy Kennels Drain areas are not covered by the SEPA flood mapping. There is an area at medium to high fluvial flood risk to the north of the Cramock Burn, which overlaps the proposed link road. The exact source and mechanism of this flooding was unclear. To determine the current fluvial flood risk, and assess the impact of the CTLR project, fluvial modelling was undertaken for each of the four watercourses identified. River Tay Hydraulic modelling was undertaken for the River Tay using a revised and updated version of a previous 1D ISIS model developed by Halcrow. The model was used to assess existing fluvial flood risk, and then to evaluate the impact of the new bridge. The baseline modelling predicted a peak design water level, based on the critical 1:200- year return period event inclusive of climate change, of 10.095 mAOD at the cross section where the new bridge is proposed. The flood extent was consistent with the SEPA flood mapping, in that the design floodplain of the River Tay is predicted to be contained within the near-bank area in the vicinity of the CTLR crossing. The bridge has been designed such that the soffit has a freeboard of approximately 1.6 m above the peak design water level and is compliant with DMRB standards. The eastern pier of the proposed bridge was found to be within the floodplain. Further post-development modelling predicted that the proposed bridge arrangement will have a negligible impact on flood levels upstream and downstream with a maximum increase of 1 mm, which is within model tolerances. A negligible floodplain displacement of 4 m3
1 Scottish Government (2014). Scottish Planning Policy.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 3 of 112 was estimated for the eastern pier and, following consultation with SEPA, it was agreed that compensatory storage would not to be required. Cramock Burn Hydraulic modelling was undertaken for the Cramock Burn to clarify the flood risk to the CTLR, and to determine the impact of the replacement Stormontfield Road bridge culvert. The predicted 1:200-year (including a climate change allowance) baseline flood extent differed from that shown in the SEPA Flood Map. Unlike the SEPA flood map, the baseline model did not predict flooding to the north of the Cramock Burn near the proposed CTLR route. The baseline model predicts flooding from the southern bank of the Cramock Burn at three locations between the Stormontfield Road bridge and upstream of the culvert adjacent to the caravan park. Floodwaters then head south along the eastern perimeter of Perth Racecourse. Because of the proximity to CTLR route, and the presence of flood receptors, sensitivity analysis was undertaken to ensure that model results were robust. The replacement of the Stormontfield Road culvert with a 2.5 m wide by 2.0 m high box culvert was predicted to provide over 300 mm freeboard both upstream and downstream. This was agreed with SEPA to be appropriate given the road constraints and would provide a net betterment compared with the existing culvert, which surcharges for the 1: 200-year (including a climate change allowance) event. Further analysis indicated that the new culvert is predicted to have no significant impact on flood risk or flows downstream. Bertha Loch Burn Modelling was undertaken for the Bertha Loch Burn to determine the existing flood risk, the impact of the new CTLR culvert, and to identify the need for mitigation. The baseline modelling predicts flooding for the 1:200-year design event (including a climate change allowance) along the southern bank of the Burn upstream of the two A9 culverts. The realigned A9 occupies part of the floodplain resulting in an increased flow southwards. The flow follows the embankment of the realigned A9 towards the Bertha Park access road, increasing flow onto the tie-in point between the realigned and existing A9 road. Without mitigation the design is potentially non-compliant with Scottish Planning Policy (2014). The mitigation strategy, agreed with SEPA and PKC, replaces the two existing culverts under the A9 with a larger rectangular culvert to prevent backing-up upstream. Embankment along the southern bank to contain floodwaters will also be provided. These measures prevent flooding from the southern bank but lead to a small increase in water depths within the Perth-to-Inverness railway culvert although a freeboard of over 400mm is still retained. The new re-aligned A9 box culvert, with dimensions 2 m by 2 m, has a freeboard of 750 mm and allows for the provision for a mammal ledge within the culvert. The replacement box culvert for the two A9 culverts has a freeboard of 300 mm, which was agreed with SEPA to be appropriate given the design constraints. The replacement culvert would still provide a net betterment compared with the existing culvert, which is predicted to surcharge for the 1: 200-year (including a climate change allowance) event.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 4 of 112 Broxy Kennels Drain Modelling was undertaken for the Broxy Kennels Drain to determine the existing flood risk, determine the impact of the new CTLR culvert, and identify the need for mitigation. The baseline modelling predicted a small amount flooding for the 1:200-year event (including a climate change allowance). This was associated with the undersized 500 mm box culvert upstream of the A9 culvert, which was in poor condition. The open sections of watercourse and 500 mm box culvert will be replaced with a 1.2 m box culvert. The model predicts that the proposed culvert would accommodate a 1:200-year flood (with a climate change allowance) and provide a freeboard greater than 600 mm. The impact upon the Perth-to-Inverness railway culvert, which will be retained, was found to be minimal (<5 mm). Climate change All of the 1:200-year peak flow estimates used in the modelling include a 20% allowance to account for climate change (with the exception of the Bertha Loch Burn embankment which includes an allowance of 35%). For all culverts, climate change effects have been explicitly incorporated with suitable freeboard allowances following discussions with SEPA. Additional Sources of Flood Risk The CTLR crossing is beyond the tidal limit, with flood risk being fluvially rather than tidally dominant. SEPA mapping indicates that there is a very low risk of surface water flooding within the study area. The CTLR includes provision of Sustainable Drainage Systems (SuDS’) to capture and attenuate runoff from the new road surfaces before discharging to nearby watercourses. The CTLR intersects the catchments of several watercourses, earthwork drainage and conveyance are provided to ensure that connectivity between watercourses and their catchments are maintained and prevent pooling. These have generally been designed to work with the existing topography however where there will be a change in catchment area, as with the Broxy Kennels Drain and Bertha Loch Burn, this has been considered in the hydraulic modelling. There is likely to be a net increase in flows to the Whiggle Burn. However, this is unlikely to alter the predicted flood extents and risk downstream given the confined nature of the channel and lack of nearby receptors. British Geological Survey (BGS) mapping indicates that the susceptibility to groundwater flooding is variable along the route of the CTLR. There is the potential for groundwater flooding to occur at the surface during periods of extended intense rainfall at the junction between the realigned A9 and the new link road (west of the River Tay), as well as from the eastern bank of the River Tay to the junction with Stormontfield Road. There is also moderate risk between the Stormontfield Road junction and the A93 junction, as well as at the connection with the A94. There are however no known reported historical instances of groundwater flooding in the area and areas of high risk are not coincident with the proposed cutting locations. SEPAs Reservoir Inundation Map indicates that the River Tay and Bertha Loch Burn would be affected by the uncontrolled release of water in the event of a dam failure. The Bertha Loch is classified by SEPA as ‘high risk’.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 5 of 112 River Almond Temporary Bridge Crossing The impact of a temporary bridge located downstream of the Perth-Inverness railway bridge was assessed. Modelling predicts that the proposed bridge soffit will accommodate the 1:200-year design water level with over 600mm freeboard. Both the abutments and the northern embankment are predicted to be located within the floodplain. The compensatory storage volume was estimated to be 750 m3 and two potential sites for compensatory storage were identified. SEPA have agreed to forgo this requirement due to the severe environmental impacts associated with the provision of compensatory storage.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 6 of 112 Table of contents 1 Introduction...... 12 2 Legislative Framework ...... 13 2.1 Flood Risk Management (Scotland) Act 2009 ...... 13 2.2 Scottish Planning Policy (2014) and SEPA Technical Guidance (2015)...... 13 2.3 Climate Change ...... 14 3 Site Location and proposal...... 15 3.1 Hydrological Characteristics of the Study Area ...... 19 3.1.1 The River Tay ...... 19 3.1.2 Minor watercourses (Cramock Burn, Bertha Loch Burn and Broxy Kennels) ...... 20 3.1.2.1 Cramock Burn...... 20 3.1.2.2 Bertha Loch Burn...... 21 3.1.2.3 Broxy Kennels Drain...... 22 3.2 Land/forestry drains ...... 23 3.3 Geology and hydrogeology...... 23 3.4 Scottish Environmental Agency (SEPA) Flood Map...... 26 3.4.1 Fluvial Flood Risk ...... 26 3.4.2 Coastal Flood Risk ...... 27 3.4.3 Surface Water (pluvial) flood risk ...... 27 3.4.4 Reservoir Flood Risk ...... 30 3.5 Sewer Flooding ...... 30 3.6 Groundwater Flood Susceptibility ...... 31 3.7 Historical Flood Event ...... 34 3.7.1 River Tay ...... 34 3.7.2 Historical flooding of the Cramock, Bertha and Broxy Kennels Drains ...... 35 3.8 Formal Flood Prevention Schemes ...... 36 3.9 Summary of Previous Studies ...... 36 3.9.1 River Tay Modelling...... 36 3.9.2 Minor watercourse Modelling ...... 40 4 River Tay Modelling ...... 42 4.1 Model Revision and updates ...... 42 4.1.1 1D Model Updates...... 42 4.1.2 Updates to the Model Boundary Conditions...... 43 4.1.2.1 Model Upstream Boundaries...... 43
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 7 of 112 4.1.2.2 Model Downstream Boundary ...... 44 4.1.3 Model Calibration...... 44 4.2 Baseline Model Predictions...... 44 4.3 Post Development Modelling Results ...... 45 4.4 Implications upon the CTLR Design ...... 47 5 Minor Watercourse Modelling ...... 48 5.1 Cramock Burn ...... 48 5.1.1 Methodology ...... 48 5.1.1.1 1D-2D Build ...... 48 5.1.1.2 Hydraulic Structures ...... 49 5.1.1.3 Roughness Values ...... 50 5.1.1.4 Upstream Boundary...... 50 5.1.1.5 Downstream Boundary...... 51 5.1.2 Baseline Model Predictions ...... 51 5.1.2.1 Sensitivity Analysis ...... 52 5.1.3 Post Development Scenario...... 55 5.1.4 Implications upon the CTLR design and mitigation requirement ...... 57 5.2 Bertha Loch Burn ...... 58 5.2.1 Baseline Model Methodology ...... 58 5.2.1.1 1D-2D Model Build ...... 58 5.2.1.2 Hydraulic Structures ...... 59 5.2.1.3 Upstream Boundary conditions ...... 59 5.2.1.4 Downstream Boundary Conditions...... 61 5.2.2 Baseline Model Predictions ...... 61 5.2.2.1 Sensitivity Analysis ...... 63 5.2.3 Post Development Scenario...... 63 5.2.4 Post Development (no mitigation) Scenario Predictions ...... 65 5.2.5 Implications upon the CTLR Design and mitigation requirement...... 68 5.2.5.1 Bertha Loch Burn Mitigation ...... 68 5.3 Broxy Kennels Drain ...... 72 5.3.1 Baseline Modelling Methodology...... 72 5.3.1.1 1D-2D Build ...... 72 5.3.1.2 Hydraulic Structures ...... 73 5.3.1.3 Upstream Boundary Conditions ...... 73
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 8 of 112 5.3.1.4 Downstream Boundary Conditions...... 75 5.3.2 Baseline Model Predictions ...... 75 5.3.1 Post Development Scenario...... 76 5.3.1.1 Post-Development Hydrology...... 77 5.3.2 Post Development Scenario Predictions ...... 77 5.3.3 Sensitivity analysis ...... 78 5.3.4 Implications upon the CTLR and mitigation requirement ...... 78 6 Additional analysis ...... 79 6.1 Outfalls from the proposed drainage ...... 79 6.1.1 Whiggle Burn ...... 79 6.2 River Almond Temporary Access Bridge...... 80 6.2.1 Methodology ...... 82 6.2.2 Results...... 82 6.2.3 Implication upon the CTLR project and mitigation requirements ...... 83 7 Conclusion...... 85
Table of figures Figure 3-1 Overview of the proposed CTLR scheme...... 16 Figure 3-2 Schematic of bridge crossing over the River Tay including deck levels...... 17 Figure 3-3 Location of new and replacement culverts...... 18 Figure 3-4 River catchments as defined in the Tay Local Plan District: Flood Risk Management Strategy ...... 19 Figure 3-5 Overview of the Cramock Burn and catchment defined on FEH website...... 21 Figure 3-6 The Bertha Loch Burn catchment as defined on the Flood Estimation Handbook website ...... 22 Figure 3-7 Overview of the Broxy Kennels Drain estimated catchment areas and flow pathways delineated in ArcGIS ...... 23 Figure 3-8 Overview of superficial deposits within the area of the CTLR route (source: British Geological Survey) ...... 25 Figure 3-9 SEPA Fluvial Flood Map...... 26 Figure 3-10 SEPA Tidal Flood Risk Map ...... 27 Figure 3-11 SEPA Surface Water Flood Map ...... 28 Figure 3-12 Sources of flood risk, based on comparison of CTLR route and SEPA Flood Maps ...... 29 Figure 3-13 SEPA Reservoir Inundation Map showing the indicative area that may flood from an uncontrolled release of water from all possible dam failure scenarios...... 30 Figure 3-14 Groundwater flood risk map. The area in blue denotes locations where groundwater could influence the duration and extent of flooding from other sources...... 31 Figure 3-15 BGS Groundwater Flood Susceptibility Map ...... 33
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 9 of 112 Figure 3-16 Picture of flood event along the River Tay and Cramock Burn (Image provided by SEPA)...... 35 Figure 3-17 Existing flood prevention scheme in the River Tay, Almond ...... 36 Figure 3-18 River Tay at Ballathie return period hydrographs obtained by scaling the December 2006 event...... 38 Figure 3-19 River Almond at Almondbank hydrographs obtained by scaling the December 2006 event...... 38 Figure 3-20 River Earn at Forteviot return period hydrographs obtained by scaling the December 2006 event ...... 39 Figure 3-21 1:200-year flood extents predicted for the Bertha Park FRA (Kaya Consulting Ltd, 2015)...... 41 Figure 4-1 Bathymetric-topographic survey cross-sections conducted for CTLR, noting that a bathymetric survey patch was conducted near the proposed crossing to facilitate any future refinement...... 42 Figure 4-2: Predicted versus observed water levels in the River Tay at the Perth gauge for the December 2006 flood event...... 44 Figure 4-3 Overview of the proposed River Tay bridge in relation to the 1:200-year (including climate change) water level...... 46 Figure 5-1: Modelled extents of the Cramock Burn, including drainage ditch around caravan site...... 49 Figure 5-2: Synthetic design 1:200-year (including climate change) hydrograph for the main channel and tributary...... 51 Figure 5-3 Peak 1:200-year flood depth at cross-section 8 along the caravan park drainage ditch ...... 52 Figure 5-4 Overview of the predicted 1:200-year flood event (with a 20% allowance for climate change) for the Cramock Burn...... 53 Figure 5-5 1:200-year flood extents for the Cramock Burn with the Stormonfield Road culvert removed, the model truncated, and inflows increased by 70%...... 54 Figure 5-6 Peak 1:200-year water level upstream of the replacement Stormontfield Road Bridge Culvert (2.5m by 2m)...... 56 Figure 5-7 Overview of Bertha Loch Burn Baseline Model ...... 58 Figure 5-8 Inflow hydrographs used for the Bertha Loch Burn baseline hydrology...... 61 Figure 5-9 1:200-year (including climate change uplift) baseline model flood extents for Bertha Loch Burn without sediment included within the A9 culverts...... 62 Figure 5-10 1:200-year (including climate change uplift) baseline model flood extents for Bertha Loch Burn with sediment included within the A9 culverts...... 62 Figure 5-11 Cross-section upstream of the proposed new culvert ...... 64 Figure 5-12 Longitudinal section showing where new culvert is proposed...... 64 Figure 5-13 Estimated areas which will either be removed or incorporated into the Bertha catchment...... 65 Figure 5-14 Maximum Flood extent for 1:200-year (including climate change) post development scenario. For this scenario a 2.0m rectangular culvert was added for the new CTLR crossing. 66 Figure 5-15 Overview of the predicted flow pathways along the right-hand bank of the Bertha Loch Burn for the baseline scenario ( 1:200-year including climate change event) ...... 67 Figure 5-16 Overview of the predicted flow pathways along the right-hand bank of the Bertha Loch Burn for the post development scenario ( 1:200-year including climate change event) .... 67 Figure 5-17 Predicted flows onto the A9 road tie-in-point for the baseline and post development scenarios ...... 68
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 10 of 112 Figure 5-18 Overview of extension to cross-sections to embankment crest ...... 69 Figure 5-19 Overview of river cross-section 7 with embankment represented...... 70 Figure 5-20 Post development with mitigation 1: 200-year (+ climate change) Flood Extent..... 71 Figure 5-21 Overview of the Broxy Kennels Baseline Model showing the cross-sections and drainage network...... 72 Figure 5-22 Modelled cross-section upstream of the A9 culvert...... 73 Figure 5-23 1:200-year Inflow FEH hydrographs used for Broxy Kennels Drain...... 74 Figure 5-24 Water depths upstream of the rectangular culvert upstream of the A9 in relation to culvert height ...... 75 Figure 5-25 1:200-year (including 20% climate change) flood extent for the Broxy Kennels watercourse ...... 76 Figure 5-26 Overview of the new culverted section of the Broxy Kennels Drain in the post development model ...... 77 Figure 6-1 Overview of the areas discharging to the Whiggle Burn from the CTLR drainage system ...... 80 Figure 6-2 Schematic of the proposed temporary access bridge over the River Almond...... 81 Figure 6-3 Representation of the temporary bridge over the River Almond in relation to the 1:200-year predicted peak flood depth...... 82
Appendices Appendix A – Overview of watercourses in the nearby area Appendix B – Overview map of topographic data and LiDAR Appendix C – Model 2D Domains Appendix D – Cramock Burn Model Results 96 Appendix E – Bertha Loch Burn Model Results Appendix F – Bertha Loch Burn SEPA Correspondence Appendix G – Broxy Kennels Drain Model Results Appendix H – SEPA Correspondence on the River Almond Temporary Bridge
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 11 of 112 1 Introduction Where the Planning Authority considers that there might be a risk of flooding to a development site, it has a statutory duty to consult SEPA for advice and guidance on flood risk, with the requirement that a Flood Risk Assessment (FRA) is conducted in support of the planning application. Sweco have been commissioned by Perth and Kinross Council to undertake a FRA in support of the planning application for the proposed Cross Tay Link Road (CTLR) project. The aim of this FRA is to demonstrate that the development proposal, inclusive of any mitigation elements, is compliant with Scottish Planning Policy (SPP, 2014)2 and the Flood Risk Management (Scotland) Act (2009). The CTLR involves the construction of a new 6-kilometre link road running west to east connecting the existing A9 road, to the north of Perth, with Stormontfield Road, the A93 and A94 which are located north of Scone and to the east of the River Tay. The design elements also include the realignment of a 2 km section of the A9 between Perth and Luncarty, as well as the provision of a new bridge over the River Tay and the Perth to Inverness railway line which will provide access to and from the new link road. This report presents the methodology and outcomes of a flood risk assessment conducted for the development site, based on SEPA’s Technical Flood Risk Guidance for Stakeholders (2015) 3. The report presents: · An overview of potential sources of flood risk to the proposed development site, based on SEPA’s indicative flood mapping and site characteristics. · Information on historic flooding instances and measures taken to reduce flood risk in the area. · Details of the modelling methodology used in this assessment, including input data. · “Baseline” predictions of peak water level and flood inundation extents for existing conditions. · Assessment of the flood risk impact of the proposed development, with and without the incorporation of mitigation measures. The fluvial modelling component of the study is divided according to the work undertaken on the River Tay, and that for the minor watercourses including the Cramock Burn, Bertha Loch Burn and Broxy Kennels Drain.
2 Scottish Government (2014). Scottish Planning Policy. 3 SEPA (2015). Technical Flood Risk Guidance for Stakeholders (SS-NFR-P-002), v9.1.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 12 of 112 2 Legislative Framework
2.1 Flood Risk Management (Scotland) Act 2009 In terms of flood prevention legislation, the binding document is now the Flood Risk Management (Scotland) Act 2009, which replaces the Flood Prevention (Scotland) Act 1961, which in turn was amended by the Flood Prevention and Land Drainage (Scotland) Act 1997. The Flood Risk Management (Scotland) Act 2009 sets in place a statutory framework for delivering a sustainable and risk-based approach to managing flooding. It places a duty on Scottish Ministers, SEPA, local authorities, Scottish Water and other responsible authorities to exercise their functions with a view to managing and reducing flood risk and to promote sustainable flood risk management. Under Section 42 of the Act, planning authorities will require applicants to provide an assessment of flood risk where a development is likely to result in a material increase in the number of buildings at risk of being damaged by flooding. Flood prevention schemes are confirmed by Scottish Ministers and financially supported by the Scottish Government if they comply with the approved cost/benefit ratio.
2.2 Scottish Planning Policy (2014) and SEPA Technical Guidance (2015) In Scotland, the requirements for a Flood Risk Assessment are set out in sections 254 to 268 of the Scottish Planning Policy (2014), which supersedes the SPP of 2010 and earlier iterations. This document sets out the Scottish Government’s national policies on different aspects of land use planning in Scotland. The SPP document provides the framework within which development proposals are assessed in terms of their vulnerability to, and potential to cause, flooding. While the focus is on new development, flood prevention measures for existing properties use equivalent risk assessment techniques. The policy defines the 1:200-year annual probability flood event as a key risk level, advising that infrastructure and buildings should be designed to be free from surface water flooding in relation to rainfall events of this (or greater) frequency of occurrence. The functional floodplain (i.e. the area of land where water flows in times of flood which should be safeguarded from further development because of their function as flood water storage areas) is also defined as the area which floods in response to events of 1:200-year probability. Section 263 of SPP (2014) sets out that the following developments may be suitable within the functional floodplain: · residential, institutional, commercial and industrial development within built-up areas provided flood protection measures to the appropriate standard already exist and are maintained, are under construction, or are a planned measure in a current flood risk management plan; · essential infrastructure within built-up areas, designed and constructed to remain operational during floods and not impede water flow; · some recreational, sport, amenity and nature conservation uses, provided appropriate evacuation procedures are in place; and · job-related accommodation, e.g. for caretakers or operational staff. Where built development is permitted, SPP (2014) requires that measures are taken to protect against or manage flood risk, and any loss of flood storage capacity mitigated to achieve a neutral or better outcome.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 13 of 112 2.3 Climate Change Section 264 of the SPP (2014) requires that climate change effects should be taken into account in flood risk assessments, in addition to an allowance for freeboard. SEPA’s Technical Flood Risk Guidance for Stakeholders (2015) recommends that an allowance of +20% should be added to the 200-year peak flow estimate to account for climate change, with this allowance being over and above a separate freeboard allowance of a minimum 500-600 mm.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 14 of 112 3 Site Location and proposal The Cross-Tay Link Road project is located in southeast Perthshire within the local authority of Perth and Kinross Council. The scheme concerns a new link road which will connect the A9, north of Perth, with Stormontfield Road, the A93 and the A94 which are located to the east of the River Tay and north of Scone. An overview of the proposal is shown in Figure 3-1. To the east of the River Tay the new link road runs west to east through primarily agricultural land, as well as the forested area of Highfield Plantation, approximately 650 m west of the A94. Along the eastern section of the new link road roundabouts are proposed at the junction to Stormontfield Road, the A93 and the A94. Excavations and cutting will be required south of the grade separated junction and between the A93 and A94 to maintain a suitable road gradient. To the west of the River Tay a 2 km section of the A9 will be realigned further to the west of the existing A9 and will connect to the new link road via a grade separated junction consisting of two roundabouts. This section is located primarily on agricultural fields but will also intersect an area of woodland between the Broxy Kennels Drain and Bertha Loch Burns (see Figure 3-3). The design includes a new bridge crossing over the River Tay the details of which are provided in Figure 3-2. As part of the project Stormontfield Road will be upgraded and the existing bridge culvert along the Cramock Burn will need to be replaced. Furthermore, new culverts will be required along the Bertha Loch Burn and Broxy Kennels Drain where the new realigned A9 road overlaps the watercourses. These will need to be appropriately designed and suitably sized. An overview of the watercourses in relation to the CTLR design is shown in Figure 3-3.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 15 of 112 Figure 3-1 Overview of the proposed CTLR scheme
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 16 of 112 Figure 3-2 Schematic of bridge crossing over the River Tay including deck levels
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 17 of 112 Figure 3-3 Location of new and replacement culverts.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 18 of 112 3.1 Hydrological Characteristics of the Study Area
3.1.1 The River Tay The CTLR project is within the catchment area of the River Tay which has a total contributing area of 5,088 km2 and is approximately 190 km in length. The watercourse originates in western Scotland, close to the mountain of Ben Lui, and flows in an easterly direction through the highlands before flowing south easterly through Perth and discharging to the Firth of Tay, south of Dundee. According to the Tay Local Plan District Flood Risk Management Strategy 4 the catchment can be divided according to the area north and south of the Highland boundary fault, which stretches west to east from Buchantley to Kirriemuir. To the north the catchment is upland in nature with steep slopes and a high average annual rainfall of 1500-3000 mm. The main land cover is moorland followed by rough grassland and woodland. To the south the catchment is lowland in nature with gentle slopes and a more meandering watercourse. Similarly, the rainfall to the south is lower with an annual average of 800-1000 mm and arable farming in the predominant form of land use. There are a series of lochs in the catchment the most notable of which is Loch Tay which is 23 km long by 1.25 km wide.
Figure 3-4 River catchments as defined in the Tay Local Plan District: Flood Risk Management Strategy
There are a series of river gauges along the River Tay notably at Ballathie and Caputh. Further gauges are positioned along tributaries to the River Tay include those at
4 http://apps.sepa.org.uk/FRMStrategies/pdf/lpd/LPD_08_Sources.pdf
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 19 of 112 Almondbank (the River Almond), Luncart and Jackstone along the Ordie Burn. An overview of the hydrology used in the River Tay modelling is presented in Section 3.9.1 The River Tay has several major tributaries most notably the Isla, the River Tummel, the Almond, the Lyon and the Earn. The River Almond drains an area of 227 km2 and flows in an east to west direction from hills close to Loch Tay before flowing through the village of Almondbank and joining the River Tay approximately 1km downstream of the proposed bridge crossing. Both the existing A9 and Perth to Inverness Railway line passes over the watercourse close to confluence with the River Tay. The River Earn discharges into the River Tay ~6 km south east of Perth and has a catchment area of 973 km2, and includes the tributaries of Ruchill, Machanay Water and River Farg.
3.1.2 Minor watercourses (Cramock Burn, Bertha Loch Burn and Broxy Kennels) There are several minor tributaries which connect into the River Tay. The Cramock Burn is in close proximity to the route of the proposed link road to the east of the River Tay, while the newly realigned section of the A9 crosses the Bertha and Broxy Kennels watercourses (See Appendix A). These minor watercourses are therefore of relevance to the study and a general overview of these is provided below:
3.1.2.1 Cramock Burn The Cramock Burn is a tributary of the River Tay located to the north east of Perth. The watercourse drains in an east to west direction through rural land composed of agricultural fields as well as significant areas of woodland to the east and south most notably Highfield Plantation. The watercourse is culverted at several sections, including where Stormontfield Road and the A93 crosses. In addition, the burn is culverted for approximately 280 m as it passes between a caravan site and Perth Racecourse before returning to open watercourse and discharging to the River Tay. To the north of the caravan site is a ditch which connects to the Cramock Burn downstream of the Perth Racing Course culvert outlet. According to the FEH Online Service the Burn has an approximate catchment area of 3.72 km2 (see Figure 3-5).
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 20 of 112 Figure 3-5 Overview of the Cramock Burn and catchment defined on FEH website.
3.1.2.2 Bertha Loch Burn The Bertha Loch Burn is a tributary of the River Tay located to the north of Perth. The watercourse drains in a west to east direction through rural land composed primarily of woodland and agricultural fields. The watercourse is heavily modified most notably with the Bertha Loch and has several culverted sections. Bertha Loch is approximately 7.7 ha in area with a maximum depth of 12 m and has an estimated upstream contributing area of 1.8 km2. The Loch has two main dams and overflows located to the east and south, as well as two emergency overflows. Each of the main overflows discharges to a separate watercourse which eventually merge ~600 m east of the Loch. The watercourse is culverted as it passes under the A9 via two 900 mm conduits before entering a small (30 m) open section and then entering a larger arch culvert under the existing railway. After the railway the watercourse reverts to an open channel with a small footpath bridge approximately 75m upstream of the confluence with the River Tay. According to the FEH Online Service the Burn has a total catchment of 2.87 km2 from the confluence with the River Tay as shown in Figure 3-6.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 21 of 112 Figure 3-6 The Bertha Loch Burn catchment as defined on the Flood Estimation Handbook website
3.1.2.3 Broxy Kennels Drain The Broxy Kennels Drain is located to the north of Bertha Loch Burn. The watercourse drains in a west to east direction through rural land composed of agricultural fields. Most of the watercourse is culverted with two (70 m and 30 m) sections open immediately upstream of the A9. A review of historical mapping indicates that previously two open land-drains ran east to west and south to north connected into the main burn however the site survey and review of historical mapping indicates that these have been converted into pipes sometime after 1966. The open channel of the Broxy Kennels Drain discharges to a rectangular culvert 500 mm by 500 mm in size before it connects into a larger 1.1 m diameter circular culvert at manhole 972202 which passes under the A9 road. This then connects into a larger 1.43 m high masonry arched culvert under the Perth to Inverness Railway which then discharges into an open channel immediately upstream of the confluence with the River Tay. Between the A9 and railway culvert, as well as at the end of the railway culvert are two open air chambers. An overview of the culverted sections and open channel location can be seen in Figure 5-21. The catchment is not covered by the Flood Estimation Handbook Online service however a hydrological assessment has been undertaken using ArcGIS and available LIDAR data to estimate probably flow paths (see Figure 3-7). From this assessment it was estimated that an area of 0.44 km2 drains directly into the open sections of Broxy Kennels, while a further 0.45 km2 drains towards the current A9 and would potentially be intercepted by the A9 drainage system and then enter the Broxy Kennels close to the inlet into the A9 culvert.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 22 of 112 Analysis of the drainage system indicates that 0.08 km2 the A9 and Perth to Inverness railway may discharge into the Broxy Kennels Drain at three connection points.
Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions
Figure 3-7 Overview of the Broxy Kennels Drain estimated catchment areas and flow pathways delineated in ArcGIS
3.2 Land/forestry drains The CTLR route also crosses a series of small land-drains including the Sherrifton Wood Drain, located to the north west of the Caravan Park, as well as the Highfield Plantation Drain, located between the A93 and A94 junctions. These are not captured within the FEH Online service, are not showing in 1:50k OS maps for the area and appear to be largely manmade. These are therefore considered as part of the drainage design element of the scheme and are not covered as part of this FRA.
3.3 Geology and hydrogeology The bedrock underlying the route of the CTLR is the Scone Sandstone which forms part of the Arbuthnott-Garvock Group. This formation is classified as sedimentary rock of fluvial origin. The British Geological Survey (BGS) describes this formation as detrital, ranging from course to fine grained, and which typically form lenses which reflect pre- existing channels or floodplains of a river or estuary. This formation is a moderately productive aquifer with fracture permeability dominating groundwater flow5. According
5 Centre for Ecology and Hydrology. Accessed https://fehweb.ceh.ac.uk/GB/map.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 23 of 112 to Dochartaigh et al (2006)6 groundwater levels across the low-lying parts of Strathmore are generally less than 10 m below ground levels but may be less than 5 m close to major rivers or on lower ground in coastal areas. The superficial deposits vary along the route of the CTLR. To the west of the River Tay the realigned section of A9 is underlain by glaciofluvial sheet deposits to the north and raised marine deposits (Devensian) to the south, around the proposed junction with the link road. The marine sediments are described by the BGS as detrital, generally coarse grained and composed of a mixture of clay, silt, sand and gravel. To the south of Bertha Loch Burn, and from the eastern bank of the Tay to the eastern section of the Caravan Park, north of Cramock Burn, raised tidal flat deposits of mainly intertidal silts and clay are the dominant formations. Further to the east the proposed link road passes over a band of coarse glaciofluvial sheet deposits of sand and gravel, and then glacial till between the A93 to the A94 roads. These deposits are described as coarse grained, granular and generally less than 10 m thick tending to form beds, channels and plains associated with the action of glacial meltwater. According to Dochartaigh et al (2006)7 the glaciofluvial deposits are likely to form highly productive aquifers whilst the intergranular raised marine deposits form low to moderately productive local aquifers. The deposits of till tend to be thin and contain little groundwater and the fine-grained raised tidal flat deposits are considered not to be a significant aquifer. A more detailed assessment of the underlying geology and hydrogeology, as well as groundwater levels observed within the area can be found in the Ground Investigation Report8 and Preliminary Sources Study Report9 undertaken as part of the project.
6 Dochartaigh et al (2006) Baseline Scotland: the Lower Devonian aquifer of Strathmore. BGS. Accessed from http://nora.nerc.ac.uk/id/eprint/10299/1/CR06250N.pdf 7 Dochartaigh et al (2006) Baseline Scotland: The Lower Devonian aquifer of Strathmore. BGS. Accessed from http://nora.nerc.ac.uk/id/eprint/10299/1/CR06250N.pdf 8 Sweco Ground Investigation Report. Document 119046-SWECO-VGT-000-RP-GE-00001 9 Sweco Preliminary Sources Study Report. Document 119046-SWECO-HGT-000-RP-GE-00003
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 24 of 112 Figure 3-8 Overview of superficial deposits within the area of the CTLR route (source: British Geological Survey)
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 25 of 112 3.4 Scottish Environmental Agency (SEPA) Flood Map SEPA provides indicative flood extent maps for high (1:10 year return period), medium (1:200-year return period) and low (1:1000-year return period) risk bands to assist in determining the need for more detailed, site-specific assessment of flood risk. Shown below are SEPA flood map extracts for the study area in relation to (i) river (fluvial) flood risk, (ii) coastal (tidal) flood risk, (iii) surface water flood risk and (iv) a composite map 1:200-year return period extent for all flood types. 3.4.1 Fluvial Flood Risk A review of the SEPA fluvial flood mapping, shown in Figure 3-9, indicates that the 1:200-year fluvial flood extent (medium flood risk) is contained within the banks of the River Tay near the Bridge crossing. The area along the eastern bank of the River Tay near to the crossing is classified as being at low flood risk (1 in 1000-year event) and this zone extends southwards to Perth Racecourse. Further south of the Racecourse is an area at high risk which extends from the bank of the River Tay to Scone Palace. The north east of Perth (between the Perth to Inverness railway and the River Tay) is classified as being at low risk, whilst the area to the west of the railway is at medium risk, potentially from flooding from the River Almond and the Tay. The Bertha Loch Burn and Broxy Kennels Drains do not appear to have been included in the SEPA modelling. There is however an area at medium to high fluvial flood risk to the north of Cramock Burn which overlap with the proposed route of the link road, as can be seen in Figure 3-12. Similarly, there is a flow path which extends from the southern bank of the Cramock Burn along the eastern boundary of Perth Racecourse, before merging with the extensive area of inundation along the River Tay.
© Crown Copyright. SEPA Licence Number 100016991 (2016)
High flood risk north of Cramock Burn Area of inundation
Figure 3-9 SEPA Fluvial Flood Map
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 26 of 112 The route of the CTLR is also not within the predicted high, medium or low flood risk zones associated with the Whiggle and Annaty Burns. This is, therefore, considered to be negligible risk of flooding to the CTLR from these watercourses.
3.4.2 Coastal Flood Risk The SEPA Tidal Flood Risk Map (Figure 3-10) shows that the proposed bridge crossing is approximately 800 m upstream of the tidal boundary, which is predicted to be located close to the confluence of the River Almond, adjacent to Perth Racecourse.
© Crown Copyright. SEPA Licence Number 100016991 (2016)
Tidal boundary
Figure 3-10 SEPA Tidal Flood Risk Map 3.4.3 Surface Water (pluvial) flood risk Figure 3-11 shows the predicted pluvial flood extents which indicate that there is a very low risk of surface water flooding within most of the study area. There are however isolated areas at high risk to the east of the River Tay which are close to, but not directly intercepted by, the proposed link road (see Figure 3-12).
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 27 of 112 © Crown Copyright. SEPA Licence Number 100016991 (2016)
Figure 3-11 SEPA Surface Water Flood Map
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 28 of 112 Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions. SEPA Licence number100016991 (2016)
Figure 3-12 Sources of flood risk, based on comparison of CTLR route and SEPA Flood Maps
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 29 of 112 3.4.4 Reservoir Flood Risk SEPA have produced a Reservoir Inundation Map an extract of which is shown in Figure 3-13. This indicates that the River Tay would be affected by the uncontrolled release of water in the event of a breach of Loch Ericht and Loch An Daimh (both designated as high risk). Flooding is predicted along the River Tay downstream of the proposed bridge affecting the north east of Perth, as well as the area to the south of Perth Racecourse and west of Scone Palace. However, floodwaters appear to remain in-bank or close to the channel upstream of Perth near the new bridge crossing and the link road. The Bertha Loch Reservoir is designated as being at high risk. A breach of Bertha Loch is predicted to result in flooding from the southern bank of the Bertha Loch Burn with flows directed southwards towards the River Almond. This flood extent intersects the location where the realigned section of A9 road is proposed and affects the current A9 road. It should however be noted that these maps are designed to be used for emergency planning purposes and show only the worst-case scenario in which all reservoirs simultaneously fail, which is considered extremely unlikely.
Figure 3-13 SEPA Reservoir Inundation Map showing the indicative area that may flood from an uncontrolled release of water from all possible dam failure scenarios.
3.5 Sewer Flooding A review of available information indicates that there is no evidence of sewer flooding having occurred within the study area. The TAYPlan and the Scone Potentially
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 30 of 112 Vulnerable Area (PVA, 0811) note evidence of sewer flooding within Perth and Scone10, however the proposed drainage for the CTLR does not add any flow into nearby sewer systems and therefore will not exacerbate urban flooding. The CTLR project includes a comprehensive drainage system which includes SuDS to capture and attenuate runoff from the new road surfaces before discharging to nearby watercourses. Earthwork drainage is also outlined which will maintain connectivity between the watercourse and its catchment and prevent surface water accumulation.
3.6 Groundwater Flood Susceptibility The groundwater vulnerability maps produced by SEPA and shown in Figure 3-14, indicate that the Perth, as well as the region to the east of the River Tay and south of the proposed link road is an area where groundwater could affect the duration or extent of flooding from other sources. Most of the proposed CTLR route is however not within this groundwater susceptibility zone, with the exception of a small area at the connection between the new link road and the A94.
Figure 3-14 Groundwater flood risk map. The area in blue denotes locations where groundwater could influence the duration and extent of flooding from other sources.
10 http://apps.sepa.org.uk/FRMStrategies/pdf/pva/PVA_08_11_Full.pdf
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 31 of 112 The BGS produce detailed ‘susceptibility to groundwater flooding’ maps which were purchased for the study area. Figure 3-15 indicates that the level of susceptibility to groundwater flooding is variable along the route of the CTLR. There is the potential for groundwater flooding to occur at the surface during periods of extended intense rainfall at the junction between the realigned A9 and the new link road (west of the River Tay), as well as from the eastern bank of the River Tay to the junction with Stormontfield Road and around the junction with the A93. There are also areas at moderate risk between the Stormontfield Road Junction and the A93 junction, where there is the potential to experience groundwater flooding for any property or asset located below ground level. However, the areas where significant excavations are proposed (south of the grade separated junction and between the A93 and A94) are both classified as having a low risk of groundwater flooding. These predictions contrast with the SEPA vulnerability mapping (see Figure 3-14) which indicates that there is a low likelihood of groundwater contributing to flooding for most of the planning area. Similarly, there are no known reported historical instances of groundwater flooding although this may reflect the sparsely populated nature of the area, and the difficulty identifying this source of flooding.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 32 of 112 Figure 3-15 BGS Groundwater Flood Susceptibility Map
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 33 of 112 3.7 Historical Flood Event 3.7.1 River Tay The River Tay catchment has a long history of flooding with Perth, Aberfieldy, Dunkeld and Pitlochry previously affected. Some of the higher magnitude low frequency events and their impacts are summarized in Table 3.1.
Table 3.1 Historical instances of flooding along the River Tay
Year Flood Details Impacts Heavy rainfall and ice Areas of Perth flooded including North Inch, blockages of Smeatons Rose Terrace, Barossa Street, North Port, 1814 Bridge at the end of a severe Castle Gable, South Inch. Several ships were winter also washed inland. Areas of Perth flooded including North and South Inches, Rose Terrace, North Port, Lower Commercial St, Princes St., Nelson St., Scott Heavy rains and strong gales. St., James St., King St., Edinburgh Rd, Marshall 31/02/1903 Heavy rain and snowmelt Pl, Moncrieff Island. from the Grampians Meikleour, Dunkeld, Pitlochry, Ballinluig, and Dalguise were also affected. Many roadways were noted to be impassable. Several areas of Perth flooded including North and South Inches, Marshall Pl (from James to Princes St.) McQuibbans Bldgs, Moncrieff Heavy rain and snowmelt Island, Commercial St. Bridgend, Edinburgh affected large areas of Tay Rd., North Port, Tay St, George Hotel's stables catchment. River Tay was and kitchens. 21/12/1912 abnormally high for 2 weeks Further locations affected included Lower before floods. Incoming tides Strathern, Dunkeld, Dalguise, Aberfeldy, Castle increased flooding Menzies, Weem Mid Atholl District, Logierait, Moulinearn, Blairgowrie District, St Fillians, Crieff, Tay Farm, Delvine House, Pitlochry, Meikle and Fardle. Flooding affected areas of Perth including Commercial St., Bridgend, basements in Marshall Pl, cellars near North Inch, Barossa Pl, Wettest January on record. Shore Road, Mun Golf Course. 22/01/1928 Snowmelt Outside OF Perth Strathearn, Comrie, Crieff, Aberfeldy, Logieriate, Dowally, Caputh, Meikle Fardle, Pitlochry, Meikleour, Strathblane, Dalguise, Meigle, Blairgowrie were affected. Approximately 780 properties were flooded, Heavy rainfall and snow melt 1993 railway damaged and transport links disrupted. in Tay and Isla Catchments. Total damage was more than £10 million.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 34 of 112 Year Flood Details Impacts 13th Flooding affected Aberfeldy. Dunkeld, Logierait December and Dalguise. The railway at Dalguise was 2006 washed out and rural areas impacted. 50 properties in the Springbank Road area were 17th July Alyth flooded after heavy rain evacuated. Prieston Road and Bankfoot 2005 flooded from Garry Burn. Storm Desmond and Frank Several areas impacted including properties Dec 2015 resulted in prolonged rainfall and road infrastructure in Aberfeldy and across Perth and Kinross. Pitlochry.
3.7.2 Historical flooding of the Cramock, Bertha and Broxy Kennels Drains There are no recorded instances of flooding along the Cramock, Bertha or Broxy Kennels Drains based on a review of available literature. There is however anecdotal evidence that the lodge to the south of Bertha Loch Burn and 150 m west of the existing A9 road has experienced flooding, although the exact source and nature of this is unknown. During discussions with SEPA pictorial evidence of flooding near the Cramock Burn was presented and is shown in Figure 3-16. However, it is strongly considered that the figure does not show flooding from the Cramock Burn, but rather surface water pooling within the field to the north.
Figure 3-16 Picture of flood event along the River Tay and Cramock Burn (Image provided by SEPA)
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 35 of 112 3.8 Formal Flood Prevention Schemes Figure 3-17 shows locations where Flood Prevention Schemes (FPS) have been implemented nearby on the River Tay and River Earn. Summary details are as follows: · Kirriemuir Flood Prevention Scheme (Scheme 12, 1986): Removal of obstructions, streambed regrading, installation of walls along north bank. · Perth Flood Prevention scheme (Scheme 45, 1995): Major flood defenses including culvert improvements, embankments, walls, sluice gates, raise ground levels, pipes, ponds and pumping stations. · Bridge of Earn Flood Prevention Scheme (Scheme 49, 1998). Features include a flood berm on the left bank, channel re-grading and the installation of flood embankments and walls on the right bank. There are also two pumping stations to pump secondary flooding from behind the flood barriers into the Black Cart. · Aberfeldy (Scheme 115): Features included an embankment to provide flood protection from the River Tay and a new pump for surface water.
Figure 3-17 Existing flood prevention scheme in the River Tay, Almond
3.9 Summary of Previous Studies
3.9.1 River Tay Modelling The present project is the latest in a series of modelling studies conducted to examine flood risk along the River Tay and its major tributaries. The model upon which the current study is based was constructed by Halcrow in 2013 using the ISIS modelling package. The Halcrow (2013) model represents the River Tay downstream of the Ballathie gauge,
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 36 of 112 the River Almond downstream of the Almondbank gauge and the River Earn downstream of Forteviot. Halcrow undertook work on the model in two stages. Stage 1 was undertaken in May 201211 and involved the initial model construction using a data file containing river cross- sections, as well as structural and flood plain data of the River Tay and its tributaries provided by Perth and Kinross Council. Stage 2 was undertaken in June 201312 and involved data gathering and the development of a more detailed 1D model in ISIS, as well as the calibration and validation of the model using an observed flood event. Data used to construct the model included: · Model data in Floodtide format including model cross-sections; · Perth flood prevention scheme drawings; · Bridge of Earn flood prevention scheme drawings; · Existing telemetry data for the River Earn; and · Available 1 m resolution LiDAR (DTM) and digital surface models (DSM) for areas in and around Perth. For the Stage 2 hydrological assessment, previous studies undertaken by Mouchel and JBA were utilized to aid the calculation of inflows at the model upstream boundaries. An overview of the data sources and the inflow hydrology are provided in the following: (1) For the River Tay a study undertaken by Mouchel in December 2011 for the area around Caputh Spittalfield and Meikleour was adopted13. This study involved a review of hydrometric information gathered by SEPA (rainfall gauges, flow gauges, AMAX, POT, time series data for various storm events). Mouchel set the upper boundary inflow at Ballathie on the River Tay with the hydrograph shape derived from data provided by SEPA for a flood event in December 2006. The hydrograph was then scaled to match peak flows calculated using the Flood Estimation Handbook Statistical Approach (single site), as agreed in consultation with SEPA. The hydrographs obtained can be seen in Figure 3-18.
11 Halcrow (May 2012) River Tay Model Development, stage 1 data file conversion. Technical Note. 12 Halcrow (June 2013) River Tay Modelling Study (version 2): Stage 2 Report. Perth and Kinross Council. 13 Mouchel (January 2012) River Tay (Caputh to Meikleour), Flood Study, Draft Report.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 37 of 112 Figure 3-18 River Tay at Ballathie return period hydrographs obtained by scaling the December 2006 event
(2) For the River Almond a previous study by Mouchel undertaken in 2010 for the Almond Bank Flood Mitigation Scheme14 was adopted. The upper boundary inflow at Almondbank on the River Almond was based on a hydrograph shape derived from the December 2006 flood event and the peak flows scaled to values obtained using the FEH statistical approach in agreement with SEPA. This included a 15% uplift to ensure 1:200-year peaks matched SEPA’s own estimates. An overview of the inflow hydrographs generated can be seen in Figure 3-19.
Figure 3-19 River Almond at Almondbank hydrographs obtained by scaling the December 2006 event
14 Mouchel (September 2010) Almondbank Flood Mitigation Scheme, Hydraulic Modelling and Option Assessment Report.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 38 of 112 (3) The River Earn inflows were adopted from an assessment by JBA in 200615. The upper boundary inflow at Forteviot on the River Earn was based on a hydrograph shape derived from the December 2006 flood event. The hydrograph was then scaled to match peak flows expected at Fortevoir (where the River Earn in the model starts), which were themselves calculated by scaling the peak flows observed at the Bridge of Earn Gauging station which is further downstream. The River Earn inflow hydrographs used are shown in Figure 3-20.
Figure 3-20 River Earn at Forteviot return period hydrographs obtained by scaling the December 2006 event
Multiple minor ungauged watercourses along the River Tay (between Ballathie and Dundee, and the River Earn (between Forteviot and the River Tay confluence) were represented as FEH inflow units, based on FEH-CDROM Version 3 catchment descriptors. An overview of minor watercourses included are shown in Table 3.2.
Table 3.2 Minor watercourses included in the River Tay Halcrow modelling study
River Tay River Earn · Shoshie Burn · St. Martins Burn, · Gelly Burn · Redgorton Burn · Water of May · Bertha Loch Burn · Miltown Burn · Cramock Burn · Deich Burn · Catmoor Burn · Yellow Burn · Annaty Burn · River Farg · Craigie Burn · Ballo Burn · Kinfaus Burn · Nethy Burn · Cairnie Pow Burn · numerous other unnamed minor · Pow of Erroll Burn burns · Pow of Lindores · Grange Pow · Bogmill Pow Burn · Huntly Burn · Invergowrie Burn
15 JBA (2006) Condition Assessment and Database of Flood and Coastal Defences, Bridge of Earn Flood Defence Scheme 1998. Final Report. Scottish Executive.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 39 of 112 The East Pow Burn and Gelly Burn were represented using the FEH rainfall-runoff method, which was found to be conservative relative to the FEH statistical. Climate change was represented using a 20% allowance.
3.9.2 Minor watercourse Modelling No evidence was found of previous modelling having been undertaken for the Broxy Kennels Drain. Similarly, whilst the SEPA flood maps indicates flooding along the northern bank of the Cramock Burn at the Caravan Park, it is unclear as to whether this Burn has been explicitly modelled or the flooding is from the River Tay. For the Bertha Loch Burn previous modelling has been undertaken by Kaya Consulting in 201416 as part of the Bertha Park Regeneration Scheme. This followed on from a Level 1 Flood Risk Assessment carried out for the site undertaken by Arup in February 2014. As part of this study hydrological analysis was undertaken using both FEH and IH124 methodologies. These flows were routed through a simplified reservoir model using ISIS river modelling software to account for the storage provided by Bertha Loch. This method provided a design flow (1:200-year) estimate of 3.34 and 2.71 m3/s for the FEH and IH124 methodologies. A 1D hydraulic model was built using 45 surveyed cross sections undertaken in July 2014 and including the two-upper branched of the Burn into which Bertha Loch discharges, as well as the combined watercourse to its confluence with the River Tay. The model included several culverted sections which were covered in the survey and included the two 900 mm culverts under the existing A9. The open sections between the A9 and railway culvert, as well as that downstream of the railway do not appear to have been represented within the model. The model predicted flooding at the confluence of the two upstream watercourses as well as further downstream along the southern bank which extends to the A9 culvert. The inundation extents were inferred by projecting peak water level predictions onto contour mapping of the surrounding area and indicated that floodwaters extend ~ 45m from the southern bank at the downstream section. An overview is shown in Figure 3-21.
16 Kaya Consulting Limited (2015) Bertha Park, Almondbank Flood Risk Assessment. Springfield Properties PLC.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 40 of 112 Figure 3-21 1:200-year flood extents predicted for the Bertha Park FRA (Kaya Consulting Ltd, 2015).
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 41 of 112 4 River Tay Modelling Fluvial flood modelling was undertaken using an updated version of the 1D River Tay model previously developed by Halcrow (2013) as the cross-sectional data on which this model was based is outdated with the measurement date unknown. The modelling is used to determine the existing flood risk and design water levels based on the critical 1:200-year return period event (inclusive of climate change) and then predict the impact of the new bridge crossing. The predicted water levels are used to determine the appropriateness of the design in terms of deck level, placement of abutments and piers, and the requirement for compensatory storage where floodplain storage is lost. This section presents an overview of the refinements made to the Halcrow (2013) model followed by an analysis of the baseline results and the impacts of the new crossing.
4.1 Model Revision and updates
4.1.1 1D Model Updates Bathymetric-topographic surveying at locations corresponding to existing model cross- sections was conducted (in the reach within 3 km either side of the proposed crossing), and additional cross-sections were added to improve resolution and to ensure that the model representation of the local river reach is up to date and accurate. Cross-sections were extended beyond the bankside area using LiDAR for Scotland Phase 1 DTM (2011-12) data (see Figure 4-1).
Figure 4-1 Bathymetric-topographic survey cross-sections conducted for CTLR, noting that a bathymetric survey patch was conducted near the proposed crossing to facilitate any future refinement.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 42 of 112 Comments within the Halcrow (2013) model build for cross-section RT13 indicate that elevations within the cross-section have been lowered for unknown reasons, but likely due to low confidence, or to improve model stability. Bathymetry data obtained as part of the current project substantiate that local bed levels are higher in this location, possibly due to sediment deposition from the adjacent outfall of the Cramock Burn. The arbitrary lowering of bed levels in the Halcrow (2013) model at this location results in an underestimation of peak water levels at the CTLR crossing location which has been corrected in the updated model build. An in-line weir significantly upstream of the crossing (at RT05) is represented in the model as a cross-section hence sensitivity analysis was conducted to remove this weir, as well as represent it explicitly as an irregular weir (spill) unit. The results showed that the model predictions near the CTLR are not sensitive to the default representation, with variations of 1 mm or less predicted for peak water levels between the different representations. The existing model contains wetted sections separated from the main river channel by high intervening ground, as well as some sections which are wetted at either or both ends (i.e. cross-section is insufficiently long to capture floodplain, causing “glass walling”). Analysis was conducted for two scenarios to assess the impact of these limitations: · A worst-case, in which cross-sections were shortened to remove separated wetted sections (thus resulting in peak water levels which are the same or higher than the default model build). · A best-case, in which cross-sections where water levels exceed end elevations were artificially extended (thus resulting in peak water levels which are the same or lower than the default model build). Model predictions near the CTLR were shown to be insensitive to these limitations in 1D modelling, with the variance in peak water level predictions between the best-case and worst-case scenarios being less than 30 mm in the vicinity of the CTLR. While the model build is one-dimensional, ISIS software permits inclusion of panels within a cross-section to represent portions of the cross-section with differing roughness’s (such as between the river channel and its floodplains) or vastly different flow depth. Average conveyance, flow depth and velocity can then be determined from an appropriate aggregation of flow properties in each panel. The Halcrow (2013) build contained no panels within cross-sections; adding panels was shown to reduce peak water level predictions near the CTLR by approximately 200 mm, therefore their inclusion is an important improvement in the model build.
4.1.2 Updates to the Model Boundary Conditions
4.1.2.1 Model Upstream Boundaries The upstream boundary conditions employed in the Halcrow (2013) study have been maintained for the current assessment. These are outlined in Section 3.9.1. Consideration was given to updating design flow values for the River Tay, based on annual maxima flow data for the Ballathie gauge provided by SEPA (covering hydrological years 1953 to 2017). However, compared to the 1:200-year (statistical) peak flow estimate of 2518.7 m3/s determined by Mouchel (2012) and employed by
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 43 of 112 Halcrow (2013), a revised estimate based on a generalised logistic fit is lower (2414.4 m3/s). It was therefore, conservatively, decided to continue to use design peak flow values employed in previous studies.
4.1.2.2 Model Downstream Boundary The Halcrow (2013) model build applied a climate change uplift to inflows, but not to the tidal lower boundary condition. While the reach of the River Tay in the vicinity of the CTLR crossing is beyond the tidal limit, with flood risk being fluvially rather than tidally dominant, tides may still have some impact upon peak water levels in the area of interest. Based on the UKCP09 50 percentile predictions for the high emission scenario for 2080 at Dundee Harbour, a 313 mm uplift was applied to the lower boundary condition of the model, resulting in a very marginal increase in peak water levels of up to 1 mm in the vicinity of the CTLR.
4.1.3 Model Calibration The Halcrow (2013) model was validated against the December 2006 flood event on the recommendation of SEPA. The observed peak water level at the Perth gauge for the event (6.663 mAOD) was predicted to within 0.1 m by the model (i.e. 6.763 mAOD predicted peak level). The updated model was re-validated against the same event, with the revised predicted peak water level at the Perth gauge (6.642 mAOD) indicating improved correlation with observations. The timing of the predicted peak is within 1 hr of the timing of the observed peak, as per Halcrow (2013) predictions.
Figure 4-2: Predicted versus observed water levels in the River Tay at the Perth gauge for the December 2006 flood event. 4.2 Baseline Model Predictions The Halcrow (2013) model does not utilise a cross-section at the proposed CTLR crossing location however a prediction can be inferred by linear interpolation between the two closest cross-sections, RT12 and RT13. Based on this the best estimate of the 1:200-year (including climate change) water level at the CTLR crossing is 9.967 mAOD.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 44 of 112 By comparison, the updated model predicts a higher peak water level at the cross- section added at the proposed crossing location (RT12.3) of 10.094 mAOD. With a 300 mm uplift applied to the downstream tidal boundary the water level increases slightly to 10.095 m AOD. A comparison of the peak water levels at cross-sections nearby is shown in Table 4.1.
Table 4.1 Overview of predicted peak 1: 200-year flood levels (including 20% climate change) for the original Halcrow River Tay Model and the revised and updated model. Original Halcrow Revised model Revised model predicted 1: Cross-section (2013) predicted 1: predicted 1: 200-year 200-year peak water level (m Label 200-year peak water peak water level (m AOD) with tidal boundary level (m AOD) AOD) uplifted for climate change RT10 13.771 13.144 13.144 RT11 12.458 12.192 12.192 RT11.1 - 11.418 11.418 RT11.2 - 11.105 11.105 RT12 10.209 10.62 10.62 RT12.1 - 10.399 10.399 RT12.2 - 10.36 10.36 RT12.3 9.967 (interpolated) 10.094 10.095 RT12.4 - 9.907 9.908 RT12.5 - 9.704 9.705 RT12.6 - 9.245 9.246 RT13 9.58 8.794 8.797 RT13.1 8.77 8.773 RT14 9.52 8.951 8.954 RT15 9.52 8.951 8.954
The design and functional floodplains of the River Tay are predicted to be contained within the near-bank area near the CTLR crossing, such that river flooding will only impact upon design of the crossing and associated structures. For the 1:200-year event without climate change, predictions have similarly increased from 9.453 mAOD in Halcrow’s (2013) study to 9.655 mAOD.
4.3 Post Development Modelling Results An overview of the proposed bridge crossing is presented in Figure 3-2. The bridge has been designed based on the design water levels outlined above with the bridge soffit level having a freeboard allowance of approximately 1600mm above the peak 1:200- year (including climate change) water level (11.695m AOD). Based on the design 1:200-year (including climate change) level of 10.095 mAOD, the western pier is not within the floodplain, and therefore does not require any compensatory storage. The eastern pier is on land with an approximate elevation of 7.4 mAOD, so the compensatory storage volume will be 2.255 times the plan area of the
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 45 of 112 pier. For the 1.5 diameter circular pier the compensatory storage volume needed would therefore be 4 m3.
Figure 4-3 Overview of the proposed River Tay bridge in relation to the 1:200-year (including climate change) water level.
A comparison of the predicted peak water levels with and without the CTLR crossing represented is shown in Table 4.2. The results indicate that the bridge will have a negligible impact with a maximum increase of 1 mm at the bridge cross-section (RT12.3) predicted. This value is within model tolerance and further mitigation is unlikely to be required.
Table 4.2 Comparison of pre and post development water levels near the proposed bridge crossing. The results show levels for the 1: 200-year (including climate change) event without the 300 mm uplift applied to the tidal downstream boundary noting that this has negligible impact on water levels in the area of interest.
1:200-year with climate Post Development 1:200- Cross-section Difference in change maximum stage year (with climate change) Label water level (m) (mAOD) maximum stage (mAOD) RT10 13.144 13.144 0 RT11 12.192 12.192 0 RT11.1 11.418 11.418 0 RT11.2 11.105 11.105 0 RT12 10.62 10.62 0 RT12.1 10.399 10.399 0 RT12.2 10.36 10.36 0 RT12.3 10.094 10.095 0.001 RT12.4 9.907 9.907 0
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 46 of 112 RT12.5 9.704 9.704 0 RT12.6 9.245 9.245 0 RT13 8.794 8.794 0 RT13.1 8.77 8.77 0 RT14 8.951 8.951 0 RT15 8.951 8.951 0
4.4 Implications upon the CTLR Design The proposed bridge is not predicted to significantly alter flood levels near the crossing and the underside of the deck is approximately 1600 mm above the 1:200-year (including climate change) flood level. This is significantly greater than the 600 mm DMRB requirement. The only elements of the crossing that will cause a negligible (4m3) displacement of floodplain is the 1.5 m diameter eastern pier. Following discussions with SEPA (outlined in Appendix H) it was agreed that provision of this storage would not be required.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 47 of 112 5 Minor Watercourse Modelling Section 3 identified that the Cramock Burn, Bertha Loch Burn and Broxy Kennels Drain may present a source of flood risk to the route of the CTLR and will require the installation of new culvert crossings. Explicit fluvial modelling is required to determine the current level of flood risk, as well as to ensure that hydraulic structures are suitably designed and to determine the mitigation requirements needed. The following section provides an overview of the modelling methodology and results for each of the minor watercourses. For each watercourse an assessment of the baseline and the predicted post development flood risk without mitigation is presented. This is followed by an overview of the ramifications these have upon the CTLR design and the proposed mitigation strategy if required.
5.1 Cramock Burn Fluvial inundation from the Cramock Burn is predicted by SEPA mapping to extend onto the proposed route of the CTLR north of Perth Racecourse, even for the 1 in 10-year event. This may result in a substantial volumetric displacement of the functional floodplain, requiring compensatory storage, as well as floodplain relief culverts to maintain connectivity for flood and drainage flows. There is however uncertainty regarding the accuracy of the SEPA flood mapping, as outlines in Section 3.4.2, hence explicit modelling is required to assess the worst-case flood risk from the burn to inform the final road route and inform any mitigation requirements. Furthermore, as part of the project the Stormontfield Road Bridge culvert will be replaced and therefore modelling will be required to ensure that the new culvert is suitably sized and to ensure that there is no flood detriment to nearby receptors. 5.1.1 Methodology
5.1.1.1 1D-2D Build A new 1D-2D model build of the Cramock Burn and its floodplain was created in Infoworks ICM. The model is composed of 74 cross-sections informed by topographic surveying obtained as part of the current project, extended using LiDAR for Scotland Phase 1 DTM data where necessary. The survey was undertaken in March 2018. The model extends from a point upstream (east) of Stormontfield Road to the confluence with the River Tay and includes a minor tributary around the northern and western perimeter of the caravan park. The 1D river cross-sections were clipped to their banks such that flow within the hydraulic model between the 1D sections and 2D domain occurred at the point where flow left the river channel and entered the floodplain (Figure 5-1).
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 48 of 112 Figure 5-1: Modelled extents of the Cramock Burn, including drainage ditch around caravan site.
The 2D domain was meshed using the same topographic survey data used to construct the 1D cross-sections and was also supplemented with 1 m resolution LiDAR where there were gaps in the coverage, namely to the south of Cramock Burn. An overview of the topographic data used in the study can be viewed in Appendix B. The minimum meshing element size was 8 m2. The maximum height variation between elements was 0.25 m, with terrain sensitive meshing applied. The boundaries of the 2D zone have been set to a normal depth condition. While the 2D zone has been designed to cover all areas where out-of-bank flows are expected, should any water reach the boundary this setting ensures that no glass wall effects are generated. The 2D area is linked to the one-dimensional river channel through the bank lines, where flow is passed between the 1D and 2D computational model elements. All river banks can exchange flow between domains with a discharge coefficient of 1 and a modular limit of 0.7 applied.
5.1.1.2 Hydraulic Structures The modelled extent contains the Stormontfield Culvert and the culverted sections adjacent to the caravan park, surveying of these structures was included in survey scoping and explicitly represented in the model build. Condition assessment was also conducted for culverted sections, with estimated depths of sediment/debris blockage accounted for in modelling.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 49 of 112 5.1.1.3 Roughness Values The Manning’s ‘n’ roughness values used in the model is shown in Table 5.1:
Table 5.1 Manning’s n values used in the model Domain Feature Mannings n 1D Channel Bed 0.035 1D Channel Banks 0.06 2D 2D Mesh 0.045 2D Buildings 1.0 2D Roads 0.02
5.1.1.4 Upstream Boundary The Cramock Burn is ungauged and small in terms of catchment area therefore inflows into the model were calculated based on catchment descriptors purchased from the FEH Online service. The catchment extent was defined based on the FEH definition (area of 3.72 km2 to the River Tay confluence) and increased to 3.78 km2 following further GIS topographic analysis. For comparative purposes and in-line with SEPA guidance two methods were used to derive the design 1:200-year (including climate change) hydrographs for the whole catchment: · FEH rainfall-runoff, using the hydrograph generation tool within ISIS (Flood Modeller) · ReFH2, using the integrated tool within Infoworks ICM The full catchment hydrograph was apportioned between the main channel and minor tributary based on the estimated portion of the total catchment area for the tributary to its confluence point with the main Cramock Burn channel.
The critical duration for the 1:200-year winter storm event was calculated for both the FEH and ReFH2 inflows based on the maximum flooded area. These were calculated to be 8 hours for ReFH2 and 10 hours for FEH. An overview of the inflow hydrographs used for the analysis is shown in
Figure 5-2.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 50 of 112 Figure 5-2: Synthetic design 1:200-year (including climate change) hydrograph for the main channel and tributary.
5.1.1.5 Downstream Boundary A fixed level lower boundary condition of 9.58 mAOD was used in the modelling, based on the predicted peak 1:200-year (including climate change) water levels in the River Tay cross-section adjacent to the confluence (RT13.1). This level was applied to the model outfall on the left-hand bank of the River Tay. This assumption is conservative, since the probability that the peak 1:200-year (including climate change) conditions in the Cramock Burn catchment will coincide with the peak 1:200-year (including climate change) conditions in the larger River Tay catchment will be much greater than the 1:200-year return period interval associated with it. Nonetheless, this simplifying and conservative assumption was considered suitable for assessing a worst-case flood risk.
5.1.2 Baseline Model Predictions Figure 5-4 shows the maximum predicted fluvial inundation extents for the 200-year storm (0.5% AEP with a climate change uplift of 20% applied) using both ReFH2 and FEH inflows. The diagram indicates that the predicted fluvial flood extents differ significantly from those shown in the SEPA Flood Map with no flooding indicated to the north of the burn near the proposed CTLR, irrespective of whether ReFH2 or FEH inflows are used. Instead, flooding is predicted to occur from the left-hand (southern) bank at three locations between the Stormontfield Road Culvert and upstream of the inlet to the culvert adjacent to the caravan park. Floodwaters are then directed southwards along the eastern perimeter of Perth Racecourse. No flooding is predicted to occur from the caravan site ditch despite the fact that disproportionate amount of overall catchment flows are assumed to inflow into the upstream end of the ditch (See Figure 5-3.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 51 of 112 Figure 5-3 Peak 1:200-year flood depth at cross-section 8 along the caravan park drainage ditch
While FEH flow peaks are significantly higher than ReFH2 peaks, predicted flood extents are only slightly larger to the south of the burn, which suggests that even higher inflows (at higher return periods, or for most pessimistic climate change assumptions) would result in greater overtopping of the southern bank rather than affecting the area to the north of the burn.
5.1.2.1 Sensitivity Analysis A series of additional model runs were undertaken to evaluate the sensitivity of the model to different factors. These included: · Altering the channel roughness values by +/- 20%; · Removing Stormonfield Road Culvert and reducing the upstream extent of the model to enable for greater forward flows; · Increasing the 200-year (including climate change) flows by 70%; and · Both above two combined (not including roughness change).
The analysis indicates that the model results are not sensitive to the removal of constraints along the Cramock Burn or changed in roughness (see Table 5.2). Similarly, whilst increasing the 1:200-year inflows by 70%, as requested by SEPA, resulted in slight overtopping of the caravan park ditch, floodwaters were predicted to flow southwards and accumulate within the caravan park. Figure 5-5 shows the maximum flood extent with the 1:200-year flows uplifted by 70% and the Stormontfield Road culvert removed. All predicted peak flows are provided in Appendix D.
Table 5.2 Impact of variations in the in-channel roughness on river water levels and maximum flood extent.
Sensitivity Maximum difference in Difference in maximum water level (m) flood extent (ha) +20% roughness 0.09 0.16 (+1.5%) -20% roughness 0.11 0.23 (-2.2%)
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 52 of 112 Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions
Figure 5-4 Overview of the predicted 1:200-year flood event (with a 20% allowance for climate change) for the Cramock Burn.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 53 of 112 Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions
Figure 5-5 1:200-year flood extents for the Cramock Burn with the Stormonfield Road culvert removed, the model truncated, and inflows increased by 70%.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 54 of 112 5.1.3 Post Development Scenario As part of the CTLR project it is proposed that Stormontfield Road will be upgraded, and the existing culvert replaced. Ecological surveys indicate that mammals may use the existing culverts hence either a ledge within the culvert or an external mammal passage will need to be provided as part of the design. A range of box culverts were modelled at the bridge to determine the appropriate dimensions with the upstream and downstream invert levels set to 300mm below the nearest river cross-section to provide a suitable bedding layer. A Manning’s ‘n’ roughness of 0.013 was applied assuming that the conduit would be concrete, straight but contain some debris17. The results, shown in Table 5.3 and Table 5.4, indicate that a culvert with a width of 2.5 m and a height of 2 m would provide 300 mm freeboard and a height of 2.4 m would be needed for 750 mm freeboard. However, the constraints require that the existing road level is maintained and, as the lowest part of the road is 13.598 m AOD, a 2.4 m culvert would leave only 468 mm to the road surface. This would render the culvert vulnerable to traffic and temperature effects and require modifications to the road, which would affect access to nearby properties. A meeting was held with representatives from SEPA on the 6th December 2018 to discuss the specific culvert issues and the freeboard allowance. For the Stormontfield Road culvert it was decided that, as this is a replacement of an existing asset which is undersized relative to the 1:200-year (including a climate change allowance) event, and that the 750 mm freeboard would have significant repercussions for the design, that providing a lower 300 mm freeboard would be suitable so along as a separate mammal passage is provided. This would still provide betterment in comparison to the existing arrangement and minimise disruption to nearby receptors.
Table 5.3 Overview of estimated upstream freeboard in the Stormontfield Road culvert Culvert Dimensions Upstream Width (m) Height (m) Soffit Level (mAOD) 200-year water level (mAOD) Freeboard (m) Baseline 11.930 12.209 -0.279 2.5 1.5 12.23 12.269 -0.039 2.5 1.8 12.53 12.250 0.280 2.5 2 12.73 12.250 0.480 2.5 2.1 12.83 12.250 0.580 2.5 2.2 12.93 12.250 0.680 2.5 2.3 13.03 12.250 0.780 2.5 2.4 13.13 12.250 0.880
17 http://www.fsl.orst.edu/geowater/FX3/help/FX3_Help.html#8_Hydraulic_Reference/Mannings_n_Tables.htm
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 55 of 112 Table 5.4 Overview of estimated downstream freeboard in the Stormontfield Road culvert
Culvert Dimensions Downstream Width (m) Height Soffit Level (mAOD) 200-year water level (mAOD) Freeboard (m) (m) Baseline 12.109 12.234 -0.125 2.5 1.5 12.13 12.234 -0.104 2.5 1.8 12.43 12.234 0.196 2.5 2 12.63 12.234 0.396 2.5 2.1 12.73 12.234 0.496 2.5 2.2 12.83 12.234 0.596 2.5 2.3 12.93 12.234 0.696 2.5 2.4 13.03 12.234 0.796
Figure 5-6 Peak 1:200-year water level upstream of the replacement Stormontfield Road Bridge Culvert (2.5m by 2m)
Further analysis (shown in Table 5.5) shows that increasing the size of the culvert is not predicted to affect the maximum flood extent or the peak flow and volume of floodwaters directed southwards through the Scone Estate.
Table 5.5 Overview of baseline and post-development flooding indicators
Flood Indicators Baseline scenario Post-Development scenario Max Area Flooded (ha) 10.709 10.709 Peak flow southwards (m3/s) 3.333 3.333 Total volume from southern bank (m3) 65670.910 65670.990 Peak flow in CS 31.5 downstream (m3/s) 4.213 4.213 Peak level in CS downstream (m AOD) 12.137 12.137
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 56 of 112 5.1.4 Implications upon the CTLR design and mitigation requirement Based on the worst-case flood modelling conducted for the 1:200-year including climate change event for the Cramock Burn, the design floodplain of this watercourse is not predicted to interact with the proposed CTLR road infrastructure. Therefore, no compensatory storage mitigation is required. These findings are reinforced by the fact that Perth and Kinross Council are not aware of any historical instances of flooding in this area, including the caravan park site. This suggests that the SEPA’s mapping may be incorrect or that this modelling significantly over-estimates flood risk from the burn. For the proposed upgrade to Stormontfield Road a box culvert 2.5m wide and 2m high would provide over 300mm freeboard which was agreed with SEPA to be appropriate given the design constraints. Further modelling predicts that this new culvert arrangement will have negligible impact in terms of flood risk hence no mitigation measures are required.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 57 of 112 5.2 Bertha Loch Burn The proposed realigned section of the A9 crosses the Bertha Loch Burn and will require the installation of a new culvert approximately 60 m in length to the west of the two existing A9 culverts. Fluvial modelling is required to assess the existing 1: 200-year (inclusive of climate change) flood risk from the burn and then to predict the impact of the new culvert and realigned A9. The modelling is used to determine the appropriateness of the design of the culvert in terms of ensuring that sufficient freeboard is provided and identifying the need for suitable mitigation.
5.2.1 Baseline Model Methodology
5.2.1.1 1D-2D Model Build A 1D-2D model build of the Bertha Loch Burn and its floodplain was created in Infoworks ICM. The model is composed of 51 cross-sections and extends 650 m upstream from the existing A9 culvert to the confluence with the River Tay. As with the Cramock Burn the 1D river cross-sections were created from topographic survey data and supplemented with 1 m resolution LiDAR where necessary. The 1D river sections were clipped to their banks such that flow within the hydraulic model between the 1D sections and 2D domain occurred at the point where flow leaves the river channel and entered the floodplain. An overview of the model is shown in Figure 5-7.
Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions
Figure 5-7 Overview of Bertha Loch Burn Baseline Model
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 58 of 112 The 2D domain was meshed using the same topographic survey data used to construct the 1D cross-sections and was also supplemented with 1 metre resolution LiDAR where there were gaps in the coverage. The minimum meshing element size was 1 m2. The maximum height variation between elements was 0.25 m, with terrain sensitive meshing applied. The boundaries of the 2D zone have been set to a normal depth condition. While the 2D zone itself has been designed to cover all areas where out-of-bank flows are expected, should any water reach the boundary this setting ensures that no glass wall effects will be generated. This 2D area is linked to the one-dimensional river channel through the bank lines, where flow is passed between the 1D and 2D computational model elements. All river banks can exchange flow between domains with a discharge coefficient of 1 and a modular limit of 0.7 applied. Roughness values applied are the same as those used for the Cramock Burn, presented in Table 5.1. Existing builds and roads are not explicitly represented in the 2D domain as mesh and roughness zones noting that there is only a small number of building nearby which could potentially be impacted, and that terrain sensitive meshing has been used.
5.2.1.2 Hydraulic Structures Relevant structures on the river have been modelled where these are considered to have an impact on flow (either under typical flow conditions or during extreme events). Upstream and downstream invert levels for the culverted sections of watercourse as it passes under the A9 and railway were set based on a culvert survey data. Sediment depths were added based on the results of a culvert conditional survey with the baseline results presented with and without sediment within the existing A9 culverts. A culvert upstream of the A9 was included in the model as it was felt that this could provide a constraint on flows passing forward. The invert levels of this conduit were set based on the upstream and downstream cross-sections which were themselves cut from the topographic survey data. The size of the culvert was estimated to be 420 mm based on images from the site survey. Inline banks were included for this feature to enable flows to pass over the crossing if the water levels exceed the bridge crest.
5.2.1.3 Upstream Boundary conditions There are no gauging stations along the Bertha Loch Burn, hence design flows were estimated using catchment descriptors from the FEH website. SEPA’s Technical Flood Risk Guidance for Stakeholders (2018) cautions against the use of ReFH2 for catchments with significant lochs and reservoirs, as indicated by having a FARL value less than 0.9. As the Bertha Loch catchment has a FARL of 0.868, the FEH rainfall- runoff methodology was instead applied to generate 1:200-year hydrographs for a variety of storm durations. In all cases, calculated flow values were raised by 20% to account for climate change, in compliance with SEPA’s (2018) guidance. The FEH rainfall-runoff method produced a peak 1:200-year (inclusive of climate change) runoff value of 5.44 m3/s. Noting that the FEH rainfall-runoff method does not account for the attenuation effect of the loch, the following methods were applied to obtain a more appropriate peak flow estimate:
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 59 of 112 1. a statistical pooling group analysis using WINFAP-FEH; 2. scaling the 200-year flows based on the difference in QMED values with a FARL value of 1 and 0.868; and 3. routing unattenuated FEH rainfall-runoff method hydrographs (calculated for the 1.8 km2 area upstream of the Loch) through a simplified storage model of the Bertha Loch, with the loch’s outflows represented as a series of weirs. The resultant outflow was then merged with the FEH rainfall-runoff hydrograph generated for the remaining area downstream of the Loch for the same return period. Predicted peak flow estimates from these methods are shown in Table 5.6. The peak estimate obtained for the Bertha Park FRA (Kaya Consulting, 2015) is also shown for comparison. Based on this analysis, routing FEH flows through a simplified storage model (method 3) produced a similar value to that estimated in the Bertha Park FRA and was more conservative compared to the other two methods. This was judged to be the appropriate and was used for the subsequent analysis. The inflow hydrographs used in the hydraulic modelling can be seen in Figure 5-8.
Table 5.6 Comparison of different flows estimated for the Bertha Loch Burn 1:200-year (with climate change Method allowance) flows in m3/s FEH flows from the Bertha Park FRA (Kaya 3.72 Consulting, 2015) Original FEH rainfall-runoff calculated using 5.44 Flood Modeller FEH Rescaled based on QMED 3.34 WINFAP-FEH Pooling group analysis 1.93 FEH flows routed through a simplified storage 3.52 model of the Bertha Loch
An estimated critical duration of 16 hours winter storm with a climate change uplift of 20% was used based on the maximum area flooded.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 60 of 112 4 FEH Downstream Inflow 3.5 Loch Outflow
3 Combined
2.5
2
Flow (m3/s) Flow 1.5
1
0.5
0 0 10 20 30 40 Duration (hours) Figure 5-8 Inflow hydrographs used for the Bertha Loch Burn baseline hydrology
5.2.1.4 Downstream Boundary Conditions A fixed level lower boundary condition was used in modelling, based on predicted peak 1:200-year including climate change water levels in the River Tay cross-section adjacent to the confluence (RT13.1) of 9.58 mAOD. This level was applied to the model outfall on the right-hand bank of the River Tay. Note that this assumption is conservative, since peak 1:200-year (including climate change) conditions in the Bertha catchment coincident with peak 1:200-year (including climate change) conditions in the larger River Tay catchment will have a much greater than 1:200-year return period interval associated with it. Nonetheless, this simplifying and conservative lower boundary assumption was considered suitable for assessing a worst-case flood risk.
5.2.2 Baseline Model Predictions For the baseline scenario the 1:200-year (including climate change) storm event is predicted to result in flooding over the southern) bank of the Bertha Loch Burn upstream of the two A9 culverts (see Figure 5-9). The floodwaters extend southwards and accumulate to the north of the public road to Bertha Park affecting the lodge to the north of the Y-junction. Floodwaters are also predicted to flow onto the A9 road north of the junction with the public road. Removing the observed sediment from the existing A9 culvert was found to reduce the volume of floodwater from the southern bank and onto the A9 as shown in Figure 5-10.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 61 of 112 Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions
Figure 5-9 1:200-year (including climate change uplift) baseline model flood extents for Bertha Loch Burn without sediment included within the A9 culverts.
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Figure 5-10 1:200-year (including climate change uplift) baseline model flood extents for Bertha Loch Burn with sediment included within the A9 culverts.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 62 of 112 The baseline model flood extents differ from those predicted in the Bertha Park FRA outlined in Section 3.9.2. Following discussions with Kaya Consulting it was found the roughness values used and the predicted flood levels upstream of the A9 culverts were very similar. The reason for the difference in flood extents therefore likely relate to the fact that the Bertha Park model is 1D with the flood extents inferred beyond the river banks by projecting peak water level onto a ground elevation model (thought to be created from LiDAR). This method does not explicitly portray the movement of floodwaters within the floodplain whereas the 1D-2D model undertaken for this study shows floodwaters flowing parallel to the channel before a branch is able to flow southwards through a topographic low point. Furthermore, for this study the elevations for the adjacent floodplain is based on more recent topographic survey data.
5.2.2.1 Sensitivity Analysis No recorded flood water level or flow data was available at the site and therefore model calibration was not possible. To gain further confidence in the model sensitivity analysis was undertaken using the baseline 200-year (including climate change) flood event. The assessed hydraulic parameters were: Manning’s ‘n’ roughness coefficients and hydrological inflows which were adjusted by 20%. The results shown in Table 5.7 and Table 5.8 demonstrate that the in-channel water levels are relatively insensitive to changes in both the channel roughness and inflows. However, due to the sloping topography of the area, small changes in channel water levels was found to have a significant impact in the total flood extent. It should be noted that for both sensitivity scenarios flooding along the southern bank of the burn still occurs. The predicted peak levels at each cross-section are provided in Appendix E.
Table 5.7 Impact of variations in the in-channel roughness on river water levels and maximum flood extent Sensitivity Maximum difference in Difference in maximum water level (m) flood extent (ha) +20% roughness 0.06 0.9 (+18%) -20% roughness 0.07 1.5 (-30%)
Table 5.8 Impact of variations in flow on river water levels and maximum flood extent Sensitivity Maximum difference in Difference in maximum water level (m) flood extent (ha) +20% inflow 0.08 1.43 (+29%) -20% inflow 0.26 2.22 (-45%)
5.2.3 Post Development Scenario As part of the CTLR project it is proposed that a section of the Bertha Loch Burn would be culverted. Ecological surveys indicate that otters use the existing culverts hence a mammal ledge will be incorporated into the design of the culvert. The ledge will be positioned 150 mm above the design 1:200-year (including climate change) flood level and a 600 mm freeboard retained above the ledge along the length of the culvert.
For the post-development scenario, a cross section immediately upstream and downstream of the realigned A9 was created (see Figure 5-11) and the reach in-
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 63 of 112 between removed and replaced with a culvert link. In addition, the ground model was updated to include the new road crossing and the 2D zone re-meshed. The upstream and downstream invert levels of the new culvert were set to 0.3 m below the lowest ground elevation of the closest cross section and 300 mm of sediment added to the culvert to ensure that an appropriate bedding layer as well as the watercourse gradient is maintained. The proposed arrangement can be seen in Figure 5-12.
Figure 5-11 Cross-section upstream of the proposed new culvert
Figure 5-12 Longitudinal section showing where new culvert is proposed
The model was run with the sediment removed for the existing A9 culverts with the assumption that these conduits will be cleared as part of the CTLR project works. The hydrology was amended to include the new SuDS pond (Pond A9-3 shown in Figure 5-13) which will receive and attenuate runoff from a section of the new link road (currently within the Broxy Kennels catchment area) before discharging into the Bertha Loch Burn. For the remaining catchment there was estimated to be a small (~3 ha) net loss in the area downstream of Bertha Loch due to a section of the new link road (~3.8 ha) which crosses the catchment and will drain to a SuDS pond before connecting into
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 64 of 112 River Almond, as well as 1.4 ha which will be redirected into a ‘cundy’ (drainage feature associated with transport systems) and discharging into the River Tay (See Figure 5-13). Given the small change relative to the overall catchment area it was conservatively decided to retain the original FEH inflows used in the baseline scenario (shown in Figure 5-8) with the northern SuDS pond represented using an inflow file set to a constant peak outflow of 6.1 l/s over the duration of the simulation.
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Figure 5-13 Estimated areas which will either be removed or incorporated into the Bertha catchment
5.2.4 Post Development (no mitigation) Scenario Predictions Analysis for this scenario is shown in Table 5.9 and indicated that a new box culvert 2 m wide by 2m high would be able to transfer the estimated 1: 200-year flows (including a climate change allowance of 20%) with a freeboard exceeding 750 mm. This would be compliant with DMRB standards and allow for a mammal ledge to be incorporated in the culvert. This sized culvert was used for the post development scenario (without mitigation) flood analysis.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 65 of 112 Table 5.9 Upstream and downstream depths in the CTLR culvert Upstream Downstream Culvert size (m) Max water Freeboard (m) Max water Freeboard (m) level (mAOD) level (mAOD) 2.0 * 2.0 11.408 1.181 11.183 0.842 1.9 * 1.9 11.422 1.067 11.183 0.743 1.8 * 1.8 11.437 0.952 11.183 0.643 1.5 * 1.5 11.498 0.591 11.184 0.342 1.2 * 1.2 11.593 0.196 11.181 0.045
Figure 5-14 shows the maximum predicted flood extent for the post development 1:200- year (including climate change) event scenario. The results show that the realigned section of the A9 occupies part of the floodplain, displacing floodwater. The proposed crossing alters the out of bank flow paths which in the baseline scenario flow eastwards and partially re-enter the Burn. Instead flow is channelled southwards along the embankment of the realigned A9 towards the Bertha Park access road. An overview of the change in flow patterns can be seen in Figure 5-15 and Figure 5-16. The modelling indicated that flows onto the tie-in point between the realigned and existing A9 road are predicted to increase from 0.002 m3/s to 1.005 m3/s for the scenario without sediment retained in the A9 culverts (see Figure 5-17).
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Figure 5-14 Maximum Flood extent for 1:200-year (including climate change) post development scenario. For this scenario a 2.0m rectangular culvert was added for the new CTLR crossing.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 66 of 112 Contains OS data © Crown copyright and database right (2017).
Figure 5-15 Overview of the predicted flow pathways along the right-hand bank of the Bertha Loch Burn for the baseline scenario ( 1:200-year including climate change event)
Contains OS data © Crown copyright and database right (2017).
Figure 5-16 Overview of the predicted flow pathways along the right-hand bank of the Bertha Loch Burn for the post development scenario ( 1:200-year including climate change event)
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 67 of 112 1.2
1 Baseline (no sediment in A9) 0.8 Baseline (with sediment in A9) 0.6 Post Development
Flow Flow (m3/s0) (no mitigation) 0.4
0.2
0 0 5 10 15 20 25 30 Time (hours)
Figure 5-17 Predicted flows onto the A9 road tie-in-point for the baseline and post development scenarios
5.2.5 Implications upon the CTLR Design and mitigation requirement The results of modelling of the Bertha Loch Burn indicate that incorporating a DMRB- compliant, 2 m diameter box culvert through the realigned A9 road section, with no other design elements, will result in flooding around the southern tie-in point with the existing A9. An increase in flood risk compared to current conditions is also predicted for a single residential building west of the realigned road, as well as the adjacent public road. Mitigation is therefore required to ensure the CTLR proposals satisfy Scottish Planning Policy (2014) requirements in relation to flood risk.
5.2.5.1 Bertha Loch Burn Mitigation Further modelling has been undertaken, and a series of discussions held with representatives from SEPA and PKC to discuss mitigation strategies and model outcomes. Through these discussions it was agreed that compensatory storage would not be suitable given the sloping topography and that the appropriate strategy was to contain floodwaters upstream of the realigned A9 crossing within the Bertha Loch Burn. This, however, results in flooding between the realigned A9 and the two existing A9 culverts. It was therefore agreed that the two A9 culverts would also be replaced with a culvert which would be designed to prevent backing up upstream of the existing A9. An overview of the correspondence with SEPA is provided in Appendix F. In terms of the design the mitigation proposal includes the incorporation of a 160 m embankment upstream of the realigned A9 designed with a 1 in 5 slope and a maximum height of 0.82 m which provided 600mm freeboard (details can be seen in Table 5.11 and Figure 5-19). The embankment has been sized for the 1:200-year event with a higher 35% allowance for climate change to provide an additional level of resilience. The location of the embankment can be seen in Figure 5-18.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 68 of 112 The new realigned A9 culvert was designed as a box culvert 2 m wide by 2 m high to ensure 750 mm freeboard was retained within both the upstream and downstream sections of the culvert (including a 300 mm bed layer) as shown in Table 5.10.
Table 5.10 Freeboard within the new realigned A9 culvert with the mitigation strategy for the 1:200-year event (with a 20% climate change allowance)
Upstream Downstream
Culvert size (m) Max water level Freeboard (m) Max water Freeboard (m) (mAOD) Level (mAOD) 2.0 * 2.0 11.530 1.059 11.257 0.769
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Figure 5-18 Overview of extension to cross-sections to embankment crest
Table 5.11 Overview of the embankment parameters Embankment 1:200-year Current Distance Cross- crest level (m (+ 35% CC) right-hand Embankment from bank to section AOD including water level bank level height (m) embankment ID 600 mm (m AOD) (m AOD) crest (m) freeboard) CS 5 13.034 12.993 13.6 0.64 3.205 CS6 12.723 12.534 13.3 0.79 3.945 CS 7 12.301 12.092 12.9 0.81 4.045 CS 8 11.762 11.547 12.4 0.82 4.075
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 69 of 112 Embankment
Figure 5-19 Overview of river cross-section 7 with embankment represented
The two existing A9 culverts are constrained due their shallow depth relative to the existing A9 road surface, therefore providing 750 mm freeboard within the replacement culvert would require significant modifications to the road. The replacement rectangular culvert has been designed to be 2 m wide and 1.6 m high, which provides 407 mm freeboard upstream and 346 mm downstream within the culvert. During discussions with SEPA it was agreed that, as this new culvert replaces two undersized culverts predicted to surcharge for the 200-year flood event, this arrangement would provide a net betterment and was therefore considered appropriate. Similarly, as the road would be used as a cycle path with only limited vehicle access, and that the depth between the peak 200-year (including 20% climate change) water level and the road surface exceeds 600 mm, there is low risk to nearby receptors. The mitigation strategy prevents flooding upstream of the realigned A9 crossing and also any backing up upstream of the existing A9 culvert as can be seen in Figure 5-20. There is however a small reduction in the freeboard within the Perth-to-Inverness railway culvert from 481 mm (baseline with sediment removed) to 457 mm upstream. This equates to a decrease in freeboard of between 4.7 and 5.2 %, which is within model tolerance and thus judged to be insignificant.
Table 5.12 Comparison of maximum water level in the Perth to Inverness railway culvert for the baseline and post development (with mitigation) scenarios ( 1:200-year with 20% climate change event)
Upstream Level in Downstream Freeboard Freeboard Scenario railway culvert (m Level in railway (m) (m) AOD) culvert (m AOD)
Baseline 10.506 0.481 10.455 0.447 Post-Development 10.530 0.457 10.475 0.427 (with mitigation)
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 70 of 112 Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database rights 2018 OS 100016971. Use of this data is subject to terms and conditions
Figure 5-20 Post development with mitigation 1: 200-year (+ climate change) Flood Extent
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 71 of 112 5.3 Broxy Kennels Drain The proposed realigned A9 road and graded junction crosses the Broxy Kennels Drain and will require the installation of a new culvert crossing which will replace the open sections to the west of the existing A9 road. Fluvial modelling is required to assess the existing 1: 200-year (inclusive of climate change) flood risk from the burn and then to predict the impact of the new culvert and realigned A9. The modelling is used to determine the appropriateness of the design of the culvert in terms of ensuring that sufficient freeboard is provided and identifying the need for suitable mitigation.
5.3.1 Baseline Modelling Methodology 5.3.1.1 1D-2D Build A new 1D-2D model build of the Broxy Kennels Drain was created in Infoworks ICM using a similar methodology to that applied to the Cramock and Bertha Loch Burns. The model was constructed using topographic survey data which was used to create 14 river cross sections for the open watercourse sections. An overview of the model is shown in Figure 5-21. The model extends from the beginning of the open watercourse, approximately 170m west of the existing A9, to the outfall along the right-hand bank of the River Tay, a typical river cross-section can be seen in Figure 5-22.
Figure 5-21 Overview of the Broxy Kennels Baseline Model showing the cross-sections and drainage network
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 72 of 112 Figure 5-22 Modelled cross-section upstream of the A9 culvert
The 2D domain was meshed using the same topographic survey data used to construct the 1D cross-sections and the boundaries have been set to a normal depth condition to prevent any glass-walling effects. The minimum meshing element size was 1 m2. The maximum height variation between elements was 0.25 m, with terrain sensitive meshing applied. This 2D area is linked to the one-dimensional river channel through the bank lines, where flow is passed between the 1D and 2D computational model elements. All river banks can exchange flow between domains with a discharge coefficient of 1 and a modular limit of 0.7 applied. Roughness values applied are the same as those used for the Cramock Burn presented in Table 5.1.
5.3.1.2 Hydraulic Structures Structures along the watercourse have been modelled where these are considered to impact flow (either under typical flow conditions or during extreme events). The A9 and railway culverts have been included with invert levels applied based on a culvert survey. Sediment levels were also included along the watercourse based on the culvert condition survey. Furthermore, a culvert between the two open sections of watercourse was included with a weir unit added to allow for overtopping of the path. A section of the A9 and railway drainage system was represented in the model with manhole invert and cover levels based on a road drainage survey. All manholes associated with the road drainage were set to stored.
5.3.1.3 Upstream Boundary Conditions The Broxy Kennels catchment was not defined in the FEH Online Service therefore hydrological analysis was undertaken in ArcGIS which identified three main contributing areas: 1. An area of 0.44 km2 was estimated to drain directly into the open section of the Broxy Kennels Drain; 2. A secondary area of 0.4 km2 was estimated to drain eastwards towards the existing A9 and would be intercepted by the drainage system running along the western
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 73 of 112 perimeter of the A9 and then into the Broxy Kennels at a connection point located close to the inlet to the A9 culvert (Manhole 972202, see Figure 5-21). Similarly, 0.05 km2 between the A9 and railway was also estimated to generate runoff which may discharge to the Broxy Kennels Drain between the inlet to the A9 and the outlet from the railway culvert. 3. 0.035 km2 of road surface was estimated to generate runoff which discharges to the A9 at three inflow points between the inlet to the A9 and the outlet from the railway culvert. An overview of these areas can be seen in Figure 3-7. There is no catchment descriptor for the Broxy Kennels therefore to estimate inflows the Bertha Loch Burn profile was used as a donor. In accordance with SEPA guidelines (2018) the descriptor was altered to better reflect the characteristics of the Broxy Kennels catchment with the FARL altered from 0.868 to 1 as there are no reservoirs within the catchment. The FEH rainfall-runoff methodology was then applied using the hydrograph generation tool within Flood Modeller to generate a hydrograph for the 200- year event (including 20% for climate change) and then this was linearly scaled based on the differences in area to produce two hydrographs for the main and the secondary contributing areas (of 0.45 km2). For the secondary contributing area, the 0.05 km2 between the A9 and railway was also counted in the total area. The two hydrographs were then inputted as inflow files with one directed to the node at the start of the watercourse and the other into manhole 972202.
An overview of the FEH inflows for 1:200-year events are shown in Figure 5-23. A critical duration of 6 hours winter storm was used based on the value calculated in Flood Modeller with a climate change uplift of 20%.
Figure 5-23 1:200-year Inflow FEH hydrographs used for Broxy Kennels Drain
For the 0.035 km2 of A9 road the integrated tool within Infoworks ICM was used and inflows were distributed between the 25 manholes along the A9 road drainage system. ReFH2 rainfall was applied to the sub-catchment and the Wallingford runoff routing model was utilised to better represent runoff from impervious surfaces.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 74 of 112 5.3.1.4 Downstream Boundary Conditions The model was run both with and without a fixed level lower boundary condition of 10.36 mAOD applied based on the predicted peak 1:200-year (including climate change) water levels in the River Tay cross-section adjacent to the confluence. Whilst this assumption is conservative it was considered suitable for assessing a worst-case flood risk.
5.3.2 Baseline Model Predictions For the baseline scenario, there is predicted to be a backing up of water upstream of the 500 mm by 500 mm culvert which connects into the A9 culvert. This results in the northern bank of the open section of channel overtopping although the extent of the floodwaters is limited (Figure 5-25), and water drains back into the watercourse after ~ 10 hours. The predicted peak levels at all cross-sections are provided in Appendix G. From Figure 5-24 it can be seen that the rectangular culvert is undersized thereby creating a throttling effect. The railway culvert was predicted to have a freeboard of 0.269 m upstream however, with the peak water level applied to the model outflow, there was predicted to be significant inflows from the River Tay resulting in the conduit surcharging at the downstream end. Without a level applied to the outfall of the model the railway culvert has a freeboard of over 800 mm both at the upstream and downstream ends.
Figure 5-24 Water depths upstream of the rectangular culvert upstream of the A9 in relation to culvert height
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 75 of 112 Figure 5-25 1:200-year (including 20% climate change) flood extent for the Broxy Kennels watercourse
5.3.1 Post Development Scenario As part of the CTLR project it is proposed that the two open sections of the Broxy Kennels Drain to the west of the existing A9 will be culverted. Therefore, for the post- development scenario, the river reaches upstream of the A9 were removed and replaced with a new culvert from the initial upstream stream node to manhole 972202. The upstream node was moved 20 m to the north west away from the new road embankments with the assumption that the existing land drains which currently drain into the open watercourse would be integrated into the pre-earthwork drainage and directed into the watercourse at the inlet. An overview can be found in Figure 5-26. In line with the Design Manual for Roads and Bridges (DMRB) (2004, volume 4 page 11)18 the culvert was set to a diameter of 1.2 m since it is greater in length than 12 m. The upstream invert level of the new culvert was set to the lowest ground elevation of the previous upstream cross section and the downstream invert level was set to the existing invert of manhole 972202 to ensure that the watercourse gradient is maintained. The inlet was positioned at the point of lowest elevation to capture surface water flows and A manhole was added between the existing and realigned A9 roads to allow for access with the invert level interpolated based on distance from the upstream and downstream nodes.
18 The Highways Agency et al (2004). Design Manual for Roads and Bridges: Volume 4 Geotechnics and Drainage, Section 2 Drainage. Accessed from http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol4/section2.htm.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 76 of 112 Figure 5-26 Overview of the new culverted section of the Broxy Kennels Drain in the post development model
5.3.1.1 Post-Development Hydrology The realigned A9 will result in a net reduction of the catchment area which discharges into the Broxy Kennels Drain with the runoff from the grade junction (~3.8 ha) and some of the pre-earth work drainage being directed southwards towards Bertha Loch Burn (see Figure 5-13 for overview). This is equivalent to approximately 14% of the main catchment area identified in Figure 3-7. However, given that the DMRB states that the culvert, since it is greater in length than 15 metres, will need to be a minimum of 1.2 m in diameter, and that there is uncertainty regarding the contributing area of the three pipes which connect into the drain, it was conservatively decided to retain the original FEH inflows used in the baseline scenario (Shown in Figure 5-23). Similarly, whilst the new link road to the north of the grade junction and the proposed new cycle path both transect the Broxy Kennels catchments, the contributing areas are within the Broxy catchment and surface runoff will be attenuated to the greenfield runoff rate via a SuDS pond.
5.3.2 Post Development Scenario Predictions The results indicate that a box culvert of 1.2 m diameter would be sufficient to transfer the estimated 1:200-year flows (including a climate change allowance of 20%) with a freeboard of 0.816 m predicted within the upstream section and 0.655 m downstream (at connection to manhole 972202). In addition, the removal of the existing 0.5 m rectangular culvert prevents the backing-up in the culvert upstream noted in the baseline scenario.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 77 of 112 A comparison of the upstream and downstream water depths in the railway culvert with and without the River Tay 1: 200-year peak level applied at the model outfall is shown in Table 5.13 and Table 5.14. The results indicate that the incorporation of the new culvert will have a negligible impact (< 5%) upon water levels within the railway culvert with over 0.8 m of freeboard remaining for the scenario without a downstream boundary applied. This increase is within model tolerance.
Table 5.13 Comparison of 1:200-year (with climate change) flood depths in the railway culvert for the baseline and post development scenarios with the River Tay peak level applied to the downstream outfall Upstream Freeboard Downstream Freeboard Scenario depth (m) (m) depth (m) (m) Baseline 1.205 0.225 1.686 -0.256 Post Development 1.214 0.216 1.692 -0.262
Table 5.14 Comparison of 1:200-year (+ climate change) flood depths in the railway culvert for the baseline and post development scenarios without a downstream level applied (free outfall) Upstream Freeboard Downstream Freeboard Scenario depth (m) (m) depth (m) (m) Baseline 0.627 0.803 0.627 0.803 Post Development 0.634 0.796 0.634 0.796
5.3.3 Sensitivity analysis An additional model run was undertaken with the combined total flows from the main and secondary areas directed into the upstream of the model. The analysis indicates that with these higher inflows that 0.65 m of freeboard would remain within the new realigned A9 culvert and 0.8 m freeboard would remain in the railway culvert downstream. Increasing the roughness of the new culvert from 1.5 to 15 was also found to have no significant impact on the results with a freeboard of over 600 mm retained within the upstream and downstream sections of the new culvert.
5.3.4 Implications upon the CTLR and mitigation requirement Based on the worst-case flood modelling conducted for the 1:200-year (including climate change event) for the Broxy Kennels the incorporation of the 1.2 m culvert will be suitably sized with a freeboard of over 600mm predicted. Similarly, the model predicts that the realigned A9 will remove a significant constraint along the Broxy Kennels and would have a negligible impact on flood depths within the railway culvert downstream. There is therefore no significant increase in flood risk from the Broxy Kennels Drain resulting from the new CTLR proposal hence mitigation is not required.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 78 of 112 6 Additional analysis
6.1 Outfalls from the proposed drainage The CTLR project will result in a net increase in impermeable surfaces within the catchments through which it passes. This has the potential to increase peak runoff rates to nearby watercourses and thereby exacerbate flood risk. The new road surfaces will however be served by a SuDS treatment system which will attenuate runoff to the two- year greenfield runoff rate before discharging to nearby watercourses (see Table 6.1 below for outfall locations). These SuDS ponds will ensure that the peak runoff rates are not detrimentally affected by the new road. The SuDS will be maintained and periodically inspected to avoid failure and reduce the risk of sub-optimal performance, blockage and flooding. Velocities from the new outfalls will also be limited to ensure that impact on flow dynamics downstream are minimised. The CTLR works intersect the catchments of several watercourses and may therefore provide a barrier to, or alter the patterns of, runoff pathways in the area. To mitigate this risk a comprehensive earthwork drainage system and drainage pipes are proposed as part of the design to ensure that connectivity between watercourses and their catchments are maintained and prevent pooling. These have generally been designed to work with the existing topography to ensure that catchments remain connected to their natural watercourses. Where there will be a change in catchment area, as with the Broxy Kennels Drain and Bertha Loch Burn, this has been considered in the hydraulic modelling.
Table 6.1 Overview of the new outfall locations to each of the watercourses Watercourse Description Redgorton Drain 2 drainage outfalls 1 drainage outfalls to Caravan Site Ditch and Cramock Burn one to the Balboughty Cottage Ditch Broxy Kennels Drain 2 drainage outfalls 1 drainage outfall into Bertha Railway Bertha Loch Burn Culvert River Almond 2 drainage outfalls 2 drainage outfalls into existing Sheriffton Sheriffton Wood Drain ditch and into River Tay Whiggle Burn 1 drainage outfall Highfield Plantation Drains 1 drainage outfall Annaty Burn 1 drainage outfall
6.1.1 Whiggle Burn The Whiggle Burn is located to the north of the Cramock Burn and drains in an east to west direction forming the Gelly Burn before discharging to the River Tay 2 km upstream of the Cramock Burn confluence. A section of the link road from the A93 roundabout to 200 m west of the A94 will drain into a SuDS pond before discharging to the Whiggle Burn at an attenuated rate of 11.5 l/s. Similarly, 9 ha of agricultural land to the north of this section of link road will also discharge into the Whiggle Burn via a drainage ditch with a potential discharge rate of 131 l/s to the Burn. Topographic analysis indicates that these areas currently drain
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 79 of 112 southwards and are within the Cramock Burn catchment. Although there are two receptors downstream of the discharge points, including a small footbridge and Stormontfield Road, these are 300 m and 2 km downstream. Furthermore, as the total catchment area of the Gelly/Whiggle Burn is 7.64 km2 a total peak discharge of 0.14 m3/s is unlikely to significantly alter the predicted flood extents and increase flood risk to the footbridges downstream. A survey undertaken in March 2019 noted the presence of a secondary terrace along the southern bank of the Burn which would likely retain any residual floodwaters from the main channel.
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Figure 6-1 Overview of the areas discharging to the Whiggle Burn from the CTLR drainage system
6.2 River Almond Temporary Access Bridge During the construction phase of the project a temporary bridge will be required over the River Almond for approximately two years to allow for construction vehicle access. An overview of the proposed temporary bridge can be seen in Figure 6-2. This will consist of two abutments placed approximately 50 m apart. The underside of the deck has been designed with an assumed water level of 9 mAOD, however the northern abutment and a section of embankment appears to occupy part of the 200-year floodplain. Given that this structure will be in place for approximately two years, it was considered appropriate that the evaluation of the impact of this feature be included within the FRA.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 80 of 112 Figure 6-2 Schematic of the proposed temporary access bridge over the River Almond
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 81 of 112 6.2.1 Methodology The River Tay model outlined within Sections 3.9.1 and 4 was used for the analysis and this covers the River Almond downstream of the Almondbank gauge to the confluence with the River Tay. The geometries of the existing cross-sections within the model were retained in their original state, noting that the model received has previously been calibrated. An additional cross-section at the location of the bridge crossing was then added based on the section drawing (shown in Figure 6-2) and this was cross referenced using topographic data, commissioned as part of the project. This section was not thought to have captured the bed level, which was portrayed as a straight gradient, hence this was interpolated based on the bed level of the upstream and downstream sections. The model was run with the same hydrology as outlined in Section 4.1.2, without an allowance for climate change (due to the temporary nature of the bridge).
6.2.2 Results The baseline modelling predicts a peak 200-year water level at the bridge crossing of 8.562 mAOD, which is below the 9 mAOD estimate shown in Figure 6-2. Incorporating the cross-section at the bridge location increases water levels upstream along the River Almond. However, this amounts to a percentage change of less than 0.2%. The soffit of the bridge has an elevation of 9.2 mAOD, hence there is approximately 600 mm freeboard retained within the design, and the structure is compliant with DMRB standards. From Figure 6-3 the southern bank, where the access road, abutments and embankment are positioned, are located within the 200-year floodplain. Similarly, the northern abutment is also located within the predicted is also within the 200-year floodplain..
Bridge opening 200-year peak water level
Figure 6-3 Representation of the temporary bridge over the River Almond in relation to the 1:200-year predicted peak flood depth.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 82 of 112 Further hydraulic analysis was undertaken to determine the impact of the temporary Bridge for the 1:200-year flood event in the River Almond and the median flood in the River Tay. As part of this, cross-sections and bridge geometries along the River Almond were extracted from the full 1D ISIS model (Halcrow, 2013) and imported into Infoworks ICM. Since the original ISIS cross-sections extended beyond the bank top, the sections were truncated. Floodwaters were allowed to flow around the existing Almond Bridge, based on a review of surrounding topography. A comparison of water levels along the River Almond indicates that the full ISIS, and truncated ICM, models produce similar results in the area downstream of the Almond Bridge (<5%) with only minor deviation upstream (<10%) for the combined 1:200-year scenario. A constant downstream boundary was applied to the outfall of the truncated model with the level of 7.068 mAOD set. This value was derived using the full 1D ISIS model which was run with a constant QMED flow applied to the upstream boundary of the River Tay and all lateral inflows set to the 2.5 year FEH design event. The truncated model predicts that the temporary crossing would result in an increase in water levels immediately upstream of the crossing, which would extend up to the upstream face of the existing Almond bridge.
Table 6.2 Predicted impact of the temporary bridge crossing over the River Almond on water levels at nearby cross-sections for the 1:200-year event without a climate change allowance. Post- Baseline Development Difference Label Bridge ID level (m Level (m (m) AOD) AOD) RA01 21.99 21.99 0.00 RA02 15.14 15.14 0.00 RA03 11.48 11.48 0.00 RA03u Almond 8.67 8.95 0.28 RA03d Bridge 8.62 8.96 0.34 RA04 8.27 8.71 0.44 RA04u 8.15 8.40 0.24 A9 Bridge RA04d 8.11 8.35 0.25 RA04d-RA04BR Interpolation 7.82 8.15 0.33 RA04BR_us Railway 7.58 8.04 0.46 RA04BR_ds Bridge 7.50 8.00 0.50 Temporary RA04A 7.34 7.77 0.43 Bridge RA05 7.07 7.07 0.00
6.2.3 Implication upon the CTLR project and mitigation requirements The modelling indicates that the temporary bridge has a suitable freeboard for the 200- year event, however the northern and southern abutments and associated embankments occupy part of the floodplain. As outlined in SEPAs Technical Flood Risk Guidance for Stakeholders (2018) volumetric compensatory storage was calculated for the area of raised ground and a total value of 750 m3 was estimated. However, the site is heavily constrained with flood embankments located along the southern bank of the River Almond and a scheduled
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 83 of 112 Monument located to the east of the railway bridge. Sweco identified two sites to the west of the temporary bridge where compensatory storage could potentially be provided. These were identified due to their elevation (higher than area of floodplain loss relating to the temporary crossing), proximity to the proposed temporary bridge, and location outside of the 200-year floodplain shown on the SEPA flood mapping. A comprehensive assessment was undertaken where it was identified that both options would require the removal or ancient woodland and are in close proximity to the River Tay/Almond Special Area of Conservation. Following consultation with SEPA (outlined in Appendix H) it was agreed that the anticipated environmental impacts would be disproportionate to the requirement to provide compensatory storage for a temporary bridge, and that this requirement was omitted.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 84 of 112 7 Conclusion Sweco were commissioned by Perth and Kinross Council to undertake a flood risk assessment in support of the planning application for the proposed Cross Tay Link Road (CTLR) project. The CTLR proposal involves the construction of a new 6km link road running west to east connecting the A9 with Stormonfield Road, the A93 and A94. The design also includes the installation of: · a new bridge over the River Tay and associated infrastructure; · a culvert crossing along the Bertha Loch Burn; · a culvert along the Broxy Kennels Drain; · replacement of the Stormontfield Road bridge culvert along the Cramock Burn. All sources of flooding were assessed, fluvial flood was identified as the primary risk for all of the watercourse crossings. To determine the current fluvial flood risk and assess the impact of the CTLR project fluvial modelling was undertaken for each of the watercourses. A climate change allowance of 20% was used for all modelling, with the exception of the Bertha Loch Burn embankment which includes an allowance of 35%. River Tay The hydraulic modelling undertaken for the River Tay used a revised and updated version of a previous 1D ISIS model developed by Halcrow. The baseline modelling predicted design water levels (1:200-year return period event inclusive of climate change), of 10.095 m AOD where the new bridge is proposed. The bridge has been designed such that the soffit has a freeboard of approximately 1.6m above the peak design water level and is compliant with DMRB standards. The proposed bridge was predicted to have a negligible impact on flood levels upstream and downstream of the crossing with a maximum increase of 1mm. The design floodplain of the River Tay is contained within the near-bank area in the vicinity of the CTLR crossing, such that river flooding will only impact upon design of the crossing and associated structures. The eastern abutment is the only component of the bridge within the floodplain and was estimated to result in a negligible displacement of 4 m2. Cramock Burn A 1D-2D model build of the Cramock Burn and its floodplain was built in Infoworks ICM. The predicted 1: 200-year (including climate change) baseline fluvial flood extent differed from those shown in the SEPA Flood Map with no flooding indicated to the north of the burn near the proposed CTLR route. Instead, flooding is predicted to occur from the southern bank and flows southwards adjacent to Perth Racecourse. The replacement of the Stormontfield Road culvert with a 2.5 by 2.0m box culvert provides over 300 mm freeboard which was agreed with SEPA to be appropriate given the road constraints and would provide a net betterment. Further analysis indicates that the new culvert has negligible impact on flood risk hence no mitigation is required. Bertha Loch Burn A 1D-2D model build of the Bertha Loch Burn and its floodplain was built in Infoworks ICM. The baseline modelling predicted flooding for the 1:200-year event (including climate change) along the southern bank of the Burn upstream of the two A9 culverts.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 85 of 112 The realigned A9 is predicted displace some of the baseline floodwater. Flow is channelled southwards along the embankment of the realigned A9 towards the Bertha Park access road and increases flows onto the tie-in point between the realigned and existing A9. Following discussions with SEPA and PKC it was agreed that the preferred mitigation strategy was to replace the two existing culverts under the A9 with a larger (2.5 m by 1.6 m) box culvert to prevent backing-up upstream, and to install a 160m embankment (<1m in height) along the southern bank to contain floodwaters. This prevents flooding from the southern bank but leads to a small increase in water depths within the Perth- to-Inverness railway culvert however a freeboard of over 400 mm is still retained. Broxy Kennels Drain A 1D-2D model build of the Cramock Burn and its floodplain was created in Infoworks ICM. The baseline modelling predicted a small amount flooding for the 1:200-year event (including climate change allowance). This was associated with the undersized 500 mm box culvert upstream of the A9 culvert, which was in poor condition. For the CTLR project the open sections of watercourses and 500 mm box culvert will be replaced with a 1.2 m box culvert. The model predictions show that this would be sufficient for the 1:200-year flood event (with climate change) with a freeboard greater than 600 mm. Similarly, the impact upon the Perth-to-Inverness railway culvert downstream was found to be minimal (<5 mm) both with and without a downstream boundary applied at the confluence with the River Tay. Additional sources of flood risk The reach of the River Tay near the CTLR crossing is beyond the tidal limit, however coastal influences were taken into account by applying a downstream boundary condition to the River Tay model. This was based on tidal gauge data at Dundee Harbour and was uplifted to account for climate change. The BGS ‘susceptibility to groundwater flooding’ map indicates that the level of susceptibility to groundwater flooding is variable along the route of the CTLR with the potential for groundwater flooding to occur at the surface during periods of extended intense rainfall at the junction between the realigned A9 and the new link road (west of the River Tay), as well as from the eastern bank of the River Tay to the junction with Stormontfield Road. There is also moderate risk between the Stormontfield Road Junction and the A93 junction. There are however no known reported historical instances of groundwater flooding in the area and these areas of high risk are not coincident with cutting locations. There is a very low risk of surface water and sewer flooding within the study area. The CTLR project includes a SuDS drainage system to capture and attenuate runoff from the new road surfaces before discharging to nearby watercourses. The CTLR project intersects the catchments of several watercourses however an earthwork drainage system is included as part of the design to ensure that connectivity between watercourses and their catchments are maintained and prevent pooling. SEPAs Reservoir Inundation Map indicates that the River Tay and Bertha Loch Burn would be affected by the uncontrolled release of water in the event of a dam failure. River Almond Temporary Crossing
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 86 of 112 During construction a temporary bridge will be required over the River Almond. As this will be in place for two years hydraulic modelling was undertaken using the River Tay model to determine the impact. The baseline model predicts a peak 200-year water level at the crossing of 8.562 mAOD and the bridge was estimated to have a freeboard of over 600mm. Post-development modelling predicts that the bridge will impact on water levels along the River Almond. Both abutments and the northern embankment are located within the 200-year floodplain and an estimated total compensatory storage volume of 750m3 was estimated. Sweco evaluated potential sites however following consultation with SEPA it was agreed that the anticipated environmental impacts would be disproportionate to the requirement to provide compensatory storage for a temporary bridge, and this requirement was therefore omitted.
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 87 of 112 Appendix A – Overview of watercourses in the nearby area
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 88 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 89 of 112 Appendix B – Overview map of topographic data and LiDAR
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 90 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 91 of 112 Appendix C – Model 2D Domains
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 92 of 112 Cramock Burn
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FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 93 of 112 Bertha Loch Burn
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FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 94 of 112 Broxy Kennels Drain
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FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 95 of 112 Appendix D – Cramock Burn Model Results
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 96 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 97 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 98 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 99 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 100 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 101 of 112 Appendix E – Bertha Loch Burn Model Results
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 102 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 103 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 104 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 105 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 106 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 107 of 112 Appendix F – Bertha Loch Burn SEPA Meeting Minutes
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 108 of 112 Minutes of Meeting
PROJECT NAME: Cross Tay link Road PROJECT MANAGER: Chris Cardno MINUTES BY: Emma Cooper DATE: 17th December 2018 PROJECT REF: 119046 OUR REF: [0000000000]
LOCATION SEPA Office, Strathearn House, Broxden DATE 6th December 2018 TIME 13.30
PRESENT Ross Fletcher PKC Debbie Crichton SEPA Sara Alexander SEPA Alistair Cargill SEPA Denise Ritchie Sweco Ryan Lockhart Sweco Iain Struthers Sweco James Walker Sweco Emma Cooper Sweco
APPOLOGIES Jillian Fergusson PKC Chris Cardno Sweco
Cross Tay Link Road (CTLR)
4th Stage 3 Consultation – SEPA
1 Introductions Action Introductions were made and DR outlined the main purpose of the meeting was to address the project CAR licensing requirements. 2 Previous Meeting Minutes
AC noted a typo in the minutes of the previous meeting on Bertha Loch Burn. IS The reference to the Reservoirs Act should relate to ‘25000 m3’. 3 Project Update RF provided an update of the project progress noting the following: · The CTLR was not included in the recently announced Tay Cities Deal funding allocation; however, senior management in PKC are working with the Scottish Government to investigate alternative funding routes and given the importance of the scheme, PKC still expect the scheme
Sweco UK Limited Sweco UK Limited Suite 4.2, City Park Reg.no 2888385 368 Alexandra Parade Reg. office: Leeds Glasgow, G31 3AU Grove House +44 141 414 1700 Mansion Gate Drive LS7 4DN www.sweco.co.uk to proceed as per the current timescales; · The CPO process is ongoing and whilst the report will not go before the council for approval in December it is expected to be submitted to the council for approval early in 2019; and · The plan is still to commence on site in 2021. DR provided a further update stating: · A draft Specimen Design to be completed by the end of the year and to be finalised late Spring / Summer; · An EIA and planning application will be submitted in the summer of 2019; · The SUDs design is still subject to some landowner discussions; and · The design is currently investigating the possibility of combining the DR/RL three SuDS basins at Scone Estate into one to create a wetland area. DC expressed concern that the SUDs basins should provide the required treatment volume / functionality and that any overspill into the wider wetland area should not be detrimental to this. 3 CAR Licencing Requirements DR noted that the licencing requirements is essentially in four categories: Culverts; Works in Floodplain; Channel Realignments and Discharge Regime. 3.1 Culverts Stormontfield Road DR noted that the existing culvert is being replaced due to its condition and the need to widen the road to improve access to Scone Palace. IS explained that the existing culvert is undersized. The replacement is to be sized appropriately however providing the recommended 600mm freeboard would lead to a constraint as the road level would require to be raised. However, the proposed 300mm freeboard would still be an improvement to the existing situation. AC advised that culvert maintenance would need to be discussed with PKC Flooding team to avoid blockages. DR to send information on Stormontfield DR culvert freeboard issue to RF so it can be sent to PKC Flood team. SA confirmed that 300mm is an acceptable freeboard level subject to justification to be provided within the license application.
Bertha IS highlighted that Bertha Loch Burn does not feature in the SEPA flood risk maps so therefore the analysis has been from scratch. The flood risk is located at the existing A9. Through previous discussions with SEPA it was agreed that compensatory storage is not required at this location. However, SA requested that it is stated in the FRA (submitted for planning) that no
2 (6) detrimental impact to flood risk occurs. IS explained the proposal to provide a new culvert under the realigned A9 and a 0.7m high embankment along the southern edge of the burn to convey flood waters towards the R Tay. AC noted that it would be possible to do something with compensatory storage, however the embankment would be suitable subject to PKC maintaining it. RF to discuss with the PKC flood team. IS advised that the Lodge and access road are the only receptors at risk of RF flooding, which the embankment would mitigate. SA noted that confining flows within the channel could increase velocities and this should be explored further along with any proposed mitigation. IS explained that the existing A9 culvert is twin 900mm pipes. To achieve the desired 600mm freeboard, the existing A9 would have to be raised locally over the culvert. IS noted that the preferred solution would be to provide 300mm freeboard with a separate mammal passage. SA stated this would be acceptable if this does not result in an increased flood risk. AC noted that freeboard flexibility would be acceptable subject to PKC maintaining invert to minimise sediment build up. DC noted that nothing has been approved yet in terms of the sewage works or Bertha Park development in that location therefore the embankment is a suitable flood protection for the culvert. SA confirmed that the Bertha IS/RF embankment would reduce the risk to the lodge and cause a potential benefit to the proposed sewage works. PKC Flooding team to confirm they are content with this approach. This informal defence can be seen as being required for essential road access development. IS confirmed that it has been designed to 200 year + climate change with scour protection on both banks of the watercourse. SA confirmed that a maintenance regime would be required after high flows. IS confirmed that the embankment would be clay cored. Both SA and AC stated that SEPA could not support new development behind this informal scheme. It was thought that the Berth Park proposals would not extend down to this reach of the Berth Loch Burn. *NB – post meeting note* – DC and AC checked masterplan which did show proposed housing plots in this area. RF has subsequently raised this with the Council’s Flooding Team and this issue is to be explored further with the PKC Flooding and Planning Teams. It was agreed that a complex CAR licence would be required as the embankment is 150m in length and 0.7m high. The culverts, bank modification and scour protection would all come under the same authorisation as the bank modification and scour protection are dependent activities due to the culvert provision. Two license fees would apply in this case (for bank modification and culverts but no fee for scour protection as dependent activities generally have no fee applied). Sunken inverts would be provided. IS stated that the culvert under the realigned A9 is oversized with 750mm freeboard to accommodate mammal ledges.
3 (6) IS advised that scour protection is being looked into for culverts generally.
DR to finalise schedule of applications and activities for agreement with DC. DR DC confirmed that in due course the draft CAR licence applications can be sent to SEPA for comment prior to formal submission.
Broxy IS explained that the new realigned A9 crossing would be a ~200m culvert. The existing culvert has two open channel inlets which are considered to be field drains. JW confirmed that this waterbody is not mapped on the 1:50 000 OS map. IS confirmed that there is no increase to the risk of flooding with a freeboard of over 600mm. It was further explained that all existing culverts would be replaced and upgraded and no mammal passage is being recommended here. DC noted that SEPA will have to look into this watercourse and to clarify if this culvert is for land gain. DC confirmed that if authorisation is required then it would be a simple CAR licence. Tay Crossing DR explained that the piers are set back from the channel approximately 8- 10m from the bank of the River Tay. DR to confirm exact dimensions of setback. One of the piers is clear of the 1:200 yr flood level and the other is DR not. Scour protection is being considered on the piers, however this is unlikely to be required for structural integrity. *NB post-Meeting Note* - DC provided clarification on recommended distance of abutments/ piers. Such works would fall under, Guidance of activities in the vicinity of inland waters and activities affecting surface water dependent wetlands, if the works are within 10m or 2 channel widths (whichever is shorter).
AC noted that the piers are not a concern as they are outwith the channel. IS confirmed that the piers have been designed for the 1:200 yr event. IS queried if there would be a requirement for compensatory storage due to the pier in the flood zone. AC confirmed that given the expected pier dimensions it isn’t considered to be an issue, but recommended they are designed to be as hydro-dynamic as possible to reduce pressure during large out of bank floods 3.2 Channel Realignments DR queried the general rules to manage field ditches and minor channels. DC confirmed that any watercourse realignment works would require a CAR licence regardless of the dimensions. DR to send DC map (OS map with DR NGRs) showing each drainage ditch for confirmation of whether the ditches constitute watercourses. RL confirmed that there is no impact to properties as a result of the realigned ditches. SA advised that clarity and justification would be required to explain that the property at Highfield would not be impacted. DR queried if the cuttings at A9 and at Highfield need to be considered an abstraction issue. Preliminary assessment indicates that a volume of
4 (6) <200m3/day would be drained. DC noted that further information would be required and an assessment of associated effects would need to be carried out. DC stated that status should be assumed as GBR at present. 3.3 Discharge Regime Construction Site DC advised that a project based construction licence could be progressed by PKC or alternatively a construction licence could be prepared and submitted by the eventual contractor. The timescale is 4 months from application therefore PKC may want to progress if this is critical to the construction programme. DC noted that there is a requirement for land for construction SuDS. DR confirmed that land has been allocated for construction SuDS. RF advised that the type of licence would depend on the procurement process. DC advised that it is possible to set out the licence then complete the Pollution Prevention Plan (PPP) at the time of construction. However, the timescales for this is on a case by case basis, noting that the PPP takes the most time. The PPP considers soil, flow velocity and area which is not considered in the SWP. DC confirmed that the CEMP is a separate process. RF highlighted for Sweco to remember the Construction / CAR licences are to be allocated to the contractor in the contract and not PKC. Sweco 3.4 Drainage Outfalls DR explained that two are over 1km in length, therefore a simple licence would be required. The rest of the drainage is considered to be GBR. SA queried whether there would be a change to the discharge points compared with the existing overland flow regime. RL explained that the drainage in both existing and proposed regimes goes from a high point at the A93 to the low point at the SuDS pond at Stormontfield Roundabout so no significant change. SA requested that this is clarified in the application. Sweco 4 Drainage Proposals DR advised that the design provides two levels of treatment where feasible. The SuDS ponds have been designed to be a less uniform shape where land permits. DR further explained that there are two ponds proposed and the rest are basins. DC advised that the SuDS ponds/drains designs are to be picked up in the next stage as the requirement goes through planning. 5 AOB RF noted that for Phase 1 the flood model used was a ‘difference model’ and therefore was different to other adjacent development flood models (it was more conservative). This resulted in some confusion and an objection from SEPA to the planning application for Phase 1. RF queried if there was anything that could be done to minimise the chances of this happening again. AC considered that this is something the PKC Flooding team and Planning department should be able to advise on rather than SEPA. AC considered it
5 (6) was up to PKC to ensure that all of these developments had a consistent and joined up approach. It was agreed that this should not be as significant an issue for Phase 2 as it was for Phase 1. RF will discuss this with PKC RF planning team however it was noted that flood models and FRA’s for these adjacent developments are ultimately the responsibility of the developers and not PKC.
Copies: all attendees, Jillian Ferguson, and Chris Cardno Enclosed:
6 (6) Appendix G – Broxy Kennels Drain Model Results
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 109 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 110 of 112 FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 111 of 112 Appendix H – SEPA Correspondence on the River Almond Temporary Bridge
FRA Report, Cross Tay Link Road 119046-SWECO-EWE-000-RP-EN-20035, Rev.: P02, 22 October 2019 112 of 112
FLOOD RISK HYDROLOGY RESPONSE TO REQUEST FOR INFORMATION RELATING TO FLOOD RISK
SOUTH EAST AREA (PERTH OFFICE)
Site: Cross Tay Link Road
Documents Reviewed: Cross Tay Link Road Flood Risk Assessment (Sweco, 1 May 2019)
Executive Summary Outlining Policy Context
If we received the FRA as part of a formal consultation we would object to the proposed development on the grounds that it may place buildings and persons at flood risk contrary to Scottish Planning Policy.
In summary we wish to receive clarification on the proposals for the proposed temporary river crossing over the River Almond to avoid any potential objection at planning application stage:
• Can compensatory storage be provided to replace the 1355 m3 of lost floodplain storage volume? • Are there any alternative bridge crossing designs available to reduce the volume of floodplain storage lost? • Is there a risk that the Perth Flood Prevention Scheme defence might be reduced during the construction and operation phases of the temporary bridge? • We recommend further modelling of the River Almond to compare pre and post bridge scenarios to help determine the risk to channel bed and bank stability particularly given the closeness to the Perth flood defence and an important cultural heritage site.
Please note our detailed comments below.
Technical Report
1. SEPA has been asked to review the flood risk assessment (FRA), prepared by Sweco, for the Cross Tay Link Road (CTLR). SEPA also met with Sweco (Meeting No. 5) on 4 July 2019 to discuss the contents of the FRA.
2. SEPA has previously agreed the hydrology inputs for the key watercourses in this study. The FRA advises that the proposed bridge over the River Tay will have a negligible impact of flood levels upstream and downstream of the bridge and will result in a loss of floodplain storage volume of only 4 m3. On the scale of the River Tay this loss of storage is negligible and SEPA is not insisting that this is replaced by the provision of compensatory storage.
3. The route of the CTLR has been finalised and it follows the original proposed alignment. We can confirm that SEPA is satisfied with the proposal to replace the Stormontfield Road culvert on the Cramock Burn with a 2.5 metre wide by 2.0 m high box culvert and also satisfied with the proposed new culvert.
4. SEPA can also confirm that it is satisfied with the proposal to replace the two existing Bertha
Loch Burn culverts under the A9 with a larger 2 m by 2 m culvert and construct an embankment along the southern bank to contain floodwaters. The larger culvert can pass more flows downstream but we are satisfied that this will not impact on any receptors before it discharges to the River Tay a short distance downstream. We would advise that the construction of this embankment should not be misinterpreted as freeing up existing floodplain for potential future built development.
5. The CTLR will also impact on the Broxy Kennels Drain. Sweco propose to replace an existing 500 mm box culvert upstream of the A9 with a 1.2 m box culvert. We confirm that we are satisfied that the additional pass forward flows will not result in any significant impacts on receptors before the watercourse discharges to the River Tay.
6. It is proposed to construct a temporary bridge over the River Almond to provide access to construction traffic. It is proposed that this bridge will be located between the A9 and the railway line at NGR NO 09684 26607. In this lower reach of the River Almond flood levels can be dominated by high water levels in the River Tay. High river levels on the River Tay can back up this lower section of the River Almond even when it is in flood. The temporary bridge will likely be in place for two to three years.
7. Sweco advised that it has carried out some modelling of the lower River Almond using a model created by the consultant Mouchel for the Almondbank Flood Protection Scheme. We would advise that the Almondbank FPS is located approximately 3 km upstream of the temporary bridge location and as such the consultant may not have focussed attention on the calibration of its model in that area. Sweco has got an estimated 0.5% AP (1:200) level of 8.562 mAOD from this model. The Babtie Shaw & Morton “Perth Flood Study – Report on Flood Mitigation Measures” (September 1993) gives the January 1993 flood level as 8.573 mAOD, and the level for the combined the 1% AP (1:100) floods on both the Rivers Tay and Almond as 8.716 mAOD. At the meeting on 4 July Sweco advised that its design water level represented a 0.5% AP (1:200) flood on the River Almond coinciding with a 0.5% AP (1:200) flood on the River Tay. There is therefore a bit of discrepancy between the modelled flows. The proposal is that the bridge will have a soffit level of 9.2 mAOD.
8. The modelling by Sweco suggests that the temporary bridge may result in a small increase in water levels of approximately 4 mm. The potential impact of the bridge is probably small due to the domination of the River Tay in this location. The FRA advises that there will be approximately 1355 m3 of floodplain storage lost during the 0.5% AP (1:200) flood due to the bridge structure. This is a significant volume of flood storage to lose but Sweco argue that as it is a temporary structure providing compensatory storage for a 3.33% AP (1:30) flood may be adequate enough rather than providing storage for the full 1355 m3. It is our view that temporary structures or build should be treated in the same manner as any permanent development. However we note that there may be some difficulties in providing compensatory storage close to the proposed temporary bridge. The Perth Flood Prevention Scheme is located on the south bank of the River Almond and should not be considered at all as a location for compensatory storage. On the north bank there is Bertha, the site of a Roman Fort, which is unlikely to be available for excavation to provide compensatory storage. We would suggest that consideration is given to potential locations for compensatory and if no appropriate site can be found then justifiable reasons based on practical, environmental and cultural heritage should be provided. We would also ask if there might be alternative structure deigns that might be considered for the river crossing that do not encroach quite so much into the floodplain.
9. We request additional information on the temporary crossing and in particular about its construction. The bridge cuts across the line of a flood embankment that forms part of the Perth Flood Prevention Scheme. We would be concerned if the defence structure should
require to be breached during the construction and/or operation of the bridge as this would create a weak point in Perth’s flood defences.
10. At the meeting on 4 July 2019 SEPA advised that it was concerned about the potential for hydrorphological impacts in the channel due to the significant narrowing of the floodplain at the proposed temporary bridge location. SEPA suggested that pre and post development modelling with a 0.5% AP (1:200) flow in the River Almond combined with a median flood in the River Tay may assist SEPA’s hydromorphology staff assess the risks of bed and bank scour and in particular damage to the Perth flood defences.
Caveats & Additional Information for Applicant
1. Please note that we are reliant on the accuracy and completeness of any information supplied by the applicant in undertaking our review, and can take no responsibility for incorrect data or interpretation made by the authors.
2. The advice contained in this letter is supplied to you by SEPA in terms of Section 72 (1) of the Flood Risk Management (Scotland) Act 2009 on the basis of information held by SEPA as at the date hereof. It is intended as advice solely to Perth and Kinross Council as Planning Authority in terms of the said Section 72 (1). Our briefing note entitled: “Flood Risk Management (Scotland) Act 2009: Flood risk advice to planning authorities” outlines the transitional changes to the basis of our advice inline with the phases of this legislation and can be downloaded from http://www.sepa.org.uk/environment/land/planning/guidance- and-advice-notes/
Briefing Note
Project Name: Cross Tay Link Road Project Author: James Walker Project Reference: 119046 Date: 12/08/2019 Project Manager: Chris Cardno Document Reference: [XXX] Revision: 1
Rev. Date Reason for issue Prepared Reviewed Approved [1] 12.08.19 First issue JJW 25.07.19 JP 12.08.19 JPF 12.08.19 [2] [00.00.00] [Text] [XX] [00.00.00] [XX] [00.00.00] [XX] [00.00.00] [3] [00.00.00] [Text] [XX] [00.00.00] [XX] [00.00.00] [XX] [00.00.00]
Sweco response 1 Introduction Following a meeting with SEPA on the 4th July 2019, several points were raised regarding the preliminary flood risk assessment for the Cross Tay Link Road Project, most notably in relation to the temporary crossing over the River Almond. This document contains the response by Sweco’s flood consultants to the requested information relating to flood risk.
1.1 Specific SEPA Flood Risk Comments to address 1. SEPA confirm that it is satisfied with the proposal to replace the two existing Bertha Loch Burn culverts under the A9 with a larger 2 m by 2 m culvert and construct an embankment along the southern bank to contain floodwaters. SEPA however advise that the construction of this embankment should not be misinterpreted as freeing up existing floodplain for potential future built development. 2. SEPA highlight that a previous study by Babtie Shaw & Morton “Perth Flood Study – Report on Flood Mitigation Measures” (September 1993) gives the January 1993 flood level as 8.573 mAOD, and the level for the combined 1% AP (1:100) floods on both the Rivers Tay and Almond as 8.716 mAOD. Sweco estimate a level of 8.562 mAOD at the proposed bridge crossing with the model representing a 0.5% AP (1:200) flood on the River Almond coinciding with a 0.5% AP (1:200) flood on the River Tay. SEPA therefore highlight a discrepancy between the modelled flows. 3. SEPA “request additional information on the construction of the temporary crossing as the bridge cuts across the line of a flood embankment that forms part of the Perth Flood Prevention Scheme. “We would be concerned if the defence structure should require to be breached during the construction and/or operation of the bridge as this would create a weak point in Perth’s flood defences. “ 4. “At the meeting on 4 July 2019 SEPA advised that it was concerned about the potential for hydromorphological impacts in the channel due to the significant narrowing of the floodplain at the proposed temporary bridge location. SEPA suggested that pre and post development modelling with a 0.5% AP (1:200) flow in the River Almond combined with a median flood in the River Tay may assist SEPA’s hydromorphology staff assess the risks of bed and bank scour and in particular damage to the Perth flood defences.” 5. SEPA request further information regarding whether alternative structural designs could reduce encroachment into the flood plain and whether compensatory storage would be feasible and potential sites identified. 6.
Briefing Note, Project Name: [XXX], Rev.:1, 4 September 2019 1 of 7 1.2 SWECO Response
1. Bertha Loch Burn flood mitigation Sweco highlight that the proposed embankment along the southern bank of the Bertha Loch Burn has been designed with a 35% climate change uplift applied to the 200-year inflows into the model. This provides an additional level of resilience and is in-line with the most recent SEPA technical guidelines1. We would therefore envisage that the additional design consideration of the embankment would be taken into account in any future planning applications which may occur behind this embankment
2. Water levels at the proposed River Almond Temporary Bridge Sweco do not have access to a copy of the Babtie Report from 1993, which was referenced during the meeting, and we therefore cannot identify fully the reason for the perceived disparity in water levels. It is unclear where the reported 100-year water level of 8.573m AOD was taken from, how the return period was identified or how it was recorded, noting that there are no known monitoring stations in the immediate area. The joint 100-year flood levels of 8.716m AOD was most probably identified by Babtie via a simplified modelling approach. However, we note that given this report is over 25 years old, it would not incorporate the more recent advances in hydrological techniques or modelling and would not include any new developments along the Rivers Tay and Almond. Sweco’s inflow hydrographs into the model were provided within the original Halcrow model, which were taken from the Almondbank Flood Protection Scheme project (Mouchel 2013). These estimates were found to be more conservative when compared with the FEH statistical analysis employed with the more up-to-date NRFAP (V7) dataset. We highlight that the 200-year SEPA flood outline (medium risk zone) closely matches the elevation contour between 8.4 and 8.8mAOD, as derived using 1 m resolution LiDAR (See Figure 1-1). Similarly, the modelling for the Flood Risk Assessment undertaken for DMRB Stage 2 CTLR Report (CH2M, 2016) also predicted a 200-year water level of approximately 8.5m AOD at the confluence with the Tay. We are therefore confident that the estimated 200-year water level is robust and defendable.
1 https://www.sepa.org.uk/media/426913/lups_cc1.pdf Figure 1-1 SEPA 1:200-year flood extent in relation to the 8-9mAOD contour derived using LiDAR.
We also note that, in the response from SEPA, it was stated that the temporary crossing is located between the A9 and the Perth to Inverness railway. The proposed crossing is located approximately 50 m downstream of the Perth to Inverness railway bridge (NGR NO 09820 26652, Figure 1-2). We highlight that our model predicts 200-year water levels of 8.798-8.764mAOD between the existing A9 and railway which is greater than the 100- year Babtie recorded value, which we assume was taken at this location.
Figure 1-2 Location of proposed temporary bridge over the River Almond
3. Information regarding the construction of the River Almond Crossing The proposed arrangement for the temporary bridge across the River Almond is shown in Drawing CRSE18014C-010 Rev P02 (a copy is issued with this memo). The nature of the proposed construction is: a sheet piled abutment to the north and a reinforced earth abutment to the south with a prefabricated ‘bailey bridge’ type deck. The longitudinal section on the drawing demonstrates the following:
· The south abutment position (including the reinforced earth extents) does not encroach on the flood bund crest;
· The access road approaching the bridge from the south sits above the crest level of the bund by approximately 1m. The flood bund is therefore not breached by the proposed bridge.
The intention is to make it a condition of the CTLR Contract that the flood protection is maintained throughout the works.
Historic Environment Scotland have been consulted on the proposed temporary crossing and access road design which interfaces with the Scheduled Ancient Monument (SAM). They have agreed in principle with the proposed design.
4. Further modelling of impact of the bridge Hydraulic analysis was undertaken to determine the impact of the temporary Bridge for the 1:200 year flood event in the River Almond and the median flood in the River Tay. The full 1D ISIS model (Halcrow, 2013) was built primarily to evaluate water levels along the River Tay. Cross-sections along the River Almond are long in this model, made up of survey and LiDAR, extending well beyond the bank top. This arrangement does not produce worst-case water levels and hence Sweco exported and updated the River Almond sections of the 1D model into Infoworks ICM The model cross-sections were truncated to the top of bank, where no overtopping was predicted. Floodwaters were allowed to flow around the existing Almond Bridge, based on a review of surrounding topography. A comparison of water levels along the River Almond indicates that the full ISIS, and truncated ICM, models produce similar results in the area downstream of the Almond Bridge (<5%) with only minor deviation upstream (<10%) for the combined 1:200 year scenario originally reported. A constant downstream boundary was applied to the outfall of the truncated model with the level of 7.068 mAOD set. This value was derived using the full 1D ISIS model which was run with a constant QMED flow applied to the upstream boundary of the River Tay and all lateral inflows set to the 2.5 year FEH design event. The truncated model predicts that the temporary crossing would result in an increase in water levels immediately upstream of the crossing, which would extend up to the upstream face of the existing Almond bridge. As SEPA suspected, this difference was not apparent previously as the downstream boundary on the River Tay was causing backing up along the River Almond and masking the effect of the bridge.
Baseline level (m Post-Development Difference Label Bridge ID AOD) Level (m AOD) (m) RA01 21.99 21.99 0.00 RA02 15.14 15.14 0.00 RA03 11.48 11.48 0.00 RA03u Almond 8.67 8.95 0.28 RA03d Bridge 8.62 8.96 0.34 RA04 8.27 8.71 0.44 RA04u 8.15 8.40 0.24 A9 Bridge RA04d 8.11 8.35 0.25 RA04d-RA04BR 7.82 8.15 0.33 Interpolation RA04BR_us Railway 7.58 8.04 0.46 RA04BR_ds Bridge 7.50 8.00 0.50 Temporary RA04A 7.34 7.77 0.43 Bridge RA05 7.07 7.07 0.00 Table 1-1 Comparison of baseline and post development water levels for the 200 year flood event
5. Impact on River Almond Floodplain
a. Loss of floodplain
Sweco have assessed the likely loss of floodplain storage associated with the proposed structure and embankment solution (per Drawing CRSE18014C-010 Rev P02). Based on the embankment footprint shown with approximately 1:2 slopes between the north abutment and the scarp slope the loss of floodplain storage is estimated at approximately 1,355 m3. The design solution proposed to the north of the River Almond has been led by the objective to keep any structures as remote as possible from the Scheduled Ancient Monument. However, an alternative arrangement would be to provide structural support to the embankment instead of side slopes which would reduce the storage requirement to approximately 750 m3. This would be subject to agreement with Historic Environment Scotland.
.b. Sites of potential compensatory storage
Sweco identified two sites to the west of the temporary bridge where compensatory storage may potentially be provided, and these are shown indicatively in Figure 1-3. These were identified due to their elevation (higher than area of floodplain loss relating to the temporary crossing), proximity to the proposed temporary bridge, and location outside of the 200-year floodplain shown on the SEPA flood mapping. These areas were identified from their topographical characteristics and consideration also has to be given to the wider issues of storage provision on these sites, including land ownership and the impact on the environment. Sweco note that both are close to existing road infrastructure and would require the removal of existing vegetation. Of the two sites Sweco consider Site A to be more appropriate given the proximity of Site B to the Scheduled Ancient Monument (SAM) and the existing A9 embankment. There is an objective to minimise the impact on the SAM, particularly given that the design proposal also includes the construction of a temporary floating road across it. In addition, it would involve excavation in the vicinity of the A9 road embankment with the concern that there may potentially be a detrimental effect on the integrity of the trunk road embankment. For these reasons, Site B has been discounted and is not accommodated in the land which has been identified for inclusion in the upcoming Compulsory Purchase Order (CPO).
Site A – whilst this site has been included within the CPO land take as the most feasible location identified, the following concerns are raised in relation to the environmental assessment which would be required:
1. The area is covered by ancient woodland of plantation origin. This will need to be removed and replanted and it will take many years to get back to maturity. 2. Whilst the area has been included in some environmental surveys carried out for the project additional surveys would be required including a detailed bat survey. There is potential therefore for issues with protected species. 3. It borders the River Tay / Almond Special Area of Conservation therefore disturbance should be avoided. Whilst the contract would include mitigation measures the potential for siltation / polluting runoff would be introduced.
In summary, whilst Site A is the only feasible location identified for compensatory storage in the upstream vicinity of the temporary bridge, the anticipated detrimental environmental impact is out of proportion to the requirements to provide compensatory storage for a temporary bridge (reduced to from 1355 to 750m3). We would therefore ask SEPA to consider omitting the requirement for compensatory storage in light of this information
B A
Figure 1-3 Overview of potential sites for compensatory storage Figure 1-4 Ground elevations in relation to the two sites of potential compensatory storage.
FLOOD RISK HYDROLOGY RESPONSE TO REQUEST FOR INFORMATION RELATING TO FLOOD RISK
SOUTH EAST AREA (PERTH OFFICE)
Site: Cross Tay Link Road. Temporary bridge crossing over the River Almond.
SEPA Ref: PCS/167509 Planning Ref: CTLR
Documents Reviewed: Briefing Note from Sweco dated 12 August 2019
Technical Report
1. Sweco has prepared a briefing note to respond to a couple of issues raised by SEPA in relation to the Bertha Loch Burn flood mitigation proposals and the temporary bridge crossing over the River Almond required for construction traffic.
2. While we acknowledge that the proposed embankment along the south bank of the Bertha Loch Burn will be designed to protect land behind it up to an estimated 0.5% AP (1:200) plus 35% climate change uplift standard we will not support proposals for building on land behind it. The proposed embankment does not form part of a formal flood protection scheme. SEPA’s current position is set out in SEPA Planning information Note 4 which states that any protection offered by informal flood defences (ie. not forming part of a flood protection scheme) would not be taken into account when considering development behind or benefitting from them.
3. We note Sweco’s comments on the predicted 0.5% AP (1:200) flood levels at the confluence of the Rivers Tay and Almond and also the location of the proposed temporary bridge. We also acknowledge that we understand that the bridge and access road will not encroach upon the Perth Flood Prevention Scheme bund or require it to be breached.
4. Following previous comments by SEPA additional model runs have been undertaken to determine if the proposed temporary bridge will impact on flood levels in the River Almond when there is no coinciding significant flood flows in the River Tay. Results indicate that under this scenario upstream water levels would be elevated up to the existing Almond Bridge. This will not result in a flooding issue but may result in an increase in scour and bank erosion should this scenario occur while the bridge is in place. The potential impacts may require some mitigation and should be considered.
5. Sweco has estimated that the temporary bridge could result in a loss of up to 1,355 m3 of floodplain storage volume. Sweco note that an alternative arrangement for the bridge design could reduce the loss to about 750 m3 but this would need to be subject to agreement with Historic Environment Scotland. Sweco has identified two potential sites for compensatory storage but these are far from ideal. Both sites are close to the existing road infrastructure and require the removal of existing vegetation and are also close to the flood protection scheme and the Scheduled Ancient Monument (SAM). Given the potential for damage to important infrastructure, the SAM, the loss of mature trees and the temporary nature of the proposed bridge and associated impacts SEPA are not insisting that compensatory storage be provided under these circumstances.
Flood Risk Technical Response Template SS-NFR-T-002 v4.0 10/12/10
Caveats & Additional Information for Applicant
6. Please note that we are reliant on the accuracy and completeness of any information supplied by the applicant in undertaking our review, and can take no responsibility for incorrect data or interpretation made by the authors.
Flood Risk Technical Response Template SS-NFR-T-002 v4.0 10/12/10 CHAPTER 15 Road Drainage and the Water Environment
Appendix 15.3 – Water Quality Calculations
Cross Tay Link Road
Revision Date Version Author Technical Reviewer Checker Approver Number P01.1 29.04.19 DRAFT J MOORE E COOPER R McLEAN D. RITCHIE
P01.2 15.07.19 FINAL J MOORE R McLEAN R McLEAN D. RITCHIE
BIM Reference: 119046-SWECO-EWE-000-RP-EN-20030
This document has been prepared on behalf of Perth and Kinross Council by Sweco for the proposed Cross Tay Link Road Project. It is issued for the party which commissioned it and for specific purposes connected with the above-captioned project only. It should not be relied upon by any other party or used for any other purpose. Sweco accepts no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties.
This document contains confidential information and proprietary intellectual property. It should not be shown to other parties without consent from Perth and Kinross Council.
Prepared for: Prepared by: Perth and Kinross Council Sweco Pullar House Suite 4.2, City Park 35 Kinnoull Street 368 Alexandra Parade Perth Glasgow PH1 5GD G31 3AU
CONTENTS
1 WATER QUALITY CALCULATIONS ...... 1 1.1 Introduction ...... 1 1.2 Routine Runoff Assessment Methodology ...... 1 1.3 Accidental Spillage Risk Assessment Methodology ...... 3 1.4 HAWRAT Assessment Parameters ...... 4 1.5 HAWRAT Calculations Sheets/outputs ...... 13
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
1 WATER QUALITY CALCULATIONS
1.1 INTRODUCTION
This appendix provides detailed information on the methodology and calculations used to inform the water quality assessment during the operational phase of the proposed CTLR Project, as reported in Volume 2 Chapter 15: Road Drainage and the Water Environment.
As part of the water quality assessment, routine runoff and accidental spillage risk to the watercourses predicted to receive road drainage were assessed using the Highways Agency’s (now Highways England’s) Water Risk Assessment Tool (HAWRAT), in line with DMRB HD45/09 guidance 1 . The approach and methods used in these assessments are described, followed by the input parameters and calculation sheets for the routine runoff and accidental spillage risk assessments, respectively.
Only drainage from the carriageways of the realigned A9 trunk road and grade-separate junction is subject to this DMRB HD45/09 risk assessment. The CTLR carriageway from the River Tay Crossing Bridge to the tie-in at the A94 junction is not a trunk road and thus has a lower pollution hazard level. This section of new carriageway is therefore subject to a simpler assessment following CIRIA’S Simple Index Approach (SIA), as prescribed in CIRIA’s Sustainable Drainage Systems (SuDS) Manual2 and has been previously agreed with SEPA. Refer to Chapter 15 for more details of the SIA methods and results. The indicative drainage outfall locations are shown on Figure 15.3: Water Environment and Mitigation.
1.2 ROUTINE RUNOFF ASSESSMENT METHODOLOGY
HAWRAT was developed to assess potential effects of routine runoff on surface waters. Runoff Specific Thresholds (RST) were developed to protect aquatic ecology in watercourses, which relate to the intermittent nature of road runoff (i.e. contaminants washed off the road surface in a rainfall event), over a typical exposure period of six hours (RST 6 hour) and for a worst-case scenario of 24 hours (RST 24 hour). Dissolved copper and dissolved zinc are used as indicators of the level of impact as they can result in acute toxic effects to aquatic life in certain concentrations. Table 1.1 summarises the RSTs for dissolved copper and dissolved zinc.
Table 1.1: RST for short-term exposure (cited within DMRB HD45/09)
Hardness Threshold Zinc (ug/l) Hardness Copper (ug/l) Low (<50mg Medium (50- High (>200mg CaCO3/l) 200mg CaCO3/l) CaCO3/l) RST 24 hour 21 60 92 385
RST 6 hour 42 120 184 770
HAWRAT also assesses chronic impacts (i.e. can persist for weeks or months) associated with sediment- bound pollutants on aquatic ecology within watercourses. Two standards are used for metal and polycyclic aromatic hydrocarbon (PAH) concentrations within sediment; Threshold Effects Levels (TELs) (i.e. the concentration below which toxic effects are very rare) and Probable Effects Levels (PELs) (i.e. the concentration above which toxic effects are observed on most occasions). Table 1.2 summarises some of the key sediment-bound pollutant thresholds used within HAWRAT.
1 Highways Agency et al. (2009) HD 45/09: Design Manual for Roads and Bridges (DMRB), Volume 11, Section 3, Part 10, Road Drainage and the Water Environment, 2009. The Highways Agency, Scottish Executive Development Department, The National Assembly for Wales and The Department of Regional Development Northern Ireland. Accessed 21/02/2019 [http://www.standardsforhighways.co.uk/ha/standards/dmrb/vol11/section3/hd4509.pdf] 2 CIRIA (2015) The SUDS Manual, C753
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Table 1.2: Sediment Concentration TELs and PELs (cited within DMRB HD45/09)
Parameter Threshold Effects Level Probable Effects Level Copper 35.7 mg/kg 197 mg/kg
Zinc 123 mg/kg 315 mg/kg Total polycyclic aromatic 1,684 µg/kg 16,770 µg/kg hydrocarbon (PAH)
HAWRAT also estimates longer-term in-river annual average concentrations for soluble pollutants (dissolved copper and dissolved zinc) which includes the contribution from road runoff. These concentrations are compared against published Environmental Quality Standards (EQS) for freshwaters to assess whether there is likely to be a long-term impact on ecology (see Table 1.3).
Table 1.3: EQS for the Protection of all Freshwater Life (DMRB HD45/09)
Parameter Hardness Range (mg/l CaCO3) Freshwater EQS (ug/l) (annual average) Dissolved Copper 0 – 50 1
>50 – 100 6
>100 – 250 10
>250 28 Dissolved Zinc 0 – 50 >50 – 100 7.8 >100 – 250 >250
HAWRAT uses a tiered approach (three-step) to assess the impacts of both soluble and sediment-bound pollutants. A ‘Pass’ or ‘Fail’ result is recorded depending on whether the risk is within or exceeds the HAWRAT thresholds. The impact of routine runoff to each receiving watercourse is summarised by a ‘traffic light’ reporting approach, whereby:
• Red = unacceptable impact (i.e. one or more pollutant concentrations exceed thresholds and therefore incur a Fail result) or there is a need to carry out further stages of assessment. • Green = no significant impact (i.e. pollutant concentrations are within thresholds and therefore incur a Pass result) with no need for further assessment. • Amber = for assessment of sediment-bound pollutants, an ‘Alert’ result indicates the presence of a protected nature site and/or a downstream structure impacting on flow velocity, which may require further site-specific consideration.
Where a Fail result is recorded for one or more of the pollutant types, the next step is required based on increasing levels of inputs and assessment. The three-step approach comprises:
• Step 1: Runoff Quality (predicts the concentrations of pollutants in untreated and undiluted highway runoff prior to any treatment and dilution in a watercourse). • Step 2: In-River Impacts (predicts the concentrations of pollutants after mixing within a watercourse). At this stage, the ability of the receiving watercourse to disperse sediments is considered and, if sediment is predicted to accumulate, the potential extent of sediment coverage (i.e. the deposition index, DI) is also considered. • Step 3: In-River Impacts with mitigation. Steps 1 and 2 assume the proposed CTLR Project drainage system does not include mitigation to reduce the risk. Step 3 takes into account SuDS (and the risk reduction factor associated with these systems) of any existing and/or any proposed measures in the drainage design. SuDS are required for new developments under the Water Environment (Controlled Activities) (Scotland) Regulations 2011 (as amended) (CAR) for new
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developments (even if the results produce a ‘Pass’ result at Step 2, prior to any mitigation). Refer to Chapter 15 for details on SuDS measures for the proposed CTLR Project.
Step 2 also incorporates two ‘tiers’ of assessment for sediment accumulation, based on different levels of input parameters. If one or more risks are defined as unacceptable at Tier 1, then a more detailed Tier 2 assessment is undertaken, which requires further information relating to the receiving watercourse including bed width, Manning’s ‘n’ (roughness), bank slope and channel gradient.
As well as assessing the risk of routine runoff from each drainage outfall individually, an in-combination assessment is undertaken where more than one outfall discharges into the same reach of watercourse. This is the ‘worst-case’ scenario as the combined effects could be more significant. To aggregate the assessments, the total impermeable and permeable carriageway areas to be drained are added together, and the low flow of the watercourse is taken at the outfall location furthest downstream (this is the assessment point of the combined outfall assessment). For drainage outfalls positioned between 100m and 1km apart, the cumulative assessment is for soluble pollutants only, whilst for outfalls positioned closer together (within 100m), the combined assessment includes soluble and sediment pollutants (at distances greater than 100m, it is considered that sediments are sufficiently dispersed to not cause a significant cumulative effect).
1.3 ACCIDENTAL SPILLAGE RISK ASSESSMENT METHODOLOGY
Along a road there is a risk of vehicular collision that could result in the spillage of fuels, oils or chemicals, particularly if tankers and heavy goods vehicles (HGVs) are involved. A risk assessment of a serious spillage causing a pollution incident was undertaken using the methodology outlined in DMRB HD45/09.
The risk is calculated assuming that an accident involving spillage of pollutants onto the carriageway would occur at an assumed frequency (expressed as an annual probability), based on calculated traffic volumes and the type of road/junction (Table 1.4). The annual probability of a serious accidental spillage also depends upon the emergency services response time, based on the location (i.e. urban, rural or remote location) and type of receiving water body (surface or groundwater) (Table 1.5).
Table 1.4: Serious Accidental Spillages per Billion HGV (km/year)
Road/Junction Type Motorway Rural Trunk Road Urban Trunk Road No Junction 0.36 0.29 0.31
Slip Road 0.43 0.83 0.36
Roundabout 3.09 3.09 5.35
Crossroad n/a 0.88 1.46
Side Road n/a 0.93 1.81
Note: Risk factor applies to all road lengths within 100m of these junction types
Table 1.5: Probability of a Serious Accidental Spillage Leading to a Serious Pollution Incident
Urban (response Rural (response time Remote (response Receiving Waterbody time to site <20 to site <1 hour) time to site >1 hour) mins) Surface Watercourse 0.45 0.6 0.75
Groundwater 0.3 0.3 0.5
Where spillage risk is calculated as less than 1% Annual Exceedance Probability (AEP) (less frequent than 1:100 years), the spillage falls within acceptable limits and no mitigation (SuDS) is required. Where road runoff discharges to, or shortly upstream, of designated conservation sites (e.g. Special Area of Conservation, salmonid waters, or a potable water supply), a more stringent threshold of 0.5% AEP (less frequent than 1:200 years) is used. As the proposed road drainage of the proposed CTLR Project is to November 2019 PAGE 3 OF APPENDIX 15.3
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tributaries a short distance upstream of the River Tay SAC, the 0.5% AEP threshold was used for this assessment. Indicative pollution risk reduction factors associated with the SuDS systems proposed for the proposed CTLR Project is shown in Table 1.6.
Table 1.6: Indicative Pollutant Risk Reduction Factors (DMRB HD45/09)
SuDS System Risk Reduction Factor Filter Drain 0.6 (40%)
Pond/Swale 0.5 (50%)
Using the same process as for the routine runoff assessment, a combined spillage risk assessment is undertaken where more than one outfall discharges into the same reach of watercourse. To aggregate the assessments, the total length of road drained (split into each road/junction type from Table 1.4) is combined for all outfalls and the highest AADT and %HGV values are taken for each road/junction type.
1.4 HAWRAT ASSESSMENT PARAMETERS
The routine runoff and accidental spillage risk assessment parameters (and sources) for each outfall proposed to drain the realigned A9 and new grade-separated junction carriageways of the proposed CTLR Project are provided in Tables 1.7 to 1.14. Individual outfall assessments are considered first followed by two cumulative assessments for two drainage outfalls draining to Redgorton Drain (networks A9-1A and A9-1) and the River Almond (networks A9-5 and A9-4/A9-5A).
Table 1.7: Drainage Network A9-1 (single outfall assessment)
Parameter Value Source Receiving Watercourse Redgorton Drain
Assessment (outfall) location Easting: 309364 Northing: Scheme drainage design 728664 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 16,516 (Sweco traffic model, 2019) %HGV 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 0.004m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.560 No descriptor available therefore used Bertha Loch as a proxy (from FEH catchment descriptors)
Impermeable road area 0.278ha Scheme drainage design drained Permeable area drained 0.001ha
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Parameter Value Source Length of road drainage to 221m (no junction) outfall Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Drain is culverted under existing structure, lake, pond or canal A9 carriageway and railway line that reduces the velocity before discharging to Tay – no within 100m of the point of proposed changes to this (not discharge? assumed to restrict flow/reduce velocity)
Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 0.5m Site visit information (Tier 1) Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 40% 1 level of treatment – filter drains solubles (indicative pollution mitigation Proposed attenuation Unlimited indices from CIRIA’s SUDS Manual and DMRB HD45/09) Proposed settlement of 40% sediments
Table 1.8: Drainage Network A9-1A (single outfall assessment)
Parameter Value Source Receiving Watercourse Redgorton Drain
Assessment (outfall) location Easting: 309316 Northing: Scheme drainage design 728748 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 15,480 (Sweco traffic model, 2019) %HGV 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 0.004m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.560 No descriptor available therefore used Bertha Loch as a proxy November 2019 PAGE 5 OF APPENDIX 15.3
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Parameter Value Source (from FEH catchment descriptors)
Impermeable road area 0.432ha Scheme drainage design drained Permeable area drained 0.092ha Length of road drainage to 294m (no junction) outfall Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Drain is culverted under existing structure, lake, pond or canal A9 carriageway and railway line that reduces the velocity before discharging to Tay – no within 100m of the point of proposed changes to this (not discharge? assumed to restrict flow/reduce velocity)
Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 0.5m Site visit information (Tier 1) Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 40% 1 level of treatment – filter drains solubles (indicative pollution mitigation Proposed attenuation Unlimited indices from CIRIA’s SUDS Manual and DMRB HD45/09) Proposed settlement of 40% sediments
Table 1.9: Drainage Network A9-2 (single outfall assessment)
Parameter Value Source Receiving Watercourse Broxy Kennels Drain
Assessment (outfall) location Easting: 309246 Northing: Scheme drainage design 727863 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 31,996 (Sweco traffic model, 2019) %HGV 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest
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Parameter Value Source SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 0.002m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.560 No descriptor available therefore used Bertha Loch as a proxy (from FEH catchment descriptors)
Impermeable road area 3.864ha Scheme drainage design drained Permeable area drained 1.142ha Length of road drainage to 893m (no junction) outfall Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Drain is culverted under existing structure, lake, pond or canal A9 carriageway and railway line that reduces the velocity before discharging to Tay – no within 100m of the point of proposed changes to this (not discharge? assumed to restrict flow/reduce velocity)
Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 0.5m Site visit information (Tier 1) Bed width (Tier 2) 0.5m Site visit information
Manning’s n (Tier 2) 0.03 Straight, uniform, grass some weeds Source: Chow (1973), cited in DMRB HD45/09
Side slope (m/m) (Tier 2) 0.5 Site visit information
Long slope (m/m) (Tier 2) 0.001 Site visit information
Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 65% 2 levels of treatment – filter solubles drains and detention basin Proposed attenuation 7.0 l/s (indicative pollution mitigation indices from CIRIA’s SUDS Proposed settlement of 65% Manual and DMRB HD45/09). sediments Outflow from SuDS restricted to the Greenfield (1 in 2 year) runoff rate.
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Table 1.10: Drainage Network A9-3/A9-7 (single outfall assessment)
Parameter Value Source Receiving Watercourse Bertha Loch Burn
Assessment (outfall) location Easting: 309560 Northing: Scheme drainage design 727099 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) No junction: 29,946 (Sweco traffic model, 2019) Roundabouts: 7,083 Slip roads: 6,244 Link road: 6,180 %HGV No junction: 10% Roundabouts: 10% Slip roads: 9% Link road: 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 0.005m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.560 FEH catchment descriptors
Impermeable road area 4.038ha Scheme drainage design drained Permeable area drained 6.652ha Length of road drainage to No junction: 226m outfall Roundabouts: 217m Slip roads: 523m Link road: 140m Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Burn is culverted under railway structure, lake, pond or canal line before discharging to Tay that reduces the velocity but this is not assumed to restrict within 100m of the point of flow/reduce velocity discharge? Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 0.75m Site visit information (Tier 1) Bed width (Tier 2) 0.7m Site visit information
Manning’s n (Tier 2) 0.03 Straight, uniform, grass some weeds
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Parameter Value Source Source: Chow (1973), cited in DMRB HD45/09
Side slope (m/m) (Tier 2) 0.5 Site visit information
Long slope (m/m) (Tier 2) 0.001 Site visit information
Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 65% 2 levels of treatment – filter solubles drains and detention basin Proposed attenuation 5.2 l/s (indicative pollution mitigation indices from CIRIA’s SUDS Proposed settlement of 65% Manual and DMRB HD45/09). sediments Outflow from SuDS restricted to the Greenfield (1 in 2 year) runoff rate.
Table 1.11: Drainage Network A9-4/A9-5A (single outfall assessment)
Parameter Value Source Receiving Watercourse River Almond
Assessment (outfall) location Easting: 309640 Northing: Scheme drainage design 726626 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 41,478 (Sweco traffic model, 2019) %HGV 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 1.062m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.466 Taken from National River Flow Archive (NRFA) gauge at Almondbank
Impermeable road area 3.318ha Scheme drainage design drained Permeable area drained 1.306ha Length of road drainage to 842m (no junction) outfall
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Parameter Value Source Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Burn is bridged under railway structure, lake, pond or canal line before discharging to Tay that reduces the velocity but this is not assumed to restrict within 100m of the point of flow/reduce velocity discharge? Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 8.0m Site visit information (Tier 1) Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 65% 2 levels of treatment – filter solubles drains and detention basin Proposed attenuation 5.7 l/s (indicative pollution mitigation indices from CIRIA’s SUDS Proposed settlement of 65% Manual and DMRB HD45/09). sediments Outflow from SuDS restricted to the Greenfield (1 in 2 year) runoff rate.
Table 1.12: Drainage Network A9-5 (single outfall assessment)
Parameter Value Source Receiving Watercourse River Almond
Assessment (outfall) location Easting: 309595 Northing: Scheme drainage design 726609 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 20,589 (Sweco traffic model, 2019) %HGV 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 1.062m3/s Wallingford HydroSolutions (WHS) software
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Parameter Value Source Baseflow Index (BFI) 0.466 Taken from National River Flow Archive (NRFA) gauge at Almondbank
Impermeable road area 0.336ha Scheme drainage design drained Permeable area drained 0.016ha Length of road drainage to 269m (no junction) outfall Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Burn is bridged under existing structure, lake, pond or canal A9 and railway line before that reduces the velocity discharging to Tay but this is not within 100m of the point of assumed to restrict flow/reduce discharge? velocity
Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 8.0m Site visit information (Tier 1) Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 40% 1 level of treatment – filter drains solubles (indicative pollution mitigation Proposed attenuation Unlimited indices from CIRIA’s SUDS Manual and DMRB HD45/09). Proposed settlement of 40% sediments
Table 1.13: Drainage Networks A9-1A and A9-1 downstream (cumulative outfall assessment including sediments – outfalls less than 100m apart)
Parameter Value Source Receiving Watercourse Redgorton Drain
Assessment (outfall) location Easting: 309364 Northing: Scheme drainage design 728664 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 31,996 (Sweco traffic model, 2019) %HGV 10% Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site
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Parameter Value Source geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 0.004m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.560 No descriptor available therefore used Bertha Loch as a proxy (from FEH catchment descriptors)
Impermeable road area 0.71ha (0.278+0.432) Scheme drainage design drained Permeable area drained 0.093ha (0.001+0.092) Length of road drainage to 515m (221+294) (no junction) outfall Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Drain is culverted under existing structure, lake, pond or canal A9 carriageway and railway line that reduces the velocity before discharging to Tay – no within 100m of the point of proposed changes to this (not discharge? assumed to restrict flow/reduce velocity)
Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 0.5m Site visit information (Tier 1) Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 40% 1 level of treatment – filter drains solubles (indicative pollution mitigation Proposed attenuation Unlimited indices from CIRIA’s SUDS Manual and DMRB HD45/09) Proposed settlement of 40% sediments
Table 1.14: Drainage Networks A9-5 and A9-4/A9-5A downstream (cumulative outfall assessment including sediments – outfalls less than 100m apart)
Parameter Value Source Receiving Watercourse River Almond
Assessment (outfall) location Easting: 309640 Northing: Scheme drainage design 726626 AADT (vehicles/day) (range) >10,000 and <50,000 Scheme design year 2038 AADT (vehicles/day) 41,478 (Sweco traffic model, 2019) %HGV 10%
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Parameter Value Source Climatic Region Colder/Wet HAWRAT Help Manual v1.0 (2009)
Rainfall Site Keighley (SAAR 1000mm) Catchment descriptors taken from CEH website at Almondbank (SAAR 1394mm) and Luncarty (SAAR 933mm) gauges. Although Keighley is not the closest site geographically, it has the closest SAAR value to these flow gauges in the CTLR study area.
Low flow (Q95) 1.062m3/s Wallingford HydroSolutions (WHS) software
Baseflow Index (BFI) 0.466 Taken from National River Flow Archive (NRFA) gauge at Almondbank
Impermeable road area 3.654ha (3.318+0.336) Scheme drainage design drained Permeable area drained 1.322ha (1.306+0.016) Length of road drainage to 1,111m (842+269) (no junction) outfall Is the discharge in or within Yes River Tay Special Area of 1km upstream of a protected Conservation site for conservation? Is there a downstream No Burn is bridged under railway structure, lake, pond or canal line before discharging to Tay that reduces the velocity but this is not assumed to restrict within 100m of the point of flow/reduce velocity discharge? Hardness Low <50mg CaCO3/l No site data, low hardness is worst-case (precautionary)
Estimated river width at Q95 8.0m Site visit information (Tier 1) Existing treatment of solubles 0% Only partial treatment on Existing attenuation Unlimited existing A9 – assume no existing treatment (worst-case) Existing settlement of 0% sediments Proposed treatment of 65% 2 levels of treatment – filter solubles drains and detention basin Proposed attenuation 5.7 l/s (indicative pollution mitigation indices from CIRIA’s SUDS Proposed settlement of 65% Manual and DMRB HD45/09). sediments Outflow from SuDS restricted to the Greenfield (1 in 2 year) runoff rate.
1.5 HAWRAT CALCULATIONS SHEETS/OUTPUTS
The routine runoff output tables and accidental spillage calculation sheets provided in HAWRAT are provided in Tables 1.15 to 1.30, and the results are summarised in Volume 2 Chapter 15.
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Table 1.15: Routine Runoff Assessment – Network A9-1 (single outfall assessment)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0 0 Velocity 0.11 m/s Tier 1 is used for the calculation No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0 0 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.08 0.29
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.38 1.25 90%ile 0.89 2.69 95%ile 1.42 5.05 99%ile 3.91 15.67
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.05 0.17
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.23 0.75 90%ile 0.53 1.61 95%ile 0.85 3.03 99%ile 2.35 9.40
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
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Table 1.16: Routine Runoff Assessment – Network A9-1A (single outfall assessment)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0 0.1 Velocity 0.11 m/s Tier 1 is used for the calculation No. of exceedances/worst year 0 1 No. of exceedances/summer 0 0.1 DI - No. of exceedances/worst summer 0 1 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0 0 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.12 0.44
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.57 1.85 90%ile 1.36 4.07 95%ile 2.16 7.61 99%ile 5.77 22.71
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.07 0.26
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.34 1.11 90%ile 0.82 2.44 95%ile 1.29 4.56 99%ile 3.46 13.63
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
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Table 1.17: Routine Runoff Assessment – Network A9-2 (single outfall assessment)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 3 4.8 Velocity 0.11 m/s Tier 2 is used for the calculation No. of exceedances/worst year 7 7 No. of exceedances/summer 1.7 2.5 DI - No. of exceedances/worst summer 4 6 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.3 1 No. of exceedances/worst year 2 2 No. of exceedances/summer 0.1 0.4 No. of exceedances/worst summer 1 2
Annual average concentration (ug/l) 1.30 4.46
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 5.10 15.90 90%ile 12.53 38.34 95%ile 18.08 57.52 99%ile 31.18 120.71
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.10 0.50 No. of exceedances/worst year 1 2 No. of exceedances/summer 0 0.2 DI - No. of exceedances/worst summer 0 2
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.45 1.59
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 1.75 5.43 90%ile 4.16 13.21 95%ile 6.22 20.04 99%ile 10.71 40.50
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
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Table 1.18: Routine Runoff Assessment – Network A9-3 (single outfall assessment)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.7 1.7 Velocity 0.14 m/s Tier 2 is used for the calculation No. of exceedances/worst year 2 3 No. of exceedances/summer 0.3 0.7 DI - No. of exceedances/worst summer 1 2 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.1 0.3 No. of exceedances/worst year 1 1 No. of exceedances/summer 0.1 0.1 No. of exceedances/worst summer 1 1
Annual average concentration (ug/l) 0.68 2.31
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 2.70 8.50 90%ile 6.64 20.76 95%ile 9.69 31.43 99%ile 17.82 69.09
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.24 0.85
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.86 2.69 90%ile 2.09 6.47 95%ile 3.05 9.93 99%ile 5.27 21.02
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
November 2019 PAGE 17 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.19: Routine Runoff Assessment – Network A9-4/A9-5A (single outfall assessment)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0 0 Velocity 0.29 m/s Tier 1 is used for the calculation No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0 0 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.00 0.01
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.02 0.06 90%ile 0.04 0.12 95%ile 0.07 0.24 99%ile 0.19 0.74
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.00 0.00
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.01 0.02 90%ile 0.01 0.04 95%ile 0.02 0.07 99%ile 0.04 0.17
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
November 2019 PAGE 18 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.20: Routine Runoff Assessment – Network A9-5 (single outfall assessment)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0 0 Velocity 0.29 m/s Tier 1 is used for the calculation No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0 0 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.00 0.00
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.00 0.01 90%ile 0.00 0.01 95%ile 0.01 0.02 99%ile 0.02 0.08
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.00 0.00
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.00 0.00 90%ile 0.00 0.01 95%ile 0.00 0.01 99%ile 0.01 0.05
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
November 2019 PAGE 19 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.21: Routine Runoff Assessment – Network A9-1A and A9-1 (combined outfalls)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.2 0.2 Velocity 0.11 m/s Tier 1 is used for the calculation No. of exceedances/worst year 1 1 No. of exceedances/summer 0.1 0.1 DI - No. of exceedances/worst summer 1 1 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0 0 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.19 0.69
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.88 2.85 90%ile 2.12 6.40 95%ile 3.42 11.82 99%ile 8.68 33.80
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.10 No. of exceedances/worst year 0 1 No. of exceedances/summer 0 0.1 DI - No. of exceedances/worst summer 0 1
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.11 0.42
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.53 1.71 90%ile 1.27 3.84 95%ile 2.05 7.09 99%ile 5.21 20.28
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
November 2019 PAGE 20 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.22: Routine Runoff Assessment – Network A9-5 and A9-4/A9-5A (combined outfalls)
Summary of predictions Soluble - Acute Impact Sediment - Chronic Impact Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene
Prediction of impact Step1
Step2
Step3 DETAILED RESULTS
In Runoff Step 1 Step 1
Copper Zinc Copper Zinc Cadmium Total PAH Pyrene Fluoranthene Anthracene Phenanthrene RST24 Toxicity Threshold Allowable Exceedances/year 1 1 1 1 1 1 1 1 1 1 No. of exceedances/year 39.50 39.40 52.50 71.40 1.00 30.30 72.50 30.30 14.40 59.40 No. of exceedances/worst year 53 50 65 81 3 37 81 37 21 66
RST6 Allowable Exceedances/year 1 1 No. of exceedances/year 11.40 14.20 No. of exceedances/worst year 18 19
(ug/l) (ug/l) (mg/kg) (mg/kg) (mg/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) (ug/kg) Toxicity Thresholds RST24 197 315 3.5 16770 875 2355 245 515 21 60 Threshold Thresholds RST6 42 120
Event Statistics Mean 23.22 68.11 305 1133 1 15615 2701 2592 166 731 90%ile 45.10 142.80 690 2629 1 35481 6138 5890 376 1661 95%ile 57.14 182.03 869 3668 2 35481 6138 5890 376 1661 99%ile 91.61 388.90 1221 6393 3 89125 15419 14795 945 4171
In River (no mitigation) Step 2 Step 2
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0 0 Velocity 0.29 m/s Tier 1 is used for the calculation No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0 % settlement needed - % RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0 0 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.00 0.01
(ug/l) (ug/l) Thresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.02 0.07 90%ile 0.05 0.13 95%ile 0.07 0.27 99%ile 0.21 0.82
In River (with mitigation) Step 3
Copper Zinc RST24 Allowable Exceedances/year 1 1 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 DI - No. of exceedances/worst summer 0 0
RST6 Allowable Exceedances/year 0.5 0.5 No. of exceedances/year 0.00 0.00 No. of exceedances/worst year 0 0 No. of exceedances/summer 0 0 No. of exceedances/worst summer 0 0
Annual average concentration (ug/l) 0.00 0.01
(ug/l) (ug/l) ThresholdsThresholds RST24 21 60 Thresholds RST6 42 120
Event Statistics Mean 0.01 0.02 90%ile 0.01 0.04 95%ile 0.02 0.07 99%ile 0.04 0.18
Details of the chosen rainfall site SAAR (mm) 1000 Altitude (m) 200 Easting 4060 Northing 4410 Coastal distance (km) 70
November 2019 PAGE 21 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.X CROSS TAY LINK ROAD APPENDIX TITLE EIA REPORT (VOLUME 2)
Accidental Spillage Risk Assessment – Calculation Sheets
Table 1.23: Spillage Risk Assessment – Network A9-1 (single outfall assessment)
November 2019 PAGE 22 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.24: Spillage Risk Assessment – Network A9-1A (single outfall assessment)
November 2019 PAGE 23 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.25: Spillage Risk Assessment – Network A9-2 (single outfall assessment)
November 2019 PAGE 24 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.26: Spillage Risk Assessment – Network A9-3 (single outfall assessment)
November 2019 PAGE 25 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.27: Spillage Risk Assessment – Network A9-4/A9-5A (single outfall assessment)
November 2019 PAGE 26 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.28: Spillage Risk Assessment – Network A9-5 (single outfall assessment)
November 2019 PAGE 27 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.29: Spillage Risk Assessment – Networks A9-1 and A9-1A (combined outfall assessment)
November 2019 PAGE 28 OF APPENDIX 15.3
CHAPTER 15 APPENDIX 15.3 CROSS TAY LINK ROAD WATER QUALITY CALCULATIONS EIA REPORT (VOLUME 2)
Table 1.30: Spillage Risk Assessment – Networks A9-5 and A9-4/A9-5A (combined outfall assessment)
November 2019 PAGE 29 OF APPENDIX 15.3
CHAPTER 15 Road Drainage and the Water Environment Appendix 15.4 – Engineering in the Water November 2019 PAGE 1-0 OF CHAPTER 6 Environment
Cross Tay Link Road
Revision Date Version Author Technical Reviewer Checker Approver Number P01 23.04.19 DRAFT E REID J MOORE R. McLEAN D. RITCHIE
P02 02.09.19 DRAFT E REID J MOORE R. McLEAN D. RITCHIE
BIM Reference: 119046-SWECO-EWE-000-RP-EN-20033
This document has been prepared on behalf of Perth and Kinross Council by Sweco for Cross Tay Link Road. It is issued for the party which commissioned it and for specific purposes connected with the above- captioned project only. It should not be relied upon by any other party or used for any other purpose. Sweco accepts no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties.
This document contains confidential information and proprietary intellectual property. It should not be shown to other parties without consent from Perth and Kinross Council.
Prepared for: Prepared by: Perth and Kinross Council Sweco Pullar House Suite 4.2, City Park 35 Kinnoull Street 368 Alexandra Parade Perth Glasgow PH1 5GD G31 3AU
November 2019 PAGE 1-0 OF CHAPTER 6
CONTENTS
1 ENGINEERING IN THE WATER ENVIRONMENT ...... 1 1.1 Introduction ...... 1
CHAPTER 15 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER EIA REPORT (VOLUME 2) ENVIRONMENT
1 ENGINEERING IN THE WATER ENVIRONMENT
1.1 INTRODUCTION
This appendix provides additional information on the watercourse crossings (bridges and culverts) that are to be constructed or modified as part of the proposed CTLR Project. Table 1.1 provides information on the watercourse crossing proposals as well as justification for each engineering option (as well as reasons for rejection of alternatives). Photographs are also provided of the affected watercourses in the vicinity of the proposed engineering works. This appendix is supported by Figure 2.3: Bridge and Culvert Locations.
Mitigation to reduce any predicted significant effects resulting from these in-channel engineering works is detailed in Section 15.7 (Mitigation and Enhancement) of Volume 2, Chapter 15: Road Drainage and the Water Environment.
November 2019 PAGE 1 OF APPENDIX 15.4
CHAPTER 15 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Table A15.1: Watercourse Engineering Activities
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) River Tay Option 1: River Tay A large structure (bridge) is required to Crossing Cast-in-place (approx. cross the River Tay (Photo 1) and existing Bridge concrete 115m wide) Highland Mainline Railway, to link the new structure segmental CTLR carriageway to the A9 interchange. cantilever box type structure. Four options (including the DMRB Stage 2 Preferred Option) were considered, Deck comprising: supported on piled, • Option 1: A 3-span low-level bridge with reinforced western back span crossing the railway concrete line and piers out with the normal abutments extents of the River Tay but in the and piers. floodplain. • Option 2: A 4-span low-level bridge with 155m main a central pier in the River Tay and Photo 1: River Tay in vicinity of River Tay span, 92m remaining piers within the floodplains. Crossing Bridge side spans • Option 3: A 5-span low-level bridge with (total length two piers in the River Tay and 339m) remaining piers within the floodplains. • Option 4: A 2-span cable stayed bridge with the intermediate pier situated within the eastern floodplain.
Key environmental considerations included the River Tay SAC (and qualifying interests) and Scone Palace Garden and Designed Landscape (GDL), as well as buildability, capital and maintenance costs. Optioneering was also informed by consultation with key stakeholders
November 2019 PAGE 2 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) including SEPA, SNH, HES and the Tay District Salmon Fisheries Board (TDSFB).
Options 2 and 3 were discounted due to the requirement for no in-channel piers, following discussions with SEPA, SNH and TDSFB. Construction of in-channel piers could potentially have a significant impact on water quality and sensitive aquatic species, such as freshwater pearl mussels, which are part of the SAC designation.
Option 4 was discounted due to the potential significant visual intrusion of a statement structure design on Scone Palace GDL, which was the main concern of HES.
Option 1 was therefore the preferred choice as it had least impact on environmental receptors, no in-channel piers and was most cost effective. Refer to Chapter 2 (Project Description) for more information. River Tay Bridge River Tay Scour protection of the piers is required to Crossing foundations (approx. prevent undermining of the foundations Bridge – pier approx. 15m x 115m wide) during flooding conditions over the design scour 15m. life of the bridge. protection Piers set-back Green bank protection measures have 10.7m from been discounted (e.g. geotextile west bank membrane) due to the high flows and and 8.2m velocities predicted during flood conditions. from east Two alternative grey bank measures are bank with being considered at present: foundations
November 2019 PAGE 3 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) set-back 4.5m • Rip-rap protection and 2.0m • Temporary sheet piled cofferdams from each required for construction of the piers bank could be cut to 300mm below finished respectively. ground level to retain scour protection.
The preferred option will be subject to further design and agreement with SEPA during the detailed design phase. Stormontfield Replacement Cramock Cramock Burn (Photo 2) is currently Road Culvert box culvert Burn (approx. crossed by Stormontfield Road through a 2.5m wide). masonry arch culvert. The road is being Internal widened and therefore requires dimensions: modification or replacement of the existing 2.35m high culvert. (above 0.3m bedding It was considered that full replacement of layer), 2.5m the existing culvert, instead of retention and wide and extension, would provide the best approx. 10m engineering solution, particularly from a long maintenance and durability perspective.
A number of replacement crossing options were considered including: Photo 2: Cramock Burn in vicinity of Stormontfield Road crossing • Portal frame culvert/overspan bridge • Box culvert • Pipe culvert
A portal frame culvert/bridge structure was discounted due to risk of differential settlement under poor ground conditions. In order to mitigate this, a piled solution would likely be required, particularly to allow the river bed to remain undisturbed. The
November 2019 PAGE 4 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) Cramock Burn has been assigned low/medium sensitivity for geomorphology and water quality respectively and is not known to support any fish populations this far upstream. Therefore, the relatively high costs of this engineering solution is considered to be disproportionate to the environmental benefits.
A pipe culvert was rejected due to the geometry/height required to maintain the current channel width, whilst minimising the structure height to lower the finished road level above and avoid a crest in the road. A pipe culvert is also the worst environmental option for a crossing structure.
A box culvert was the preferred choice as it allows relatively simple/quick installation whilst minimising costs, particularly through repeat fabrication of precast units. Load can also be spread more easily over an integral box which is preferable due to the poor underlying ground conditions. This option requires disturbance of a short section of the river bed during construction; however this is difficult to avoid without a substantially overdesigned structure, and due to the watercourse being of low/medium quality and is not known to support any fish populations, this would be at disproportionate cost. Broxy Burn New box Broxy Burn Broxy Burn (Photo 3) has a small poorly Culvert culvert and (approx.1.2m defined channel which conveys water from channel wide) existing field drainage pipes to the River
November 2019 PAGE 5 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) realignment Tay. This burn will be crossed by the (approx. 200m realigned A9 and associated slip roads and long) therefore requires a culvert to connect into an existing manhole directly to the west of Internal the existing A9. An approximate 200m dimensions: channel realignment is required to provide 1.2m high the shortest length possible beneath the (above 0.2m carriageways and providing manholes at bedding standard intervals. layer), 1.2m wide and A number of crossing options were approx. 200m considered including: long Photo 3: Broxy Burn in vicinity of proposed • Portal frame culvert/overspan bridge A9 realignment, facing downstream towards • Box culvert existing A9 road embankment • Pipe culvert
A portal frame culvert/bridge structure was discounted due to risk of differential settlement under poor ground conditions. In order to mitigate this, a piled solution would likely be required, particularly to allow the river bed to remain undisturbed. Broxy Burn has been assigned low sensitivity for geomorphology/water quality and does not support any fish populations. Therefore the relatively high costs of this engineering solution is considered to be vastly disproportionate to the environmental benefits.
A pipe culvert is a reasonable alternative for this structure, but a box culvert has been selected for this crossing.
November 2019 PAGE 6 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) A box culvert was the preferred choice as it allows relatively simple/quick installation whilst minimising costs, particularly through repeat fabrication of precast units. Load can also be spread more easily over an integral box which is preferable due to the poor underlying ground conditions. This option requires disturbance of the river bed during construction; however this is difficult to avoid without a substantially overdesigned structure, and due to the watercourse being of low quality and does not support any fish populations, this would be at vastly disproportionate cost. Bertha Loch New box Bertha Loch The realignment of the A9 to the west will Burn Culvert culvert Burn (approx. require a new structure to cross Bertha (new) 2.0m wide) Loch Burn (Photo 4). Internal dimensions: A number of crossing options were 2.0m high considered including: (above 0.3m bedding • Portal frame culvert/overspan bridge layer), 1.5m • Box culvert wide and • Pipe culvert approx. 70m long A portal frame culvert/bridge structure was discounted due to risk of differential settlement under poor ground conditions. In order to mitigate this, a piled solution would Photo 4: Bertha Loch Burn in vicinity of the likely be required, particularly to allow the proposed A9 realignment river bed to remain undisturbed. Bertha Loch Burn has been assigned low/medium sensitivity for geomorphology and water quality respectively and no aquatic species have been recorded. Therefore the
November 2019 PAGE 7 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) relatively high costs of this engineering solution is considered to be disproportionate to the environmental benefits.
A pipe culvert was rejected as it does not provide mammal passage beneath the realigned A9 and is the worst environmental option for a crossing structure.
A box culvert was the preferred choice as it allows relatively simple/quick installation whilst minimising costs, particularly through repeat fabrication of precast units. Load can also be spread more easily over an integral box which is preferable due to the poor underlying ground conditions. This option requires disturbance of a short section of the river bed during construction; however this is difficult to avoid without a substantially overdesigned structure, and due to the watercourse being of low/medium quality and no recorded presence of fish, this would be at disproportionate cost. The box structure is also sized to allow installation of a mammal ledge to DMRB requirements.
November 2019 PAGE 8 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) Bertha Loch Replacement Bertha Loch Bertha Loch Burn (Photo 5) is currently Burn Culvert box culvert Burn (approx. crossed by the existing A9 carriageway in (replacement) 2.0m wide) twin 900mm pipe culverts. These pipe Internal culverts are proposed to be replaced by a dimensions: single structure to remove the susceptibility 2.1m high to predicted flooding upstream. (above 0.3m bedding A number of crossing options were layer), 2.0m considered including: wide and approx. 40m • Portal frame culvert/overspan bridge long • Box culvert • Pipe culvert Photo 5: Bertha Loch Burn – existing twin pipe culvers under A9 carriageway A portal frame culvert/bridge structure was discounted due to risk of differential settlement under poor ground conditions. In order to mitigate this, a piled solution would likely be required, particularly to allow the river bed to remain undisturbed. Bertha Loch Burn has been assigned low/medium sensitivity for geomorphology and water quality respectively and no aquatic species have been recorded. Therefore the relatively high costs of this engineering solution is considered to be disproportionate to the environmental benefits.
A pipe culvert was rejected due to the geometry/height required to maintain the current channel width, whilst minimising the structure height to lower the finished road level above and avoid a crest in the road. A
November 2019 PAGE 9 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
Affected Construction Engineering Watercourse detail Justification for proposal Photograph Activity (banktop (dimensions) width) pipe culvert is also the worst environmental option for a crossing structure.
A box culvert was the preferred choice as it allows relatively simple/quick installation whilst minimising costs, particularly through repeat fabrication of precast units. Load can also be spread more easily over an integral box which is preferable due to the poor underlying ground conditions. This option requires disturbance of a short section of the river bed during construction; however this is difficult to avoid without a substantially overdesigned structure, and due to the watercourse being of low/medium quality and no recorded presence of fish, this would be at disproportionate cost. This section of the existing A9 will be closed to traffic and therefore no provision for a mammal ledge in the structure has been made.
November 2019 PAGE 10 OF APPENDIX 15.4
CHAPTER 13 APPENDIX 15.4 CROSS TAY LINK ROAD ENGINEERING IN THE WATER ENVIRONMENT EIA REPORT (VOLUME 2)
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November 2019 PAGE 11 OF APPENDIX 15.4