Received 21.08.2020 East Derwent Highway Upgrade Development Application

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Hydraulics Impact Assessment Received 21.08.2020

East Derwent Highway Upgrade Hydraulic Impact Assessment

IS262318 | A 14 August 2020

Department of State Growth

Hydraulic Impact Assessment Department of State Growth

Received 21.08.2020

Hydraulic Impact Assessment

East Derwent Highway Upgrade

Project No: IS262318 Document Title: Hydraulic Impact Assessment Revision: A Date: 28 April 2020 Client Name: Department of State Growth Project Manager: Clayton Johnston Author: Clayton Johnston File Name: IS262318-0000-CH-RPT-0001

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Document history and status

Revision Date Description Author Reviewed Approved

A 28/04/2020 Draft for internal review SF AK AK

B 14/08/2020 Final Draft for DSG review CJ AK AK

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Contents 1. Introduction...... 5 1.1 Project Background ...... 5 1.2 Existing Conditions ...... 6 1.2.1 Highway trunk drainage, Debomfords Lane ...... 7 1.2.2 Dumbarton Drive trunk drainage...... 7 1.2.3 Llenroc Street trunk drainage ...... 8 1.2.4 Overall Context ...... 8 1.3 Design Intent ...... 9 1.4 Previous Flood Records ...... 10 2. Assessment Criteria ...... 11 2.1 Overview ...... 11 2.2 Local Authority Planning Codes ...... 11 2.2.1 Context Setting ...... 11 2.2.2 Interpretation ...... 11 2.3 Flood Immunity Standards...... 12 2.3.1 Terminology ...... 12 2.3.2 Road Authority Standards ...... 13 2.3.3 Austroads Guidelines ...... 13 2.3.4 Overview of Standards ...... 14 3. Assessment Methodology ...... 15 3.1 Overview ...... 15 3.2 Existing Conditions ...... 15 3.2.1 Inputs and Assumptions ...... 15 3.2.2 Design Flood Events ...... 15 3.2.3 Hydrologic Modelling Approach ...... 15 3.2.3.1 Catchment and Model Development ...... 15 3.2.3.2 Model Validation ...... 17 3.2.4 Hydraulic Modelling Approach...... 17 3.2.4.1 Model and Terrain Development ...... 17 3.2.4.2 Boundary Conditions ...... 17 3.2.4.3 Roughness and Losses ...... 18 3.2.4.4 Model Validation ...... 18 3.2.4.5 Critical Storms ...... 18 3.2.4.6 Existing Case Flood Modelling Results ...... 18 3.3 Proposed Drainage Design...... 19 3.3.1 Inputs and Assumptions ...... 19 3.3.2 Modelling Approach ...... 19 3.3.3 Hydrologic Modelling Approach ...... 19

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3.3.4 Hydraulic Modelling Approach...... 20 3.3.4.1 Modelled Components ...... 20 3.3.4.2 Model Development ...... 21 3.3.5 Results ...... 22 3.4 Water Sensitive Urban Design ...... 22 3.4.1 Water quality target reductions ...... 22 3.4.2 Proposed treatments ...... 22 3.4.3 MUSIC modelling results ...... 24 3.4.4 Scheme Compliance ...... 24 4. Conclusion ...... 25 4.1 Compliance with E7.0 Stormwater Management Code ...... 25 4.2 Compliance with E11.0 Waterway and Coastal Protection Code ...... 27 4.3 Compliance with E16.0 Coastal Erosion Hazard Code ...... 28 5. References ...... 30

Appendix A. Existing Case Model Development ARR2016 Data Hub Data A.1 Design Rainfalls A.2 Pre-Burst Rainfall A.3 Areal Reduction Factors A.4 Manning’s n Roughness and Rainfall Losses Appendix B. Existing Case Preliminary Flood Mapping Appendix C. Preliminary Drainage Design Drawings Appendix D. MUSIC Model Treatment Train

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Important note about your report

The sole purpose of this report is to assess the immunity and adequacy of the proposed drainage design for the East Derwent Highway, between Golf Links Road and Sugarloaf Road, in terms of flooding.

In preparing this report, Jacobs has relied upon, and presumed accurate, any information (or confirmation of the absence thereof) provided by the Department of State Growth, Clarence City Council and/or from other sources. Except as otherwise stated in the report, Jacobs has not attempted to verify the accuracy or completeness of any such information. If the information is subsequently determined to be false, inaccurate or incomplete then it is possible that our observations and conclusions as expressed in this report may change.

Jacobs derived the data in this report from information sourced from the Department of State Growth, Clarence City Council and/or available in the public domain at the time or times outlined in this report. The passage of time, manifestation of latent conditions or impacts of future events may require further examination of the project and subsequent data analysis, and re-evaluation of the data, findings, observations and conclusions expressed in this report. Jacobs has prepared this report in accordance with the usual care and thoroughness of the consulting profession, for the sole purpose described above and by reference to applicable standards, guidelines, procedures and practices at the date of issue of this report. For the reasons outlined above, however, no other warranty or guarantee, whether expressed or implied, is made as to the data, observations and findings expressed in this report, to the extent permitted by law.

Limitations recognised in available data and the assumptions adopted in the study are listed below:

• Quality of the available pipe information inherently limits the accuracy of the assessment due to the possibility of incorrect, missing or interpolated invert levels and pipe sizes.

• As-constructed drawings and survey were not available for much of the broader stormwater network not captured by the field survey undertaken for the project. Invert levels and pit and pipe details were interpolated by way of information provided by Clarence City Council. Given minimal information, this limits the accuracy of the assessment.

• The model did not include the water retention on roof tops or underground carparks. This was considered as a conservative approach and in large flood events it was assumed the roof tops and underground carparks would be overwhelmed.

• Lack of model calibration – No verification or calibration to historical rainfall events was undertaken as part of this study due to lack of suitable streamflow gauged data or historic flood levels.

• This report should be read in full and no excerpts are to be taken as representative of the findings. No responsibility is accepted by Jacobs for use of any part of this report in any other context.

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1. Introduction

1.1 Project Background

The East Derwent Highway is being upgraded between Golf Links Road and Sugarloaf Road in Geilston Bay to increase road capacity and improve traffic flow along the highway. This 1.4 kilometre section is the only length of single carriageway highway between the Tasman Highway interchange and Grasstree Hill Road in Risdon.

The upgrade will primarily involve duplication of the existing highway to provide four lanes (two in each direction) and installation of a new traffic signals at the existing Geilston Bay Road / Clinton Road intersection. The project will also improve safety for all road users, including pedestrians and cyclists, particularly through improved access to Lindisfarne North Primary School and the Geilston Bay Recreation Area.

The proposed improvements are a component of the overall commitment by the Tasmanian Government to improve the East Derwent Highway between Lindisfarne and Grasstree Hill Roundabout.

Key aspects of the upgrade will include: • Widening of the highway to accommodate four lanes of traffic – two in each direction • New traffic signals for an upgraded intersection at Clinton Road and Geilston Bay Road • New access roads off the highway to tie-in with the new signalised intersection and provide safer access to Lindisfarne North Primary School and the Geilston Bay Recreation Area. • New concrete raised central median and safety barrier to reduce the risk of collisions • Provision of an off-road shared path and dedicated on-road cycling lanes • Improvements to the left-turn movement from Sugarloaf Road intersection

Figure 1-1: Project location Map

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As the upgrade will involve an increase in impervious pavement area as a result of the road duplication and addition of new access roads, as well as the crossing of a watercourse, Faggs Gully Creek, a robust assessment of proposed hydrological and hydraulic conditions has been undertaken to address the requirements of the Clarence Interim Planning Scheme 2015 (the Scheme).

The objective of this report is to detail the hydraulic assessments that have been undertaken and to outline key stormwater management features and appropriate risk mitigation treatments, where relevant.

1.2 Existing Conditions

At the southern end of the project, the East Derwent Highway slopes gently downhill from the Golf Links Road intersection at a maximum gradient of 4% before flattening out into an elongated sag curve in vicinity of Araluen Street. Here, the highway reaches a low point as it traverses Faggs Gully Creek. The highway curves north-west as it passes through the existing Geilston Bay Road / Clinton Road intersection and straightens, rising steeply uphill at a maximum grade of 7% near the Sugarloaf Road intersection.

Faggs Gully Creek is conveyed under the highway via a twin 2100mm x 2400mm concrete box culvert. The catchment area of Faggs Creek is approximately 6 km2 and flows east to west, draining to the River Derwent Estuary. The catchment is steeply sloped with large hills defining the catchment boundaries, including Natone Hill and Fishers Hill to the west, and Sugarloaf Hill and Caves Hill to the east, with elevations more than 200m in some instances. The upper catchment is heavily forested with incised channel reaches. The lower catchment is predominantly a residential urban landscape comprising the community of Geilston Bay. It is in this part of the catchment that Faggs Creek intersects the East Derwent Highway, 600 m before the Creek’s confluence with the River Derwent at the Geilston Bay Boat Club.

The creek is densely vegetated in some locations, including weed infestations, particularly at the concrete box culvert inlet and outlet. There are also signs of scour along the steep banks and in areas in which the banks are less densely vegetated. There are a number of existing stormwater outlets which drain directly into the creek within the project area, three on the upstream side of the highway box culvert, and five on the downstream side.

Figure 1-2: (left) Twin concrete box culvert outlet under highway, looking upstream, is densely overgrown (right) Looking upstream towards the highway culvert, the banks of Faggs Creek are incised and show signs of erosion.

Whilst there are a number of relatively small sub-catchments draining directly into the creek, there are three sizeable systems discharging to the creek, as outlined in the sections below.

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1.2.1 Highway trunk drainage, Debomfords Lane

Between Golf Links Road and Araluen Street, highway runoff is captured in a pit and pipe network and conveyed via a trunk main (diameter varies between DN 900 and DN1050) which crosses the highway south of Araluen Street and runs along Debomfords Lane, across the Tennis Club car park and discharges into Faggs Gully Creek approximately 400m west of the highway. The catchment for this system commences south of the project area along the highway to Araluen Street, and also includes contributions from networks on Golf Links Road and Derwent Avenue.

Figure 1-3: (left) Looking south from Debomfords Lane, towards the sag curve in the highway and stormwater manholes directing the trunk main down Debomfords Lane (right) Trunk main discharging to the Faggs Gully Creek via a 900mm diameter RCP, 400m west of the highway.

1.2.2 Dumbarton Drive trunk drainage

Another significant sub-catchment which outlets to Faggs Gully Creek is the trunk drainage system near the intersection of Dumbarton Drive and Geilston Bay Road via 3 x 600mm diameter RCPs.

There are two large pits upstream of the multi-pipe outfall on the corner of Geilston Bay Road and Dumbarton Drive, which assist in detaining flows from the catchment and reducing outlet velocities into the creek (refer Figure 1-4). Upstream of these pits, a single 1200mm diameter RCP conveys incoming flows via a large, incised earthen channel between Dumbarton Drive and the highway.

This earthen channel, shown in Figure 1-5, has recently been stripped of vegetation, likely as result of works being undertaken upstream for the new residential subdivision at the north-western end of the project area. This channel will capture new flows from the subdivision (which includes stormwater detention features), as well as flows conveyed from the eastern side of the highway. These flows from the east include highway runoff and urban stormwater from the pit and pipe network along the northern end of Clinton Road, which discharges into a tributary of the roadside drain opposite 34A Clinton Road. These flows from the eastern catchments are conveyed under the highway via a 1800m diameter RCP.

A recent site visit in March 2020 highlighted that the existing 1200mm diameter RCP inlet was significantly blocked with debris and sediment, reducing the flow conveyance capacity.

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Figure 1-4: (left) One of two drainage pits, detaining flows from the catchment, prior to creek discharge (right) Overgrown multi-pipe outfall discharging attenuated flows into the lower reaches of Faggs Gully Creek.

Figure 1-5: (left) Inlet to a 1200mm diameter RCP, which conveys flows into the drainage pits in the figure above, shown blocked with debris in March 2020 (right) Large earthen drain, channelises flows into the 1200mm culvert from an upstream subdivision and from across the highway.

1.2.3 Llenroc Street trunk drainage

The trunk drainage line for the Llenroc Street catchment, off Clinton Road, also runs down into Faggs Gully Creek through 3 Llenroc Street and across Geilston Creek Road.

The outlet point intersects with the creek approximately 70 m east of the highway.

1.2.4 Overall Context

A summary of the key discharge points described above, in addition to the overall creek catchment, is provided by the topographical map in Figure 1-6.

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Sugarloaf Road

Minor creek tributary Faggs Gully Creek

Key stormwater discharge location #1

Key stormwater discharge location #2 Key stormwater discharge location #3

Golf Links Road / Derwent Avenue

Figure 1-6: Hydrology within the project area, including key discharge locations (Source: ListMap, 2020)

1.3 Design Intent

The upgrade will involve an increase in impervious area due to the duplication of the highway and addition of new local roads. The adequacy of the existing stormwater capture and conveyance system therefore required hydraulic assessment to determine the requirement for any supplemental additional drainage infrastructure.

The overall design intent involves using the existing stormwater system and discharge points as much as possible. Kerb and channel has been incorporated alongside the highway in a number of locations, and along the new proposed local roads to minimise impacts on adjacent land and reduce the amount of earthworks required in cutting drains. Due to the introduction of these hard drainage features, use of Water Sensitive Urban Design (WSUD) principles has been considered in the design to reduce impacts of increased catchment imperviousness on peak flows and water quality.

Minimal changes have been made to the existing vertical and horizontal geometry of the Highway to minimise the disruption to existing overland flow paths . Retaining existing flowpaths, where possible, assists with minimising changes to flood levels. New stormwater infrastructure was introduced to assist in capturing flows from the overland flow paths. The most prominent examples of this new infrastructure being grated strip drains along the central median and a new water crossing over Faggs Gully Creek accommodated by a new box culvert.

In keeping with Acceptable Solution A1 of the Scheme’s Stormwater Management Code (Clause E7.7.1), any new additions to the stormwater network, including kerbs and channels, pits and pipes, swales and drains, have all been designed and graded to capture runoff from the highway and local roads, and discharged by gravity into public stormwater infrastructure, which includes existing systems and/or watercourses.

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Proposed new local road infrastructure west of the highway and on the southern side of Faggs Gully Creek requires the introduction of new discharge points to the creek. This will assist in avoiding a lengthy trunk drainage system connecting to an existing outlet. As such Performance Criteria P4 must be relied upon in the Scheme’s Waterway and Coastal Protection Code (Clause E11.7.1). New discharge points have been proposed near the downstream extension of the twin box culvert under the highway where rock protection will be provided to prevent scour at the outlet. Positioning new discharge outlets in a location where scour protection is already proposed helps mitigate the risk of further erosion. It also confines discharge points to one location so that ‘natural’ stream bed conditions can be maintained downstream as much as possible.

Although there may be an opportunity to further rationalise discharge points in the Detailed Design phase of the project, most of the new outlet points comprise relatively small catchments. The outlet with the largest catchment has also been designed to incorporate a bioretention basin upstream, which will detain and slow flow velocities and help with improving the water quality of flows discharging into the creek.

1.4 Previous Flood Records

No previous flood study of the project area is known to have been undertaken, however, high level flood modelling of the area is currently being completed for Council as part of the Rosny to Otago Stormwater System Management Plan (SSMP). An advanced draft of the SSMP, provided by Council in December 2019, indicated that there exists (according to that particular flood model), an overland flowpath across the highway in the vicinity of Debomfords Lane, at Faggs Gully Creek and north of Clinton Road, particularly in a 1% Annual Exceedance Probability (AEP) event.

A review of the River Derwent Flood Data book was completed in 2000 by the Department of Primary Industries, Water and Environment. This document was reviewed but no information was found regarding previous flood events or flood risk at Faggs Gully Creek, particularly in regards to overtopping of the highway. The only known and recorded flooding event in the project area occurred in May 2018 at the Geilston Bay Boat Club, during which greater Hobart experienced flash flooding. There is no record of the highway overtopping and becoming impassable during this event.

A Council development engineer previously noted the trunk drainage system across Dumbarton Drive and Geilston Bay Road intersection has overtopped on occasion, refer Section 1.2.2 for further details about this system.

Due to limited data being available to set up or calibrate a project flood model, particularly since the SSMP flood model was completed at a higher level and for a much broader catchment with a vastly different model build to the current project, new hydrology and hydraulic models were established to assess flood impacts for the project, using available asset data from State Growth and Council.

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2. Assessment Criteria

2.1 Overview The stormwater system for the project has been assessed and design in accordance with a number of relevant standards and guidelines, including: • Clarence Interim Planning Scheme 2015 • Department of State Growth Drainage Standards and Specifications • Australian Standards • Australian Rainfall and Runoff (2019) • Austroads Guide to Road Design – Part 5 (2013) • Local Government Association Tasmanian (LGAT) Standards • Other urban drainage design manuals (e.g. Queensland Urban Drainage Design Manual, QUDM 2016)

2.2 Local Authority Planning Codes

2.2.1 Context Setting

The project requires assessment under various Codes within the Clarence Interim Planning Scheme 2015 (the Scheme) which are relevant to this Hydraulic Impact Assessment. Specifically, these codes include: • E7.0 Stormwater Management Code – the purpose of this Code is to ensure that stormwater disposal is managed in a way that furthers the objectives of the State Stormwater Strategy. No development is exempt from this Code.

• E11.0 Waterway and Coastal Protection Code – the purpose of this Code is to manage vegetation and soil disturbance in the vicinity of wetlands, watercourses and the coastline. This Code applies in locations where the works partly impact a designated Waterway and Coastal Protection Area, generally around Faggs Gully Creek and its tributaries. Some works may be considered exempt from this Code where they involve clearing or modification of vegetation and soils within a public park, and if works are within 2m from existing infrastructure (roads, tracks, etc.).

• E16.0 Coastal Erosion Hazard Code – The purpose of this Code is to facilitate sustainable development on coasts vulnerable to erosion, identify areas which are vulnerable to current and anticipated coastal erosion and provide for development responses that appropriately respond to coastal erosion hazard. The works are not considered exempt from this Code, however they only impact a very small fraction of the identified Coastal Erosion Hazard Area, near the intersection of Dumbarton Drive and Geilston Bay Road, and will be assessed accordingly.

2.2.2 Interpretation

E7.0 Stormwater Management Code requires an assessment of a proposed development’s minor and major stormwater drainage system in various storm events. For the purposes of this assessment, these drainage systems are defined below (verbatim from the Scheme):

• Minor stormwater drainage system - the stormwater reticulation infrastructure designed to accommodate more frequent rainfall events (in comparison to major stormwater drainage systems) having regard to convenience, safety and cost.

• Major stormwater drainage system - the combination of overland flow paths (including roads and watercourses) and the underground reticulation system designed to provide safe conveyance of stormwater runoff and a specific level of flood mitigation.

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For the purposes of this hydraulic assessment, the above definitions in the context of the Scheme have been interpreted as follows:

• An effective and economical minor stormwater drainage system, accommodating more frequent rainfall events, may allow for encroachment of flows within a road reservation provided that the spread of flow widths is kept to an acceptable, safe level. Given there are no metrics provided in the Scheme, refer flow width, depth and velocity design criteria that have been adopted for a minor storm (in accordance with Queensland Urban Drainage Design Manual 2016)

• Safe conveyance of flows as part of a major stormwater drainage system, can include overland flow paths across roads and watercourses in the instance that the minor stormwater drainage system capacity may be exceeded in large storm events. Given there are no metrics provided in the Scheme definitions for what constitutes safe conveyance and a specific level of flood mitigation, a risk-based approach has been adopted. This approach aims to ensure that for specified design flood events, impacts (increase in design flood levels) will be negligible on private dwellings and minimised elsewhere.

2.3 Flood Immunity Standards

2.3.1 Terminology

This report uses the terminology Annual Exceedance Probability (AEP) to define the likelihood of design flood events occurring, that is the probability of a flood event occurring or being exceeded within a year. Average Recurrence Interval (ARI) was a term used previously to define the probability of design flood events (IEAust, 1987) and was defined as the average period between occurrences equalling or exceeding a given value.

In the revision of Australian Rainfall and Runoff (AR&R) (Engineers Australia, 2019), the adopted terminology to define design flood probabilities has been changed to AEP. The Bureau of Meteorology (BoM) has similarly adopted this terminology in publishing the revised rainfall Intensity-Frequency-Duration (IFD) curves for Australia (BoM, 2019).

For clarity of understanding, the adopted and previously used terminology is shown in Table 2.1.

Table 2.1: Annual Exceedance Probability and Average Recurrence Interval Conversions

ARI (years) AEP (%)

3 month 4 E.Y. 1 1 E.Y. 1.44 50 2 39 4.48 20 5 18 10 10 20 5 50 2 100 1 200 0.5 500 0.2 2000 0.05

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2.3.2 Road Authority Standards

The Department of State Growth’s Drainage Design Standards (Professional Services Specification T8, 2012) notes road drainage design is to be in accordance in with Austroads Guide to Road Design - Drainage (Part 5) and Guide to Road Design – Roadside Environment (Part 6B).

Design flood protection standards are nominated by State Growth in Table 2.2 below for various road categories and design floods in accordance with Table 8.1 of the Drainage Standard, where Q100 equates to a design flood with a 1% AEP event. Between Golf Links Road and Sugarloaf Road, the East Derwent Highway is designated under State Growth’s road hierarchy as a Category 3 road (or Austroads functional classification of 2+3).

Table 2.2: State Growth Flood Protection Design Criteria

Austroads State Growth Bridge soffit Minimum road level functional road category level classification Catchment area

≤ 2500 ha ≥ 2500 ha

1 1 Q100 + Debris Q100 + 0.5m Q100 + 0.5m 2+3 3 Q100 Q50 Q100 4+5 4+5 Q100 Q20 Q100 6+7 2 Q100 + Debris Q50 + 0.3m Q100 + 0.3m 8+9 4+5 Q100 Q20 Q100

Based on the requirements for a Category 3 (State Growth) road above, and for a watercourse catchment area less than 2500 ha, State Growth standards require a Q50 (2% AEP) flood immunity for the road upgrade project.

Where it may not be feasible to provide freeboard to the finished road level in the required design flood (2% AEP), particularly in locations where there are existing overland path issues, ‘flood immunity’ as required by State Growth Standards is also said to be achieved if storm events can be safely conveyed by the stormwater system for convenience (i.e. traffic flow is maintained) and protection of life and property. Key design criteria to achieve immunity are thereby defined by flow width, depth and velocity limitations specified below in Section

2.3.3 Austroads Guidelines

Austroads Guide to Bridge Technology Part 8: Hydraulic Design of Waterway Structures details recommended immunities for various road classifications. This is summarised in Table 2.3.

Table 2.3: AS5100 Flood Immunity and Serviceability Limit States (SLSs)

Austroads road classification Flood immunity(1) SLS (serviceability limit states)

Controlled access highways 100 years ARI 100 years ARI Includes: motorways and (1% AEP) (1% AEP) freeways (National/State/Territory) Arterial roads classes 1 and 2(2) 50–100 years ARI (2%–1% AEP) 50–100 years ARI (2%–1% AEP) Includes: highways and urban arterial roads (National/State/Territory)

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Arterial road class 3(2) Includes: 50 years ARI (2% AEP) 50 years ARI (2% AEP) main roads (State/Territory) Local roads classes 4 and 5(2) 10–20 years ARI (9.5–4.9% AEP) 20 years ARI (4.9% AEP) Urban collector/distributor roads 10–50 years ARI (9.5–2% AEP) 20–50 years ARI (4.9–2% AEP) Urban local roads 10 years ARI (9.5% AEP) 10 years ARI (9.5% AEP)

(1) Subject to approval by relevant authority

(2) For description of road classes, refer to Austroads (2015).

The East Derwent Highway would be classified within the category of arterial road classes 1 and 2. This road class would expect to have a road immunity of 2% to 1% AEP, subject to approval by the relevant authorities. In line with State Growth’s Drainage Standards above, a minimum 2% AEP road flood immunity has been adopted.

As described in Section 2.3.2, ‘immunity’ of the road in this context is defined as either; the level of the road is equal to or above the design flood (2% AEP) event, or, the road safely conveys the design flood so that there is an acceptable level of aquaplaning risk. As the design becomes finalised, the depth-velocity product along the road will be checked to ensure it is not appreciably higher than it is currently.

2.3.4 Overview of Standards

From the above, it is apparent that for this proposed development, there are different standards, guidelines and codes to be applied for different sections of the development that have the potential to conflict with each other and confusing to the reader.

To provide clarity as to how the proposed development is impacting on the existing flooding behaviour in the vicinity of the development, the overall philosophy of at least maintaining the existing flooding characteristics of the areas in the vicinity of the development has been adopted. That is, where possible, ‘not making things worse’.

In an overall sense, the duplication of the Highway and associated infrastructure has been designed (currently at Preliminary Design) to minimise resultant flood impacts on private dwellings, while maintaining the current flood immunity of the highway and associated infrastructure as closely as possible. While it is expected that there will be minor variances in design flood levels (either up or down) due to differences in elevations because of the new proposed infrastructure, no flood impacts to private dwellings are expected and newly created infrastructure will be designed in accordance with the above guidelines.

For the purposes of assessing the ability of new infrastructure to accommodate major storms and to assess the impact of the upgrade of existing infrastructure at the Preliminary Design stage of the project, the 1% AEP and the 5% AEP design floods have been modelled. Analysing these events at this stage of design affords an understanding of the likely flood impacts that require mitigation through subsequent stages of design.

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3. Assessment Methodology

3.1 Overview

The Hydraulic Impact Assessment undertaken for the proposed highway upgrade involved assessment of pre- development (existing) and post-development (proposed design) conditions, through the following tasks; • Existing conditions o Review of the project site and available background information o Establishment of a hydrological model for derivation of streamflow inputs; o Development of a hydraulic flood model utilising field survey data and inputs from the hydrologic model to estimate existing flood levels, depths and velocities; o Production of existing case mapping and flood data to use in drainage design inputs. o Validation of results through ground-truthing the site • Proposed drainage design o Utilisation of existing condition flood mapping results and field survey data to develop the road surface drainage design o Development of a new stormwater network in 12d (Version 14) to capture and convey runoff generated from new local roads and the increased width of the highway to supplement the existing network o Use of hydraulic analytical tools to assess and ensure adequacy of the proposed network in minor and major storm events with respect to the Scheme and flood immunity requirements o Assessment of the suitability of network discharge locations, including requirements for scour protection o MUSIC modelling of incorporated Water Sensitive Urban Design (WSUD) elements to determine water quality impacts attributed to increased impervious area as part of the project

3.2 Existing Conditions

3.2.1 Inputs and Assumptions

The following data was used in existing case model development for Faggs Gully Creek and associated tributaries: • 2013 LiDAR (1m grid from Geosciences Australia) which extended over the full catchment area; • Topographical engineering field survey undertaken in the highway road corridor, October 2019; • Spatial layers of the pit and pipe network from Clarence City Council (CCC) in September 2019; • Data from the ARR 2016 Data Hub.

3.2.2 Design Flood Events

For the purposes of assessing the ability of new infrastructure to accommodate major storms and to assess the impact of the upgrade of existing infrastructure at the Preliminary Design stage of the project, the 1% AEP and the 5% AEP design floods have been modelled. Analysing these events at this stage of design affords an understanding of the likely flood impacts that require mitigation through subsequent stages of design.

3.2.3 Hydrologic Modelling Approach

3.2.3.1 Catchment and Model Development

Hydrological modelling of Faggs Gully Creek and it’s minor tributaries was conducted in XP RAFTS to estimate the design flood events (refer Section 3.2.2) within the project area. The model was parametrised with catchment

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areas, fractions impervious, Manning’s n values and lag times, which are further detailed in Appendix A. The creek catchment area and modelled sub catchment areas are shown in Figure 3-1.

Figure 3-1: Faggs Gully Creek catchment for hydrologic assessment

The model was further developed with ARR 2016 rainfall data, pre-burst rainfall, temporal patterns, areal reduction factors and initial and continuing losses from the ARR 2016 Data Hub (refer Appendix A).

The RAFTS model was then simulated using Storm Injector, a post processing and analysis tool.

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3.2.3.2 Model Validation

No calibration or validation to historic events was undertaken within the hydrological model due to lack of quality- controlled streamflow gauges with a sufficiently long-term record of data within the catchment to perform flood frequency analysis.

Attempts to validate flows using the online RFFE (Regional Flood Frequency Estimation) tool and Rational Method formula produced peak flow estimates less than those produced by the hydrological model. Hydrological modelling is a more reliable flow estimation method, so the XP-Rafts flows were taken forward for use in large- scale hydraulic modelling. Additional model calibration was conducted after a site visit was performed by the hydrologists to inform detailed design drainage modelling.

3.2.4 Hydraulic Modelling Approach

3.2.4.1 Model and Terrain Development

A TUFLOW 2D hydrodynamic model was built to undertake a hydraulic assessment of the existing drainage networks and watercourses in Faggs Gully Creek catchment. The modelling approach utilised rain on grid for the lower catchment which included the full hydraulic model extent. This allowed for the identification of minor flow paths entering Faggs Gully Creek. The model consisted of one domain with a regular grid cell size of 3 m. This grid size was deemed appropriate as it enabled a reasonable representation of the area while maintaining a practicable simulation time using TUFLOW HPC (Heavily Parallelised Computation).

The design flood hydrographs from the hydrology model were input to the hydraulic model to account for inflows from the upper reaches of the catchment that were not accounted for by the rain on grid area.

The 2D model topography was prepared using the 2013 LiDAR data to create a 1m resolution digital elevation model (DEM) as well as information from a detailed site survey of the highway corridor (which replaced the lidar data where available). Key transverse culverts, including the twin concrete box culvert under the highway at Faggs Gully Creek were included in the modelling. A small triangulated grid was used to apply approximate bed levels of the River Derwent Estuary at the Faggs Gully Creek catchment outfall location.

3.2.4.2 Boundary Conditions

Upstream boundaries were sourced from the hydrologic XP-RAFTS model. Total flows from sub catchment G (Faggs Creek), local flows from sub catchment J (northern tributary) and sub-catchment H (urban drainage catchment) were converted to .ts1 files for TULFOW input. The sub-catchment G and J hydrographs were applied to the TUFLOW model as 2D QT (flow over time) type inflows. The sub catchment H hydrograph was applied to the pit and pipe network as 1D QT (flow over time) type inflows. This hydrograph was applied at three separate locations at the upstream ends of the pit and pipe network. Each 1D inflow boundary applied a factor to the hydrograph proportional to the runoff area upstream. These runoff areas were initially intended to be within the rain on grid area but the ability of runoff to enter the network was limited due to the coarseness of available data.

Rainfall was applied to the active 2D domain using an RF rainfall area. This approach applies rainfall directly to the 2D surface of the hydraulic model from a hyetograph of rainfall depth over time. The rainfall data applied to the hydraulic model was the same used for the hydrology model inputs.

The downstream boundary at the River Derwent Estuary was modelled as a stage-time fixed water level (HT). The level chosen was based on Highest Astronomical Tide (HAT) at Hobart (0.86 m AHD). This was sourced from the Tasmanian Government’s Department of Primary Industries, Parks, Water and Environment.

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3.2.4.3 Roughness and Losses

Manning’s n roughness values were adopted in the TUFLOW model surface, consistent with land use mapping, aerial imagery and based on the recommendations of ARR2016. Rainfall losses for the rain on grid component of the hydraulic mode have been adopted in line with data from the ARR2016 Data Hub, with initial losses adjusted down to account for pre-burst rainfall.

3.2.4.4 Model Validation

Due to limited historic flood level data in the study area, no additional calibration or validation of the TUFLOW models to observed flood levels has occurred.

A mass balance calculation was undertaken to confirm the validity of the modelling results. These calculations indicated a mass balance error of less than 1% which was deemed acceptable.

The total peak flow at the catchment outfall from the hydraulic model was also compared to the total from the hydrology model (sub-catchment M). The hydraulic model flow in the 1% AEP was found to be a good match to the hydrology.

3.2.4.5 Critical Storms

As per ARR2019 guidance, each AEP event was assessed for a range of durations and temporal patterns to identify the critical storm. Eight design rainfall storm durations (45, 60, 90, 120, 180, 270, 360 & 540 minutes) were modelled. For each duration, ten temporal patterns were assessed to determine the median temporal pattern at the area of interest. The hydraulic model allowed for hydraulic model routing for the rain on grid portion of the catchment. All durations and temporal patterns were simulated using the hydraulic model. The resulting water surfaces were post processed to identify the critical storm for each event, in particular at the point at which East Derwent Highway crosses over Faggs Gully Creek. The critical storms were identified as: • 1% AEP; 270-minute critical duration and median temporal pattern 9 • 5% AEP; 180 minute critical duration and median temporal pattern 6

Notably, the critical storms are location specific, and the temporal patterns taken forward were relevant for the Faggs Gully Creek crossing. Overtopping locations associated with minor flow paths in other locations would likely have different critical storms that may result in slightly higher flood levels and flows.

3.2.4.6 Existing Case Flood Modelling Results

For each of the critical storms identified for the Design Flood Events, flood modelling of existing conditions has shown that the East Derwent Highway at Faggs Gully Creek has sufficient immunity to the road level in the events tested. Locations to the north and south of the crossing are likely to be impacted by shallow sheet flow over the road as the drainage network becomes overwhelmed, including south of Araluen Street at the sag point in the highway and just north of Clinton Road. It is noted that Geilston Bay Road becomes inundated as design floods break from the banks of the creek, and is likely to continue to be an issue in the proposed design without significant flood mitigation or raising of existing road levels (which will likely impact upon private dwelling(s)).

Existing case flood maps (refer Appendix B) have been used to inform the proposed drainage design, particularly where overland flow paths impact the immunity of the highway. It should be noted that these maps are preliminary and are subject to change as the design is further optimised and refined during the Detailed Design phase.

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3.3 Proposed Drainage Design

As discussed in Section 1.3, the design intent for the proposed drainage design has been to utilise the existing stormwater system and discharge points as much as possible, supplementing this network as required to capture and convey additional runoff generated from new and/or increased impervious areas. Minimal changes have been made to the existing highway’s vertical and horizontal alignment to ensure that existing overland flow paths can be maintained along their natural alignment and afflux minimised as much as possible.

3.3.1 Inputs and Assumptions

The following data was used in for development of the proposed drainage design network for the highway upgrade: • Existing case modelling results and input flows for key transverse culverts within project area; • Topographical engineering field survey undertaken in the highway road corridor, October 2019; • Preliminary road design geometry and layout information; • Flood immunity and impacts requirements, refer Section 2.3.

3.3.2 Modelling Approach

Key flow rates determined for major transverse structures in the existing drainage network were utilised in establishing the proposed drainage design network for the upgrade. Given the hydraulic assessment of the existing hydraulic network was conducted at a scale to ensure computational run times were reasonable, the same methodology cannot be undertaken to assess the performance of the proposed design at a sufficiently fine resolution, i.e. to determine the spread of flow widths from kerb and channel etc. Whilst the grid cell size for the assessment could be reduced to assess these features, this would result in excessively long run times for every design iteration required.

As such, separate hydrologic and hydraulic methodologies have been used to efficiently develop the proposed drainage design and meet the required immunity standard in Section 2.3, as detailed in the following sections. Once the drainage design is confirmed, the broader performance of the proposed system, in particularly Faggs Gully Creek, will be confirmed with the previous hydraulic modelling approach and compared to the existing case to determine afflux levels (if any).

3.3.3 Hydrologic Modelling Approach

Placement of drainage structures and their reporting catchments has been determined iteratively in 12d modelling software, using LiDAR, engineering feature survey and preliminary design road surface geometry.

Assessment of each of the catchments and associated runoff generated was undertaken in accordance with the methodology prescribed in Austroads Guide to Road Design Part 5, using the Probabilistic Rational Method for urban catchments up to 1 km2 (AGRD Part 5, Section 6.4.2).

The minimum time of concentration for each catchment was calculated in accordance with the standard inlet times, as shown in Table 3.1, or using prescribed alternative methods where these were not applicable. Runoff coefficients were selected using identified impervious fractions based on surfacing type and/or land usage, as shown in Table 3.2, and applied with factors to ascertain coefficients for the various flood frequency events being considered for assessment of minor and major stormwater systems.

Rational method computations conducted via the 12d Drainage Analysis module also utilised rainfall intensity- frequency-duration data (IFDs) extracted from the Bureau of Meteorology (BOM, 2020), to calculate peak flows across the network.

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Table 3.1: Standard Inlet Times (Austroads Guide to Road Design Part 5, Table 6.3)

Location Inlet Time (minutes)

Road surfaces and paved areas 5

Urban residential areas where average slope of land at 5 top of catchment is greater than 15%

Urban residential areas where average slope of land at 8 top of catchment is greater than 10% and up to 15%

Urban residential areas where average slope of land at 10 top of catchment is greater than 6% and up to 10%

Urban residential areas where average slope of land at 13 top of catchment is greater than 3% and up to 6%

Urban residential areas where average slope of land at 15 top of catchment is up to 3%

Table 3.2: Fraction impervious vs development category (Austroads Guide to Road Design Part 5, Table 6.8)

Development Category Fraction impervious (fi)

Significant paved areas e.g. roads and car parks 0.9 Urban residential – high density 0.9 Urban residential – low density (including roads) 0.75 Urban residential – low density (excluding roads) 0.50 Open space and parks, etc. 0.20

3.3.4 Hydraulic Modelling Approach

The proposed stormwater network incorporates components of the existing stormwater network, as well as a combination of new pits and pipes, kerbs and channels, and swales/ drains to capture and convey runoff.

3.3.4.1 Modelled Components

New pit types specified are generally side entry pits (or surface pits outside of the road reservation footprint), in accordance with the relevant road authority’s standards. Pits located within the Crown’s road reservation will be to Department of State Growth specification, whereas pits located outside of this reserve along new local / access roads, will be to LGAT specifications.

Where existing pits are being used in the upgraded network as junctions, pits will be assessed for adequacy and fitted with risers and Class B or Class D (trafficable) lids as required. This helps ensure the upgraded design is cost effective by maximising use of existing infrastructure as much as possible, whilst also considering ongoing maintenance requirements.

In locations where overland flow issues have been identified (refer Section 0), in particular sheet flow across the highway south of Araluen Street and north of Clinton Road in minor and major storms, treatments in the raised central median have been utilised to avoid localised damming of flows and to achieve flood immunity criteria. In these locations, longitudinal high capacity grated drains will be installed in the gutter in lieu of an extensive pit and pipe network.

The minimum size of new pipe sizes specified is 375mm (with the exception of subsoil and underdrainage pipe systems), in accordance with the Department’s T8 Drainage Design Standards. Minimum pipe cover requirements,

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as shown in Table 3.3, based on pipe location and likely vehicle loadings have been applied wherever possible to keep cover to minimum allowable depths and ensure the resulting required pipe classes are cost-effective.

Table 3.3: Minimum pipe cover (Austroads Guide to Road Design Part 5A, Table 6.1)

Location Minimum Cover Minimum Cover Rigid type pipes, Flexible type pipes, e.g. e.g. concrete plastic or thin metal (mm) (mm)

Residential private property, and parks not subject to 300 450 traffic Private property and parks subject to occasional traffic 450 450 Footpaths 450 600 Road carriageways, car parks and other areas subject to 600 750 regular vehicular traffic

The proposed road design footprint requires upstream and downstream extensions of the existing twin 2100 x 2400 concrete box culvert under the highway at Faggs Gully Creek, in addition to a new, second culvert crossing of the creek at the new Dumbarton Drive / Geilston Bay Road intersection.

3.3.4.2 Model Development

Pit spacings to supplement the existing network were optimised iteratively based on available gutter capacities, contributing catchment areas and imperviousness, longitudinal grades, pit capture capacities and the required flood immunity performance criteria (refer Section 2.3), which also designate specific criteria for parking lanes, bus stops and pedestrian crossings.

The 12D Drainage Analysis module combined the hydrologic and hydraulic inputs, including catchment flows, Manning’s equation to calculate flow widths, as well as pit capture curves, inlet capacities, losses and blockage factors generally in accordance with the recommended values in Austroads Guide to Road Design Part 5.

Upwelling of water within pits and velocities through culverts were controlled through iteration of pit spacings, culvert sizes and longitudinal pipe grades. Minimum self-cleansing design velocities of not less than 0.7 m/s were adopted for partial pipe flow, and a maximum flow velocity of 5.0 m/s for full pipe flow. For parts of the culvert network discharging to new or existing outlets, scour protection will be provided in locations where outlet velocities are in excess of 2 m/s, which aligns with recommended advisable velocity for a coarse gravel stream bed (Austroads Guide to Road Design Part 5, Table 3.10).

New local road infrastructure west of the highway and on the southern side of Faggs Gully Creek require a new discharge points to the creek to be introduced to avoid a lengthy trunk drainage system connecting to an existing outlet. Each of these new discharge points, comprising relatively small catchment areas, have been proposed near the downstream end of the box culvert extension under the highway where rock and batter protection will be provided to prevent scour at the culvert outlet. Locating new discharge outlets in a location where scour protection is already proposed helps mitigate the risk of erosion and confines discharge points in one location so that natural stream bed conditions can be maintained downstream as much as possible.

The new outlet with the largest catchment has also been designed to incorporate a bioretention basin upstream, which will detain and slow flow velocities and improve the water quality of flows discharging into the creek. There may be opportunities to further rationalise these creek discharge points during the Detailed Design phase.

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3.3.5 Results

The Preliminary Drainage Design resulting from the 12D Drainage Analysis and assessment against existing case flood modelling can be located in Series 1600 of the Preliminary Design Drawings provided separately as part of the Development Application documentation (extracted in Appendix C).

Although this drainage network has been designed to meet the required flood immunity standards in the minor (5% AEP) and major (1% AEP) storm events as far as reasonably practical, it is acknowledged that the network may undergo further refinement and minor changes through the Detailed Design phase of the project, including minor pit relocations due to clashes with other infrastructure and rationalisation of outlets wherever possible.

Due to the introduction of a waterway crossing under the new Dumbarton Drive / Geilston Bay Road intersection, this has implications for the flow regime and afflux within the creek, and along Geilston Bay Road where existing inundation issues in this area were identified from existing case flood modelling results (refer Section 3.2.4.6). The waterway area of the crossing has been maintained as much as possible to ensure impacts to the flow regime, and thereby erosion and natural values, have been minimised. The minor and major stormwater systems in this area (which include roads and overland flow paths), have been designed to ensure that changes to existing conditions are minimised and that required flood width, depth and flow depth-velocities can be achieved as far as reasonably practical, appreciating that there are existing inundation issues in this area.

3.4 Water Sensitive Urban Design

To address Acceptable Solution A2 under Clause E7.7.1 of the Scheme’s Stormwater Management Code, water sensitive urban design (WSUD) principles for treatment and disposal of stormwater have been applied in the design given the increase in impervious area as a result of the upgrade exceeds 600 m2.

The areas for which WSUD principles have been applied, as shown in the Drawings (Appendix C), include new side roads (Dumbarton Drive extension (MC12), Debomfords Lane connection (MC13) and Lindisfarne North Primary School access road (MC14)). Also included, are areas in which the highway has been widened, excluding the existing widened alignment at the northern end of the project north of Clinton Road (Ch. 4075) to Sugarloaf Road (Ch. 4375).

A WSUD assessment has been undertaken using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC), a nationally recognised stormwater quality modelling package, in line with the requirements of the Scheme.

3.4.1 Water quality target reductions

Water quality target reductions in line with the State Stormwater Strategy 2010 have been adopted for the project as follows: • 80% reduction in the annual average load of Total Suspended Solid (TSS) • 45% reduction in the annual average load of Total Phosphorus (TP) • 45% reduction in the annual average load of Total Nitrogen (TN)

3.4.2 Proposed treatments

A combination of treatments was adopted to meet the above objectives for runoff generated by increased impervious area within the project site including vegetated (grass-lined) swales, a bioretention swale and bio- retention basin. A summary of the treatments is provided below.

Grass lined Table Drains and Swales

Grass lined table drains, swales and vegetated earthworks batters have been included in the MUSIC model to account for treatment of stormwater runoff for catchments which do not undergo any further retention or

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treatment prior to discharge into Faggs Gully Creek. This includes the swale drain along the western side of the school access road and the drainage channels along the toe of earthworks batters north of Clinton Road between Ch. 3500 and Ch. 3800, on the western side of the highway. Vegetated swales are commonly used to assist in removal of coarse and medium sediment and to slow flows, particularly in minor storms, thus reducing the impact of increased catchment imperviousness on peak flow rates when compared to more hydraulically efficient pit and pipe drainage systems.

Bioretention Swales

A bioretention swale has been proposed on the eastern side of the highway and shared path between Ch. 3200 and Ch. 3300 to treat increased runoff generated by the widened highway cross section. The swale will assist in removing coarse to medium sediments, whilst the bioretention component in the base of the swale will help decrease flow velocities and remove finer particulates and associated contaminants in minor storm events. The catchment for this bio-swale includes the northbound lanes from Golf Links Road to Araluen Street, as well as the shared path, a total area equivalent to approximately 5000 m2. The bioretention surface treatment area is equivalent to 240 m2.

Bioretention Basin

A bioretention basin has been proposed beside the highway on the south-western corner of the new Clinton Road / Dumbarton Drive signalised intersection, approximately Ch. 3400. This basin will treat flows north of the bio- swale catchment, from south of Araluen Street to the new signalised intersection. This includes flows captured in the median grated drain, and thereby treats runoff from the entire road cross section through this area. Key design criteria for the basin are included in Table 3-4 below.

Table 3-4: Bioretention Basin Design Criteria (Sourced from Austroads Guide to Road Design, Part 5A, Section 7 – Basins (2013), where applicable)

Description Criteria

Basin Catchment Area 4270 m2

Basin Treatment Surface Area 350 m2

Storage Design Storm Event 20% AEP Spillway Design Event 1% AEP Minimum settlement storage depth 300 mm Desirable internal batter slope 1V:8H Desirable external batter slopes 1V:3H Minimum embankment crest width 3.0m

The basin will include a selection of native plantings appropriate for placement in the littoral, ephemeral and submerged zones which ideally, may be endemic to the area and selected with assistance from Council and the local Landcare or Bushcare group.

Pedestrian fencing will also enclose the perimeter of the basin with a 2m wide gravel path around the edge of the basin enabling safe access for maintenance staff through the provided gate. A maintenance track will also extend down to the filter media at no steeper than 1:6.

The basin provides an effective treatment of runoff from increased impervious areas and productively makes use of land between the highway and access road to the Recreation Area which otherwise may have been left undeveloped. In line with the principles behind the rain garden installed at Simmons Park in Lindisfarne, the basin

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is also anticipated to contribute to improved amenity and community interest in the area, mark the entrance to the Recreation Area and contribute to revegetation works proposed as part of the upgrade.

3.4.3 MUSIC modelling results

The conceptual water quality treatment train developed as part of the MUSIC model comprises a mixture of the treatment types prescribed above, as well as predominately impervious source catchments. The model, as shown in Appendix D, includes the culmination of discharge points into Faggs Gully Creek to assess the overall effectiveness of water quality treatment measures.

The performance of the bio-swale and bio-basin are provided in Table 3-5 and Table 3-6. The overall performance of the treatment train is provided in Table 3-7.

Table 3-5: MUSIC model results for the bioretention swale

Parameter Sources Residual Load % Reduction % Reduction target levels

Total Suspended Solids (kg/yr) 849 5.4 99.4 80 Total Phosphorus (kg/yr) 1.49 4.4E-02 97 45 Total Nitrogen (kg/yr) 5.71 1.2 78.9 45 Gross Pollutants (kg/yr) 64.9 0 100 N/A

Table 3-6: MUSIC model results for the bioretention basin

Parameter Sources Residual Load % Reduction % Reduction target levels

Total Suspended Solids (kg/yr) 935 30.4 96.8 80 Total Phosphorus (kg/yr) 1.52 0.197 87 45 Total Nitrogen (kg/yr) 5.15 2.51 51.2 45 Gross Pollutants (kg/yr) 78 0 100 N/A

Table 3-7: MUSIC model results for the overall (combined) treatment system

Parameter Sources Residual Load % Reduction % Reduction target levels

Total Suspended Solids (kg/yr) 3.1E+03 322 89.5 80

Total Phosphorus (kg/yr) 5.25 1.0 80.6 45

Total Nitrogen (kg/yr) 18.6 9.9 46.7 45

Gross Pollutants (kg/yr) 245 16.7 93.2 N/A

It is acknowledged that the WSUD elements outlined previously, including the sizing of swales and bioretention basin, are based on the current Preliminary Drainage Design and may undergo further refinement and optimisation as the project progresses through Detailed Design.

3.4.4 Scheme Compliance

This section has demonstrated compliance with Acceptable Solution A2, Clause E7.7.1 of the Scheme’s Stormwater Management Code, by detailing the WSUD principles that have been incorporated into the design to account for increases in impervious area, and an assessment of their effectiveness.

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4. Conclusion

This report has detailed the findings of a Hydraulic Impact Assessment undertaken for the East Derwent Highway upgrade between Golf Links Road and Sugarloaf Road, Geilston Bay. The proposed development involves upgrades to the existing drainage network to accommodate additional runoff generated as result of the highway duplication and addition of new local roads. Faggs Gully Creek and its minor tributaries is the key hydrological feature within the project area. Faggs Gully Creek is conveyed under the highway just south of the existing Clinton Road intersection via a twin 2100mm x 2400mm concrete box culvert. The catchment area of Faggs Creek is approximately 6 km2 and flows east to west, draining to the River Derwent Estuary.

An assessment of the existing Faggs Gully Creek catchment, detailed in Section 3.2, found that the twin concrete box culvert highway crossing achieved the required immunity standards in the design flood events tested. However, overland flow paths were identified across the highway in two locations for the modelled flood events; including south of Araluen Street at the sag point in the highway, and just north of Clinton Road. Flooding also encroached sections of Geilston Bay Road and the Dumbarton Drive intersection adjacent to the Creek as the drainage network became overwhelmed.

The design intent for the upgraded stormwater network along the highway and new local roads was to maximise use of the existing stormwater system and discharge points as much as possible. Additionally, horizontal and vertical changes to existing road geometry were minimised to ensure existing overland flow paths were maintained along their natural alignment and afflux minimised as much as possible.

The reports presented in this report are based on the Preliminary Drainage Design and flood modelling undertaken to date, and will be subject to further refinement, verification and optimisation as the project progresses through the Detailed Design Phase.

As the project is subject to assessment under various Codes within the Clarence Interim Planning Scheme 2015 (the Scheme), assessment against the following Codes was undertaken: • E7.0 Stormwater Management Code • E11.0 Waterway and Coastal Protection Code • E16.0 Coastal Erosion Hazard Code

Compliance statements against Development Standards for each of these Codes is summarised in Section 4.1.

4.1 Compliance with E7.0 Stormwater Management Code

Table 4.1: Response to E7.7 Development Standards (Source: Clarence Interim Planning Scheme 2015)

Objective: To ensure that stormwater quality and quantity is managed appropriately. Acceptable Solutions Performance Criteria A1 P1

Stormwater from new impervious surfaces must be disposed of by Stormwater from new impervious surfaces must be managed by any gravity to public stormwater infrastructure. of the following:

Response: a) disposed of on-site with soakage devices having regard to the suitability of the site, the system design and water sensitive As indicated in Section 1.3, any new additions to the urban design principles stormwater network have been designed and graded to b) collected for re-use on the site; capture runoff from the highway and local roads, and discharged by gravity into public stormwater infrastructure, c) disposed of to public stormwater infrastructure via a pump which includes existing systems and/or watercourses. system which is designed, maintained and managed to minimise the risk of failure to the satisfaction of the Council.

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Thus, Acceptable Solution A1 is achieved. *Acceptable solution is met, reliance on Performance Criteria is not required.

A2 P2

A stormwater system for a new development must incorporate A stormwater system for a new development must incorporate a water sensitive urban design principles R1 for the treatment and stormwater drainage system of a size and design sufficient to achieve disposal of stormwater if any of the following apply: the stormwater quality and quantity targets in accordance with the State Stormwater Strategy 2010, as detailed in Table E7.1 unless it is a) the size of new impervious area is more than 600 m2; not feasible to do so.

b) new car parking is provided for more than 6 cars; *Acceptable solution is met, reliance on Performance Criteria is not c) a subdivision is for more than 5 lots. required.

Response: The increase in impervious area for the project exceeds 600 m2. The project has demonstrated sufficient incorporation of WSUD principles in the design, verified through MUSIC modelling (refer Section 0).

Thus, Acceptable Solution A2 is satisfied. A3 P3

A minor stormwater drainage system must be designed to comply No Performance Criteria. with all of the following: a) be able to accommodate a storm with an ARI of 20 years in the case of non-industrial zoned land and an ARI of 50 years in the case of industrial zoned land, when the land serviced by the system is fully developed; b) stormwater runoff will be no greater than pre-existing runoff or any increase can be accommodated within existing or upgraded public stormwater infrastructure.

Response:

The upgraded minor stormwater drainage system has been designed to accommodate a 20 year ARI storm event (in line with the adopted flood immunity standards in Section 2.3) as far as reasonably practical. It is acknowledged in some areas, particularly at the new Geilston Bay / Dumbarton Drive intersection, there are existing inundation issues in large flood events. The design has aimed to maintain existing conditions in this location, and avoid worsening of existing conditions as much as possible.

Where there is an increase in pre-existing runoff that cannot be accommodated within existing infrastructure, new or upgraded trunk drainage systems have been recommended to cater for additional flows. This is demonstrated by use of the existing DN1050 trunk main along Debomfords Lane for a large section of the catchment north of Golf Links Road, which is then supplemented by a new underground network and roadside table drains to capture additional runoff, discharging to outlets to Faggs Gully Creek. It is noted that a bioretention basin has been included in the design to detain and slow flow velocities and improve the water quality of additional flows discharging into the creek.

Thus, Acceptable Solution A3 is satisfied.

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A4 P4

A major stormwater drainage system must be designed to No Performance Criteria. accommodate a storm with an ARI of 100 years.

Response:

The major stormwater drainage system, including the combination of the minor stormwater drainage system, roads, watercourses and overland flow paths, has been designed to accommodate a 100 year ARI storm event (in line with the adopted flood immunity standards in Section 2.3) as far as reasonably practical. As indicated in response to A2, it is recognised that there are known existing inundation issues within the project area, and wherever possible the design seeks to capture and improve these issues, and to avoid worsening of existing conditions, in particular across roads and adjacent land.

Thus, Acceptable Solution A4 is satisfied.

4.2 Compliance with E11.0 Waterway and Coastal Protection Code

Table 4.2: Response to E11.7 Development Standards (Source: Clarence Interim Planning Scheme 2015)

Objective: To ensure that buildings and works in proximity to a waterway, the coast, identified climate change refugia and potable water supply areas will not have an unnecessary or unacceptable impact on natural values. Acceptable Solutions Performance Criteria A1 P1

Building and works within a Waterway Building and works within a Waterway and Coastal Protection Area must satisfy all of the and Coastal Protection Area must be following: within a building area on a plan of subdivision approved under this a) avoid or mitigate impact on natural values; planning scheme. b) mitigate and manage adverse erosion, sedimentation and runoff impacts Response: c) avoid or mitigate impacts on riparian or littoral vegetation; Acceptable Solution A1 does not d) maintain natural streambank and streambed condition, (where it exists); apply, Performance Criteria must be e) maintain in-stream natural habitat, such as fallen logs, bank overhangs, relied upon. f) avoid significantly impeding natural flow and drainage;

g) maintain fish passage (where applicable); h) avoid landfilling of wetlands; i) works are undertaken generally in accordance with 'Wetlands and Waterways Works Manual' (DPIWE, 2003) and “Tasmanian Coastal Works Manual” (DPIPWE, Page and Thorp, 2010), and the unnecessary use of machinery within watercourses or wetlands is avoided.

Response:

Mitigations to natural values, including through implementation of water quality control measures designated in this report, are also documented separately in the Natural Values Assessment for this project.

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Construction in and around key watercourses in the project area (i.e. for the highway culvert extension and new intersection culvert) must occur in small, discrete zones of impact to mitigate erosion and sedimentation impacts, and to maintain natural in- stream habitat as much as possible.

Sizing of the new intersection culvert has been optimised to suit the required flood immunity criteria as well as maximise waterway area and to maintain existing flow paths as much as possible.

WSUD elements have also been incorporated into the drainage design to reduce the impacts of peak flows and improve the water quality of additional runoff generated. Soil, water and erosion management and controls will be a requirement of the construction contractor’s Construction Environmental Management Plan (CEMP), and to ensure that any works undertaken in watercourses are generally in accordance with the manuals prescribed in P3(i) above.

Performance Criteria P4 is satisfied.

A4 P4

Development must involve no new Development involving a new stormwater point discharge into a watercourse, wetland or lake stormwater point discharge into must satisfy all of the following: a watercourse, wetland or lake. a) risk of erosion and sedimentation is minimised;

b) any impacts on natural values likely to arise from erosion, sedimentation and runoff are Response: mitigated and managed; Acceptable Solution A2 does not c) potential for significant adverse impact on natural values is avoided. apply as the development involves a

discrete number of new discharge points, Performance Criteria must be Response: relied upon. The project’s upgraded stormwater system, as discussed in this report, involves new discharge points into Faggs Gully Creek. These new outlet points have been optimised to ensure that the additional discharge will be occurring in a location where scour and embankment protection as a result of the highway box culvert extension is already being implemented. Locating these new outlets in a location where scour protection is already proposed helps mitigate the risk of further erosion and confines discharge points in one location so that natural stream bed conditions can be maintained downstream as much as possible.

As detailed in the separate Natural Values Assessment, construction of new stormwater outlets is expected to have minimal additional or adverse effects on existing natural values along the creek, outside of what has already been assessed as lost through the construction footprint.

Re-vegetation works as part of an overall landscaping plan will occur along the creek lines, as well as scour protection through rock pitching and batter stabilisation, as previously described.

Additionally, soil, water and erosion management and controls will be a requirement of the construction contractor’s Construction Environmental Management Plan (CEMP).

Performance Criteria P4 is satisfied.

4.3 Compliance with E16.0 Coastal Erosion Hazard Code

Table 4.3: Response to E16.7 Development Standards (Source: Clarence Interim Planning Scheme 2015)

Objective:

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To ensure that development in Coastal Erosion Hazard Areas is fit for purpose and appropriately managed based on the level of exposure to the hazard. Acceptable Solutions Performance Criteria A1 P1

No Acceptable Solution. Buildings and works must satisfy all of the following:

a) not increase the level of risk to the life of the users of the site or of hazard for adjoining or Response: nearby properties or public infrastructure; Performance Criteria P1 must be b) erosion risk arising from wave run-up, including impact and material suitability, may be relied upon. mitigated to an acceptable level through structural or design methods used to avoid damage to, or loss of, buildings or works; c) erosion risk is mitigated to an acceptable level through measures to modify the hazard where these measures are designed and certified by an engineer with suitable experience in coastal, civil and/or hydraulic engineering; d) need for future remediation works is minimised; e) health and safety of people is not placed at risk; f) important natural features are adequately protected; g) public foreshore access is not obstructed where the managing public authority requires it to continue to exist; h) access to the site will not be lost or substantially compromised by expected future erosion whether on the proposed site or off-site; i) provision of a developer contribution for required mitigation works consistent with any adopted Council Policy, prior to commencement of works; j) not be located on an actively mobile landform.

Response:

The works impact a very small fraction of the mapped Coastal Erosion Hazard Area near the confluence of Faggs Gully Creek with the River Derwent. Although the susceptibility of this coastal area to erosion is unknown due to the uncertainty in underlying information, the area is classed as an acceptable hazard zone (all soft sediment shores), and is landwards of likely and possible natural recession limits (ListMap, 2020). Given the existing coastal hazard level is acceptable and works are occurring in the section mapped furthest upstream along the creek (landwards of likely recession limits), the risk to life and natural features posed by erosion hazard and erosion risk from wave run-up is said to be unchanged by the proposed works. Additionally, access to and through the area and public foreshore will not be lost, compromised or obstructed by any future coastal erosion in the works area. The works are also not being undertaken in a known actively mobile landform. Erosion mitigations for flows within Faggs Gully Creek, particularly either side of the new waterway crossing under the Dumbarton Drive / Geilston Bay Road intersection, will be implemented by the developer. The measures being implemented will assist in reducing downstream erosion risk around the confluence of the creek with the coastline, and have already been addressed in responses to Development Standards for E11.0 Waterway and Coastal Protection Code, both through design features and implementation of a CEMP.

Based on the above, Performance Criteria P1 is satisfied.

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Hydraulic Impact Assessment

5. References

Australian Rainfall and Runoff, 2016 A Guide to Flood Estimation, Commonwealth of Australia Available at: http://book.arr.org.au.s3-website-ap-southeast-2.amazonaws.com/

Austroads 2013 Guide to Road Design Part 5 – Drainage – General and Hydrology Considerations

Austroads 2013 Guide to Road Design Part 5A – Drainage – Road Surfaces, Networks Basins and Subsurface

Austroads 2019 Guide to Bridge Technology Part 8 – Hydraulic Design of Waterway Structures

Brisbane City Council (2004) Natural Channel Design Guidelines, Brisbane City Council Brisbane

City of Hobart (2019,) [Available online] Storm Surge and Flood Prone Land, https://www.hobartcity.com.au/Development/Planning/Storm-surge-and-flood-prone-land

Clarence City Council (2014), Clarence Interim Planning Scheme, Available at: https://www.iplan.tas.gov.au/pages/plan/book.aspx?exhibit=claips

Department of State Growth (2012), Professional Services Specification – T8 Drainage Design Standards, Available at: https://www.transport.tas.gov.au/__data/assets/pdf_file/0008/111401/T8_- _Drainage_Design_Standards_-_June_2012.PDF

Derwent Estuary Program (2012), Water Sensitive Urban Design – Engineering procedures for stormwater management in Tasmania, Available at: https://www.derwentestuary.org.au/water-sensitive-urban-design/

Engineers Australia (2015) Revision of Australian Rainfall and Runoff

Fallon, L. et al. (2000), River Derwent Flood Data Book, Department of Primary Industries, Water and Environment, Tasmanian Government.

GeoScience Australia (2010), Digital Elevation Data from FraserCoast2010 Available at: http://www.ga.gov.au/scientific-topics/national-location-information/digital-elevation-data

Local Government Association Tasmania (2013), Tasmanian Standard Drawings Available at: https://www.lgat.tas.gov.au/__data/assets/pdf_file/0021/321348/LGAT-Standard-Drawings-Release-Version- Dec-2013.pdf

McInnes KL, Monselesan D, O’Grady JG, Church JA and Xhang, X, (2016), Sea-Level Rise and Allowances for Tasmania based on the IPCC AR5, CSIRO Report 33 pp.

Office of the Surveyor General (2019), [Available online] Coordinate, Height and Tide Datums – Tasmania, Department of Primary Industries Parks Water and Environment, Tasmanian Government, https://dpipwe.tas.gov.au/land-tasmania/geospatial-infrastructure-surveying/geodetic-survey/coordinate- height-and-tide-datums-tasmania

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Hydraulic Impact Assessment

Appendix A. Existing Case Model Development

ARR2016 Data Hub Data

A.1 Design Rainfalls

The ARR2016 Intensity-Frequency-Duration (IFD) rainfalls depths that have been adopted in this analysis are shown in Table 5-1.

Table 5-1: Adopted ARR2016 IFD Rainfall Depth data

Duration Rainfall Depth (mm) 63.2% 50% AEP 20% AEP 10% AEP 5% AEP 2% AEP 1% AEP AEP 30 min 7.78 8.83 12.4 15 17.7 21.6 24.8 45 min 9.33 10.6 14.7 17.7 20.7 25 28.4 1 hour 10.6 12 16.6 19.9 23.2 27.7 31.3 1.5 hour 12.8 14.5 19.9 23.6 27.3 32.2 36.1 2 hour 14.7 16.6 22.7 26.8 30.9 36.2 40.3 3 hour 17.8 20.1 27.5 32.4 37.2 43.3 48 4.5 hour 21.5 24.5 33.5 39.5 45.2 52.6 58.3 6 hour 24.6 28.1 38.6 45.5 52.2 60.9 67.6 9 hour 29.5 33.8 47 55.6 64 75.2 83.8 12 hour 33.3 38.3 53.6 63.8 73.7 87.2 97.6

Spatial variation of design rainfall was investigated to ensure that applying a single set of IFD data for all the catchments was appropriate. Taking into consideration the catchment size (6 km2) it was deemed appropriate to adopt a single set of IFD data for all sub-catchments in the XP-RAFTS model.

A.2 Pre-Burst Rainfall

The ARR2016 Data Hub provided median pre-burst depths for the location of interest as shown in Table 5-2.

Table 5-2: ARR 2016 Data Hub Median Pre-Burst Depths

Duration (min) Pre-Burst Depth (mm) 50% AEP 20% AEP 10% AEP 5% AEP 2% AEP 1% AEP 60 4.1 5.2 6.0 6.8 5.4 4.4 90 3.6 4.1 4.4 4.8 4.8 4.9 120 3.0 4.6 5.6 6.6 6.5 6.4 180 5.4 6.2 6.6 7.1 13.6 18.4 360 3.6 6.7 8.7 10.7 18.0 23.5 720 2.3 5.9 8.2 10.5 13.0 14.9 1080 0.5 3.9 6.2 8.3 8.1 8.0 1440 0.2 3.2 5.1 7.0 6.2 5.6 2160 0 2.1 3.5 4.9 2.9 1.4

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2880 0 1.8 3.1 4.2 5.2 5.9

The median pre-burst depths were taken away from the datahub initial losses via storm injector for all simulated events. Pre-burst rainfall was not added to the storm if the pre-burst was found to exceed the initial loss. In these instances, the initial loss was set to zero.

Temporal Patterns for the southern Slopes of Tasmania were used for the assessment including all ten temporal patterns from the AEP bins. Intermediate temporal patterns were used for the 5% AEP events and the rare temporal patterns were used for the 1% AEP.

A.3 Areal Reduction Factors

The Southern Slopes of Tasmania Areal Reduction Factor (ARF) parameters were used to define the areal reduction factor for the study catchment of 6.012 km2, these are outlined in Table 5-3.

Table 5-3: ARF Parameters

ARF Parameter Value

A 0.06050

B 0.34700

C 0.20000

D 0.28300

E 0.00076

F 0.34700

G 0.08770

H 0.01200

I -0.00033

A.4 Manning’s n Roughness and Rainfall Losses

The 2D Manning’s n layers were delineated based on land use mapping and aerial imagery. Table 5-4 shows the adopted manning’s n roughness parameters for the TUFLOW model surface. These values are consistent with those recommended by ARR2016. Note that the buildings and urban block Manning’s n layers have been given a depth varying manning’s n to account for roof areas. For example, the buildings Manning’s n value will be 0.015 at depths below 30mm to account for flow quickly running off building roofs. Above 100 mm a manning’s n value of 1.0 was applied to flow. At these depths flow is likely to be at ground level and obstructed by the building. The Manning’s n values between these depths are interpolated. All 1D culverts were assigned a Manning’s n value of 0.015.

The manning’s n definitions file also defined the rainfall losses for the rain on grid component of the model. These are also shown in Table 5-4 and have only been applied to permeable land use areas. Note that urban blocks have been assumed 66% permeable. The losses have been adopted from the ARR2016 Data Hub, with the initial losses adjusted down to account for pre-burst rainfall.

Table 5-4 : 2D Hydraulic Model Roughness Parameters

Land use/ Vegetation Type Adopted Depth Varying Initial and Continuing Losses Manning’s n Manning’s n (mm)

Roads and hardstand 0.020 NA IL-0.0, CL-0.0

Maintained grass 0.040 NA IL-4.5, CL-3.7

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Land use/ Vegetation Type Adopted Depth Varying Initial and Continuing Losses Manning’s n Manning’s n (mm)

Unmaintained grass 0.070 NA IL-4.5, CL-3.7

Urban Block 0.100 0.025<30mm, 0.10>100mm IL-3.0, CL-2.3

Medium Vegetation 0.090 NA IL-4.5, CL-3.7

Open Water 0.020 NA IL-0.0, CL-0.0

Vegetated Channel 0.050 NA IL-0.0, CL-0.0

Low Vegetated Channel 0.035 NA IL-0.0, CL-0.0

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Hydraulic Impact Assessment

Appendix B. Existing Case Preliminary Flood Mapping

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6 0 0.1 0.2 0.4 Kilometers 0

96 E 85 a 5 8 9 s 0 8 8 3 H t 8 6 9 i D 8 7 g 6 1 e 2 7 h 6 r 6 w w 5 d 9 51 7 5 a a 6 e o 5 51 y 2 n 5 R 5 3 8 44 t 7 f 4 4 4 5 6 a 9 0 1 46 lo 5 r 4 4 1 38 2 a 4 9 g 3 u 35 0 7 4 S 3 5 8 3 3 3 3 36

4 3 2 6 3 1 35 5 1 4 5 3 0

3 3 8

6 1 4 3 2 3 6 29 3 6 3 5 5 8 28 3 3 6 28 3 2 1 97 2 9 8 23 29 2 28 26 6 22 2 9 8 7 27 1 21 60 2 6 1 2 38 4 5 8 47 5 4 2 9 3 76 1 3 41 6 2 5 9 5 8 4 4 6 1 21 35 0 7 2 4 1 2 32 6 19 2 6 3 1 0 15 17 2 2 7 15 4 14 19

4 3 3

3 4

1

4

1 8 31 7 k 3 2 e 3 re 7 2 C 0 s Faggs Creek 1 1 g 18 ag 6 F Hwy Crossing 9 6 1 4 9 6 7 3 1 4 3 5

4 5 3 4 11 1 67 6 4 3 3 5 7 8 9 12 3 55 1

45 3

1

0 1 5 9 2 2 2 0 0 2 1 2 1 2 2 2 3 6 7

1 1 9 4 7 2 9 1 1 2 1 1 7 1 4 7 6 6 8 1 1 2 7 9 9 9 2 5 2 5

9 1 8 0 4 2 1 3 4 3 1 1 1 1 1 2 9 1 1 3 3 12 0 2 4 2 1 1 2 3 2 4 1 4 2 1 1 2 14 6 1 1 5 2 2 16 8 16 oad 1 4 6 R 7 1 16 17 nks 4 7 3 Li 6 1 olf 1 8 G 1 5 18 7 8 4 1 19 20 6 9 11 8 4 10 1 17 31 25 26 27 River 27 Derwent 48 Estuary

Service Layer Credits: Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong), Esri Korea, Esri (Thailand), NGCC, (c) OpenStreetMap contributors, and the GIS

Map 2 Legend Depth (m) Existing Case 1% AEP Model Extent < 0.01 Depth & Contour Water Level (m) 0.0 - 0.25

0.25 - 0.5 1:6,800 East Derwent Highway 0.5 - 1 Job No: IS262318 By: M Lovell 1 - 2 Last Modified: 29/04/2020 ± > 2 MGA Zone 55 W:\IS262318\Spatial\ArcGIS\MXD\Maps\E021\Map2_EX_01P_Dep_Cont.mxd Received 21.08.2020

6 0 0.1 0.2 0.4 Kilometers 0

E 85 a 6 8 2 s 9 H t 1 8 i D 6 g 7 6 1 e 2 7 h r 5 6 w w 1 d 6 56 a 7 a e 5 o y 3 0 n 5 5 R 8 44 t 52 f 4 6 a 4 o 46 rl 3 4 4 38 2 a 9 g 3 35 u 7 9 S 3 5 3 3 3 3 37 3 3 3 6 1 3 40 3 3

5 1 8

3 4 9 36 6 2 1 3 3 5 5 4 28 3 27 4 3 1 25 2 8 23 27 9 2 6 22 2 69 80 21 59 2 64 0 47 8 5 7 4 1 7 3 6 9 5 3 4 4

1 7

7 4 1 3 6 9 2 1 1 3 0 25 17 2 7 14

3

1

1 3 7 k 3 e 7 re 2 C 1 s Faggs Creek 11 agg 6 F Hwy Crossing 9 16 1 8 6 3 1

5 3 3 4 1 1 5 6 101 4 6 7 8 9 12 1 2 13 2 45 2 2 8 2 2 2 1 0 4 1 2 8

0 5

2 1 1

4 5 1 2 6 1 2 6 7 2 7 8 5 2 9 4 9 1 4 0 3 8 1 3 3 1 9 1 1 1 1 1 1 2 1 3 1 2 4 13 4

1 1

1 15 8 ad 2 4 s Ro 1 3 16 ink 8 L 5 olf 1 5 G 7 8 4 19 20 6 9

3 9 1 11 13 25 26 27 River Derwent Estuary

Service Layer Credits: Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong), Esri Korea, Esri (Thailand), NGCC, (c) OpenStreetMap contributors, and the GIS

Map 4 Legend Depth (m) Existing Case 5% AEP Model Extent < 0.01 Depth & Contour Water Level (m) 0.0 - 0.25

0.25 - 0.5 1:6,800 East Derwent Highway 0.5 - 1 Job No: IS262318 By: M Lovell 1 - 2 Last Modified: 29/04/2020 ± > 2 MGA Zone 55 W:\IS262318\Spatial\ArcGIS\MXD\Maps\E021\Map4_EX_05P_Dep_Cont.mxd Received 21.08.2020

Hydraulic Impact Assessment

Appendix C. Preliminary Drainage Design Drawings

*Please refer to the Development Application’s overall drawing set for preliminary drainage design drawings.

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Appendix D. MUSIC Model Treatment Train

Figure B-1: MUSIC model overall treatment train