BRIDGE MILLS, TINTWISTLE, DERBYSHIRE
14/12/2016 FLOOD RISK ASSESSMENT
The Alan Johnston Partnership LLP
Ref: 216-232 216216----232232 Bridge Mills, Tintwistle, Derbyshire Prepared By
Document Owner(s) Project/Organization Role
Kenny O’Brien Partner
John Speers Civil Engineer
Project Closure Report Version Control
Version Date Author Change Description
P1 09/12/2016 J.Speers First Issue
P2 14/12/2016 J.Speers Minor Amendments
Record of Approval
Originator Approved Date J.Speers K.O’Brien 09/12/2016 J.Speers K.O’Brien 14/12/2016 ------
Page 1 216216----232232 Bridge Mills, Tintwistle, Derbyshire Contents
1.0 INTRODUCTION ...... 444
2.0 SITE DESCRIPTIODESCRIPTIONNNN ...... 555
2.1 Geology ...... 5
2.2 Hydrogeology ...... 6
2.3 Hydrology ...... 6
3.0 PROPOSED DEVELODEVELOPMENTPMENT ...... 666
4.0 PLANNING POLICY ...... 777
4.1 Sequential Test ...... 7
4.2 Exception Test ...... 7
5.0 FORMS OF FLOODIFLOODINGNG ...... 777
5.1 Flooding from Rivers ...... 7
5.2 Flooding from the Sea ...... 9
5.3 Flooding from Land ...... 9
5.4 Flooding from Groundwater...... 10
5.5 Flooding from Sewers ...... 11
5.6 Flooding from Reservoirs, Canals and Other Artificial Sources ...... 11
6.0 DEVELOPMENT AND DRAINAGE STRATEGY ...... 121212
6.1 Effect of Development on Flood Risk ...... 12
Page 2 216216----232232 Bridge Mills, Tintwistle, Derbyshire 6.2 Existing Drainage Systems ...... 12
6.3 Proposed Drainage Strategy...... 12
6.3.1 Surface Water Drainage ...... 13
6.3.2 Foul Water Drainage ...... 15
6.4 Managing Residual Flood Risk ...... 16
7.0 CONCLUSIONS ...... 161616
8.0 RECOMMENDATIONS ...... 171717
Appendix A Site Location Plan ...... 18
Appendix B Aerial Photograph ...... 19
Appendix C Topographic Survey ...... 20
Appendix D Proposed Site Plan ...... 21
Appendix E Flood Risk Maps & EA Correspondence ...... 22
Appendix F Hydraulic Modelling Sections Drawing ...... 23
Appendix G Envirocheck Flood Screening Report ...... 24
Appendix H United Utilities Sewer Records ...... 25
Appendix I Greenfield Runoff Assessment ...... 26
Appendix J Attenuation Storage Assessment ...... 27
Appendix K Drainage Strategy Drawing ...... 28
Page 3 216216----232232 Bridge Mills, Tintwistle, Derbyshire
1.0 INTRODUCTION
This Flood Risk Assessment (FRA) has been prepared on behalf of BXB Tintwistle Ltd in connection with an outline planning application for a residential development, consisting of approximately 165 residential units, with associated works and vehicular and pedestrian access off New Road. The proposed site is located adjacent to New Road, Tintwistle. This FRA is required as the total area of the proposed development site is over 1.0ha and a part of the site lies within Flood Zones 2 and 3.
This FRA has been prepared in accordance with the general requirements of the National Planning Policy Framework and the Technical Guidance to the National Planning Policy Framework, both published by the Department for Communities and Local Government. This involves the identification of flood risk to new development(s) on the site, the possible effect of this development on flood risk elsewhere and the investigation of the impact on the development as a result of increased sea levels, fluvial flows and larger pluvial events due to increased impermeable areas and climate change.
The purpose of this report is to present a site specific assessment of flood risk based on available information, to identify relevant flood levels affecting the site with respect to a particular probability of flooding. The flood levels identified are dependent on the correctness of the current conditions, as stated in this report and the prediction of future climatic conditions implicit in the National Planning Policy Framework.
For any size of storm event, the procedure will always yield a probability that the event will occur in any year, albeit the probability is smaller for larger events. The predicted storm event has a probability of occurrence in any year and the derived flood level has an associated probability of exceedance. This means that there is always a risk that property flooding could occur one or more times in any year.
Therefore, this report should not be interpreted or relied upon as providing a guarantee against flooding. There is always a residual risk that flooding will occur and it is not possible to predict a zero risk of flooding.
In accordance with National Planning Policy Framework (NPPF) and the associated Technical Guidance document, flood risk must be assessed for all sources including tidal (from the sea), fluvial (from rivers), pluvial (from land), groundwater, sewer and artificial water bodies (e.g. reservoirs, canals, major water supply infrastructure etc.).
More specifically, the development of any site must be carried out in such a way as to mitigate any potential flood risk, both on and off site from all sources of flooding.
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2.0 SITE DESCRIPTION The existing site is located adjacent to New Road, Tintwistle. The site is located at National Grid reference SK 01915 96910. The location of the existing site is illustrated in Appendix A and the extent of the area to which the application relates is shown on the enclosed aerial photograph in Appendix B.
The existing site covers a total area of approximately 4.01ha and is a brownfield site. The existing site is currently disused but has been previously developed and used for industrial purposes, most recently as a storage yard and filling out area for pre-fabricated living units. An aerial photograph indicating the current site is included within Appendix B.
To the north, the site is bounded by open undeveloped land and an existing residential development, to the west a waste water treatment works and to the east by the existing highway, New Road. To the south the site is bounded by the River Etherow.
The site slopes from east to west with levels varying from approximately 136.50mAOD in the east to approximately 132.00mAOD in the west of the site. Steep banks are located along the western and southern boundaries of the proposed site. The bank along the western boundary falls from approximately 135.00mAOD to approximately 128.00mAOD at the waste water treatment works.
The bank along the southern boundary forms the bank of the River Etherow and falls from approximately 137.00mAOD to approximately 132.00mAOD at the south-east corner of the site. The same bank falls from approximately 131.50mAOD to approximately 127.50mAOD at the south-west corner of the site.
An aerial photo is included in Appendix B to illustrate the site at the present day and the topographic survey is included in the existing site plan, attached in Appendix C.
2.1 Geology
The British Geological Survey 1:50,000 Bedrock geology maps show that the site has alluvium superficial deposits, with bedrock reported to be of the Upper Kinderscout Grit Formation, comprising of Sandstone.
The Tier Consult, Stage 1 Geo-environmental Appraisal for Land at New Road, Tintwistle Report, T/13/1256/GA, dated February 2015 supports this information with exploratory hole records indicating strata comprising of made ground to depths of up to 7.60mbgl, underlain by river terrace (gravelly clays) superficial deposits to depths of up to 11.45mbgl, underlain by a light brown/grey sandstone (Kinderscout Grit Sandstone).
Page 5 216216----232232 Bridge Mills, Tintwistle, Derbyshire 2.2 Hydrogeology
The Environment Agency groundwater maps show that the superficial deposits and bedrock are both classified as Secondary A Aquifers, defined as permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. These are generally aquifers formerly classified as minor aquifers.
The Environment Agency groundwater maps also indicate that the development site is not situated within a groundwater Source Protection Zone (SPZ).
The Tier Consult, Stage 1 Geo-environmental Appraisal for Land at New Road, Tintwistle Report, T/13/1256/GA, dated February 2015 indicates that groundwater was encountered in a number of exploratory holes across the site. It indicates that due to the cohesive nature of the made ground and alluvium strata, the shallow groundwater encountered is likely to be perched water with the actual groundwater table at depths of between 5.07mbgl and 7.90mbgl.
It is therefore likely that the groundwater level is dependent upon the level of the adjacent River Etherow. Therefore, a shallow groundwater body is unlikely to be present at this site
2.3 Hydrology
The development site is situated in the direct catchment of the River Etherow. The River Etherow is located approximately 1.0m south of the site. The site is located mainly within Flood Zone 1 but a small part of the site, along the southern boundary is located within Flood Zones 2 and 3.
Flood Zone 2 is categorized by the Environment Agency as land assessed as having between a 1 in 100 and 1 in 1,000 annual probability of river flooding (1% – 0.1%) and Flood Zone 3 is categorized as land assessed as having a 1 in 100 annual probability of river flooding (1% ).
Therefore, the small section of the site, along the southern boundary is identified by the Environment Agency as at risk of flooding from rivers and streams but the majority of the site is identified as having a low risk of flooding from rivers and streams.
However, no works associated with the proposed development fall within Flood Zones 2 or 3.
3.0 PROPOSED DEVELOPMENT The proposed development will occupy the entire 4.01ha site and will consist of 165no. residential units with associated gardens and access roads. To enable a viable scheme, the proposed site levels are constrained by the existing highway to the east and the existing slopes at the site boundaries and therefore, site levels are likely to remain similar to the existing. A proposed site plan is attached in Appendix D.
As can be seen from the proposed site plan, the Trans Pennine Trail is included as a part of the proposed site works. It is proposed that the trail will run along the existing river bank to the south of the proposed site.
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4.0 PLANNING POLICY The National Planning Policy Framework (NPPF) sets out the Government’s policy on meeting the challenges of climate change, flooding and coastal change. The NPPF states that:
“Planning plays a key role in helping shape places to secure radical reductions in greenhouse gas emissions, minimising vulnerability and providing resilience to the impact of climate change, and supporting the delivery of renewable and low carbon energy and associated infrastructure. This is central to the economic, social and environmental dimensions of sustainable development. Local planning authorities should adopt proactive strategies to mitigate and adapt to climate change taking full account of flood risk, coastal change and water supply and demand consideration.”
This Flood Risk Assessment proposes recommendations to facilitate the proposed development so that it takes into account flood risk at all stages of the development.
4.1 Sequential Test Based on the sites location, mostly within Flood Zone 1 but partially within Flood Zones 2 and 3, the Sequential Test would generally be required. However, as discussed further in Section 5.1 below, the modelled flood level data suggests that the minimum site level is approximately 2.30m above the 1 in 1000 year modelled flood level and therefore the site would be classified as falling within Flood Zone 1.
Therefore, the development is deemed appropriate, in Flood Zone 1 and a sequential test is not required.
4.2 Exception Test NPPF classifies the development (dwelling houses) as ‘More Vulnerable’ but as outlined above within Section 4.1, the actual development on the site is located within Flood Zone 1, with the areas within Flood Zone 2 and 3 being undeveloped, and therefore the Exception Test is not required.
5.0 FORMS OF FLOODING The Technical Guidance to the National Planning Policy Framework requires all forms of flooding to be considered.
5.1 Flooding from Rivers The Flood Risk map is included as Appendix E. It can be seen from the map that the majority of the site is located in Flood Zone 1 with a chance of flooding of less than 0.1% (or 1 in 1000) and only a the strip along the southern boundary is located within Flood Zones 2 and 3.
Page 7 216216----232232 Bridge Mills, Tintwistle, Derbyshire The Environment Agency have been contacted and asked to provide product 4 flood level information for the River Etherow, adjacent to the proposed site, but to date no response has been received. Therefore, the design flood levels have been taken from the previous flood risk assessment for the site; Mixed Use Development, Bridge Mills, Tintwistle, Flood Risk Assessment, August 2011, prepared by Fairhurst.
The flood levels within the Fairhurst flood risk assessment report have been developed using hydraulic modelling and the results are summarised in the table below;
Table 4.7 – Design Scenario Flood Levels - Bridge Mills, Tintwistle, Flood Risk Assessment D/I/D/90486/01, August 2011 by Fairhurst
The sections drawing from the Fairhurst flood risk assessment report has been included within Appendix F, to indicate the location of the design flood levels outlined above. The 1000 year design flood levels have been compared to the lowest proposed site level, at the location of each of the cross sections identified above.
The incorporation of the Trans Pennine Trail within the site means that this is the lowest proposed site level along the southern boundary of the site and therefore these levels have been compared to the 1000 year peak water level. It must be noted though that the trail (outdoor sports and recreation) is classified within the NPPF as ‘water-compatible’ and is therefore a suitable development within Flood Zone 3.
The results are indicated in the table below;
1000 year Peak Lowest Proposed Level Cross Section Water Level (mAOD) Trail Level (mAOD) Difference (m) 0 133.90 136.50 +2.60
1 133.00 136.60 +3.60 2 132.40 135.40 +3.00 3 131.20 132.40 +1.20
4 130.30 130.40 +0.10 5 130.30 N/A N/A
As can be seen from the levels comparison above, the lowest proposed trail level is a minimum of 0.10m above the 1000 year flood level and as such can be classified as being located within Flood Zone 1. Although the minimum 600mm freeboard has not been achieved on the western
Page 8 216216----232232 Bridge Mills, Tintwistle, Derbyshire section of the trail it does not matter as the trail is classified as a ‘water-compatible’ development and is therefore suitable within Flood Zone 3.
The lowest finished floor level at each cross section on the proposed site has also been compared to the 1000 year peak water level below;
1000 year Peak Lowest Proposed Level Cross Section Water Level (mAOD) FFL (mAOD) Difference (m) 0 133.90 137.00 +3.10
1 133.00 135.60 +2.60 2 132.40 135.15 +2.75 3 131.20 133.50 +2.30
4 130.30 133.35 +3.05 5 130.30 N/A N/A
As can be seen from the levels comparison above, the lowest proposed finished floor level is a minimum of 2.30m above the 1000 year flood level and as such the proposed development can be classified as being located within Flood Zone 1 and not Flood Zones 2 or 3. The undeveloped river bank to the south of the site is classified as being located within Flood Zones 2 and 3 but as no built development is being undertaken within this area.
Therefore, the risk of flooding to the new development from rivers is considered to be low.
The Environment Agency have been contacted regarding flood level information for the adjacent River Etherow, within the immediate vicinity of the site but to date no response has been received. The correspondence is attached in Appendix E.
When received the flood level information provided by the Environment Agency should be compared to the proposed site levels and modelled flood level data identified above, to confirm the proposed developments classification within Flood Zone 1. The Environment Agency should then be consulted to confirm that the proposed development is classified within Flood Zone 1 and therefore appropriate as currently detailed.
5.2 Flooding from the Sea The site is not at risk of flooding from the sea. The lowest level of the site is approximately 127.50mAOD, i.e. well above tidal flood levels.
5.3 Flooding from Land Intense rainfall, often of short duration, that is unable to soak into the ground or enter drainage systems can run quickly off land and result in local flooding.
The Surface Water Flooding map is included in Appendix E. It can be seen from the map that the majority of the site is classified as having a very low risk of surface water flooding. Only, small
Page 9 216216----232232 Bridge Mills, Tintwistle, Derbyshire areas to the north of the site, are classified as having a medium to high risk of surface water flooding.
However, as a part of the new development, new surface water sewers are to be constructed and these should collect the surface water runoff from the proposed development, therefore mitigating the surface water flood risk.
The ground levels on the proposed site are to raised higher above the existing levels but will still maintain the overall site levels falling towards the River Etherow. As such, any overland flows will be conveyed through the site and towards the River Etherow to the south. Finished floor levels of the proposed housing units should be set above external ground levels to protect against potential inundation from any possible overland flows.
The Environment Agency have been contacted regarding historical flooding incidents in the immediate vicinity of the site but to date no response has been received. The correspondence is attached in Appendix E. However, the EA/NRW Historic Flood Map, located within Appendix G indicates that there are no recorded historic flood events from local drainage/surface water within the site boundary.
As a result of the mitigation measures outlined above and no recorded historic flood events resulting from surface water, the proposed development is considered to be at low risk of flooding from surrounding land.
5.4 Flooding from Groundwater As outlined in Section 2.2, the Tier Consult, Stage 1 Geo-environmental Appraisal for Land at New Road, Tintwistle Report, T/13/1256/GA, dated February 2015 indicates that groundwater was encountered in a number of exploratory holes across the site. It indicates that due to the cohesive nature of the made ground and alluvium strata, the shallow groundwater encountered is likely to be perched water with the actual groundwater table at depths between 5.07mbgl and 7.90mbgl.
It is It is therefore likely that the groundwater level is located between 5.07mbgl and 7.90mbgl and is dependent upon the level of the adjacent River Etherow.
Also, the ESI groundwater flood data contained within the Envirocheck Flood Screening Report, attached in Appendix G, indicates that there is a low to moderate risk of groundwater flooding within the site area but the EA/NRW Historic Flood Map, also located within Appendix G indicates that there are no recorded historic flood events from groundwater/high water table within the site boundary. As such, the site is considered to be at low risk of flooding from groundwater.
Page 10 216216----232232 Bridge Mills, Tintwistle, Derbyshire 5.5 Flooding from Sewers A record of Public Sewers surrounding and serving the development site has been obtained from United Utilities and this is included in Appendix H.
As can be seen from the United Utilities map, the only public sewer within the immediate vicinity of the site is the existing 300mm diameter combined water sewer. The sewer runs east to west through the north of the proposed development site, prior to discharging to the waste water treatment works, located to the west of the site.
All of the adjacent sewers located to the east of the site, are situated within the highways and prior it entering the proposed site boundary, connect to the existing 300mm diameter combined water sewer which crosses the site. Therefore, due to the lack of existing sewers within the proposed site boundary is it concluded that there is a minimal risk of flooding from existing sewers.
The EA/NRW Historic Flood Map, located within Appendix G indicates that there are no recorded historic flood events from local drainage/surface water, mechanical failures or obstruction blockages within the site boundary. Therefore, this coupled with the minimal risk of flooding from existing sewers, leads to the conclusion that the site itself is considered to be at low risk of flooding from sewers.
5.6 Flooding from Reservoirs, Canals and Other ArtificiArtificialal Sources The Environment Agency Flood Reservoir Flood Risk map is included in Appendix E. It can be seen from the map that the site is at risk of flooding from reservoirs. According to the Environment Agency map the reservoir which may cause flooding in this area is Arnfield Reservoir, which is located approximately 450m north-west of the site.
Bottoms Reservoir is also located approximately 350m east of the site and this reservoir is the last in a series of five reservoirs, known as the Longdendale Chain, which impound the River Etherow upstream of the site. In the event of an extreme flood occurring within the catchment upstream of the site, the five reservoirs would provide a degree of attenuation which should reduce the peak flow rate of the flood to the site.
Due to the introduction of the Reservoirs Act 1975 in England, the Environment Agency must ensure that all large reservoirs are inspected and supervised by reservoir panel engineers and that the reservoirs are inspected regularly and essential safety work is carried out. Therefore, the likelihood of reservoir flooding occurring is low and therefore the site is considered to be at low risk of flooding from reservoirs.
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6.0 DEVELOPMENT AND DRAINAGE STRATEGY
6.1 Effect of Development on Flood Risk
As previously outlined in Section 2.0, the existing site covers a total area of 4.01ha and is currently a brownfield site. However, the introduction of the new roof areas and car parking areas from the proposed development, will still lead to an increase in impermeable areas.
Therefore, the changes to the existing site will increase the volume of impermeable areas and as such, the proposed development will lead to an increase in the;
• Volume of surface water ponding on the site • Volume of surface water runoff leaving the site or discharging into surrounding areas • Peak discharge rate from the site.
Accordingly, site-wide drainage systems are required to drain the foul and surface water flows arising from the proposed development. Where possible, any existing drainage networks should be utilised. Appropriate design and construction of these systems as set out in Section 4.3 should ensure that there is no increase in offsite flood risk that would otherwise impact downstream areas.
6.2 Existing Drainage Systems
As can be seen from the United Utilities map, attached in Appendix H, the only public sewer within the immediate vicinity of the site is the existing 300mm diameter combined water sewer which runs east to west through the north of the proposed development site, prior to discharging to the waste water treatment works, located to the west of the site.
It is envisaged that the existing 300mm diameter combined water sewer running through the site will be maintained with the required sewer easement put in place. Whilst no existing connections from the site to the adjacent public sewer or river are expected, the site should be surveyed and any existing connections located, should be surveyed to determine if they are suitable for re-use for the proposed development.
6.3 Proposed Drainage Strategy
As outlined above in section 6.1, site-wide drainage systems are required to drain the foul and surface water flows arising from the proposed development. The proposed drainage systems must ensure that there is no increase in offsite flood risk that would otherwise impact downstream areas.
Page 12 216216----232232 Bridge Mills, Tintwistle, Derbyshire 6.3.1 Surface Water Drainage The Building Regulations - Approved Document H (2002) details a hierarchy of potential methods for disposing of surface water as shown below in order of preference:
• Discharge via infiltration • Discharge to watercourse • Discharge to sewer
Considering the hierarchy above, the surface water network for the retained area of the existing site should infiltrate where possible.
As indicated in Section 2.1, the Tier Consult, Stage 1 Geo-environmental Appraisal for Land at New Road, Tintwistle Report, T/13/1256/GA, dated February 2015 indicates strata comprising of made ground and gravelly clay superficial deposits (river terrace) to depths of up to 11.45mbgl, underlain by a light brown/grey sandstone (Kinderscout Grit Sandstone).
The site is also highlighted on the Environment Agency maps as a landfill site and was registered as such from 1983 to 1989. Therefore, as a result of the underlying strata, it is envisaged that infiltration drainage is not a viable option for the discharge of surface water. This is due to the potential for inundation settlement and also leaching of contaminants through the made ground as a result of infiltration systems in made ground.
As the possibility of infiltration has been ruled out by the site investigation, the intention for the development would be to discharge to watercourse. As indicated in Section 5.1 the nearest watercourse is the River Etherow, which is located approximately 1.0m south of the site and lies approximately 5m – 6m below the site. Therefore, the intention would be for the development to discharge to watercourse.
The drainage systems should be designed to suit the proposed site layout and topography which aims to provide an efficient design.
In line with Sewers for Adoption (6th Edition), the requirements for the design of a new surface water drainage systems are as follows:
• Below ground piped drainage to be sized to accommodate the 1 in 2 year (50% AEP) design storm without surcharge. • System to be designed not to flood any part of the site in a 1 in 30 year (3% AEP) design storm. • For events in exceedance of the 1 in 30 year design storm and up to and including the 1 in 100 year event, site drainage and topography should be designed where practicable to route surface water run-off away from buildings to safe above-ground storage areas on site, thereby preventing this run-off from leaving the site and increasing flood risk elsewhere.
Page 13 216216----232232 Bridge Mills, Tintwistle, Derbyshire For each design case described above, the design storm is the critical storm duration for the site conditions. In the case of the 1 in 100 year design storm, a 30% increase in the peak rainfall intensity is applied to allow for the estimated worst case impacts of climate change. This is in accordance with Table 5 of the Technical Guidance to the National Planning Policy Framework.
Suitable systems of below ground drainage will be required to contain as a minimum requirement, the 1 in 30 year event. Additionally, surface water run-off from events that exceed the design capacity of the new drainage system, up to and including the 1 in 100 year (+30%) event, will be retained on-site in safe storage areas.
As there are minimal car parking areas or other areas which would contribute to a substantial risk of contamination, it is envisaged that no measures to prevent oil and other contaminants being passed forward to the existing watercourse are required. Therefore, no oil separators or other appropriate pre-treatment methods have been included within the design of the surface water drainage system.
It is proposed that the surface water discharge from the proposed development should be designed to mimic that from the existing site. Therefore, even though the site is brownfield, it is envisaged that the surface water discharge rate will be restricted to the existing greenfield runoff rate, as we are discharging to the adjacent watercourse.
An assessment of the existing surface water run-off from the site was completed using MicroDrainage. MicroDrainage uses the method outlined within the Institute of Hydrology Report 124 (IH124) ‘Flood Estimation for Small Catchments’ to calculate the greenfield runoff rate and uses a catchment area of 50ha, with the results interpolated to the actual area of the development site.
Therefore, for the existing 4.01ha site, the following existing discharge rates have been calculated;
Greenfield Runoff Allowable Discharge Return Period Rate for 50ha (L/s) Rate (L/s) 1 year 256.4 20.6
Qbar 294.7 23.6 30 year 499.7 40.1 100 year 613.1 49.2
The greenfield runoff assessment is attached in Appendix I. The calculated discharge rates above have been checked using the ICP SuDS method which is also available in MicroDrainge. This method uses the IH124 method and automatically interpolates the results for the identified site area. The check using the ICP SuDS method in MicroDrainage is also attached in Appendix I.
Page 14 216216----232232 Bridge Mills, Tintwistle, Derbyshire Using a post development impermeable area of 2.15ha and the restricted discharge rate of
23.6 L/s (Qbar) for the 1 in 100 year + 30% return period design storm, a storage volume of 1007.5m3 is required. The attenuation storage assessment is attached in Appendix J.
As can be seen from the drainage strategy drawing, attached in Appendix K, it is anticipated that the attenuation storage will be provided in the form of an attenuation basin, located in the south-west corner of the site and oversized pipes, located along the proposed access roads. It is envisaged that a vortex flow control device will be used to restrict the surface water discharge to the required rate.
This surface water drainage proposal and discharge rate should be confirmed via discussions with the Lead Local Flood Authority and United Utilities prior to the commencement of any works.
Due to the invert levels of the existing River Etherow, it is envisaged that the proposed surface water system will be discharged via gravity, with no need for pumping.
6.3.2 Foul Water Drainage Foul water drainage disposal is set out in Part H of the Building Regulations in order of priority the preferred methods are;
1. Public sewer 2. Septic tank 3. Cesspool.
The foul water system shall be designed in accordance with;
• BS EN 752:2008 (Drain and sewer systems outside buildings) • Sewers for Adoption (6th Edition) • Technical Guidance to the National Planning Policy Framework document (Department for Communities and Local Government, March 2012). • BS EN 12056-2:2000 (Drainage systems inside buildings) • Building Regulations Approved Document H, Drainage and waste disposal. (Office of the Deputy Prime Minister, December 2010).
As can be seen from the drainage strategy drawing, attached in Appendix K, it is envisaged that the foul water discharge from the proposed development will be designed to discharge to the existing 300mm diameter combined water sewer which runs east to west through the north of the proposed site, as this is the only realistic option of discharging the foul water runoff.
Again, the location and rate of discharge for the proposed foul sewer should be confirmed via discussions with the Lead Local Flood Authority and United Utilities prior to the commencement of any works.
Page 15 216216----232232 Bridge Mills, Tintwistle, Derbyshire Due to the anticipated invert levels of the existing United Utilities 300mm diameter combined water sewer running through the proposed site, it is envisaged that the proposed foul water system will be discharged via gravity, with no need for pumping.
6.4 Managing Residual Flood Risk
The mitigation measures identified previously, in Section 5.0 ensure that the site is actually located within Flood Zone 1 with only the adjacent river bank to the south of the site and the far western sections of the proposed Trans Pennine Trail at risk of flooding during the extreme 1 in 1000 year flood event.
As the site and adjacent highways to the east are located within Flood Zone 1, it is envisaged that should it be required access and egress will be achievable throughout the site and adjacent highways during the extreme 1 in 1000 year flood event.
There is a finite risk that the design flood conditions required by current design standards are exceeded by an extreme and rare event. However, with the design and mitigation measures reported such a risk is low and the exceedance flow path routes required to channel flows around the development buildings will be such that no additional flood risk mitigation measures are envisaged to be necessary.
7.0 CONCLUSIONS
• As a result of the comparison between the flood level information provided within the previous flood risk assessment report (Bridge Mills, Tintwistle, Flood Risk Assessment D/I/D/90486/01, August 2011 by Fairhurst), the minimum finished floor level is approximately 2.30m above the 1 in 1000 year flood level, therefore indicating that the proposed development is located within Flood Zone 1.
• The proposed Trans Pennine Trail is located to the south of the site and as a result of the comparison between the flood level information provided within the previous flood risk assessment report (Bridge Mills, Tintwistle, Flood Risk Assessment D/I/D/90486/01, August 2011 by Fairhurst), the minimum level is approximately 0.10m above the 1 in 1000 year flood level, therefore indicating that the proposed trail is located within Flood Zone 1. Although the minimum freeboard has not been achieved the trail is classified as a ‘water-compatible’ development and therefore in compliance with NPPF is suitable in this area.
• As the proposed development is located within Flood Zone 1 it has a low probability of flooding.
• The site is at a low risk of flooding from surface water.
• The site is at a low risk of flooding from other sources.
Page 16 216216----232232 Bridge Mills, Tintwistle, Derbyshire • The identified strata indicate that infiltration SUDS techniques are not feasible and therefore surface water discharge from the proposed site should discharge to the adjacent River Etherow, to the south of the site.
• Even though the site is brownfield, the surface water discharge is to be restricted to the greenfield runoff rate of 24.1 L/s, during the 100 year plus 30% climate change design storm. Discussions with the Lead Local Flood Authority and United Utilities should be completed to ensure agreement of this figure and the overall approach.
• Foul water discharge from the proposed site should discharge to the existing United Utilities 300mm diameter combined water sewer which runs east to west through the north of the proposed site. Discussions with the Lead Local Flood Authority and United Utilities should be completed to ensure agreement of the foul water discharge rate and the overall approach.
8.0 RECOMMENDATIONS
• An attenuation basin, oversized pipes and a flow control device should be utilised to ensure surface water flows are limited to the specified discharge rate.
• The external ground levels around proposed residential units shall fall away from the proposed residential buildings with any new levels being designed to ensure any overland flood routes, for events in excess of the 100 Year + 30% CC, exit towards the surrounding highways, for use in exceptional circumstances.
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Appendix A Site Location Plan
Page 18 Contract Number T/13/1256
Contract Land at New Road, Tintwistle
Client S.C.C. Ltd
Site Location Plan
THE SITE
Scale NTS Reproduced from the Ordnance Survey map with the permission of The Controller of Her Majesty’s Stationary Office, © Crown Copyright. All rights reserved. Drawn by AR Approved JB
Drawing Number T/13/1256/GA/1 216216----232232 Bridge Mills, Tintwistle, Derbyshire
Appendix B Aerial Photograph
Page 19
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Appendix C TTTopographicTopographic Survey
Page 20
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Appendix D Proposed Site Plan
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AAAppendixAppendix E Flood Risk MapMapssss & EA Correspondence
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John Speers
From: GMMC Info Requests
Dear John
Thank you for your email regarding flood risk assessment which was received on 8 December 2016.
Your new reference number is: GMMC 31701 ANB
The Freedom of Information Act and Environmental Information Regulations state that a public authority must respond to requests for information within 20 working days however we always aim to provide an answer as soon as we can.
I have spoken to Katie, as she advised I have asked the officer to prioritise your enquiry and we will get the data to you as soon as it is ready.
For further information on what you can expect from us and our full service commitment to you then please visit our website: http://www.environment-agency.gov.uk/aboutus/149626.aspx
If you need to contact me in the meantime, please do not hesitate to do so using the details below and quoting reference number GMMC 31701.
Yours sincerely,
Andrew Barkhouse Customer and Engagement Officer Greater Manchester, Merseyside and Cheshire Direct email: [email protected] Office address: Richard Fairclough House, Knutsford Road, Latchford, Warrington, WA4 1HT
Environmental Incident
If your email was to report an environmental incident i.e. pollution, fish in distress, dumping of hazardous waste etc then please call our freephone 24 hour Incident Hotline on 0800 80 70 60. For more details about incident reporting please see our website: https://www.gov.uk/report-an- environmental-incident
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Appendix F Hydraulic Modelling Sections Drawing
Page 23