Proposed Mixed Use Development Land at Newton on Trent, Lincolnshire Flood Risk Assessment For Furrowfresh
13480/FRA Original April 2014 Rev A May 2014 Rev B June 2015 Rev C September 2015
BSP Consulting
13480/FRA/Rev C
Land at Newton on Trent, Lincolnshire
Flood Risk Assessment
Flood Risk Assessment Land at Newton on Trent, Lincolnshire Prepared By:
Tony Goddard BEng (Hons) CEng MICE Associate Director
Checked By:
Matthew Viggars MEng (Hons) CEng MICE Chartered Civil Engineer
Revision: C Contents Executive Summary 1.0
Introduction
2.0
Background Information
3.0
Flood Risk Assessment
4.0
Conclusions and Recommendations
Appendix A
Detailed Site Location Plan
Appendix B
Topographical Survey
Appendix C
Proposed Site Layout
Appendix D
Correspondence Received
Appendix E
Surface Water Drainage Strategy
Appendix F
JBA Hydraulic Modelling Study
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Land at Newton on Trent, Lincolnshire
Flood Risk Assessment
Executive Summary Introduction
Existing Site Conditions
Development Description and Planning Context
Definition of Flood Hazard Probability
Climate Change
Detailed Development Proposals
Flood Risk Management Measures Off Site Impacts
BSP Consulting has been commissioned by Furrowfresh to undertake a Flood Risk Assessment for a proposed mixed use development at a site at Newton on Trent. This Flood Risk Assessment has been prepared in accordance with the Department for Communities and Local Government (DCLG) publication ‘Technical Guidance to the National Planning Policy Framework’. The site occupies an area of approximately 17.7 hectares and is located to the north of Newton on Trent. Open fields lie to the north, west and south of the site. Residential development lies to the east. The site is currently used for rearing livestock. The site falls from 8m AOD in the south west to 6m AOD in the east. A raised earth bund with a crest level at or about 8m AOD forms the western site boundary. The development proposals are for a mixed use residential, community and employment development together with associated hard and soft landscaping. In accordance with the National Planning Policy Framework, the proposed residential use falls under the more vulnerable category and the community/employment uses fall under the less vulnerable category in terms of flood risk. The River Trent is the primary source of flood risk for the locality of Newton on Trent. The EA flood mapping indicates that the west of the site is at risk in a 0.1% (1:1000 year - Flood Zone 2) probability flood event and that east of the site would be at risk of flooding in a 1.0% (1:100 year - Flood Zone 3a) probability flood event due to out of bank flooding from the River Trent. This map shows the indicative extent of the natural floodplain, if there were no flood defences or certain other manmade structures, such as surface water sewers, canals and channel improvements. Land to the west of the site that is served by the Fenton Marsh Drain is noted to be functional floodplain. Further detailed hydraulic modelling has been undertaken to assess the probability of flooding at this site. This includes a re-evaluation of the EA model using detailed topographical survey data, breach assessments and a land raising assessment. For development the critical flood event is noted to be the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event. The post development key event modelling indicates flood levels at the site varying from 7.91m AOD in the west to 6.76m AOD in the east. The 1:1000 year overtopping modelling indicates flood levels at the site varying from 8.10m AOD in the west to 7.66m AOD in the east. The implications of climate change have taken into account in the discussions on probability. The recent detailed modelling of the local catchment area accurately quantifies the potential effects of climate change for the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event. The National Planning Policy Framework states that developments of a more vulnerable category, such as the proposed residential use, are appropriate within Flood Zone 3a subject to the sequential and exception tests being passed by the Local Planning Authority. Developments of a less vulnerable category, such as the proposed community and industry uses, are appropriate within Flood Zone 3a subject to the sequential test being passed by the Local Planning Authority. Mitigation measures in relation to site development levels, flood resilience, SuDS drainage, flood warning and evacuation are proposed to be employed at this site. The proposals will incorporate attenuation and SuDS principles therefore there will be a reduction in the rate of surface water leaving the site as a result of the development proposals. This will therefore reduce the risk of flooding to others as a result of the development proposals. The potential adverse effects of the proposed land raising have been assessed and through detailed hydraulic modelling the proposals have been refined to ensure that there is no detrimental impact on flood depths to third party land. 3
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Flood Risk Assessment
Residual Risks
Conclusions and Recommendations
The investigations carried out as part of this flood risk assessment and flood risk management measures proposed have demonstrated that the development will be safe, without increasing flood risk elsewhere. The following conclusions have been drawn and recommendations are made to ensure flood risk at this site is minimised: The site is noted to be at risk of flooding from the River Trent as some flows are able to flow around or overtop the existing flood defences in extreme flood events. Land raising is necessary at this site to provide a reasonable standard of protection to the proposed dwellings. It is a key constraint to the proposed development that the modelling confirms that there would be no change in flood depths to third party land following land raising for the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event. This has been proven for the proposed development extent. Additional breach and overtopping modelling has been undertaken to inform the proposed development levels. The minimum finished floor level of the proposed dwellings is proposed to be raised 300mm above the modelled 1:100 year plus climate change breach level or above the modelled 1:1000 year overtopping flood level; whichever is the highest. On the basis of the investigations undertaken to date it is clear that some infiltration potential is present but the rate of infiltration may either be too slow or insufficiently reliable to allow infiltration drainage to be promoted as the primary means of surface water drainage for this site. Offsite discharge of surface water runoff to the IDB drains to the east of the site will be restricted to a greenfield runoff rate of 1.4 l/s/ha. This equates to an overall site runoff rate of 24.8 l/s. Liaison has been undertaken with LCC Highways, as LLFA, to discuss options for the development of adoptable SuDS as source control features and this is ongoing. The aim of this is to promote an integrated SuDS scheme across the site and a comprehensive treatment train for surface water runoff. The master plan included in Appendix C has been developed on the basis of our investigations, analysis and liaison. This has ensured that sufficient space has been allocated to include the above noted SuDS features. This strategy will be developed in further detail at the detailed design stage. A safe access/egress route has been proven for this site by the use of flood hazard mapping. A Flood Emergency Plan should be drawn up in parallel with the detailed design to confirm a practicable flood emergency strategy for this proposed development.
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Flood Risk Assessment
1.0
INTRODUCTION
1.1
TERMS OF REFERENCE
1.1.1
BSP Consulting has been commissioned by Furrowfresh to undertake a Flood Risk Assessment for a proposed mixed use development at a site at Newton on Trent.
1.1.2
In the preparation of this report, consultations have been undertaken with the Environment Agency (EA), Trent Valley Internal Drainage Board (TVIDB) and Anglian Water Services (AWS). A site visit was made in November 2013, to walk the local area, to assess the local topography, significant watercourses and current site use. The West Lindsey District Council SFRA – July 2009 by AECOM has also been referred to.
1.1.3
This Flood Risk Assessment has been prepared in accordance with the Department for Communities and Local Government (DCLG) publication ‘Technical Guidance to the National Planning Policy Framework’ while retaining the layout as recommended in the superseded ‘Planning Policy Statement 25 (PPS25) Development and Flood Risk’.
1.1.4
This report has been produced on behalf of the client, Furrowfresh, and no responsibility is accepted to any third party for all or any part. This report should not be relied upon or transferred to any other parties without the express written authorisation of BSP Consulting. If any unauthorised third party comes into possession of this report, they rely on it at their own risk and the authors owe them no duty of care or skill.
1.2
NATIONAL PLANNING POLICY FRAMEWORK
1.2.1
The National Planning Policy Framework was published on 27 March 2012. This replaced Planning Policy Statement 25: Development and Flood Risk.
1.2.2
Technical Guidance to the National Planning Policy Framework has been published and this site specific FRA is written in compliance with this new guidance.
1.2.3
The National Planning Policy Framework, and supporting technical guidance, can be downloaded free of charge from the internet at the following link: http://www.communities.gov.uk/publications/planningandbuilding/nppf
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2.0
BACKGROUND INFORMATION
2.1
SITE LOCATION, DESCRIPTION AND DETAILS
2.1.1
Figure 2.1 below indicates the location of the site; a more detailed site location is included in Appendix A. A site visit was made in November 2013, to walk the local area, to assess the local topography, significant watercourses and current site use.
Dunham Bridge
Plan reproduced by kind permission of the TVIDB
Figure 2.1 Newton on Trent - Site Location and Catchment Plan 2.1.2
The approximate site boundary is hatched and outlined in red on Figure 2.1 above. The site occupies an area of approximately 17.7 hectares and is located to the north of Newton on Trent at OSNGR 483000E, 374700N. The Newton Sewer runs west to east (piped) along the southern boundary of the site. The River Trent flows 6
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Flood Risk Assessment
south to north 600m west of the site. Open fields lie to the north, west and south of the site. Residential development lies to the east. The site is currently used for rearing livestock. 2.1.3
Photographs of the site and local area are included below as Figures 2.2 to 2.7.
2.1.4
Site and Local Area Photographs
Figure 2.2 Western site boundary with raised bund to LHS. (Looking north)
Figure 2.3 Looking west from the west of the site towards the Fenton Marsh Drain.
Figure 2.4 Minor drainage ditch at the east of the site.
Figure 2.5 Torksey Main Drain near Newton on Trent. (Looking north)
Figure 2.6 River Trent right bank flood defence near Dunham Bridge. (Lkg north)
Figure 2.7 Torksey Main Drain Pumping Station near Torksey Lock. 7
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Flood Risk Assessment
2.1.5
The topographical survey of the site in Appendix B indicates that the site falls from 8m AOD in the south west to 6m AOD in the east. There are undulations in levels between these points but these levels describe the general trend. A raised earth bund forms the western site boundary. This raised bund has a crest level at or about 8m AOD.
2.1.6
Observations made by studying local mapping data and while walking the local area confirm that the site lies in a gently sloping area with a gradual fall from west to east towards the Torksey Main Drain. The western boundary lies along a high point in the local topography from which levels fall towards the Fenton Marsh Drain which lies about 330m to the west of the site. Levels rise up again from the Fenton Marsh Drain towards the EA maintained River Trent right bank defences 600m to the west of the site. The watershed lines are indicated in black on Figure 2.1 above.
2.1.7
Overall Catchment Context and Local Watercourse Classifications River Trent:
Tidal, Main River which flows from south to north 600m to the west of the site.
Fenton Marsh Drain:
TVIDB maintained watercourse which flows from south to north 330m to the west of the site and has a pumped outfall to the River Trent at Torksey Lock.
Torksey Main Drain:
TVIDB maintained watercourse which flows from south to north 1000m to the east of the site and has a pumped outfall to the River Trent at Torksey Lock.
Newton Sewer:
Minor TVIDB maintained watercourse which flows from west to east directly to the south of the site and has a gravity outfall to the Torksey Main Drain.
The locations of the above watercourses and features in relation to the site are shown on Figure 2.1. 2.2
APPROACH TO THE ASSESSMENT
2.2.1
The EA have provided modelled river flood levels for the River Trent in the reach local to the site. They have also provided localised flood mapping data. The TVIDB have provided detailed mapping and a commentary regarding their land drainage network. AWS have provided us with details of their current flood defence construction works. This assessment seeks to draw together the relevant level information from the EA and to collate this with the findings of our investigations, modelling and discussions to assess the flood risk at this site.
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3.0
FLOOD RISK ASSESSMENT
3.1
DEVELOPMENT DESCRIPTION AND PLANNING CONTEXT
3.1.1
The development proposals are for a mixed use residential, community and employment development together with associated hard and soft landscaping. A masterplan layout proposal is included as Appendix C.
3.1.2
The local area benefits from a Strategic Flood Risk Assessment. This assessment is the West Lindsey District Council SFRA – July 2009 which was undertaken by AECOM for West Lindsey District Council.
3.1.3
In accordance with the National Planning Policy Framework, the proposed residential use falls under the more vulnerable category and the community/employment uses fall under the less vulnerable category in terms of flood risk.
3.2
DEFINITION OF THE FLOOD HAZARD
3.2.1
The potential sources of flooding in the vicinity of the site are as detailed below:
3.2.2
Fluvial Flood Risk
3.2.3
The River Trent is the primary source of flood risk for the locality of Newton on Trent. The EA have provided us with updated modelled flood levels and flood extents for the River Trent and this information is reproduced in Appendix D.
3.2.4
The River Trent flows from south to north 600m to the west of the site. The River Trent was modelled by Mott MacDonald for the EA in 2013/2014 in the Tidal Trent SFRM. This data includes for the updated 2014 interim water levels.
3.2.5
The EA have also provided us with historical flood mapping. A copy of this mapping is included below in Figure 3.1 and in Appendix D. This mapping indicates flooding of the locality of Newton on Trent and the site in 1947 when the River Trent overtopped its banks. The site was not affected by flooding in the other listed flooding events in 1932, 1977, 2000, 2012 and 2013 flood events. This is particularly significant as the EA have recently noted that in 2013 the levels recorded on the Humber and Trent were the highest on record; however the site was not affected by flooding.
3.2.6
The SFRA confirms that “In 1947 fluvial flooding from the River Trent caused extensive flooding throughout West Lindsey including areas of Wildsworth, East Stockwith, Laughterton and Newton on Trent. There was overtopping of the Trent floodbank in the Torksey area causing parts of the village to be flooded.”
3.2.7
The area local to Dunham Bridge, to the south west of the site, was flooded in December 2012 making the A57 impassable for a few days. The site was not affected by flooding in this event.
3.2.8
The EA have also provided us with a copy of their flood zone mapping which indicates that the site is at risk of flooding from the River Trent. A copy of this 9
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mapping is included below in Figure 3.2 and in Appendix D. The potential for flood risk from this source is discussed in more detail in section 3.3 Probability.
Site Location
Figure 3.1 Newton on Trent – Historic Flooding, from EA Data
Site Location
Figure 3.2 Newton on Trent – Flood Zone Map, from EA Data 3.2.9
Land drainage watercourses run from south to north both to the west and east of the site. These drains are both maintained by the TVIDB and are both pumped by 10
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Flood Risk Assessment
separate pumping stations to the River Trent at Torksey Lock, to the north of the site. 3.2.10
These watercourses serve to drain the local area that lies within the natural floodplain of the River Trent. As such these drains actually reduce the risk of flooding at the site.
3.2.11
The function of these drains and the lines of flood defences are discussed further in section 3.3 Probability.
3.2.12
Tidal Flood Risk
3.2.13
The tidal flood risk at this site is in relation to the River Trent and hence it is identified above in the section on fluvial flood risk as the fluvial flow dominates.
3.2.14
Surface Water Flood Risk
3.2.15
The site is located in the valley floor of the River Trent with only gentle falls towards the IDB maintained drains.
3.2.16
The EA have provided us with copies of their surface water flood mapping and this is included in Appendix D. This mapping indicates that the site is at low risk of surface water flooding. No significant flood flows paths are indicated across the site area and only small shallow isolated ponding is indicated. This is in line with the existing topography.
3.2.17
The site is therefore not at significant risk of flooding from surface water.
3.2.18
Flood Risk from Ground Water
3.2.19
We have not been able to locate any records of significant springs or elevated groundwater in this locality. A Phase 1 and limited Phase 2 Site Investigation has been undertaken at this site. The trial pit logs and a plan indicating the depth from ground level to water are included in Appendix E. Water was found at depths in excess of 1.2m below existing ground levels, with no water strike at one of the pit locations. There are no surface indications of flooding from groundwater such as boggy ground, or springs, on the site. The site is therefore not at significant risk of flooding from ground water.
3.2.20
Flood Risk from Sewers and Infrastructure
3.2.21
The sewer records included in Appendix D confirm that there are no significant adopted sewers in the locality of the site.
3.2.22
The flood risk associated with reservoir flooding is much lower than the flood risk associated with the fluvial/tidal influence of the River Trent.
3.2.23
The site is not downstream of any significant wet process industrial works. The sewers and infrastructure flood risk source can therefore be discounted as an unacceptable source of flood risk to the site.
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3.3
PROBABILITY
3.3.1
The EA have provided an extract of their flood map for the local area and a copy of this is included above in Figure 3.2 and within Appendix D. This indicates that the west of the site is at risk in a 0.1% (1:1000 year - Flood Zone 2) probability flood event and that east of the site would be at risk of flooding in a 1.0% (1:100 year - Flood Zone 3a) probability flood event due to out of bank flooding from the River Trent. This map shows the indicative extent of the natural floodplain, if there were no flood defences or certain other manmade structures, such as surface water sewers, canals and channel improvements.
Figure 3.3 SFRA Indicative Climate Change Flood Map 3.3.2
An assessment of the potential implications of climate change was undertaken for the SFRA and a copy of the relevant mapping is included above in Figure 3.3. This mapping indicates that the west of the site at risk of flooding in a 1:1000 year plus climate change flood event and that the east of the site is at risk of flooding in a 1:200 year plus climate change flood event. This flooding appears to be associated with back flooding of the Torksey Main Drain catchment due to overtopping of the River Trent flood defences at Torksey Lock. This mapping is to be treated with some caution as it is based on assumptions rather than a climate change modelling exercise. The SFRA notes “For this SFRA it has been agreed with the Environment Agency that, taking a precautionary approach, in order to indicate the potential effect of climate change on the extent of the FZs, the planning authority should use the present day Flood Zone 2 envelope as an indication of the
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possible extent of Flood Zone 3 in 100 years time given the current predictions for climate change.” 3.3.3
The SFRA identifies the land to the west of the site that is served by the Fenton Marsh Drain as being functional floodplain. The EA note that the defence line along the right bank of the River Trent to the north of Dunham Bridge to have a design standard of 10 years. The defence line immediately to the west of the site has a design standard of 100 years. This assertion appears to be reasonable.
3.3.4
The EA have also provided us with modelled 1:100 year, 1:100 year plus climate change and 1:1000 year flood extents. This modelling indicates that the site is affected by flooding in all these modelled flood events as some flows are able to flow around or overtop the flood defences. JBA have been engaged on this project to add detail to the existing EA model and to consider how the site can be elevated to reduce flood risk while not increasing the risk of flooding to others. This modelling supersedes the SFRA mapping exercise.
3.3.5
The modelling work undertaken by JBA is included as Appendix F. This modelling includes a re-evaluation of the EA model using detailed topographical survey data, breach assessments and a land raising assessment. For development the critical flood event is noted to be the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event. The baseline scenario mapping is indicated below in Figure 3.4.
Site Location
Figure 3.4 1:100 plus Climate Change Baseline Mapping 13
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3.3.6
During the baseline modelling exercise it was noted that there is a flood water flow conveyance route at the east of the development site. This flood flow prevents flood levels rising significantly within Newton on Trent. As such in consideration of the potential adverse impact of land raising at this site it is a key constraint of the modelling that there should be no change in flood depths to third party land following land raising.
3.3.7
Working to this criteria meant that there was an iterative process to determine a “development extent” that could be raised safely. The final results indicate that the overland flow route along the eastern boundary of the site, running from southwest to north-east, has been maintained and there has been no detrimental impact on flood depths to third party land. This has been checked to a tolerance of +or30mm to allow for modelling inaccuracy. Figure 3.5 below indicates the final development extent.
Figure 3.5 Impact to Flood Depths Following Land Raising 3.3.8
Ground levels should only be raised within the boundary extent shown. Levels east of this proposed development extent should not be altered in order to maintain the existing overland flow route.
3.3.9
Breach modelling for the post development scenario has been undertaken to inform the process of assessing development levels. Breaches of the defences 14
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alongside the River Trent were found not to affect the site. Breaches of the defences adjacent the west of the site give flood levels higher than the baseline model. The peak water levels from the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event are indicated below in Figure 3.6.
Figure 3.6 Peak Water Levels from All Breach Scenarios 3.3.10
Overtopping of the flood defences in a 1:1000 year fluvial with 1:5 year tidal event flood has also been modelled to assess the impact of this event at the post development site. In this most onerous event the defences are overtopped and the land to the rear of the defences is significantly inundated. The flooded depths around the site have been modelled and the peak flood depths at the site are noted on Figure 3.7 below.
3.3.11
It is therefore concluded that the site is at risk of flooding from the River Trent in its current state. The site is defended and the defences offer a reasonable standard of protection in the present day. Firstly the site is defended by the flood defence along the west of the site. Secondly the TVIDB drains and functional floodplain to the west of the site act as a buffer to reduce the impact of any flooding at the site. It is however noted that additional protection in the form of land raising is required to provide an acceptable standard of protection for the proposed development. This is detailed further in Section 3.6 below.
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Figure 3.7 Peak Water Levels from 1:1000 Year Overtopping Scenario 3.4
CLIMATE CHANGE
3.4.1
Climate change is recognised as a factor for consideration in terms of its effects on flood risk. This issue affects the local catchment from fluvial sources in as much as fluvial flood flows are estimated to increase by up to 20% over the next 100 years.
3.4.2
The implications of climate change have taken into account in the discussions on probability noted above. The recent detailed modelling of the local catchment area, as included above, accurately quantifies the potential effects of climate change for the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event.
3.5
DETAILED DEVELOPMENT PROPOSALS
3.5.1
The proposed development is discussed in Section 3.1 above. A masterplan layout proposal is included as Appendix C.
3.5.2
The National Planning Policy Framework states that developments of a more vulnerable category, such as the proposed residential use, are appropriate within Flood Zone 3a subject to the sequential and exception tests being passed by the Local Planning Authority. Developments of a less vulnerable category, such as the proposed community and industry uses, are appropriate within Flood Zone 3a subject to the sequential test being passed by the Local Planning Authority. 16
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3.6
FLOOD RISK MANAGEMENT MEASURES
3.6.1
Existing and Proposed Flood Defences
3.6.1.1 The EA have confirmed the extent of their flood defences in the locality as part of the data request. The arrangement of the existing flood defences is indicated below in magenta on Figure 3.8. AWS have confirmed that they are constructing new defences around their water treatment works to a level of 8.67m AOD to protect against the 1:100 year plus climate change flood. They will also be raising a section of existing bund to fill in a low spot in the existing defence line. Once this is undertaken the defences protecting Newton on Trent will be continuous and at or about the 1:100 year flood level. 3.6.1.2 We are also aware that LCC Highways are initiating further flood defence works to mitigate flood risk associated with the A57 to the east of Dunham Bridge. Analysis and assessment of this defence proposal is ongoing.
Figure 3.8 Local Existing and Proposed Flood Defence Map 3.6.2
Proposed Site Development and Finished Floor Levels
3.6.2.1 The analysis undertaken in Section 3.3 Probability has confirmed that it is necessary to undertake land raising at this site to protect it from flood risk from the River Trent. It has also been demonstrated that this can be undertaken for the proposed development extent without increasing the flood risk to third party land.
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3.6.2.2 As the site lies in a defended area there is always a residual risk of defence overtopping or defence breach. As such it is prudent to set the proposed dwelling floor levels in relation to the modelled breach or overtopping flood levels. The minimum finished floor level of the proposed dwellings is therefore proposed to be raised 300mm above the modelled 1:100 year plus climate change breach level or above the modelled 1:1000 year overtopping flood level; whichever is the highest. 3.6.2.3 The land at the west of the site is noted to be between 7m and 8m AOD and raising this land above the level of the defences is viable. This area of higher ground would then provide protection to the east of the site against flooding from the west. At the east of the site the village hub is a less vulnerable use and it is therefore proposed that the minimum finished floor level of the proposed buildings is therefore proposed to be raised 300mm above the modelled 1:100 year plus climate change breach level. This will allow the village hub to relate better with the existing levels and village to the south. 3.6.2.4 On the basis of the above levels strategy and the peak flood levels indicated on Figures 3.6 and 3.7 above it is proposed that the site levels are graded across the site in line with the following levels strategy sketch plan, Figure 3.9. This meets the criteria set out in 3.6.2.2/3 above. Ground levels within the development extent will be set at 150mm below floor levels at plots and generally no lower than 300mm below minimum floor levels to give safe access and egress.
Figure 3.9 Proposed Development Finished Floor Levels 3.6.2.5 The ground levels of the landscaped areas to the north and south of the site will grade down to adjacent ground levels subject to drainage and specific local 18
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constraints. The land at the east of the site, outside of the “development extent” will remain at or about current ground levels to maintain the modelled flood conveyance pathway. 3.6.2.6 Precise detailing of the site levels will be developed and agreed at the detailed design stage. 3.6.3
Flood Resilient Construction
3.6.3.1 Flood Resilient Construction techniques could be utilised, as deemed appropriate, at this development to supplement the mitigation provided by careful setting of development levels. This should be undertaken in accordance with the CLG publication Improving the Flood Performance of New Buildings. Flood Resilient Construction. 3.6.4
Sustainable Site Drainage Systems
3.6.4.1 To aid the early stage SuDS assessment a preliminary Site Drainage Evaluation has been undertaken in accordance with the recommendations in the SuDS Manual. A copy of the evaluation report is included in Appendix E. It is noted from the generalised findings of the site investigation undertaken to date that the underlying soils will be of low permeability. 3.6.4.2 Based on the preliminary Site Drainage Evaluation it would appear that SuDS features that would appear reasonable for use at this site could be as follows: Rainwater Harvesting to reduce pass forward flow volume. Green roofs (subject to development/planning constraints) to treat runoff, slow flows and aid evapotranspiration. Pervious Pavement to attenuate flows and provide water quality improvement. Swales to act as attenuation and flow conveyance features and to improve water quality. Proprietary Treatment Systems to remove silts and floatable pollutants and to protect downstream SuDS features. Channels & rills can be used for conveyance in the more dense built up areas of the site where swales are inappropriate. Retention or detention basins/wetlands to be used as site control features to improve water quality prior to offsite discharge and to provide the final stage of attenuation back to the required discharge rate. 3.6.4.3 A full Phase 1 and limited scope Phase 2 site investigation has been undertaken for this site. The local geology has been assessed and the infiltration potential of the soils has been considered. 3.6.4.4 The soils encountered at the site ranged from sands and gravel to clays and mercia mudstone. It is reported that “Where the Holme Pierrepont Sand and Gravel or weathered Mercia Mudstone Clay was penetrated (TP1, TP2 and TP3), a very weak red-brown blocky MUDSTONE was encountered. This was proven to a maximum depth of 2.90m begl (TP3) but not penetrated.”
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Flood Risk Assessment
3.6.4.5 The Site Investigation report confirms that “A programme of soakaway testing should be undertaken at the site to assess the suitability for the discharge of surface waters via soakaways. However, the presence of the basal Mudstone will limit the effective depth of soakaways together with the presence of near surface water locally at the site.” 3.6.4.6 On the basis of the investigations undertaken to date it is clear that some infiltration potential is present but the rate of infiltration may either be too slow or insufficiently reliable to allow infiltration drainage to be promoted as the primary means of surface water drainage for this site. 3.6.4.7 The existing site can be seen to be greenfield with runoff currently tending towards the TVIDB drainage network of the Torksey Main Drain via the Newton Sewer. Ordinarily we would undertake an assessment of the greenfield runoff regime using the rural runoff SuDS calculation. However in this case the TVIDB have confirmed that the site discharge of surface water to the IDB drains to the east of the site should be restricted to a greenfield runoff rate of 1.4 l/s/ha. This equates to an overall site runoff rate of 24.8 l/s. 3.6.4.8 A preliminary surface water attenuation design has been undertaken to ensure the feasibility and deliverability of the proposed development. The proposed site control attenuation facilities balance the surface water runoff from the proposed impermeable development areas back to the runoff rate indicated above prior to discharge to the IDB drains to the east of the site. Above ground attenuation ponds are to be utilised as the site control SuDS feature ensuring that the required restriction rate can be achieved. This also provides one treatment train for water quality improvement. 3.6.4.9 A preliminary drainage strategy layout including the above noted features is included in Appendix E. The inverts of the attenuation ponds are sufficiently higher than the IDB drainage therefore discharges to this watercourse can be effected under gravity. In the unlikely event of a blockage of the outfall pipework an exceedance pathway exists towards the IDB drainage without affecting any onsite or adjacent properties. Detailed calculations are included in Appendix E indicating that the discharge rate in the critical 1 in 100 year plus 30% climate change is limited to less than 24.8 l/s. 3.6.4.10 Liaison has been undertaken with LCC Highways, as LLFA, to discuss options for the development of adoptable SuDS as source control features and this is ongoing. The aim of this is to promote an integrated SuDS scheme across the site and a comprehensive treatment train for surface water runoff. 3.6.4.11 The master plan included in Appendix C has been developed on the basis of our investigations, analysis and liaison. This has ensured that sufficient space has been allocated to include the above noted SuDS features. This strategy will be developed in further detail at the detailed design stage. 3.6.5
Flood Warning and Evacuation
3.6.5.1 The site lies close to but not within a flood warning area, as indicated below in Figure 3.10. The flood warning area to the east of the site is named River Trent at 20
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Flood Risk Assessment
Torksey including Marton and Newton. The prospective occupants should register to receive flood warnings. This is an elective scheme that requires occupants to register to receive automated warnings in times of heightened flood risk. This flood warning area is related to the wider area of the River Trent from Cromwell Weir to Gainsborough. Residents should call Floodline on 0345 988 1188, select option 1 and enter Quickdial number 05222132 to get more information. 3.6.5.2 Evacuation to areas in Flood Zone 1 on receipt of a severe flood warning must be a consideration in any Flood Emergency Plan adopted for this proposed development.
Site Location
Figure 3.10 River Trent at Torksey including Marton and Newton – EA Flood Warning Area Mapping 3.6.6
Access/Egress Arrangements
3.6.6.1 In times of flood the highest land in the locality is to be found along the line of the flood defences to the south of the site and along the line of the A57 heading east. This line of higher land provides a safe egress route as annotated on the Flood Hazard Mapping included below as Figure 3.11. The hazard rating along this route is noted to be “very low”. 3.7
OFF SITE IMPACTS
3.7.1
The proposed development surface water discharge rate will be attenuated back to less than the greenfield runoff rates in the 1:100 year plus 30% climate change rainfall event therefore the proposed development will not increase or exacerbate any flooding problems associated with rainfall runoff adjacent to or downstream of the site for the proposed scheme lifetime.
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Flood Risk Assessment
3.7.2
The proposals will incorporate attenuation and SuDS principles therefore there will be a reduction in the rate of surface water leaving the site as a result of the development proposals. This will therefore reduce the risk of flooding to others as a result of the development proposals.
3.7.3
The potential adverse effects of the proposed land raising have been assessed and through detailed hydraulic modelling the proposals have been refined to ensure that there is no detrimental impact on flood depths to third party land.
Figure 3.11 1:100 plus Climate Change Fluvial Event Combined With the 1:5 Year Tidal Event - Hazard to People Classification Mapping 3.8
RESIDUAL RISKS
3.8.1
The residual risks associated with the potential sources of flood risk have been discussed and mitigation proposed as previously noted.
3.8.2
The investigations carried out as part of this flood risk assessment and flood risk management measures proposed have demonstrated that the development will be safe, without increasing flood risk elsewhere.
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Flood Risk Assessment
4.0
CONCLUSIONS AND RECOMMENDATIONS
4.1
The following conclusions have been drawn and recommendations are made to ensure flood risk at this site is minimised: The site is noted to be at risk of flooding from the River Trent as some flows are able to flow around or overtop the existing flood defences in extreme flood events. Land raising is necessary at this site to provide a reasonable standard of protection to the proposed dwellings. It is a key constraint to the proposed development that the modelling confirms that there would be no change in flood depths to third party land following land raising for the 1:100 plus climate change fluvial event combined with the 1:5 year tidal event. This has been proven for the proposed development extent. Additional breach and overtopping modelling has been undertaken to inform the proposed development levels. The minimum finished floor level of the proposed dwellings is proposed to be raised 300mm above the modelled 1:100 year plus climate change breach level or above the modelled 1:1000 year overtopping flood level; whichever is the highest. On the basis of the investigations undertaken to date it is clear that some infiltration potential is present but the rate of infiltration may either be too slow or insufficiently reliable to allow infiltration drainage to be promoted as the primary means of surface water drainage for this site. Offsite discharge of surface water runoff to the IDB drains to the east of the site will be restricted to a greenfield runoff rate of 1.4 l/s/ha. This equates to an overall site runoff rate of 24.8 l/s. Liaison has been undertaken with LCC Highways, as LLFA, to discuss options for the development of adoptable SuDS as source control features and this is ongoing. The aim of this is to promote an integrated SuDS scheme across the site and a comprehensive treatment train for surface water runoff. The master plan included in Appendix C has been developed on the basis of our investigations, analysis and liaison. This has ensured that sufficient space has been allocated to include the above noted SuDS features. This strategy will be developed in further detail at the detailed design stage. A safe access/egress route has been proven for this site by the use of flood hazard mapping. A Flood Emergency Plan should be drawn up in parallel with the detailed design to confirm a practicable flood emergency strategy for this proposed development.
Disclaimer We would note that all comments made in this report are based on the sources stated in Section 1.1. This report and its recommendations are intended for the use of Furrowfresh for the above site only.
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Appendix A Detailed Site Location Plan
Newton on Trent – Detailed Site Location Plan
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Land at Newton on Trent, Lincolnshire
Flood Risk Assessment
Appendix B Topographical Survey
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Land at Newton on Trent, Lincolnshire
Flood Risk Assessment
Appendix C Proposed Site Layout
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Appendix D Correspondence Received
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Land at Newton on Trent, Lincolnshire
Flood Risk Assessment
Appendix E Surface Water Drainage Strategy
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Land at Newton on Trent, Lincolnshire
Flood Risk Assessment
Appendix F JBA Hydraulic Modelling Study
