Land at Bragbury End - Flood Risk Assessment by James Bompas

Document type Report Date July 2023

BRAGBURY END, STEVENAGE

FLOOD RISK APPRAISAL

BRAGBURY END, STEVENAGE FLOOD RISK APPRAISAL

Project name Bragbury End Stevenage Flood Risk Appraisal

Project no. 1620015847

Recipient RPF Developments

Document type Report

Version 1.1

Date 10/07/23

Prepared by T Cusworth

Checked by T Hillas

Approved by A Guay

Document no. 1620015847-RAM-RP-FRA-00001

Revision Date Prepared by Checked by Approved by Description

1.0 15/05/2023 TC TH AG Draft 1.1 10/07/2023 TH Final

https://uk.ramboll.com

This report is produced by Ramboll at the request of the client for the purposes detailed herein. This report and accompanying documents are intended solely for the use and benefit of the client for this purpose only and may not be used by or disclosed to, in whole or in part, any other person without the express written consent of Ramboll. Ramboll neither owes nor accepts any duty to any third party and shall not be liable for any loss, damage or expense of whatsoever nature which is caused by their reliance on the information contained in this report.

Ramboll UK Limited

Registered in England & Wales Company No: 03659970 Registered office: 240 Blackfriars Road London SE1 8NW

1. INTRODUCTION

1.1

Background

Ramboll UK Ltd (Ramboll) is commissioned by RPF Developments to provide a flood risk appraisal for land off the A602, Stevenage, SG14 3RY. The site is bordered by Stevenage Brook, a tributary of the River Beane.

A Site Redline Boundary Plan is included in Appendix 1.

1.2 Objectives

The purpose of this review is to undertake an appraisal of flood risk to the site from various sources, and in particular fluvial flooding given the proximity of the site to Stevenage Brook. The appraisal will be used to update the findings of previous report completed by Ramboll in 2013. The principle aims of the project are to provide the following:

• Updated appraisal of fluvial flood risk, based on up-to-date hydraulic modelling results;

• Develop an outline strategy identifying any mitigation measures required to manage future flood risk; and

• Outline a drainage strategy to detail a suitable approach to managing surface water.

1.3 Ramboll and Climate Change

Ramboll UK Ltd is a Partner for Sustainable Change, and as such sustainability is central to our assessments and reporting. We have made specific considerations for climate change throughout this report, to ensure that our planning and design advice is supportive of an approach to ensure robust and sustainable societies.

More information on our company-wide strategy and our commitment to being a Partner for Sustainable Change can be read here https://ramboll.com/strategy-2022.

1.4 Limitations

This report has been prepared by Ramboll exclusively for the intended use by the client in accordance with the fee proposal document dated December 2022 between Ramboll and the client defining, among other things, the purpose, the scope and the terms and conditions for the services. No other warranty, expressed or implied, is made as to the professional advice included in this report or in respect of any matters outside the agreed scope of the services or the purpose for which the report and the associated agreed scope were intended or any other services provided by Ramboll.

Ramboll’s services are not intended as legal advice, nor an exhaustive review of site conditions and/or compliance. This report and accompanying documents are initial and intended solely for the use and benefit of the client and may not be used by or disclosed to, in whole or in part, any other person without the express written consent of Ramboll. Ramboll neither owes nor accepts any duty to any third party, unless formally agreed by Ramboll through that party entering into, at Ramboll’s sole discretion, a written reliance agreement.

2. SITE DESCRIPTION

2.1 Application Site Description

The Site comprises an irregular shaped parcel of land that covers an area of approximately 8.26 hectares (ha). The site is located to the north of the A602, approximately 4.5 km to the southeast of Stevenage town centre, and is bordered by Stevenage Brook.

2.2 Existing Topography

A topographic survey was provided to Ramboll as part of the study which covers the proposed site area. This has been used alongside publicly available LiDAR data (Light Detection and Ranging Data) to provide the description of the site topography. Both the topographic survey and LiDAR Plan are included in Appendix 2

The site appears to have previously undergone cut and fill to provide levelling of the site into a series of plateaus, which have in turn been used as sports pitches. Within the developable area of the site to the east, two plateaus are observable, which are located between 71.5 and 72.5 and 70.0 to 68.0 mAOD for the west and east plateau respectively Aside from the broadly level plateaus, land falls away steeply to the north east and west towards Stevenage Brook (to around 66.0 mAOD adjacent to the Brook), and climbs steeply to the north towards the A602 to a level of around 75 mAOD.

2.3 Existing Drainage

No public or private sewer records we requested as part of this study, and instead the presence of existing drainage has been inferred from a) the presence of impermeable/built surfaces requiring positive drainage b) the presence of any above ground infrastructure that may be indicative of below ground drainage i.e. inspection chambers, gulleys etc and c) the presence of any buildings that may be served by foul drainage.

There are currently no buildings situated within the site boundary, and the land surface is entirely covered by undeveloped, permeable surfaces. It is therefore concluded that the site is not currently served by either a foul or a surface water network. There may however be existing land drains/previous drainage improvements made to mitigate waterlogging of sports pitches.

2.4 Geological Setting

The Site is shown on the British Geological Society (BGS) online GeoIndex to be underlain by superficial deposits of Alluvium (clay, silt, sand and gravel) which extend along the course of Stevenage Brook. This is designated as a Secondary (undifferentiated) Aquifer. This designation means that the layer has previously been designated as both ‘minor’ and ‘non-aquifer’ in different locations due to the variable characteristics of the rock type. Beyond the extent of the Alluvium, the site is underlain by ‘Head’ deposits of clay, silt, sand and gravel. The furthest extents of the site around Stevenage Brook are underlain by superficial Glaciofluvial Deposits consisting of sand and gravel. These are permeable layers capable of forming an important source of base flow to rivers.

The bedrock of the Site is of the Holywell Nodular Chalk Formation and New Pit Chalk Formation (undifferentiated). This is designated as a Principal Aquifer. The south of the Site is shown to lie within a Zone 3 (Total Catchment) Groundwater Source Protection Zone (SPZ). The north of the Site lies partly within a Zone 2 (Outer Zone) SPZ and partly within a Zone 1 (Inner Zone) SPZ.

2.5 Hydrological Setting

Stevenage Brook, a tributary of the River Beane, drains the catchment to north of the site, in which the majority of the Borough of Stevenage is situated. Stevenage Brook forms part of the wider River Lee catchment, which is predominantly underlain by chalk. The location of Stevenage Brook in relation to the site is indicated on the Hydrology Setting plan, Figure 2.1 below:

Figure 2 1: Hydrological Setting

The rivers and streams of chalk areas are largely fed by chalk springs and therefore have a large baseflow component. Flood risk in chalk areas includes flooding from groundwater sources, although flooding can also occur as a result of surface water runoff from glacial clay overlying the chalk or due to urban development.

3. REVIEW OF BASELINE DATA

3.1 Assessment of Flood Risk Fluvial/Tidal Flood Zone Classification

The majority of the site is shown to be located within Flood Zone 1 (Low probability). There are small areas of the site location within Flood Zones 2 and 3 (Medium and High probability respectively), and these can be broadly described as follows:

• Land immediately adjacent to and forming the northern site boundary is shown to be located within Flood Zone 3. This is the lower lying land associated with Stevenage Brook riparian area; and

• Land to the west of the site adjacent to where the new roundabout/site access is proposed

Figure 3.1 indicates the presence and extent of flood zones within the site boundary.

Figure 3.1: Flood Zone Categorisation

3.2 Modelled Flood Levels

As part of the assessment of fluvial flood risk, Ramboll UK Ltd requested Product 4, 5 and 6 information, which was duly provided by the Environment Agency (EA). The EA flood level information provided is from the Stevenage Brook hydraulic modelling study completed by Mott MacDonald in 2019. The EA has provided both mapping of predicted flood extents and peak flood levels which are stated to be “in-channel” levels. It should be noted that the in-channel levels may not directly correspond to levels on a floodplain.

To assess the potential flood depths at individual locations across the site, a number of spot levels have been read from the LiDAR survey. These have then been compared to the respective flood levels from the closest corresponding model node to provide an indication of flooding likelihood and severity i.e. for which return period flooding could occur, and the flood depth should flooding be predicted. The results of this assessment are indicated in Table 3.1 below.

Table

The use of green denotes the ground level being above a flood level

The use of red denotes the ground level being below a flood level

The comparison of the existing ground levels to modelled flood levels indicates the majority of the site, and in particular those areas designated for development, are above the peak flood levels for both the 1 in 100 year plus climate change and 1 in 1,000 year events.

It is noted however that the route of the proposed access could suffer flooding to depths 660 mm during a 1 in 1,000 year event. Whilst levels climb away towards the road and the new roundabout junction, the future planning and design of the development should consider access/egress during a flooding event; this is discussed in Chapter 5.

3.3 Surface Water Flood Risk

The EA has undertaken national-scale modelling of potential surface water flood risks (i.e. those associated with extreme rainfall events and associated overland flow rather than flooding from rivers or the sea). Such risks are categorised as High, Medium, Low or Very Low as follows:

• High – Greater than a 1 in 30 (3.33%) annual probability;

• Medium – Between a 1 in 30 and 1 in 100 (3.33% to 1%) annual probability;

• Low – Between a 1 in 100 and a 1 in 1,000 (1% to 0.1%) annual probability; and

• Very Low – Less than a 1 in 1,000 (0.1%) annual probability.

The majority of the site is shown to be at Very Low or Low risk of surface water flooding. There is, however, an area of Medium potential surface water flood risk in the northeast and an area of High potential surface water flood risk in the north. This is presented in Figure 3

These areas represent isolated low topography within the site, occupied by vehicular access or parking and landscaping, largely outside of the footprint of the buildings on-site. Only a very small area of one of the buildings is shown to be within a potential surface water flood risk area.

The land off-site to the north is also shown to be at risk of surface water flooding. However, as set out previously, ground levels to the north are shown in LiDAR to drop approximately 4 m below ground levels recorded adjacent and within the site boundary. Therefore, surface water flooding of land north of the site is unlikely to impact on the site itself.

3.4 Groundwater Flooding

The Stevenage Borough Council (SBC) Strategic Flood Risk Assessment1 (SFRA) states that groundwater flooding usually occurs in low lying areas underlain by permeable rock and aquifers that allow groundwater to rise to the surface through the permeable subsoil following long periods of wet weather. Low lying areas are more susceptible to groundwater flooding because the water table is usually at a much shallower depth and groundwater paths tend to travel from high to low ground. The Borough is situated on chalk strata which is associated with groundwater flooding.

Appendix A, Figure 5 within the SBC SFRA indicates that the site has a varying susceptibility to groundwater flooding. The majority of the site is shown to be at medium risk, this is classified within the figure as potential for groundwater flooding of property situated below ground level. An area just north of the A602 is identified as having low risk, classified limited potential for groundwater flooding to occur.

3.5 Historic Flood Records

The EA has stated in their previous consultation response (2013) that they hold records of flooding at the Site dating from 1947, 1978 and 1993. The recorded extents of flooding from 1947 are particularly coarse, however, the extent of historic flooding appears to be less extensive though still in the same areas as that which corresponds to Flood Zone 3.

The SFRA has also been reviewed for records of flooding, and sets out the same records of flooding as the EA consultation response. There are no records of flooding from other sources such as surface water (pluvial), groundwater, artificial water bodies or sewers. The extent of the historic flood events are shown in Figure 3.

1 Stevenage Borough Council, Strategic Flood Risk Assessment (2016), available at: http://democracy.stevenage.gov.uk/Data/Executive/20150609/Agenda/Level-1-SFRA-Update-June-2016.pdf

Figure 3.2: Surface Water Flooding

3.6 Flood Defences

Two weirs are identified through the reach of the brook adjacent to the Site boundary. Flood defences are also shown on either side of the brook immediately downstream of the Site boundary. The description of the upstream weir states that it is a:

“weir with Footbridge DS: Weir in situ concrete 800 mm drop. Flow runs cleanly and evenly over asset with no significant erosion present within channel.”

The role that the weir plays as a flood defence is not described. The downstream weir is described as a disused non-flood defence. There is no detail on the role played by the downstream flood defences and whether they offer protection to the Site and to what standard of protection.

3.7 Reservoir Flooding

The EA Long Term Flood Risk map indicates that the Site is modelled to be within an area with a potential flood risk associated with reservoir failure.

However, the mapping only represents the potential consequence of a catastrophic failure of a reservoir and the resultant release of the water retained within the reservoir. The mapping does not indicate the likelihood of such an event which is considered to be of a very low probability. All large reservoirs must be inspected and supervised by reservoir panel engineers. As the enforcement authority for the Reservoirs Act 1975 in England, the EA ensure that reservoirs are inspected regularly, and essential safety work is carried out.

Figure 3.3: Historic Flood Records

4. SURFACE WATER MANAGEMENT

4.1 Overview

The following section sets out a high-level strategy for the management of surface water emanating from the new development. The guidance has been developed through use of the Hertfordshire County Council Interim arrangement on Surface Water Drainage Advice2, along with the National Statutory guidance outlined in the Non-Statutory Technical Standards for Sustainable Drainage Systems3

4.2 Disposal of Surface Water

Part H of the Building Regulations establishes a hierarchy for surface water disposal, which encourages a SuDS approach. The hierarchy is that surface runoff must be discharged to one or more of the following in the following order of priority:

1. To ground via infiltration;

2. To a watercourse;

3. To a surface water sewer or highway drain; and

4. To a combined sewer.

A review of the underlying ground conditions outlined in Section 3.1 indicate that permeability is likely to be highly variable across the site; lower in the upper layers of alluvium and till, but far higher within the bedrock chalk layer. A ground investigation is required to determine the thickness of surface deposits, and whether access to lower chalk layers can be achieved. There is also the potential for deep-bore soakaways to be used, whereby a deep excavation is made to the underlying chalk, into which surface water is discharged. Should use of soakaways not be feasible, a direct connection for surface water could be made to Stevenage Brook.

4.3 Runoff Rates and Storage

The Hertfordshire County Council (HCC) guidance states that peak discharge rates from site “will not increase as a result of the proposed development for up to a 1 in 100 chance in any year including an allowance for climate change” It is expected that “all applicants to achieve greenfield runoff rates for greenfield development sites to reduce the impact of the development on the wider water environment”. MicroDrainage has been used to estimate the existing runoff rates and future storage requirements based on the introduction of the development and associated impermeable area. These are summarised as follows:

• 8.65 ha total site area;

• 23.5 l/s greenfield runoff rate for the 1 in 1 year event (using ICP method);

• 4,500 m3 of storage required, based on providing storage for events up to the 1 in 100 plus 40% climate change.

• Storage calculation based on assumed 0.65 impermeable area i.e. 5.63 ha;

• Based on the required 4,500 m3 of storage required, approx. 0.90 ha should be allocated to green-blue infrastructure, ecological enhancements and amenity

4.4 SuDS Attenuation

Table 4.3 below provides an overview of potentially suitable SuDS options available for attenuating surface water runoff.

2 https://www.hertfordshire.gov.uk/media-library/documents/environment-and-planning/water/flood-investigations/archiveconsultations/hertfordshire-interim-suds-policy-statement.pdf

3 https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/415773/sustainable-drainage-technicalstandards.pdf

Table 4.3: Sustainable Drainage Options

SUDS Group Technique

Retention

Wetland

Balancing pond

Provides both storm water attenuation and treatment. Runoff from each rain event is detained and treated in the pool. The retention time promotes pollutant removal through sedimentation

Good removal of pollutants, can be used where groundwater is vulnerable, good community acceptability, high ecological, and amenity benefits

No reduction in runoff volume, land take may limit use in high density sites

Potential for inclusion in lower-lying area along Stevenage Brook

Infiltration

Sub-surface storage

Oversized pipes, tank systems and modular geocellular systems that can be used to create a below ground storage structure

Modular and flexible, dual usage (infiltration/storage, high void ratios), can be installed beneath trafficked and soft landscaped areas

No water quality treatment

Better options available considering nature and character of the development

Shallow wetland

Extended detention

wetland

Pond wetland

Pocket wetland

Submerged gravel

wetland

Wetland channel

Infiltration trench

Infiltration basin

Wetlands provide stormwater attenuation and treatment. They comprise shallow ponds and marshy areas, covered in aquatic vegetation.

Wetlands detain flows for an extended period to allow sediments to settle and to remove contaminants They can provide significant ecological benefits

Good pollutant removal and if lined can be used where groundwater is vulnerable. Good community acceptability, ecological and amenity benefits

Land take is high, requires baseflow, little reduction in runoff volume, not suitable for steep sites

Potential for inclusion in lower-lying area along Stevenage Brook

Surface water runoff can be discharged directly to ground for infiltration by soakaways, basins, or trenches. A prerequisite is that both

Reduces the volume of runoff, effective at pollutant removal, contributes to groundwater recharge,

Requires appropriate pretreatment, basins require TBC

SUDS Group Technique

Soakaway

Porous paving

Permeable paving

Filtration

Surface sand filter

Sub-surface sand filter

Advantages

groundwater and ground conditions are appropriate to receive the quality and quantity of water generated simple and cost-effective, easy performance observation

Block or porous paving allows runoff to infiltrate through to sub base layer

Water can then be infiltrated into ground or conveyed into storage or drainage systems

Reduces the volume of runoff and if designed for infiltration contributes towards groundwater recharge. Easy to install and retrofit. Simple to manage. If lined can be used where groundwater is sensitive

Disadvantages

a large flat area, offset from foundations

Perimeter sand filter

Structures designed to treat surface water runoff through filtration using a sand bed filter medium. The filters can be designed with or without infiltration.

Temporary storage of runoff is achieved through ponding above the filter layer. They are used where particularly high pollutant removal is required

Flexibility of design, efficient in removing pollutants, suitable for retrofits and in tightly constrained urban locations

Not suitable for heavily trafficked areas or adoptable roads. Requires regular sweeping to prevent clogging with dirt

Not for high sediment content, detention times can support algae growth, minimum hydraulic head of 1.2 m required, possible odour problems, high capital and maintenance cost

Suitable for Use at Site?

To be assessed in full upon receipt of the Ground Investigation report

Bioretention /filter swale

Vegetated strips of land designed to accept runoff as overland sheet flow between a hardsurfaced area and a receiving system

Landscaping features, effective in removing pollutants, flexible layout to fit into landscape, suited for highly impervious areas, good retrofit capability, effective pre-treatment option

Requires landscaping and management, large land requirement, not suitable for steep sites; no significant attenuation or reduction of flows

Should be incorporated for all parking areas

Better options available considering nature and character of the development

Filter trench /drain

Shallow excavations filled with rubble or stone that create temporary subsurface storage for filtration of storm water runoff

Hydraulic benefits achieved with filter trenches, trenches can be incorporated into site landscaping and fit well beside roads and car parks

High clogging potential without effective pretreatment, limited to small

Better options available considering nature and

SUDS Group Technique

Detention

Detention basin

Enhanced dry swale

Enhanced wet swale

Conveyance

Conveyance swales

Advantages

Receive lateral inflow from an adjacent impermeable surface

Surface storage basins that provide flow control through attenuation. Normally dry and in certain situations the land may also function as a recreational facility.

Cater for a wide range of rainfall events, can be used where groundwater is vulnerable, potential for dual land use, easy to maintain

Disadvantages

Suitable for Use at Site?

catchments, high cost of replacing filter material character of the development

Land take, little reduction in runoff volume, detention depths constrained by levels

Better options available considering nature and character of the development

Swales are linear vegetated drainage features in which surface water can be stored or conveyed. They can be designed to allow infiltration, where appropriate.

Incorporate into landscaping, good removal of pollutants, reduces runoff rates and volumes, low cost

Not suitable for steep areas, significant land take, not suitable in areas with roadside parking

Should be used to form the spine of the blue-green network

Rills

Source control

Formal linear drainage features in which surface water can be stored or conveyed. They can be incorporated with water features such as ponds or waterfalls where appropriate.

Negate the need for underground pipework. Can provide some attenuation

Possible reduction in runoff volume via plant uptake and infiltration

Potential trip/wheel hazard, disabled access issues

Better options available considering nature and character of the development

Multi-layered system that covers the roof of a building with vegetation cover/landscaping over a drainage layer. Designed to intercept and retain precipitation, reducing the volume of runoff and attenuating peak flows.

Mimics greenfield state of building footprint for high density developments, good removal of pollutants, ecological benefits, insulates buildings, sound absorption

Additional weight, not appropriate for steep roofs, maintenance of roof vegetation

Not suitable for private dwellings

Green/blue roof

4.5 Overview of Surface Water Strategy

It is not possible at this time for a full strategy and drainage layout to be developed. However, once a site masterplan is agreed, the proposed drainage for the site should be developed in full detail, in accordance with the following:

• Use of Infiltration Testing – The HCC local SuDS policy requires that at sites where ground conditions allow, surface water drainage should be infiltration led following the hierarchy set out in Building Regulations Part H. The underlying conditions indicate that whilst the ground conditions are likely to be variable (especially in the upper layer), the presence of chalk may provide a suitable conditions for infiltration of runoff. This should be assessed in full once a ground investigation report is available and/or infiltration testing has been completed;

• Connection to Stevenage Brook – Should infiltration not be feasible, a direct connection for surface water could be made to Stevenage Brook;

• Allowable Runoff Rates – Based on the 8.26 ha site and assumed 65% developable/impermeable area, a greenfield runoff rate of 23.5 l/s for the 1 in 1 year event was calculated. The future drainage for the site, if discharging at least part of the discharge into Stevenage Brook, will need to ensure a peak discharge of no greater than 23.5 l/s.

• Required Storage Volume – As the site is greenfield, and as the wider site masterplan and landscaping is yet to be developed, there is huge potential for adding value to the development through inclusion of SuDS measures, and in doing so helping to manage water in a sustainable way. The SuDS and storage options for the site should be designed to incorporate a total of 4,500 m3 of storage. This is the calculated volume to provide storage for the 1 in 100 year event, plus 40% climate change;

• SuDS Strategy – As suggested above, the site layout should be developed to include sufficient blue-green infrastructure to promote the sustainable management of surface water. This should form part of the initial site Masterplanning considerations, which will negate the need for more costly and less sustainable underground solutions later on in the development process; and

The above strategy should be developed in full detail as part of the planning application, prior to which, the requirement for SuDS should be considered as part of the site Masterplanning and Landscaping development.

5. MITIGATION MEASURES

This assessment summarised in this report has undertaken an updated assessment of flood risk, based on updated hydraulic modelling provided by the EA. It has also undertaken an updated review of other site conditions and statutory information. As part of our development of mitigation options, and consideration of future site development, Ramboll has considered climate change in the following ways:

• Consideration of climate change allowances when considering peak fluvial flood levels – this is also a policy requirement of the EA for all NPPF-compliant FRAs;

• Consideration of greater frequency and higher magnitude of surface water flooding events and overland flow, and assessing how a site and building layout can be designed to manage this risk;

• Consideration of the likely increased risk of seasonal groundwater flooding as a result of wetter winters; and

Each of the above will be considered when assessing the mitigation measures, which are summarised as follows:

• Finished Floor Level - A review of the site levels vs. modelled flood levels indicates that most parts of the site should remain at least 2000 mm above the peak 1 in 100 year plus climate change flood level. However, whilst site levels are likely to be well above the peak fluvial flood level, all FFL and threshold levels should be at least 150 mm above surrounding ground to manage future risk groundwater flooding and surface water flooding.

• Site Access – a review of the flood mapping indicates that the proposed alignment of the access road crosses an area indicated as being within Flood Zone 3, high risk of flooding. A statutory requirement of the development in managing flood risk will be ensuring there is a permanent safe route for access/egress from/to the site. A review of the LiDAR levels along the route of the proposed road indicates a level of around 68.50 mAOD. To give 300 mm freeboard above the 1 in 100 year +30% climate change level, the new access road should be set at 68.80 mAOD, requiring raising of approx. 300 m above existing ground levels;

• Compensatory Storage – Whilst national flood risk policy requires a proposed development to demonstrate it can remain safe from flooding, it is also incumbent on a developer to ensure there is no adverse impact on flood risk downstream. In the case of site at Bragbury End, this will be providing compensatory storage to offset any lost floodplain storage resulting from the raising of levels to provide the proposed access road. This assessment should be considered as part of the site and access road design, and be reported within the Flood Risk Assessment submitted as part of the planning application;

• Updates to Hydraulic Modelling – the EA make periodic updates to their hydraulic modelling and associated flood mapping, amending climate change allowances, hydrological modelling methods, presence of new flood alleviation schemes/flood defences and improve resolution LiDAR. Whilst the most recent hydraulic modelling was undertaken relatively recently in 2019, the Flood Risk Assessment to support the planning application should request the most up-todate hydraulic modelling to be used in the support of the planning application;

• Planning for Exceedance Events - this risk relates to the occurrence of intensive rainfall events (expected to become more frequent with the advent of climate change) which could cause overland flow and surface water flooding, or cause the capacity of the site drainage system to be exceeded and result in flooding. To manage this risk, the development should consider

exceedance overland flow routes during extreme flood events, adopting the principles set out in CIRIA Report C634, Designing for Exceedance in Urban Drainage.

Along with the planning of exceedance routes, external gradients where possible, are be designed to fall away from buildings, so that any overland flow resulting from extreme events would be channelled away from building entrances The design of exceedance routes should correlate with blue-green corridors, which make highly suitable exceedance flow paths.

6. SUMMARY

It is understood that the site at Bragbury End, Stevenage is to be brough forward for residential development. An initial masterplan indicates this will consist of around nine neighbourhood plots, and include a new site access via a new roundabout junction, providing access to the A602.

A review of the EA flood risk mapping indicates that the majority of the site is situated within Flood Zone 1, low risk of flooding. The low risk parts of the site consist of the previously levelled plateaus which previously formed the network of sports pitches. These plateaus sit generally around 2000 mm+ above the 1 in 100 year plus climate change flood level. As such, unless future remodelling of the site is proposed that would reduce the levels of these plateaus, these are considered to be at low risk of flooding, and are unlikely to be subject to any development constraints in relation to flood risk.

Flood risk from all other sources including groundwater, surface water and sewer flooding was considered to be low, and furthermore there are no recorded incidents of flooding to the site.

The proposed development will be required to manage surface water in a way that does not adversely impact neighbouring sites, and adheres to the local and national statutory guidance for surface water drainage (HCC SuDS guidance for developers, in the first instance). The development must ensure a peak rate of discharge of no greater than 23.5 l/s, which is the equivalent greenfield rate for the 1 in 1 year storm event. A total of 4,500 m3 will be required to provide attenuation for the 1 in 100 year plus 40% climate change event.

The provision of SuDS measures provides the opportunities to enhance the sustainability credentials of the development, along with add value and ensuring optimal use of land for ecology enhancements and biodiversity net gain, landscaping and provision of blue-green infrastructure. There are a wide range of SuDS measures that be used at plot, neighbourhood and site-wide level, and the planning for SuDS should be incorporated from the Masterplanning outset. There is significant opportunity to provide enhancements along with

A number of mitigation measures are also outlined, which along with a robust strategy for managing surface water, should help manage the future risk of flooding. It is now known that extreme weather occurrences such as extreme rainfall are likely to become more commonplace in the advent of climate change. Whilst the mitigation measures outlined typically fall into the category of design best practice, adopted these at an early stage will help ensure the future development is resilient to climate change, and can be delivered safely and sustainably.

APPENDIX 1

SITE REDLINE BOUNDARY PLAN

Ramboll - Bragbury

APPENDIX 2

TOPOGRAPHIC SURVEY AND LIDAR PLAN

Ramboll - Bragbury

APPENDIX 3

ENVIRONMENT AGENCY PRODUCT 4, 5 & 6 DATA

Ramboll - Bragbury

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

The da ta in this map has been e xtracted from the Stevenage Brook Modelli ng Study (M ott Mac Dona ld, 2019 ) This study is a c atchment s cale mappi ng s tudy and so ma y need loca l upda te s for site s pe cific decis ions . It should be noted that it was not created to produc e flood levels for specific de velopment sites within the catchment Modelled outlines tak e into a ccount catchment wide de fe nc es. Flood risk data requests including an allo w ance for climate change w illbe based on the 1 in 100 flood plus 20% allo w ance for climate change, unless other w ise stated. You should refer to Flood risk assessments:climatechangeallo w ancestocheckifthis allo w anceisstillappropriateforthetypeofdevelopment you are proposing and itslocation You mayneed to undertakefurtherassessmentoffuturefloodriskusing differentallo w ancestoensureyourassessmentoffuture flood risk is based on best available evidence. https://www gov uk/guidance/flood-risk-assessmentsclimate-change-allowances

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

Site l ocation

Flood risk data requests including an allo w ance for climate change w illbe based on the 1 in 100 flood plus 20% allo w ance for climate change, unless other w ise stated. You should refer to Flood risk assessments:climatechangeallo w ancestocheckifthis allo w anceisstillappropriateforthetypeofdevelopment you are proposing and itslocation You mayneed to undertakefurtherassessmentoffuturefloodriskusing differentallo w ancestoensureyourassessmentoffuture flood risk is based on best available evidence. https://www gov uk/guidance/flood-risk-assessmentsclimate-change-allowances

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

(*CC ) Defende d

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

) Defende d

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

1D No d e Resu lts ! 1D n odes

Environment Agency Ref: HNL 304201 AS

The data in this map has been extracted from the Stevenage Brook Modelling Study (Mott MacDonald, 2019)

Flood risk data requests including an allowance for climate change will be based on the 1 in 100 flood plus 20% allowance for climate change, unless otherwise stated. You should refer to ‘Flood risk assessments: climate change allowances’ to check if this allowance is still appropriate for the type of development you are proposing and its location. You may need to undertake further assessment of future flood risk using different allowances to ensure your assessment of future flood risk is based on best available evidence.

https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances

All flood levels are given in metres Above Ordnance Datum (mAOD) All flows are given in cubic metres per second (cumecs)

MODELLED FLOOD LEVEL

MODELLED FLOWS

2D No d e Resu lts:

A602,

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Site l ocation

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

No d e Resu lts: Heig h ts ! 1 i n 20 (5%) Defende d

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7

2D No d e Resu lts: Heig h ts ! 1 i n 25 (4%) Defende d

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Detailed FRA centred on: land off the A602, Stevenage -

AS

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7

Defended

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023

AS

) Defende d

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

No d e Resu lts: Heig h

Defended

Detailed FRA centred on: land off the A602, Stevenage - 30/03/2023 - HNL 304201 AS

Environment Agency Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

(0.1% ) Defende d

Historic Flood Map centred on: land of f the A602, S tevenage -

304201

Environment Agency

Alchemy, Bessem er Road, Welwyn Garden City, Hertfordshire, AL7 1HE

Site l ocation

Flo od Event O utlines

The historic flood event outlines are based on a com bination of anecdotal evidence, Environment Agency staff observations and survey

Our historic flood event outlines do not provide a definitive record of flooding

It is possible that there will be an absence of datain places where we have not been able to record the extent of flooding

It is also possible for errors occur in the digitisation of historic records of flooding