Flood Risk Assessment Breakspear House, Hemel Hempstead. Kier Property. October 08

Transcription

1 Flood Risk Assessment Breakspear House, Hemel Hempstead. Kier Property October 08

2 Prepared by:... Approved by:... Simon Marcos Keith Searle Principal Engineer Associate Director Flood Risk Assessment Rev No Comments Date 1 First issue 07/12/07 2 Second Issue 10/10/08 Marlborough House, Upper Marlborough Road, St Albans, Hertfordshire, AL1 3UT Telephone: Fax: Website: Job No Reference FRA001 Date Created: 10 October 2008 This contains confidential and commercially sensitive information, which shall not be disclosed to third parties. f:\projects\c52467g breakspear house hemel hempstead\documents\reports\flood risk assessment\fra 001 second issue 10 october 2008.doc

3 Table of Contents 1 Introduction Objectives Limitations Development Description and Location The Site Proposed Development Hydrogeology Surface Watercourses Existing Site Drainage Probability of Flooding Flood Zone Strategic Flood Risk Assessment Local Development Plan Flood Risk Assessment Potential Sources of Flooding Fluvial Local and On-site Drainage System Groundwater Land Drainage Flood Risk Management Measures On-Site Mitigation Measures Off-Site Mitigation Measures Residual Risk Drainage Strategy Proposed Drainage Sustainable Urban Drainage Systems SUDS Options Grey Water and Surface Water Recycling Conclusions APPENDICES Appendix A 3965/TP01 Master Plan K52467/SK05

4 Executive Summary

5 Faber Maunsell Flood Risk Assessment Executive Summary A 1.477ha (approx) area of brownfield land located at Breakspear House within the Hemel Hempstead Industrial Estate is being re-developed by Kier Property. The current proposal for the site is a mixed-use development incorporating a hotel and an office development. Between the different buildings, there will be hardstanding areas that provide access roads and car parking. The current buildings on site have been demolished. The land lies outside of the flood plain of the local river system as defined by the Environment Agency on their internet based flood zone map and therefore lies within Flood Zone 1 i.e. low risk of flooding. There are no known flooding issues at the site. However, the Local Planning Authority has indicated that the development requires a flood risk assessment to be included as part of the planning application. This is because PPS25 requires an FRA to be undertaken for developments of 1 hectare or greater in Flood Zone 1 as it is possible that there may be a risk of flooding from other sources. Furthermore the run off from larger sites may be sufficient to cause flooding elsewhere. The Flood Risk Assessment undertaken herein is a Level 2 Scoping Study as defined in CIRIA Report 624 Development and Flood Risk. The development has a low flood risk and with appropriate drainage design to adoptable standards and flood mitigation measures employed, the flood risk can be controlled and lowered further whilst not increasing the flood risk to neighbouring property. It has been reported that the storage lagoon located some 275m from the site was the subject of a flooding incident some years ago but it is understood that the development site was not affected. Surface water run-off from the site is currently drained to a public sewer without attenuation. There is 1200 diameter surface water public sewer that runs through the site and other public sewers also run in close proximity to the site. A current requirement for managing surface water from previously developed site is that the run-off from the proposed development should be compared with existing rates and not Greenfield rates. All published guidance on sustainable drainage systems, including the latest SUDS Manual C697, indicates that as a minimum requirement drainage discharges from new developments should achieve the existing drainage conditions. However, it is proposed that the run-off from the development is reduced by 20% to satisfy Environment Agency s aspirations to achieve Greenfield run-off rate from such developments using the most appropriate SUDs techniques. On this basis an allowable surface water discharge from the site of 125 l/s has been estimated, based on the existing site impermeable area and utilising the Rational Approach of a rainfall intensity of 50mm/hr. The Environment Agency s aspiration to reduce runoff from the site to Greenfield rates is deemed to be impractical. An assessment of Greenfield runoff rates for the site utilising the ICP SUDS Methodology (Code of practice for Sustainable Drainage Systems, Chapter 6) determined that the Greenfield flow Q BAR is 0.3l/s. It would be difficult to control such low flows and require very high volumes of attenuation storage for which space on site does not permit. There are no local land drainage features at or near the site for surface water discharge. The site is underlain by Clay-with-flints which in turn overlies the Chalk. The Clay-with-flints is generally a low permeability material which is very unlikely to be suitable for shallow soakaways although deep soakaways may be considered. Attenuated surface water run-off from the proposed development will be drained to the local public sewer.

6 Faber Maunsell Flood Risk Assessment Hydraulic analysis has estimated that the required total equivalent storage volume for the site will be between m 3 for a 1 in 30 year storm rising to m 3 for the 1 in % year storm. The additional 20% is to take into account the potential for future climate change during the lifetime of the development. The actual storage required will need to be based on the final drainage layout and strategy put forward for the site. At this stage, it is envisaged that this volume can be achieved by using a combination of permeable paving with sub-base storage and cellular block type below ground storage. The site is within the Thames Water area and the development will have a right of discharge into the foul public sewerage system. Based on a Thames Water Impact Study for the site, foul flows are to be limited to a maximum of 6l/s. Calculations have assessed based on the current occupancy proposals that future average flows should not exceed 6l/s and as such foul discharges from the site should have a free discharge to public sewer without the requirement for any attenuation. From records there is an existing sewer running under the site in the area of the proposed development and there are also local public sewers in the immediate vicinity of the site; however the levels all of the local sewers are not available on received records. The development will not require sewer diversions.

7 1 Introduction

8 Faber Maunsell Flood Risk Assessment 1 1 Introduction Faber Maunsell has been commissioned by the client, Keir Property, to undertake a Site Specific Flood Risk Assessment for the proposed mixed use development at Breakspear House in Hemel Hempstead. The Environment Agency, who are a statutory consultee, need to be satisfied that the granting of planning permission will address the risk of the new development flooding and that the proposals will not in turn increase the risk of flooding to neighbouring land and property. This report will, in accordance with PPS25 (and the draft Practice Guide Companion), propose preventative measures to mitigate against flooding from any source, if found necessary. The report will also look at a drainage strategy for the proposed new development to establish constraints and design requirements. This Flood Risk Assessment seeks to satisfy the general reporting requirements as set out in PPS25 Annex E and Section 2.54 of the Practice Guide Companion to PPS25. This Flood Risk Assessment has been prepared in order to identify and evaluate the various possible sources of flood risk to which the proposed redevelopment site might be subjected to and identify any mitigation, protection or compensation measures deemed necessary or feasible. This Flood risk Assessment has been revised to reflect changes made to the masterplan and address the concerns of the Environment Agency. 1.1 Objectives The Flood Risk Assessment will undertake to: Collate and investigate site-specific historic flood data and information from recent site visits. Liaise with the Local Authority and the Environment Agency to understand their position with regard to flood risk this development. Evaluate the degree of flood risk to which the site is subject. Identify the extent to which mitigation and compensation measures are required to manage any potential flood risk. Identify Sustainable Drainage System (SUDS) methods for the control of runoff discharged from the site in line with Environmental Agency recommendations. The report will also investigate and comment on: The existing surface water drainage networks serving the site. The points and methods of discharge of the surface water runoff. The proposed methods of disposal of surface water flows and the associated discharges. Risk of flooding due to rising groundwater. Off site impacts 1.2 Limitations This document has been prepared solely as a flood risk assessment of the site proposed for the proposed development at Breakspear House, Hemel Hampstead. Faber Maunsell accept no responsibility or liability for any use which is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. The information, views and conclusions drawn concerning the site are based, in part, on information supplied to FML by other parties. FML has proceeded in good faith on the

9 Faber Maunsell Flood Risk Assessment 2 assumption that this information is accurate. FML accepts no liability for any inaccurate conclusions, assumptions or actions taken resulting from any inaccurate information supplied to FML from others. It should also be noted that the effects of ground and water borne contamination on the environment and the impacts of flooding are constantly under review, and authoritative guidance is potentially subject to change. The conclusions presented herein are based on the guidance available at the time this report was prepared, and no liability can be accepted for the retrospective effects of any changes or amendments to the legislation or guidance. The copyright in this document (including its electronic form) shall remain vested in Faber Maunsell Ltd ("FML") but the Client shall have a licence to copy and use the document for the purpose for which it was provided. FML shall not be liable for the use by any person of the document for any purpose other than that for which the same was provided by FML. This document shall not be reproduced in whole or in part or relied upon by third parties for any use whatsoever without the express written authority of FML.

10 2 Development Description and Location

11 Faber Maunsell Flood Risk Assessment 4 2 Development Description and Location 2.1 The Site The site is located at Breakspear House, Maylands Avenue, Hemel Hempstead, approximately 500m northwest of Leverstock Green. It is situated south of the Hemel Hampstead Industrial Estate and close to the junction of Maylands Avenue and St Albans Road. The National Grid Reference for the site is , The site s postcode is HP2 4SF. An extract of the Ordnance Survey map showing the location of the site is included in Figure 1. The site is bounded by Maylands Avenue to the east and St Albans Road to the south. There are residential areas to the south and west of the site and a building owned by BSI to the north of the site. The site s boundary is shown in the proposal plans in Appendix A. The site is approximately ha in size and used to accommodate an office and storage buildings with car parking areas and access road. These have now been demolished. The main entrance to the site is off Maylands Avenue via a double locked gate entrance. The site falls gently to the southeast with the central area being at a lower level than the surrounding ground to the north and south which is raised by up to 2m with soft landscaping. A topographical survey is not available yet, but an Envirocheck Master Plan shows a benchmark of mOD on Leverstock Green Road 250m to the west of the site. Figure 1 Existing Site Location 1 The development site is situated approximately 1.5km to the west of the banks of the River Gade, located 2.5 km to the west of the site and wholly lies outside of the indicative flood plain

12 Faber Maunsell Flood Risk Assessment 5 as recorded by the Environment Agency on their internet based maps and as estimated by hydraulic models. 2.2 Proposed Development The proposed development is split into two phases; Phase I and Phase II. Phase I of the development consists of a mixed-use development incorporating a hotel and offices space with associated access roads and car parking facilities. Phase II of the development will occupy the northern part of the site which shall be left as essentially green field prior to later development. This Flood Risk Assessment only covers Phase I of the development. The buildings on the site have been demolished to make way for the development. 2.3 Hydrogeology The published 1:100,000 scale Groundwater Vulnerability Map for the Thames Estuary (Sheet 40, Environment Agency, 1995) indicates that overall the site is underlain by a Major Aquifer which comprises the Upper Chalk. The west of the site lies on a Minor Aquifer. The EA publication on Groundwater Levels in the Chalk Basal Sands Aquifer of the London Basin, June 2006 indicated a groundwater level of around +70m OD in January 2006 in the area south to Hemel Hempstead, with a decrease of 4 m between 2004 and The ground conditions beneath the site, identified from the exploratory holes, comprise a variable thickness of Made Ground overlying Clay-with-Flints overlying in turn Upper Chalk Formation. The 1:50,000 British Geological survey map of the area shows that beneath the Clay with Flints is a boundary zone between Woolwich and Reading Beds and the underlying Upper Chalk. The Chalk may potentially be suitable for the disposal of surface water via soakaways. However previous boreholes sunk on the site indicate a variable thickness of Clay with Flints such that the depth to the Chalk varies between 3m and 14.5m from surface. It may not be possible therefore to reach the chalk with shallow soakaways. The Clay with Flints is generally a low permeability material which is very unlikely to be suitable for soakaways. It may be necessary to carry out soakage tests to confirm this to the satisfaction of the EA. Licensed Water Abstraction The Envirocheck report indicates that there is one water abstraction 856m to the north east of the site. The abstraction is for non-evaporative cooling purpose. The Envirocheck report indicates there is no surface water abstraction within 1km of the site. It is believed that a groundwater monitoring point was installed on the Breakspear House site following the Buncefield Fire. However details are not available. In terms of aquifer protection, the Environment Agency generally adopts a three-fold classification of Source Protection Zones for public supply abstraction wells. Zone I - or inner source protection is located immediately adjacent to the groundwater source. It is based on a 50-day travel time and is designed to protect against the effects of human activity and biological/chemical contaminants that may have an immediate effect on the source. Zone II - or outer source protection is larger than Zone I and is defined by a 400-day travel time to the source. The travel time is designed to provide delay and attenuation of slowly degrading pollutants. Zone III - or source catchment covers the complete catchment areas of a groundwater resource. Information available on the Environment Agency s web-site indicates that the site falls within the Total Catchment Zone (Zone 3) of an unnamed groundwater source. However the nearest drinking water abstraction is thought to be the Bow Bridge pumping station located some 3km to the east

13 Faber Maunsell Flood Risk Assessment Surface Watercourses The site lies within the catchment of the River Gade that flows in a south-eastern direction 2.5km to the west of the site. There is no surface watercourse within the site boundary or in close proximity to the site. The nearest water feature in the area is a balancing pond that is located approximately 275m east of the site, adjacent to St Albans Road. Surface water runoff from the site and the surrounding areas is discharged into this pond. 2.5 Existing Site Drainage The majority of the site was occupied by buildings and areas of hardstanding except the two embankments to the north and to the south of the site which are covered by dense grass. Thames Water provided a record drawing showing the location of the foul and surface water drainage in relation to the development and these are shown on Drawing No. K52467/SK05 in Appendix A. An 825 mm dia. surface water public sewer runs roughly from west to east within the site. There is a manhole (No. 9203) situated within the elevated landscape. A drainage survey of the existing sewerage on the site will be required as part of the detailed design process to assess the level of protection required. The plan also indicates that there is a 975 mm dia. surface water sewer beneath Maylands Avenue that runs in southerly direction and connects into the above 825 mm at a manhole (No. 0202) by the roundabout at the junction of St Albans Road and Maylands avenue. From Manhole No. 0202, the sewer diameter increases in size to 1225mm and runs in a westerly direction to an outfall by a pond, approximately 275 m east of the site and south of St Albans Road. The runoff from the site and surrounding areas is discharged into this balancing pond before it is pumped to a location east of the M1. A 300mm dia. foul sewer runs parallel to the above two surface water sewers until the roundabout and beyond before making a U-turn to run in the south westerly direction. The surface water runoff from the site was drained to either the 825mm dia. sewer that runs across the site or the 975mm dia. sewer beneath Maylands avenue. Thames Water do not hold details of private connections. The two buildings that used to occupy the site and the associated car parking and hardstanding areas covered an impermeable area of approximately 1.13ha. Assuming 50 mm/hr rainfall intensity, the peak flow from the site is approximately 157 l/s. Development plans aim to avoid building over the on-site public sewers but if not possible an application will have to be made to Thames Water to obtain a build over agreement or submit a proposal to divert the public sewers for Thames Water s approval as per the requirements of the Water Industry Act 1991.

14 3 Probability of Flooding

15 Faber Maunsell Flood Risk Assessment 8 3 Probability of Flooding 3.1 Flood Zone The main driver for and basis of a Flood Risk Assessment is Planning Policy Statement 25 (PPS25) Development and Flood Risk. PPS25 creates a policy framework within which all those engaged in the planning process can actively contribute to a more sustainable approach to managing flood risk. Its aim is to ensure that flood risk is taken into account at all stages of the planning process to avoid inappropriate development in areas at risk of flooding. Where new development is exceptionally necessary, the policy aim to make it safe without increasing the risk elsewhere and where possible reducing the overall risk. The approach advocated by PPS25 should, as a minimum, ensure that the negative environmental impacts of development on surface water run-off are mitigated. PPS25 requires that site-specific flood risk assessments be undertaken for all sites with an area greater than one hectare to ensure that, inter alia, downstream flooding problems are not made worse by development run-off. The actual requirements for a flood risk assessment depend significantly upon the specific location, and in particular the protection provided by permanent flood defences along the river, the type of development, and the information already available regarding flood risk. The appropriateness of siting a particular development at a specific location is defined in PPS25 using the precautionary principle and a sequential test (see Table D1, PPS25). PPS25 calls for a risk-based Sequential Test to be applied at all stages of planning so that new developments are steered towards areas at the lowest probability of flooding (i.e. Zone 1). Flood Zones are defined by the Environment Agency as areas depicting the probability or likelihood of flooding without flood defences in place. The sequential test is divided into 3 flood risk zones: Zone 1 Low Probability: This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). Zone 2 Medium Probability: This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% 0.1%) or between a 1 in 200 and 1 in 1000 annual probability of sea flooding (0.5% 0.1%) in any year. Zone 3a - High Probability: This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year. Zone 3b - The Functional Floodplain: This zone comprises land where water has to flow or be stored in times of flood. SFRAs should identify this Flood Zone (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes). Any development in Flood Zone 2 and 3 should be supported by a flood risk assessment and Developments of 1 hectare or greater in Flood Zone 1 need a FRA due to the risk of flooding from other sources. The site is located in Flood Risk Zone 1, as shown on the Environment Agency s internet-based Flood Maps. The site is approximately 1.5 ha in area (i.e. greater than 1ha); hence a sitespecific assessment is required to consider the flood risk associated with the development proposal. PPS25 superseded Planning Policy Statement Guidance Note 25 (PPG25) in December PPS 25 has subsequently been supplemented by a Practice Guide Companion which was issued as a draft consultation document in February 2007.

16 Faber Maunsell Flood Risk Assessment 9 Figure 2 EA indicative Flood Zone Map 3.2 Strategic Flood Risk Assessment A Strategic Flood Risk Assessment (SFRA) has been produced in August 2007 by four of Hertfordshire s councils (Dacorum Borough Council, St Albans City and District Council, Watford Borough Council and Three Rivers District Council). The principal aim of the SFRA is to map all forms of flood risk and use it as an evidence base to locate new development primarily in low flood risk areas. It will also enable the four councils to prepare appropriate policies for the management of flood risk, inform the sustainability appraisal so that flood risk is taken account, identify the level of detail required for site-specific FRA and determine the acceptability of flood risk in relation to emergency planning capability. The SFRA covers the area of the proposed development and considers the risk of flooding in the area to be low. 3.3 Local Development Plan The Local Development Plan designates the area as being General Employment Area. The Local Planning Authority (LPA) is yet to carry out a Sequential Test in accordance to PPS25 following the Level 1 SFRA. As the site is in Flood Zone 1 and the risk of fluvial flooding is considered to be low, it is expected that the Sequential Test will find the area to be suitable for development with respect to flood risk.

17 4 Definition of Flood Hazard

18 Faber Maunsell Flood Risk Assessment 11 4 Flood Risk Assessment Contacts were made with Dacorum Borough Council, the EA, Thames Water and the local highway department to determine if there were any flooding incidences in the area. Dacorum Borough Council has confirmed that they hold no records of flooding at the site. Potential Sources of Flooding Although there is no known flooding on the site and the surrounding area, the following potential sources of flooding have been considered:. (1) Fluvial from the River Gade, located 2.5km to the west of the site. (2) Local and On-Site Drainage Systems (3) Groundwater (4) Land Drainage Flooding A fifth possibility is a combination of sources. Each of the categories of flood risk is discussed in detail in the following paragraphs. Fluvial The River Gade flows through Hemel Hempstead and is approximately 2.5km to the west of the site. The river joins the river Bulbourne which flows through Berkhamsted and continues to flow southwards. The site is within the catchment of this river. The site lies within Flood Zone 1, with an annual probability of flooding from rivers or the sea of less than 0.1%, i.e. 1 in 1000 year return period. The Strategic Flood Risk Assessment states the following with respect to flooding in the River Gade and Bulbourne catchments: No previous fluvial flood events have been recorded by the Environment Agency in the Gade or Bulbourne, although overbank flooding from the Gade is known to occur in Gadebridge Park. Additionally, the Hemel Hempstead Gazette describes major flooding events in the Gade and Bulbourne catchments in 1879 and Maryland Avenue was not mentioned as one of the areas flooded in 1879 which was estimated to be 1 in 100 year flood event. The flood events occurred before the Hemel Hemstead flood relief scheme was constructed in The SFRA further indicate that the flood relief scheme offers significant protection to Hemel Hemstead to beyond a 100 year flood protection. The risk of flooding from the River Gade is therefore very low and can be discounted. Local and On-site Drainage System None of the different bodies contacted holds any record of historic flooding due to surcharging of the drainage systems in the area. There is however anecdotal evidence that when the sluice valve and pumping systems associated with the balancing pond in the area fails to operate, the flow backs up through the surface water sewers to an extent that it causes localised flooding around the roundabout. The depth of the two public sewers cutting across the site is in excess of 3m. The water level in the manholes of these sewers is not expected to reach ground levels unless there is blockage in the system. If the systems surcharges, it is expected that the manholes by the roundabout outside the site would flood first due to the difference in level. In the unlikely event that the sewers are blocked locally, flooding would occur within the car parking area, close to the manholes within the site. (Refer to Drawing No. K52467/SK05 in the Appendix A).

19 Faber Maunsell Flood Risk Assessment 12 Mitigation of the risk of surface water flooding from the proposed drainage system will be effected using appropriately designed drainage services. SuDS will be used in accordance with PPS25 and current standards wherever practicable. Designed and maintained to the appropriate standards, the risk of flooding due to proposed site drainage system will be low. The flood risk from local and on site drainage systems is considered to be low. Groundwater Groundwater was only encountered in one of the boreholes sunk for the recent ground investigation undertaken in Seepage of groundwater was noted at a depth of 0.7m bgl during the fieldwork. During the subsequent monitoring carried out at the site groundwater was not encountered. The EA publication on Groundwater Levels in the Chalk Basal Sands Aquifer of the London Basin, June 2006 indicated a groundwater level of around +70m OD in January 2006 in the area south to Hemel Hempstead, with a decrease of 4 m between 2004 and Thus the regional groundwater table lies some 60m below ground level and it is therefore very unlikely that groundwater will cause any risk of flooding at this site. Land Drainage There are no drainage ditches within the site. low. The risk of land drainage flooding is therefore Pluvial Flooding / Flooding from Surrounding Areas 'Pluvial' flooding is that which results from rainfall generated overland flow before the runoff enters any watercourse, drain or sewer. It is more often linked to high intensity rainfall events However it can also result from lower intensity rainfall or melting snow where the ground is saturated, frozen, developed or has low permeability. This results in overland flow and ponding in depressions in the topography. In urban areas pluvial flows likely to follow the routes of highways and other surface connectivity to low spots where flooding can occur. In some cases it can deviate from this route into adjacent developments via dropped kerbs (either for access to driveways or disability access). There is no known history of overland flows coming into the site. A site walkover around the boundary of the site was undertaken to assess the risk of pluvial flooding entering the site. The site is bounded by a residential area to the west which is served by a number of roads. These are Greenway, Maddox Road, the Flags and New Park Road. These roads are all provided with surface water drainage system. There is a narrow strip of wooded land between the residential area and the site. The risk of overland flow entering the site from the west is very low. Figure 3 Residential Area west of the site. The site is behind the trees at the back To the north of the site, a narrow alleyway separates the site from buildings owned by Schroff and BSI. The risk of pluvial flooding entering the site from this direction is considered to be low.

20 Faber Maunsell Flood Risk Assessment 13 Figure 4 Buildings to north of the site St Albans Road forms part of the southern boundary of the site. This road has a fall in the direction of the roundabout located south west of the site. The level of this roundabout is below the site level. The road is also provided kerbs and gullies and there is no risk of surface water entering the site from this end. Figure 5 St Albans Road, South of site The site is bounded by Meylands Avenue to the east. The road has a surface water drainage system with kerbs and gullies. There is a drop kerb by the entrance to the site. It is unlikely that floodwater will enter the site through this as the ground level by the main gate is higher than the road level and any overland flow not collected by the road gullies would flow southwards to the roundabout. The risk of overland flow entering the site from Meylands Avenue is considered to be low.

21 Faber Maunsell Flood Risk Assessment 14 Figure 6 Main site entrance Maylands Avenue From the above, the flood risk from pluvial flooding is considered to be low. Off site impacts The surface water run-off from the site will be reduced from the current rate by 20%. The surface water outfall and the receiving pond is 275m away from the site and there are no buildings between the site and the outfall that could be at risk of flooding. There are no records of flooding in the area. Given the above, the development will not increase the risk of flooding in other areas.

22 5 Flood Risk Management Measures

23 Faber Maunsell Flood Risk Assessment 16 5 Flood Risk Management Measures Existing flood alleviation measures associated with the principal flood risk source are identified as the local sewerage systems. 5.1 On-Site Mitigation Measures The design team for the development should, as part of detailed design, follow advice of the flood risk assessment and carry out further risk analysis to determine the appropriate sustainable urban drainage methods of runoff control and pollution control to employ at this site. All incoming services should be laid within sealed ducts or pipe work to avoid disruption to service and potential entries into the building by water. Any basements or below ground plant rooms should include appropriate sumps and bunds to make provision for pollution control and the pumping out of water. Where possible the car parks and other hard standing vehicle areas should be drained into the surface water system having passed through bypass petrol/oil interception. All plant room drainage or fuel/chemical storage areas (if any) will be discharged into the foul system. If chemical or diesel storage will not be present on this site it will not be necessary for special runoff pollution control measures other than the provision of a bypass oil/petrol interceptor on the surface water outfall from the site. The proposed drainage system should be designed to provide flood protection in line with the requirements of PPS25, Building Regulation, Sewers for Adoption (6 th Edn.) and other current standards. Thames Water have undertaken a Sewer Impact Study in July 2008 (Ref: X ) to assess the impact of the development on the public foul sewer system. Based on the study, foul flows are to be limited to a maximum of 6l/s. Calculations have assessed based on the current occupancy proposals that future average flows should not exceed this figure and as such foul flows should have a free discharge to public sewer without the requirement of any attenuation. 5.2 Off-Site Mitigation Measures With the proposed drainage system designed to the required standard, the development will not increase the risk of flooding to the neighbouring land and property. 5.3 Residual Risk The residual risks to this development are considered to be climate change resulting in higher than expected rainfall durations and intensities. Operational deficiencies within proposed sewerage structures are also considered to be a risk requiring an ongoing maintenance regime to be carried out. Further studies also need to be carried out to ascertain the flood risk posed by the operation of the local balancing pond.

24 6 Drainage Strategy

25 Faber Maunsell Flood Risk Assessment 18 6 Drainage Strategy 6.1 Proposed Drainage A current requirement for managing surface water from previously developed site is that the run-off from the proposed site should be compared with existing rates. All published guidance on sustainable drainage systems, including the latest SUDS Manual C697, indicates that as a minimum requirement drainage discharges from new developments should achieve the existing drainage conditions. However, it is proposed that the run-off from the development is reduced by 20% to satisfy Environment Agency s aspirations to achieve Greenfield run-off rate from such developments using the most appropriate SUDs techniques. The current surface water discharge from the site is estimated to be 157 l/s based on current impermeable area of 1.13 ha and a rainfall intensity of 50mm/hr. The proposed development has an impermeable area of 0.85ha. The size storage provided for the site to attenuate flows from this area will primarily depend on the allowable discharge in addition to infiltration capacity of the soil. On the basis of the above, the allowable surface water discharge is calculated as 125 l/s. Preliminary calculations have been completed using MicroDrainage s WinDES Quick Storage Estimate to determine an approximate attenuation volume for the proposed development sites. These will be re-addressed during the detailed drainage design. The following parameters were used in the calculation for the attenuation storage volume: M5-60 minute rainfall = 20mm. Ratio of M5-60 to M5-2 day rainfall, r = 0.4 Impermeable area = 0.85ha. Allowable discharge = 125 l/s.(with no allowance for infiltration) The hydraulic analysis has estimated that the required total equivalent storage volume for the site will be between m 3 for a 1 in 30 year storm rising to m 3 for the 1 in % year storm. The additional 20% is to take into account the potential for future climate change during the lifetime of the development. The actual storage required will need to be based on the final drainage layout and strategy put forward for the site. The volumes calculated take no account of storage within the site drainage and manholes or delays to the time of entry into the system. At this stage, it is envisaged that this volume can be achieved by using a combination of permeable paving with sub-base storage and cellular block type below ground storage. The Environment Agency s aspiration to reduce runoff from the site to Greenfield rates is deemed to be impractical. An assessment of Greenfield runoff rates for the site utilising the IH124 or ICP SUDS Methodology (Interim Code of practice for Sustainable Drainage Systems, Chapter 6) determined that the Greenfield flow Q BAR is 0.3l/s. It would be difficult to control such low flows and require very high volumes of attenuation storage for which space on site does not permit. Given that surface water outfall and the receiving pond is only 275m away from the site and that the development pose very low risk of flooding to other areas, a reduction of flows by 20% is considered to be appropriate. The site is underlain by Clay with Flints which in turn overlies the Chalk. The Clay with Flints is generally a low permeability material which is very unlikely to be suitable for shallow soakaways. It is therefore proposed that the attenuated surface water run-off is drained to the public sewer.

26 Faber Maunsell Flood Risk Assessment 19 It is a requirement that flood flows from storms up to 1 in 100year + 20% for climate change is contained on site and routed through storage. Where, for example, storing flood volumes generated by storms exceeding the 1 in 30 year flows in the car park is not feasible, then a storage volume will be required to cope with 1 in 100 year storm plus a 20% allowance for climate change. Further analysis will be required based on the final layout of the proposed development, drainage layout and strategy put forward for the site in order to refine the volume required. During the detail design stage, these attenuation storages may need to be increased to cater for Phase II of the development. The new drainage system should be designed in accordance with current standards. Based on a Thames Water Impact Study, foul flows are to be limited to a maximum of 6l/s. Calculations have assessed based on the current occupancy proposals that future average flows should not exceed this figure and as such foul flows should have a free discharge to public sewer without the requirement of any attenuation. A full drainage survey will be required as part of the detailed design phase.

27 7 Sustainable Urban Drainage Systems

28 Faber Maunsell Flood Risk Assessment 21 7 Sustainable Urban Drainage Systems Government guidance to developers, issued through; the Environment Agency, DEFRA with respect to land use policy and the Department of Communities and Local Government with respect to Town and Country Planning issues, recommends an integrated approach to development in areas at risk of flooding. Recommendations made in policy documents indicate land to be developed or redeveloped should implement good practice in drainage design to minimise impact on and to catchments that are sensitive to flood risk. A sustainable approach to redevelopment emphasises the need to consider the immediate environs of the site and its contribution to the wider catchment. The use of Sustainable Urban Drainage Systems (SUDS) as recommended within Planning Policy Statement Note 25 (PPS25), on Development and Flood Risk is one means of delivering this policy. Sustainable Urban Drainage Systems are currently being widely used in surface water drainage design throughout the UK. They are seen as a tool for reducing the flows by such means as infiltration, transpiration and actually slowing the flow rates of the runoff that occur from storms. Guidance is given in sections F1-14 of Annex F, Planning Policy Statement Note 25. Sustainable drainage systems can help reduce the environmental impact of development. Their use provides a significant contribution towards more sustainable development. Some SUDS techniques can provide a measure of treatment, which may remove the need for petrol interception for example, although it is noted that the compact nature of the site does not allow for large areas of natural vegetation to be used as part of the treatment process. 7.1 SUDS Options The following SUDS options below are could be considered during the design stage. Permeable Paving: Permeable paving is generally seen as an infiltration system. However, this system can be used as an attenuation tank, with the water being held in a sub-base matrix below the paved area and discharging via a pipe network. Stormwater storage can be provided within proprietary cellular units installed under the permeable paving (stormcell or similar) to slow down runoff from entering the sewerage system. A problem with permeable paving is that it can only be used in areas of car parking and is not suitable for areas with heavy traffic loads and lorry movements. This limits its use on the proposed site to surface areas designated for car parking only. Fleets/Swales: Fleet and Swales can be designed if space permits to be either an infiltration system or else used to convey surface water around the site. The advantages of using swales over conventional positive drainage systems such as underground piped networks are that they are designed to have very shallow gradients. They are constructed in this way for a number of reasons. Firstly, a shallow gradient means that the velocity of the flow is reduced to stop erosion of the swale bed. Secondly, with the water flowing at a slower rate, the pollutants in the runoff are reduced by filtration. Swales can also be used as landscape features with the introduction of grasses and plants. If the swales are to take runoff from Roads and haulage yard areas, the swale is unlikely to cope with the contaminants in the runoff. The runoff will still therefore require passing through a petrol interceptor. Careful design of proposed site levels will be required to ensure that swales can effectively operate. Retention Basins: Retention basins are structures that the surface water is discharged to via swales, underground pipes, etc. They are used to retain and slow flows down and do not necessarily contain water at all times. Basins can also be designed to incorporate landscape design. The use of oversized pipework and manholes may be used where new sewerage systems are constructed to complement SUDS techniques in reducing runoff from the new development.

29 Faber Maunsell Flood Risk Assessment Grey Water and Surface Water Recycling Grey water recycling is the term used for the reuse of rain and wastewater as a non-potable water supply. Roof runoff from new buildings at the proposed site would be reasonably clean in comparison to Road runoff, and could be suitable for the flushing of toilets and irrigation of landscaped features.

30 8 Conclusions

31 Faber Maunsell Flood Risk Assessment 24 8 Conclusions The proposed site is shown on the indicative flood plain maps to lie outside to the flood zone of the River Gade. The site is located in Flood Zone 1 (low risk). Flooding from other sources is also considered to be low. There are no known historic flooding issues at the site. The site will not increase flood risk to the property or neighbouring buildings. The current surface water discharge from the site is estimated to be 157 l/s based on current impermeable area of 1.13 ha and a rainfall intensity of 50mm/hr. It is proposed that the runoff from the site is reduced from existing levels by 20% using appropriate SuDS techniques. The proposed development has an impermeable area of 0.85ha. The size of storage provided for the site to attenuate flows from this area will primarily depend on the allowable discharge in addition to infiltration capacity of the soil. On the basis of the above, the allowable surface water discharge is calculated as 125 l/s. Further analyses based on various storm events and negotiations with Thames Water and the Environment Agency will be required to determine the final allowable discharge from the site. The Environment Agency s aspiration to reduce runoff from the site to Greenfield rates is deemed to be impractical. An assessment of Greenfield runoff rates for the site utilising the ICP SUDS Methodology (Interim Code of practice for Sustainable Drainage Systems, Chapter 6) determined that the Greenfield flow Q BAR was 0.3l/s which would be difficult to control and require very high volumes of attenuation storage for which space on site does not permit. Using an assumed allowable discharge rate of 125l/s l/s, the required attenuation storage for the 1 in 30 year storm is between m 3 rising to between m 3 for the 1 in 100 year +20% event. The actual storage required will need to be based on the final drainage layout and strategy put forward for the site. During the detail design stage, these attenuation storages may need to be increased to cater for Phase II of the development. There are no local land drainage features at or near the site for surface water discharge. The site is underlain by Clay-with-flints which in turn overlies the Chalk. The Clay-with-flints is generally a low permeability material which is very unlikely to be suitable for shallow soakaways although deep soakaways may be considered. Attenuated surface water run-off from the proposed development will be drained to the local public sewer. It is proposed that run-off from the site is attenuated by utilising appropriate SUDS techniques before it is drained to the local public sewer. At this stage, it is envisaged that this volume can be achieved by using a combination of permeable paving with sub-base storage and cellular block type below ground storage.

32 Appendices

33 Appendix A 3965/TP02 Master Plan K52467/SK05

34

35 Ri dge Lvl Eaves Lvl Project no. Drawing no. Revision 3965 TP02 A Rev... Date. Sig.. Chk.. 9 TC Lvl dge Ri 8 6 TC5 to 1 K AR RP A DC KE C DE TE SI 2 E AS PH Y BA CE I RV SE re to S t if L O D w ce ffi.w F c.w n he tc i K M re i F t xi E c w h ns Cl St on ti ra pa re P s d o Go e l Ma T I s/ r e v r Se Di n i n g r o o m e l ma Fe s Di Fo y e r e c i f Of Ba r I n n e h c t Ki Up 8 HO TEL CE FI OF Fo y e r y r e v r Se Li f t s m u i r At t n a r u a t s Re Up I n TC2 e c n a r t En D RE Y HA WA ms 5 OT 7. 2 FO E/ CL Y C ep Ke r ea Cl ST A UE EN AV DS AN YL MA Ri dge Lvl Eaves Lvl D RE Y HA WA ms 5 OT 7. 2 FO E/ CL Y C S N A B D L A O R X1 X2 GMW Architects PO Box Kensington High Street London W8 6SL T+44 (0) F+44 (0) Project Hemel Gateway Drawing Title Master Plan. Status ow l S ' GMW Architects Do not remove this panel from the drawing No dimensions to be scaled from this drawing Report all errors and omissions to the Architect Dimensions to be checked on site before fabrication North Sheet Scale Drawn A1 1/500 CH Date Checked 07/10/08 SH Project no. Drawing no. Revision 3965 TP02 A