london borough of brent strategic flood risk … point level...jacobs were appointed by london...

London Borough of Brent

Strategic Flood Risk Assessment (SFRA)

Level 2: Wembley Point Site

June 2010

London Borough of Brent Brent House

349 High Road, Wembley Middlesex HA9 6BZ

London Borough of Brent Level 2 SFRA Wembley Point Site

Intentionally left blank


Table of Contents

1 Overview ................................................................................................... 1

2 Risk of Flooding From Rivers (Fluvial) ...................................................... 2

2.1 Fluvial Flooding Zone Water Depths ................................................. 2

2.2 Flood Hazard ..................................................................................... 3

3 Risk of Flooding From Other Sources ....................................................... 4

3.1 Surface Water Flooding ..................................................................... 4

3.2 Groundwater Flooding ....................................................................... 4

Site 24: Wembley Point .................................................................................... 5

List of Figures 1 Wembley Masterplan Area Flood Zones 2 Wembley Masterplan Area Ground Level Contours

List of Appendices

A Wembley Point Site


May 2010

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1 Overview Jacobs were appointed by London Borough of Brent to carry out the Strategic Flood Risk Assessment (SFRA) Level 2 for the Wembley Point Site area in April 2010. This report references the London Borough of Brent SFRA Level 1 completed in December 2007. This report details the Level 2 SFRA for the sites defined by London Borough of Brent site plan shown in Appendix A.

For all sites, it is essential that sustainable drainage techniques are stipulated as a development control recommendation to reduce the runoff from the site, thereby not exacerbating existing localised drainage problems nearby. This is a key recommendation of the Level 1 SFRA (refer Section 6.4.8 of the Level 1 SFRA).

Figure 1 presents the flood zones within the Wembley Point Site and shows the site is affected by a potential risk of flooding from River Brent. The flood zones are obtained from the River Brent Flood Study, Addendum Hydraulic Modelling Report (Jacobs, March 2009).

The ground level contours around the Wembley Point Site are presented in Figure 2. The ground level contours are derived from the Environment Agency filtered LiDAR data of the Brent catchment flown in January 2004.


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2 Risk of Flooding From Rivers (Fluvial)

2.1 Fluvial Flooding Zone Water Depths

The Wembley Point Site is situated immediately adjacent to the River Brent, downstream of Monks Park, Tokyngton. The site covers an area of approximately 12900 m2 with a maximum length and width of approximately 200m and 80m respectively. The site comprises one commercial high building with adjacent car parking. A proportion of the proposed development area is potentially at risk of flooding from the River, as indicated in Figure 1. The severity of flood risk within the site is represented by the PPS25 defined flood zones, delineated within the Level 1 SFRA as:

Zone 3b Functional Floodplain

Land assessed as having a 1 in 20 or greater annual probability of flooding in any year i.e. 5% Annual Exceedance Probability (AEP); and Areas of the region susceptible to flooding within which “water has to flow or be stored in times of flood” (PPS25).

Zone 3a High Probability

Land assessed as having a 1 in 100 or greater annual probability of flooding in any year (i.e. 1% AEP).

Zone 2 Medium Probability

Land assessed as having between a 1 in 100 (i.e. 1% AEP) and 1 in 1000 (i.e. 0.1% AEP) annual probability of river flooding in any year.

Zone 1 Low Probability

Land assessed as having a less than 1 in 1000 annual probability of river flooding in any year (i.e. 0.1% AEP).

To provide a depiction of the distribution of flood risk from River Brent across the Wembley Point Site, a summary of the maximum predicted flood depths within the flood affected site is provided in Table 2.1. This has been calculated as the minimum site level within the flood affected proportion of the site, deducted from the anticipated peak design flood level. Selected representative points on the site from the hydraulic model flood map files were considered for flood depth estimation. The peak flood water velocities estimated for the 1 in 1000 year event are also provided in Table 2.1. Peak water levels, minimum ground levels and peak velocities are taken from the River Brent Flood Study, River Brent Improvements, Hydraulic Modelling Report Addendum (Jacobs, March 2009), carried out on behalf of the Environment Agency.


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Table 2.1: Water Levels, Depths and Velocities at the Wembley Point Site

Maximum

ground

level

within the

flood

plain

Minimum

ground

level

within the

flood plain

1% (1 in 100)

Event plus Climate

Change

1 in 1000

year

Peak

Velocity

(m/s)

5% (1 in 20) 1% (1 in 100) Event

0.1% (1 in 1000)

Event

Event

Peak

WSL Depth

Peak

WSL Depth

Peak

WSL Depth

Peak

WSL Depth

(m AOD) (m AOD) (m AOD) (m) (m AOD) (m) (m AOD) (m) (m AOD) (m)

25.6 24.4 - - 25.8 0.7 25.9 0.9 26.3 1.3 3.0

Note: Flood depths are from River Brent Improvements Hydraulic Modelling Report Addendum (March 2009) : flood outline surface water floodplain peak level The method used to calculate the Wembley point site floodplain flood depths is to subtract the floodplain water level Triangulated Irregular Network (TIN) , using the 12D Digital Terrain Model (DTM) software from the filtered DTM (ground surface TIN) at representative locations where flood depths are high. This information was calculated as part of the River Brent Flood Study Addendum Hydraulic Modelling Report (Jacobs, March 2009). The design event flood outline SHP files were also derived from Jacobs 12D analysis of the ISIS water levels to produce the floodplain water level TIN. The hydraulic model results (Jacobs, March 2009) show there is no fluvial or surface water flooding for the 1in 20 year event around the Wembley Point area. Figure 1 of this report shows that the 1 in 20 year flood event stays in-bank within the Brent and Wembley Brook watercourses. In Figure 2 the filtered LiDAR data of the ground level does not depict the River Brent top of river embankment, which is above the 1 in 100 year plus climate change. The pockets of flooding presented on the Wembley site in Figure 2 between the 1 in 100 year and 1 in 100 year +20% is a representation of the likely pluvial flooding or surface water flooding on the site, not from fluvial flooding. The Wembley Point site shows fluvial flooding during for the 1 in 1000 year flood event when the river embankment is overtopped according to the hydraulic model (Jacobs, March 2009).

2.2 Flood Hazard

In order to define flood hazard mapping several factors should be taken into account. The Defra document FD2321 explains the flood hazard formula which includes velocity, depth and a debris factor therefore flood hazard is not only dependent on velocity and depth. Also the timing of the peak velocity may be different to the timing of the peak water level, therefore multiplying the peak velocity by the peak water level and the other factors may not give a an accurate flood hazard rating. This report contains the peak depth of flooding at the site for the 1 in 20, 1 in 100, 1 in 100 + climate change and the 1 in 1000 year event along with the peak 1 in 1000 year velocity. Defining the flood hazard rating of each site requires investigating the timing of the peak velocities and the peak water levels including a debris factor which is not identified at the SFRA stage. This flood hazard rating should be assessed during the more detailed Flood Risk Assessment (FRA) stage.


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3 Risk of Flooding From Other Sources It is important to consider not only the potential risk of flooding from rivers, but also flooding from other sources, including surface water and groundwater. This is discussed further below.

3.1 Surface Water Flooding An examination of the potential risk of surface water flooding across the Borough was carried out as part of the Level 1 SFRA. Further details relating to the precise date, location and/or nature of these incidents are unknown. This does however reiterate the importance of ensuring that all development within the precinct (i.e. not simply within those sites that are situated in an area at risk of flooding) adopts sustainable drainage techniques to reduce the rate of runoff. The potential risk of surface water flooding within the site does not appear sufficient to prevent future development of the Wembley Point Area. It is important however to ensure that future stages of the planning process places upon the developer a requirement for a detailed Flood Risk Assessment (FRA), in accordance with Section 6.7.1 of the Level 1 SFRA. The FRA should be carried out at an early stage, and should guide the layout and design of the proposed development, thereby reducing the potential impacts that the modifications of the site will have upon the existing drainage regime.

3.2 Groundwater Flooding The potential risk of groundwater flooding within the Borough was reviewed as part of the Level 1 SFRA (refer Section 5.6 of that document). In summary, it was concluded that the risk of groundwater flooding within the London Borough of Brent was generally very low.


May 2010

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Site 24: Wembley Point Description of Flood Risk Fluvial Flooding (Flood Extent & Depth) The site is shown on Figure 1 and has an area of approximately 1.3 Ha. The potential risk of fluvial flooding within the site is set out in the table below.

PPS25 Flood Zone Indicative

Percentage of Site Affected

Approximate (Maximum) Flood

Depth (m)1

Zone 3b Functional Floodplain 5% (1 in 20) Design Event

0% 0m

Zone 3a High Probability 1% (1 in 100) Design Event

1% 0.7m

Zone 3a plus Climate Change 20% 0.9m

Zone 2 Medium Probability 0.1% (1 in 1000) Design Event

99% 1.3m

Zone 1 Low Probability - -

1 Based upon a minimum ground level within the flood affected proportion of the site of 24.5m AOD

Speed of Floodwaters The source of flooding within Wembley Point Site is overland flow that has exceeded the capacity of the adjacent River Brent watercourse. In the 0.1% flood event, the overland flood water peak velocity reaches 3.0 m/s. Period of Inundation The 0.1% flood event was considered in order to estimate the inundation period at Node B.162j which is located just upstream of B.162. The anticipated period of inundation for the Wembley Point site is estimated to 24 hours for the event. Flood Defences The Wembley Point Site is not offered any form of protection from raised formal defences. Flooding from Other Sources (Surface Water & Groundwater) As explained in Section 3 above, Thames Water has provided information to indicate that there have been up to 5 reported incidents of sewer related flooding in recorded history within the immediate vicinity of the Wembley Masterplan area. The details associated with these incidents are not available, however it is important to ensure that future development does not increase the rate of runoff from the site. The potential risk of groundwater flooding within the site is considered negligible. Overview of Flood Risk (PPS25) The predicted flooding regime within the site is based upon The River Brent Flood Study, Final Hydraulic Modelling Report, Revision 2 carried out on behalf of the Environment Agency (Jacobs, September 2007).


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The site boundary is within Flood Zone 3 but is afforded a degree of protection by the river wall. For the 1 in 1000 year flood event the river Brent top of bank is overtopped and the entire site floods, therefore is in mainly in flood zone 2.The degree of protection to the site by the river embankment will need to be assessed further as part of the site specific FRA. From the hydraulic model results water level hydrographs at node B.162 (Jacobs, March 2009) inundation of floodwaters is likely to be up to 24 hours during flooding at Wembley Point. Planning Recommendations Spatial Planning A relatively large proportion of the Wembley Site is affected by flooding from the River Brent, and it is recommended that future development of this site is limited to areas situated outside of Flood Zone 2. This reinforces the intent of the Sequential Test which requires Council (and developers) to steer vulnerable uses within the site to areas of lowest risk. Any proposed development within flood affected areas of the site will have to meet the requirements of the Sequential Test before any future consideration of the site can be taken forward. Throughout the site, (re)development must deliver a measurable reduction in flood risk. Risk reduction measures for consideration should include SUDS to seek a reduction in the rate of runoff from the site. Development Control All proposed redevelopment within the Wembley Site will require a detailed Flood Risk Assessment (FRA), in accordance with Section 6.7 of the Brent Borough SFRA. It is essential that the adopted design follows the ‘sequential approach’, steering development towards areas of lowest risk within the site. The following development control recommendations must be incorporated into all redevelopment within the flood affected proportion of the site;

1. A positive reduction in the risk of flooding within the Borough must be demonstrated as an outcome of the proposed development;

2. Floor levels must be situated above the 1 in 100 year fluvial predicted maximum flood

level plus climate change, incorporating an allowance for freeboard (refer to section 6.8 of the SFRA);

3. As stated in PPS25 Practice Guide (June 2008), “basement development should only

be permitted in areas at flood risk if it passes the Exception Test, so the basement will be safe”. Determining whether the basement passes the Exception Test would be assessed during the more detailed Flood Risk Assessment stage

4. Implement SUDS to ensure that runoff from the site (post redevelopment) does not

exceed Greenfield runoff rates. Any SUDS design must take due account of groundwater and geological conditions (refer Section 6.9 of the SFRA).

5. To ensure the safety of the residents and employees during a flood, access and egress

routes must be designed to meet Environment Agency defined criteria, as set out in Appendix D of the SFRA. It is essential to ensure that the nominated evacuation route does not divert evacuees onto a ‘dry island’ upon which essential supplies (i.e. food, shelter and medical treatment) will not be available for the duration of the flood event;

6. Ensure that there is no net loss in floodplain storage as a result of the proposed

development. It must be demonstrated that the proposed development does not result in an increase in maximum flood levels, within adjoining properties.


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7. A minimum buffer zone must be provided for all culverted and open waterways. The minimum buffer width is 8 metres. Consultation with the Environment Agency is recommended at an early stage to confirm the minimum buffer width required.

8. As an integral part of the government’s “Making Space for Water” agenda, the

Environment Agency is actively seeking the renaturalisation of culverted watercourses as part of any future development. Realistic opportunities to reinstate the natural open waterway within existing culverted reaches of the river(s) should be heavily promoted.


May 2010

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P:\B0565100 Wembley Area SFRA Level 2\GIS\Wembley_Point_Flood Zone Map_270410_92.mxd

DATE27/04/10

CLIENTLondon Borough of

Brent Council

TITLE

Wembley PointFlood Zones

PROJECT

Wembley Brent SFRA

This map is reproduced from Ordonance Survey material with the permission of Ordonance Survey on behalf of the Controller of Her Majesty's Stationery Office.© Crown copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings.London Bourough of Brent 100025260, 2010

FIGURE 1

0 25 50 75 10012.5Metres

�

Legend

River Brent and Wembley Brook

Wembley Point SFRA Site

Medium Probability Zone 2 (1 in 100 to 1 in 1000 annual probability of river flooding*)

Climate Change Zone 3a (1 in 100 + 20% or greater annual probability of river flooding)

High Probability Zone 3a (1 in 100 or greater annual probability of river flooding*)

Functional Floodplain Zone 3b (1 in 20 or greater annual probability of river flooding)* Note: Flood zones are from the EA Commission River Brent Improvements Addendum Hydraulic Modelling Report (March 2009)

Wembley Brook

P:\B0565100 Wembley Area SFRA Level 2\GIS\Wembley_Point_DTM_110510_92.mxd

DATE27/04/10

CLIENTLondon Borough of

Brent CouncilPROJECT

Wembley Brent SFRATITLE

Wembley PointGround Levels

FIGURE 2

This map is reproduced from Ordonance Survey material with the permission of Ordonance Survey on behalf of the Controller of Her Majesty's Stationery Office.© Crown copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings.London Bourough of Brent 100025260, 2010

0 25 50 75 10012.5Metres

�

Legend

Ground level (mAOD) From Environment Agency Filtered LiDAR data

Below 20.5

20.5 - 21

21 - 21.5

21.5 - 22

22 - 22.5

22.5 - 23

23 - 23.5

23.5 - 24

24 - 24.5

24.5 - 25

25 - 25.5

25.5 - 26

26 - 26.5

26.5 - 27

Above 27

Wembley Point SFRA Area

River Brent


APPENDIX A

Location of Wembley Point Site

24. Wembley Point.

Address:Harrow Road, HA9

Ward:Tokyngton.

Area:1.2 hectares.

Description:Large office building surrounded by a large area of flat concrete used for carparking. The site is located immediately adjacent to Stonebridge ParkUnderground and Overground Station to the south and the North Circular tothe east and Harrow Road to the north.

Core Strategy policy context:Core Policies 13, 16, 17 and 18

Planning guidance:None planned.

Planning history:None relevant.

Allocation:Subject to the sequential and exception tests, a level 2 SFRA and Flood Risk Assessment (in accordance withPPS25), redevelopment of site to retain the office building in its current use; develop part of the site not in thefunctional flood plain for a residential development and other complementary uses. Development should deliveran improved pedestrian experience and linkages to the Stonebridge Park Underground Station and provide forimproved pedestrian access across the North Circular Road. Residential development will not be permitted within30 metres of the central section of the North Circular Road and will require mitigation of noise pollution.Development should have regard for the possible Fast Bus route through Park Royal.

104 unitsIndicative development capacity

2015 - 16Indicative development phasing

Flood risk comments:Southern parts of the site are affected by flood risk zone 3b and the majority of the site lies within flood risk zone2 (see Brent’s SFRA for more information). Flood zones are subject to change and modelling and re-modellingis carried out on a quarterly basis by the Environment Agency.

Justification:Redevelopment will maximise the use of this site, with development on land currently used as a car park.

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London Borough of Brent | Site Specific Allocations - Proposed Submission June 2009

Site Specific Allocations - Proposed Submission June 2009

london borough of brent strategic flood risk … point level...jacobs were appointed by london...

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