Login

Home

Comments on the Flood Risk Assessment - Supporting Planning Application 12/P/00645

This document details issues arising from the Flood Risk Assessment presented by Odyssey Consulting Engineers in support of Planning Application 12/P/00645 (Land at Ash Green Lane West, Ash)

Odyssey Consulting Engineers FRA Statement states:

3.16 The SFRA ‘Other Sources of Flooding’ map indicates that the local area was affected by a flood event in August 2006, and although there is no evidence within the SFRA to suggest that this particular site was flooded, properties in Ash and Ash Vale were seen to flood as a result of surface water flooding. This is believed to be due to the poor maintenance regime of the ordinary watercourses and surface water sewers within the area.

Ash Residents Association Response

The area is noted for flooding during periods of high rainfall and there are Guildford Borough Council records showing more extensive flooding on a number of occasions, most recently in 2000, 2006, 2007 & 2008.  The Capita Symonds report, commissioned by Guildford Borough Council and dated January 2009, shows total flooding of the area right up to the southern border of the railway embankment, in 2000 and 2006.  Following this report serious flooding occurred again in July 2007 and August 2008 when again the fields to the south of Ash Lodge Drive were completely flooded.

The flooding was not due to poor maintenance but to sheer volume of water which the main 1220mm drain was unable to handle.

Correspondence with Thames Water has confirmed that they do not have a complete picture of the water entering the 1220mm surface water drain and agree that numerous private drains, unadopted drains and watercourses are not shown on their drawings nor included in their hydraulic calculations.

One unadopted watercourse in particular, the entry via culvert A14/1, which receives surface water from areas of Ash Green, Poyle Road and areas of the Hogs Back flows at rates up to 1m3/sec and would flow at an even greater rate if the flow were not restricted by the natural bund of the dismantled railway, where it passes through an 18” diameter pipe.

Water being restrained by the natural bund 29/04/2012
Water being restrained by the natural bund 29/04/2012

Water heading towards culvert A14/1 29/04/2012 through the fields to the rear of Ash Lodge Drive.
Water heading towards culvert A14/1 29/04/2012 through the fields to the rear of Ash Lodge Drive.

Culvert A14/1
Culvert A14/1

The 1220mm drain discharges into the River Blackwater just north of the A323/A331 junction.  Calculation of the capacity indicates that it could carry up to 3.25m3/sec.  Telemetry from the Environmental Agency River Blackwater gauging stations indicate that flooding from the Blackwater occurs with flows greater than 1.25m3/sec

Recent flood events of 2006, 2007 & 2008 have all resulted in flood water pouring out of the 1220mm drain into the roads indicating that the drain is undersized to cope with all the entries into the system.  The massive flow has resulted in flooding of the Blackwater.  Flood alerts were issued again by the Environmental Agency on 29/04/2012 when the recorded levels were only centimetres below the level at which flooding occurs.

Odyssey Consulting Engineers FRA Statement states:

10.26 In total there are four points of discharge into the ordinary watercourse adjacent to the northern boundary of the site. The first caters for the adoptable highway, the second and third cater for the surface water runoff from a small number of dwellings to the north of the development. The fourth point of discharge in the north east corner of the site caters for surface water runoff from the remaining of the development.

Ash Residents Association Response

The discharge of water into the ordinary watercourse at the north of the development is not recommended for a number of reasons.

  1. This watercourse passes through a field (adjacent to the school playing fields) which is regularly flooded.
  2. The route described in the application is incorrect.  Having turned north the water course runs alongside the field  and then turns west under the school fence and joins the school drainage gulley before descending into a chamber.  It is then routed to the northern side of Bateman Grove where it then joins the drainage system (eventually the 1220mm drain described above).  The concern here is the water is being routed into school playing fields with possible water soluble contaminants from the development.  There is no connection between this watercourse and the one described in the application which would have taken the water in to the 1220 drain near Avon Close.
  3. The watercourse runs alongside South East Water main feed pipes of up to 14” in diameter adjacent to the site.  These pipes are in poor condition and have ruptured 3 times in the last 3 years.  Each time there has been extensive flooding of the field nearest the school, Ash Green Lane West, surrounding land and the associated watercourses.  The most recent rupture occurred at 2pm on 4th January 2012.
  4. The Environment Agency recommends (in the letter to the developers) quote “We understand that there is a ditch on site adjacent to the northern boundary of the site. This is not shown on our OS map but does appear to be visible on aerial photographs of the area. This would be deemed as an ordinary watercourse and as such should be crossed by bridges (not culverts) and ideally should have a 5m buffer left along side it.”  We believe it will be impossible to achieve the 5m buffer anywhere along the route.
  5. The watercourse route shown of photographic development plan is incorrect.

 

End of watercourse in field showing continuation of flow into school playing field
End of watercourse in field showing continuation of flow into school playing field

Chamber in school field taking flow from watercourse proposed for this development.

Chamber in school field taking flow from watercourse proposed for this development.

Odyssey Consulting Engineers FRA Statement states:

8.2 Extract:  Any overland flow during extreme flood events will flow unrestricted away from residential development via the road corridors and open space at the low lying catchment boundary to the ordinary watercourse.

The above events are termed Exceedance.

Definition from water authority SUDS manual:-
“Exceedance Routes When SUDS are overwhelmed by exceptional rainfall, exceedance routes protect people and property by providing unobstructed overland flow routes from the development.”

Ash Residents Association Response

This indicates that floodwater from the development will be passed (in extreme events) directly into the watercourse.  This is wholly unacceptable.  The levels in the Blackwater are at critical levels and even the smallest increase in discharge into the river during a period of high rainfall is highly likely to cause flooding in Lakeside, and in areas of Hampshire to the west of the river.

SUDS systems in general are prone to flooding in storm events, their limited capacity being overcome by prolonged periods of rain.  One local example of this is one of the examples of a comprehensive SUDS system described in the Environmental Agencies document “SUDS a practical guide (Thames region)”.  This is the 62ha development at Elvetham Heath. The SUDS system there was overwhelmed by the rain on Friday 20th July 2007 resulting in the flooding of properties in Chineham Close and Mounts Way.

In cases such as this the discharge rate from the proposed development is likely to be between 5 to 7 times the normal Greenfield discharge rate which is wholly unacceptable.

Odyssey Consulting Engineers FRA Statement states:

10.23 It is proposed that surface water roof run-off from all private roof areas will drain to a back garden rainwater harvesting system (water butt) which will store rainwater for irrigation. An overflow pipe from the proposed water butts shall connect to a private gravity network which in turn will connect to the SuDS features in the private areas or roads (cellular storage and permeable paving) providing treatment and attenuation.

Ash Residents Association Response

The use of water butts cannot be considered as a means of preventing or reducing flooding.  Most water butts remain full from October through to May as the requirement for garden watering is nil.  As a result storm water will directly enter the SUDS pit and cause it to rapidly fill and overflow.

SUDS Manual CIRIA C697 states that water butts “Only provide marginal stormwater management benefits, and have a high risk of blockage on small throttles. The property owner is responsible for operation and maintenance, therefore cannot be guaranteed”.

Odyssey Consulting Engineers FRA Statement CSa/1820/104 Illustrative Masterplan

Ash Residents Association Response

This drawing shows the blocking off of the western end of Ash green Lane West by the development and replacing access with a narrow footpath between properties.  We believe this is unacceptable as it prevents vehicular access to Ash Green Lane West, which would be needed to service and repair the South East Water main feed pipes.

Domestic Water Supplies

This site would require a water supply of 21,330 litres per day.  It must be emphasised that the area is in a drought region and is currently under its 2nd hose pipe ban within 5 years.  The environment secretary Caroline Spelman stated on 2nd May 2012 that “councils would have to consider whether new developments can be supplied with water when making planning decisions”.

Authors: Mr. Anthony Pugh, Consultant Engineer and Ash Residents Association

Newsletter Subscription

We will, from time to time, want to communicate information to residents without it appearing on our website. Therefore, if you have not given us your e-mail address in the past, can you please click this button and supply it now. All e-mail addresses will be kept securely and will not be disclosed to anyone else. Due to a large amount of fake subscriptions we are now asking for your road name and postcode in order to validate you are a real person. This information will never be made public and will only be used to validate your subscription.
Name: Email:
Road Name: PostCode: