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Flooding Report - Greenfield Runoff Rates

Greenfield Runoff Rates

The developer has recognised that this area is subject to extensive flooding, as he has proposed raising the development site by at least 300mm in an attempt to provide some protection to the new homes.

There are however serious problems with the Flood Risk Assessment and also several of the documents cross referring to this.

Firstly the developer has used the desk based calculation on the highly inaccurate IH124 procedure to obtain a Greenfield runoff figure. The IH 124 Report is dedicated to small catchment hydrology but nevertheless its formula for QBAR, which is in widespread use for SUDS applications, is based on data of 71 catchments < 25 km2, only one of which is

The fields are low lying and have lips around the perimeter of each field between 200 – 400mm high mainly caused by the soil dug to form the watercourses being heaped along the side. Also the root structure of the trees and hedgerows has caused considerable raising of the perimeters, so each field resembles a tray. There is therefore virtually no surface water runoff from any of the fields. Bewley Homes own soil infiltration tests have also shown there to be no measurable infiltration so the only way flood water levels reduce is by evaporation. This is born out by the fact that after a period of rain the fields remain in a state of flood for months.

View along Footpath 350 (watercourse behind hedgerow) note raised bank and no runoff.

View alongside field adjoining Ash Lodge Drive note raised bank and no runoff. FP350 underwater in foreground.

View alongside field adjoining Ash Manor School (watercourse to right) note raised bank and no runoff

The existing greenfield runoff figures (desk based calculation) of between 44 and 48% claimed in section 5.11 table 2 of the developer’s FRA are simply absurd. The true value is probably around 1%. Having studied the runoff in all accessible fields during a period of rain, one can find only 2 places where any actual runoff occurs. One is at the junction of FP349 and FP351, where a deep pond area discharges a trickle through a very small gulley into a ditch. This has been scraped in the soil in a futile attempt to keep FP349 passable , which is still 200mm deep in water The other is halfway along FP351, where another very small gulley 100mm wide drains a small puddle area into another ditch at a very low trickle rate.

The QBAR figures therefore claimed in the developer’s FRA are totally invalid.

Junction of FP349 and 351. FP349 is under water, note yellow GBC notice of planning application in background – picture taken 20/12/2012

This gross overstating of the existing greenfield runoff renders their whole flood risk assessment worthless.

It would require a depth of flooding of at least 200mm in the fields before any surface water runoff into the watercourses occurred. As there is virtually no greenfield runoff on the undeveloped site any runoff from the new development would be a net gain into the surface water drains causing an increase in the flood risk to the River Blackwater and the existing properties in Ash Lodge Drive, Loddon Way, Southlands Road and Colne Way.

View back across field from FP349. – FP351 is on the raised portion to the left – picture taken 20/12/2012

Aerial view from Bing maps of typical flooding after some rain. Ash Lodge Drive is at bottom of picture.

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