Sunday, 20 March 2016

Flood risk from new housing developments

The risk of flooding caused by new developments is a serious issue, as witnessed recently in the North of England. There is increasing pressure to build on flood plains but that is not the only problem. All new developments on green field sites channel surface water rapidly into rivers instead of absorbing it into the ground and trees. This poses enormous flood risks to downstream settlements which developers will ignore if they are given half a chance. They can justify each individual small development by showing that it does not add significant risk itself, but overtime the accumulated impact of multiple developments builds up until an exceptional weather event causes severe flooding in towns elsewhere.

surface water risks around Dunton

This problem is particularly relevant in areas around Basildon where surface water drains into the Mardyke and Crouch rivers. Developments in Basildon and Billericay will be of great concern to residents of Ramsden Bellhouse and Wickford where the Crouch has come very near to bursting its banks this Winter.

Crouch near to flooding in Wickford January 2016
We need to include comments to the Basildon and Brentwood Local Plan consultations objecting to overdevelopment on the grounds of flood risk. Here as an example is my objection comment submitted to the Brentwood consultation:

Flood Risk
Development of Dunton Hills and West Horndon will pose a very high risk of flooding especially through its onward effect on the Mardyke River.
The Environment Agency flood map shows that the location for the development of Dunton Hills Garden Village is at a high risk of surface water flooding in several areas. It is a critical drainage area. This means that it presently absorbs large amounts of water including water that runs off from surrounding areas. Removal of trees and other vegetation will reduce the ability of the area to absorb rainfall. Rain that falls on roof tops and road surfaces will be quickly channelled through surface drains needed to prevent flooding in the new development area.
The altitude of the land is mostly around 40m. The A127 presents a barrier to drainage systems because it is at a lower altitude of about 20m. Therefore most of the surface water will have to be drained towards the South and West via the Mardyke tributary and into the Mardyke itself.
It will be necessary to take into account events of extreme rainfall when up to 50mm can fall in a period of 24 hours. At times of persistent rain that are common during the winter in this area this could continue over periods of several days. Rain falling on 250 hectares of developed land will have to be managed via the drainage system. SuDS will not be sufficient to mitigate the risk to the wider area along the Mardyke. This could amount to 100,000 m3  of extra rain water in a day. It is estimated that the Mardyke River has a typical capacity of 50,000 m3  per day and is already prone to flooding causing inundations around West Horndon and Tilbury.
The development of the Dunton Hills area will thus multiply by a significantly factor the amount of water entering an area that is already the scene of many past floods. Mitigation will require an extensive system of flood defences and pumping stations at enormous cost. This will have to be done before building starts and will require agreements between Thurrock and Essex that are likely to be difficult to reach agreement on. There is a danger therefore that development will go ahead without any new flood defenses and nothing will be done until the first major flooding event has already happened.
In my opinion the cost and uncertainty over the ability to control the flood risk makes the Dunton Hills Garden Village development unacceptable in its own right.

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