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Flooding Report - Omission of flows into the surface Water Drainage System

Omission of flows into the surface Water Drainage System

Due to the concern over the omission of a number of substantial flow inputs into the Thames Water drainage system shown on drawing 5136-1 an enquiry was sent to Thames Water to clarify the situation.

Extract from enquiry to Thames Water 14/02/2012

"I have been asked to contact you by Ash Residents Association with respect to the assessment of storm drain capacities and current applications for development in this area.

Because the area floods frequently and also because of the critical situation with the capacity of the River Blackwater (into which the local drains flow) I was asked to do a drains study. It is evident from past occurrences in 2007 and 2008 that the storm drains concerned have insufficient capacity during periods of very heavy rain, with storm water coming up out of the drains and raw sewage floating on top.

Having compiled the inputs and expected flows into the storm drains I have noticed that a number of inputs into the drains are not shown. One in particular that causes concern is GBC culvert A14/1 which enters the 1,220mm drain beneath Ash Lodge Drive at a point near manhole 1151. This culvert is 1,200mm x 800mm and carries flow from the region of the Hogs Back. Flow is held back by a natural bund but the water is allowed though a restricting pipe at a rate of up to 1m3/sec. Because of the large quantity of water held back by the bund this flow continues, for many days at that rate, following a storm.

"The reason I am given for their omission, is that these inputs are unadopted."

"Are the flows from these unadopted inputs taken into account when calculating the capacity of the storm drains? I am particularly concerned with the flows in the 1,220mm pipe and the 900mm pipe which run parallel with Ash Lodge Drive and join at Avon Close near manhole 9051. The drains are already capable of putting more into the River Blackwater than it can handle and hence the several recent floodings of properties alongside its course."

Response from Thames Water 23/02/2012

Extracts from Thames Water reply:

"I have to admit to having very little knowledge of the catchment to which you refer and I have had to consult with our catchment planning team for their take on the queries you raise."

" It is common that private or unadopted assets are not show on the Map of Sewers. Where we have sufficient data we represent them as unadopted sewers, but we often do not have correct location and asset data in order to map them."

"Thames Water completes various levels of hydraulic assessments on the public sewer networks under its control. This is dependant on the size of the sewer, the proposed increase in capacity as well as issues that have affected our existing customers in the vicinity (i.e. reported flooding events). In many cases we expect developers to fund investigations that we undertake to determine the impact of their proposals on the existing sewer system, as well as to identify any infrastructure enhancements that could be implemented to mitigate against a deterioration in the performance of the sewer system due to proposed development. We have not recently undertaken any detailed modeling assessments in the area of Ash that you are interested in."

"As this is a surface water system, there will be rainfall events that will exceed the design capacity of any surface water sewer system and the hydraulic capacity of the local watercourse. This has the potential to flood the surface water outfall and thereby restrict the hydraulic capacity of the public sewer system."

Comments on reply from Thames Water

It is intimated from Thames Water Authority reply that they do not have sufficient data on the unadopted inlets to enable them to carry out any detailed modeling of the area we are concerned with, and would expect the developer to fund such investigations. It is evident from recent applications for development in the area that the Flood Risk Analysis carried out by the consultants contains none of this data - the consultants are probably totally unaware of the existing drains structure and in particular the high flow rate from A14/1. In consequence the whole process and subsequent planning approvals would be based on highly flawed information.

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