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Flooding Report - Existing Surface Water Drain System based on Thames Water Drg 5136-1

Existing Surface Water Drain System based on Thames Water Drg 5136-1

The existing surface water drainage system consists of a 1,220 mm dia main drain, fed from a series of smaller diameter pipes. There is a second main drain of 900 mm dia which joins into the 1,220 mm dia drain along Ash Lodge Drive at Avon Close. The 1,220 mm dia drain then continues on its route to the Blackwater taking in further smaller diameter feeds on the way.

Culvert A14/1 joins the 1,220 mm drain at a point near manhole hole 1151 conveying the surface water from the Hogs Back area. It has been noted that this large 1,200mm x 800mm feed has been omitted from the Thames Water Waste water drawing 5136-1

In fact there are at least 4 water courses that enter this 1,220 drain system that are not shown on this drawing. Thames Water do not therefore take these figures into their calculations.

An assessment has been carried out to calculate how much water is carried in each main pipe during varying levels of rainfall.

Each area that feeds into the drains has been assessed based on the following basis:

Percentage amount of rainfall transferred to drains

Road and pavement surfacesRoofs and garage areasDrives and flagged areasGrassy Runoffs Class 4 Soil
100% 100% 100% 50%

Note that the 50% surface water value from the grassy runoff will rapidly increase to saturation excess (100%) due to the impermeable nature of the clay. Calculations for this study however have been based on 50%.

The areas calculated for each drain are as follows. (Note that the park area that slopes down from Ash Church Road to Collins Gardens has been included as this water runs down into the Collins Gardens drains).

Main Drain 1,220mm dia Areas in metres2

Road and pavement surfacesRoofs and garage areasDrives and flagged areasGrassy Runoffs x 50%Total
32,750 50,720 48,600 40,696 172,766

Secondary Drain 900mm dia Areas in metres2

Road and pavement surfacesRoofs and garage areasDrives and flagged areasGrassy Runoffs x 50%Total
16,700 21,160 22,300 25,940 86,100

The detail calculations for the above values are available on an Excel spreadsheet.

Based on the above results it is possible to calculate the flow via the drains into the River Blackwater resulting from various rates of rainfall.

As previously stated culvert A14/1 enters the main drain approximately halfway along Ash Lodge Drive. The culvert carries water from a ditch that drains the large amount of water that is held back by the natural bund formed by the disused railway embankment. The flow is fairly constant at 3600m3/hr (1m3/s). The table below therefore include a constant value add-in of 1m3/s for all the 1,220 pipe values from 20mm/hr upwards. A lower value of 0.5m3/s has been included in the 10mm/hr flow rate as this rainfall rate does not usually result in a full culvert.

Table of flow against rainfall

Rainfall10mm/hr20mm/hr30mm/hr40mm/hr50mm/hr
Flow rate in 1,200mm m3/s 0.98 1.96 2.44 2.92 3.40
Flow rate in 900mm m3/s 0.24 0.48 0.72 0.96 1.20
Combined flow after junction 1.22 2.44 3.16 3.88 4.60

The system is rapidly overwhelmed by rainfall in excess of 20mm/hr. This is confirmed during the torrential rain of August 2008 when a 30 minute rate of approximately 40mm/hr was noted with subsequent flooding of properties.

From the values noted in the flooding study the River Blackwater can take a flow of up to 1.25m3/s before flooding occurs. It can be seen from this table that the combined flow from these pipes could cause the river to flood at even the 20mm/hr rainfall rate.

What actually happens is that much of the excess surface flood water flows in torrents into the fields to the south of Ash Lodge Drive which serves as a flood plain thus saving many properties from flooding. Residents in the area will confirm the large volumes of water that sweep across their gardens and into the fields.

These fields have been 100% flooded right across to the disused railway embankment on a number of occasions, the extent being included in the Capita Symons report of January 2009 commissioned by GBC. Partial flooding occurs every year.

The continued allowing of new developments by GBC in the locality has steadily made the flooding worse. Areas that were natural flood plains have been turned over to developers with the consequential loss of natural water holding capacity and the inevitable increase of more surface water into the drains thus increasing the flood risk locally, and in areas of Surrey, Hampshire and Berkshire along the River Blackwater. Recent developments have included, Bateman Grove (20 homes), Parish Close (61 homes), Oaktrees (45 homes), Collins Gardens (13 homes), Queen Elizabeth Close (10 homes) and The Briars (53 homes). A total of 202 homes that have been added to this area.

The proposal to build on this land is not feasible even with the most sophisticated SUDS system as not only will it take away the flood plain facility, it has been shown that the development would add substantially to an already unacceptable flow into the River Blackwater causing flooding in Hampshire and as far as Frimley.

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