Flood risk along the A22
05/02/2014 08:59:00......Posted by Tim Pollard
Although the majority of Sanderstead has relatively low flood risk (with the exception of an area on the Surrey border), I thought it would be worth passing on an update on the Caterham Bourne and the heightened risk of flooding along the Bourne catchment area (which runs along the A22 down to Purley). Although this area falls outside the ward, the knock-on effects of any road closures will affect Sanderstead, as those residents who lived through the Limpsfield Road traffic when the Godstone Road was last flooded will attest.
Groundwater levels in the upper Wandle area are high, causing the Caterham Bourne to flow. The Caterham Bourne is located to the south of the borough running through both Kenley & Purley Wards then into Croham, Waddon and then into Broad Green Wards as it makes its way into Wandle Park and then into the river Wandle. The Bourne’s origin is in the upper Woldingham area and then makes its way along the valley coming into the borough along the A22 corridor. The Bourne runs parallel with the Godstone Road in the main and is a combination of open ditch and culvert.
The intense rainfall received over the past couple of months has saturated the ground causing the flow of the Bourne to increase significantly. During January the area has seen some 220mm of rain which has raised the water table immensely, to illustrate this the water table during the summer months is usually 25metres below ground, readings taken today at Kenley Treatment works recorded the water table had risen to within 0.75metres of the surface, this means surface water flooding is already being experienced along the low lying parts of the valley, in particular the Bourne Park Close recreation ground and adjoining open space heading toward the Kenley treatment works up to Bourne View. It is expected that surface flooding is likely to increase as the levels rise, this in combination of the Bourne flowing has put the area at high risk of surface flooding.
Many residents will recall the adverse weather over the Christmas of 2000/1 where the area suffered severe flooding and the closure of the A22 for a number of weeks. As part of the improvement to the Bourne during that time the council created a Bund in the Allotments at Bourne View: this enabled the area to flood within, rather than putting residential properties at risk. This flood area has put us in good stead this time as we have been able to control the flow of the Bourne and use this area to relieve the flow. The area is now approximately 75% full and unless rainfall decreases we will need to put emergency measures in place to extract water via tankers and or over-pump into sewers: the equipment needed to do this is currently being organised to ensure we can react swiftly.
The council has provided sandbags to a number of residents along Bourne Park Close and the parade of shops between the Police Station and Hayes Lane.
The trash screens are being kept clear and the flow rate is high, the culverted sections appear to be flowing strongly and this is causing surcharging of water into the water treatment works. Sutton & East Surrey Water, who manage this site, have put measure in place to protect the pump rooms but surface flooding is occurring. Just outside the borough in Whyteleafe the A22 Godstone Road has flooding at the junction of Maple Road. At the moment the road remains open and we are liaising with Tandridge council on it.
Downstream into Purley the Bourne is flowing well with no flooding to note at this stage, the Bourne then comes into Purley Oaks Depot and the balancing pond is approximately 80% full. We are working with Thames Water to ensure this is managed, we are also investigating the possibility of over pumping back into the surface water system if there is capacity to do so.
In terms of the weather, unfortunately things are not improving in the short term - the theme remains heavy and persistent rain with strong to gale force winds. The accumulative effect of the rest of the week’s rain is likely to see a further (35mm – 75mm) of rain over the next three days and the accompanying strong to gale force winds (40–60mph) may lead to significant issues.
Operationally we are clearing the trash screens each day and having a presence in the area, along with working closely with the treatment works to ensure the flow of water is managed. Council officers have pulled together a multi-agency emergency meeting tomorrow with the Environment Agency, Thames Water, Sutton & East Surrey Water and neighbouring authorities to pull together the collective risks and develop a joint action plan.
Going forward the council will continue to provide daily updates but in the meantime, the Environment Agency have advised that if residents would like to report rising groundwater levels or flooding they should call our Incident Hotline on 0800 80 70 60 or online via “my account” on the councils website.
In light of the recent severe weather, I have taken the opportunity to attach a link to information on the councils website setting out the Councils responsibilities for flooding. I would encourage members of the public to sign up to the EA’s free groundwater Flood Alert/Warning service by calling Floodline on 0845 988 1188 or registering online at http://www.environment-agency.gov.uk/homeandleisure/floods/34678.aspx?page=1&type=RiverAndTown&term=kenley