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Flooding Report - Reasons for flooding

Reasons for flooding

Assumptions

To illustrate the effects of surface water flooding, a storm rainfall rate for 1 hour of 24 mm has been shown. Records for 20th July 2007 show that 110 mm fell in 24 hours with a peak of 24mm in 1 hour.

Flooding from the north

During heavy rain storms, rivers of water from the Ash Street and Ash Church Road area flow rapidly downhill through residents’ gardens, down roads and alleyways towards the fields. Water bursts up through drain covers, out of road drains, and raw sewage is also thrust up into the torrent of water. The storm drain is rapidly overwhelmed, and so the water flows to the lowest point which is the fields south of Ash Lodge Drive and Loddon Way. Once the fields are flooded the water then continues to rise and starts flooding the homes. An estimation of the quantity of water arriving from this direction could be as much as 8,400 m3 per hour for a rainfall of only 24mm/hr. The 100year+30% rate of 52mm/hr would see around 18,200 m3 per hour crossing Ash Lodge Drive and Loddon Way into the fields.

View from Ash Street looking down the slope of Ash Lodge Drive towards the development site (behind the houses facing). Storm water rushes down this and other adjacent roads in torrents to flood the site.

Particularly bad is Ash Church Road which rises to a high point near St Peter’s Church and then descends to Ash railway station.

For example on Monday 11th October 2012 at 09.00 the writer was travelling to Woking for a meeting with Woking Borough Council. It was raining fairly heavily but not storm conditions. Considerable flooding was noted in Ash Street outside the police station, and as the writer travelled up Ash Church Road there were torrents of water gushing out of the drains. Rivers of water were running down each side of the road towards the junction with Grange Road and round into Ash Street by the police station. Passing over the brow of the hill by St Peters Church there was a similar situation with the water gushing out of the drains forming rivers running down both sides of the road towards the Ash railway station crossing.

The police station yard has indeed been flooded on a number of occasions and in one particularly bad event in 2006, the flow south from Ash Street, through the yard, was so great that after bursting through the rear fence of the police station yard, the floodwater then flowed rapidly across the properties in Southlands Close and eventually adding to the flooding in Southlands Road. Such was the ferocity of the flow that some properties in Southlands Close suffered under-floor damage to their suspended floors, which included electrical cabling being torn out. One owner still has the invoices for the repairs necessary totalling well over £1,000.

Southlands Road – South Ash (July 2007) – houses on the right all flooded internally

Southlands Road near Lea Close– South Ash (July 2007)

Map showing water courses and main directions of flooding into development site.

Map showing main areas of flooding following a storm event

Flooding from the South

The high ground to the south of the proposed development area has the potential to shed huge quantities of water during heavy rain storms. The water is bunded by the railway embankment, causing the southern side of the embankment to flood and the water rises rapidly in the area near the 18" transit pipe to depths often up to 1.5 metres.

Water from Hogs Back starting to flood fields bunded by disused railway embankment. – (1.2m deep near the embankment) - This picture April 2012 but it is a regular occurrence. This is the area to be used as the Bewley Homes SANG

The transit pipe acts as a restriction orifice and allows water through at a rate of approximately 3600m3/hr, depending on the head of water at the pipe entry. The writer has noted on many occasions the northern exit of this pipe completely flooded to a height of 300-400 mm above the top of the pipe. On some occasions it rises so high that it reaches the bottom step of the adjacent stile.

Northern exit of transit pipe (top of pipe submerged) – picture taken April 2012

The water flows rapidly along the large watercourse towards culvert A14/1. Approximately half way along it reaches a soil bridge area between 2 fields which is penetrated by 2 x 12 " pipes. These cannot cope with the flow and the water floods out of the ditch and into the fields. The water also floods over the top of the bridge area into the downstream side of the watercourse. This water flows at an alarming rate (in excess of the EA recommended maximum) and the bridge area cannot be safely crossed to get from one field to the other.

Water has overwhelmed twin 12” transit pipes and is now rapidly flowing across soil bridge. – picture taken April 2012 (similar pictures are available for January 2012 and December 2012)

The volume of water then travels rapidly along the open watercourse, and then enters culvert A14/1 and then into the storm drain which copes in normal rainfall conditions. However, as previously noted, in heavy storm conditions the Ash Lodge Drive storm drain is unable to cope with the water from the residential area north of the fields, and so flow into the drain is blocked (and may even reverse). The water in culvert A14/1 cannot then enter the storm drain and the level rapidly rises adding to the flooding in the fields, and also to the rear gardens, garages and drives of the properties on the south side of Ash Lodge Drive. It should be noted that this flow through the transit pipe continues for many days after the rain has stopped, due to the large volume of water held back by the bund effect of the railway embankment. Releasing this water at a greater rate would inevitably cause flooding downstream both locally and in the Blackwater floodplain between Tongham and Frimley Green.

Water flowing towards A14/1 – This water course is fast flowing, deep with steep sides and would be a source of considerable danger of drowning to children and adults.

Flooding of garden from excess flow into culvert A14/1

Flooding from the West

The greater part of Manor Road and Ash Manor School is also between 2 and 5m higher than the development fields and surface water also floods down into the area during storm conditions. The watercourse bordering the western side of the field is narrow and shallow and rapidly fills, allowing water to then flood into the field.

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