Contractor perspective on the Upper Mole Flood Alleviation scheme, Norwich

21 September, 2016 | 18:00 - 20:30

An aerial view of Tilgate lake.
An aerial view of Tilgate lake.

About this event

The River Mole Flood Risk Management (FRM) Strategy that encompasses this project was approved by the Environment Agency's Board of Directors (National Review Group, NRG now known as Large Project Review Group) in July 2005. This was then developed into the Upper Mole Flood Alleviation Scheme, which was approved by NRG in June 2009.

Clay's Lake flood detention reservoir is one of four such reservoirs which make up the Environment Agency's Upper Mole Flood Alleviation Scheme is approximately 4km south of Crawley, West Sussex and just west of the northern portal of the Balcombe Forest Railway tunnel (London to Brighton railway). There is an existing lake present on site, held back by a small dam on the Stanford Brook believed to have been built in the 1950s.

The works comprise:

  • Removal of the existing dam and replacement with a new higher dam. The lake will be reinstated to its existing level after the works are complete, with the increase in dam height used to provide flood storage
  • Construction of a permanent access track from Parish Lane to the dam site
  • Development of an onsite borrow area to provide fill for the dam, and additional reprofiling/ enlargement to the landowners requirements.

The new dam will replace the existing smaller dam at the lake with no change in normal reservoir water level. The dam is a 170m long embankment and approximately 11m high at its highest point where it crosses Stanford Brook. The dam will provide 350,000m3 of reservoir storage volume.

Stanford Brook is conveyed through the dam in a 1.8m diameter culvert on the right-hand side of the dam. Flows into the culvert will be throttled by a 380mm square orifice at the upstream end of the culvert to provide inherent flood attenuation. This orifice will be mounted on a penstock gate and protected from blockage by three levels of inclined trash screens within a reinforced concrete inlet structure. The design also incorporates a bypass pipe to allow the reservoir to be drained in the event that the screens block and become submerged. A 35m wide grasscrete spillway is provided over the central part of the dam. The reservoir will fill to top water level in floods greater than 1 in 1000 annual exceedance probability (AEP).

For more information please contact Andrew McCloud or Michelle Hughes e: or