Clay’s Lake enters home stretch

A flood prevention scheme located near the West Sussex town of Crawley is nearing completion, with Mackley currently on-site to carry out the final phase of works.

The flood prevention works at Clay’s Lake are being carried out by Mackley working as part of Team Van Oord on behalf of the Environment Agency.

The Clay’s Lake project forms part of the Upper Mole Flood Alleviation Scheme which was developed by the Environment Agency after floods in the Crawley area during 2000.

When complete, the scheme will reduce the risk of flood to more than 280 homes in the area, and to Gatwick Airport.

A key element of the project – the removal of an existing dam and its replacement with a larger dam to increase the lake’s capacity from 10,000 cubic metres to nearly 400,000 cubic metres – was completed in September 2017, shortly before a seasonal break in on-site activities.

Mackley returned to the site in April to carry out the following tasks before the project is handed over to the Environment Agency in September:

  • Various MEICA (mechanical, electrical, instrumentation, control and automation) works
  • Trimming the overbuild from the embankment
  • Installing grasscrete to the downstream spillway
  • Creating an access track across the dam
  • The installation of deer fencing and gates around the perimeter
  • The completion of landscaping at the borrow area
  • Installing fish brushes through the culvert
  • Grass seeding the embankment
  • Re-stocking the lake with fish

Dan Burden, Mackley’s Site Manager, said:

“Following construction of the embankment over the last two summer seasons, we mobilized for the final time in April 2018 to complete the works.  

“The dam embankment was overbuilt towards the end of 2017 to allow for settlement, which was monitored throughout the shutdown.  

“We’ve now trimmed the overbuild from the dam and the spillway and reinstated the borrow pit land with subsoil and topsoil in readiness for the Environment Agency’s planting contractor.  

“The borrow pit excavation created a void in the landscape which was shaped into a spring fed pond with an overflow bypass installed in 2017.”  

The grasscrete spillway, currently under construction, will provide scour protection to the downstream face of the dam embankment in the event of a one in a 1,000 year flood event. However, the dam is designed and constructed to withstand a one in 10,000 year ‘Probable Maximum Flood’.

Ducting and watertight drawpit installations to receive the cabling for the telemetry and power requirements at the control structures are also underway.  

Dan Burden added:

“We are constructing an operations building with the foundations, steel frame structure and cladding currently being installed. This building will eventually include an area for tools and welfare facilities for the Environment Agency personnel when they visit the site.

“Following completion of the grasscrete spillway, the secondary concrete works will be installed along with fish brushes to the culvert – prior to installation of trash screens and platforms within the control structures.  

“Once the grasscrete is signed off, sacrificial fill and topsoil will be placed downstream of the works and the gabion retaining walls will be completed.

“Finally landscaping and seeding of the embankment and demobilisation will be carried out in readiness for handover to the client in September 2018.’’

05 July 2018

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