Westbrook Weir Modelling and Outline Design

Tender description

The Environment Agency is seeking to appoint a Contractor to undertake flood risk modelling and outline designs relating to Westbrook Mill weir on the River Wey (SU9666944205) and its proposed removal. Site Background Westbrook mill has been present for approximately 300 years, and numerous weirs and control structures have been in place maintaining a head of water for its operation. The mill and associated factory have been closed for decades and there is negligible heritage value associated with the site. The weir at Westbrook Mill consists of six 1.5m wide sluice gates in steel frames. There is a difference of 1.3m between the upstream and downstream bed levels, and a maintained upstream water level 1.6m above the weir sill of 36.3m AOD. The side walls downstream of the weir are cast concrete and there is a cast concrete central pier which extends 15m downstream under the road bridge giving access to the complex car park. An access walkway is present over the weir, and the weir is manually operated. The cast concrete 'troughs' formed by the walls and the central island also have a cast concrete bed slab set at approximately 36.2m AOD which functions as a second, shallow weir at low water level conditions. Upstream of the weir the channel is confined between steel sheet pile walls with concrete capping beams for approximately 50m. There are three overspill weirs upstream that feed the Hell Ditch (SU9582944564, SU9630344350, SU9655644244), which rejoins the main river approximately 3km downstream. There is also an informal breach which introduces additional flow into the Hell Ditch (roughly SU9649544320). In 2016, the Environment Agency installed a brush-furnished fish pass at the most-upstream overspill weir, allowing the Hell Ditch to function as a fish migration route bypassing Catteshall and Westbrook weirs on the main River Wey.