Fluvial Design Guide - Chapter 10

Case study 10.2 – Weedon flood storage reservoir

Background

The Northamptonshire village of Weedon Bec, nearly at the upstream limit of the River Nene valley, suffered disastrous flooding in 1947, 1992 and 1998.

The culverts under the railway embankment downstream of the village constrict the flow and the limited channel size through the village compounds the flooding problems.

A number of options were considered to alleviate the problem by the Environment Agency and Halcrow Group. The only viable option was an upstream online storage reservoir scheme. The scheme was completed by Edmund Nuttall Ltd in autumn 2002 at a cost of £1.24M.

Weedon embankment dam under construction, looking downstream, with the borrow pit at the lower right

Summary of scheme

The project includes a 450m long, 6.5m high clay embankment across the valley, with a culvert that incorporates a low-flow fish channel on the line of the original river channel to carry the controlled outflow and provide attenuation up to a flood return period of about 50 years.

A 150m long concrete-block spillway, that forms part of the impounding embankment, carries excess flood flows up to the PMF (probable maximum flood).

The embankment site has been landscaped, retaining tree and hedge lines screening the embankment, to minimise visual impacts. The upstream borrow area has been developed into a large wetland area as a habitat for aquatic flora and fauna.

Hydro-Brake® arriving on site

Principal scheme figures

Dam crest freeboard (m) 1.60
Overflow level (m) 92.05
Surface area (ha) 37
Capacity (Ml) 810
Controlled outflow (m3/s) 8–12
PMF (m3/s) 195
Habitat created (ha) 1.5

The key component of the flow control system is a 2m diameter 6.5 tonne stainless steel Hydro-Brake® flow control device, located at the upstream end of the culvert. This was designed by Hydro-International and incorporates the facility to adjust the controlled outflow between 8 and 12 m3/s in the light of operational experience.

Because it delivers an approximately uniform discharge over a wide range of head, the Hydro-Brake helped to:

  • reduce the total upstream storage requirement and hence the land take;
  • reduce the depth and frequency of impoundment in the storage area during less severe events.

Further information: Weedon flood storage scheme - the biggest Hydro-Brake® in the World, (Boakes, Stephenson, Lowes, Morison & Usborne) in Long-term benefits and performance of dams, Thomas Telford, 2004

 

    Case study 10.1      Case study 10.2      Case study 10.3