Fluvial Design Guide - Chapter 10

Case study 10.3 – Bruton flood storage reservoir

Bruton flood storage reservoir is impounded by an earthfill dam located on the River Brue about 2km upstream of Bruton, a town at the eastern edge of Somerset with a long history of flooding. The dam was built in the early 1980s to alleviate the risks of flooding and is now owned by the Environment Agency.

The reservoir was designed to be kept empty under normal flow conditions, with a culvert under the dam conveying flows that are within the bankfull discharge capacity of the river channel through the town. During flood flows that exceed the discharge capacity of the culvert, the reservoir progressively impounds; it was originally designed to just fill in an event with a return period of 100 years. An open spillway, forming part of the dam, was provided for the discharge of larger floods.

Construction of the culvert as part of the original scheme (1980s)

Following inspections undertaken under the Reservoirs Act 1975 in 2000 and 2006 (and various studies undertaken in the intervening years), the Environment Agency implemented a £2.6 million programme of remedial and improvement works in 2008, designed by Black & Veatch.

View across the spillway and embankment dam, with impoundment area on right (2006)

2008 remedial and improvement works

The primary drivers for the 2008 scheme were as follows.

  • The peak velocity of up to 11 m/s on the spillway chute (reached at discharges of 150–200 m3/s) was much greater than the normally recognised limit of 8 m/s recommended for the cellular concrete blockwork that formed the original surface protection.
  • The trash screen at the upstream end of the culvert did not meet current standards regarding screen area or safe access.
  • The storage capacity fell well short of that required to meet the original design standard based on current hydrological appraisal methods.

Original trash screen structure at upstream end of culvert (2006)

The scheme therefore consisted of the following principal measures:

  • upgrading the surface protection on the spillway using stepped blocks (see diagram on right);
  • building a new larger screen structure at the upstream end of the culvert;
  • raising the spillway crest by about 2m, with corresponding raising of the rest of the embankment crest.

The fill material for the dam raising was obtained from a borrow area within the impoundment area just upstream of the left abutment.

Stepped block (950kg)

Upstream face of dam and new trash screen structure at upstream end of culvert (August 2009)

Stepped blocks being installed

Principal scheme figures

Parameter Original Current
Dam crest level (m) 75.30 77.30
Overflow level (m) 73.30 75.20
Surface area (ha) 13.9 17.6
Capacity (Ml) 515* 820
PMF (m3/s) 240 300–500

*  640Ml quoted by original designers

Earthmoving for the embankment raising

 

 

Earthworks largely complete, viewed from upstream near right abutment (August 2008)

New stepped block spillway protection (August 2009)

Further information: An innovative new spillway for Bruton flood storage reservoir (Pether, Marsh & Cartwright, 2009), Dams & reservoirs, Vol 19, N° 2, 67–72

 

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