Bristol Water Technical note prepared for Chew Valley Flood Forum

Chew Magna Reservoir - description and operation

Description

The Chew Magna reservoir was constructed in the 1850s as a small storage reservoir to maintain river flows during summer for the benefit of mill owners on the River Chew. It was never designed to have sufficient volume to be a water resource reservoir or mitigate flooding.

Operating licence

The licence of operation requires us to maintain statutory discharge volumes to maintain minimum flow in the Winford Brook in both summer and winter.

Dimensions

The reservoir has an average depth of approximately 3m and has a theoretical design volume of approximately 90 Ml (90,000 m3). Over the years there has been some silt build up. Our current estimate of available volume is approximately 70 Ml (70000 m3).

Operation

  • The reservoir has insufficient capacity during dry periods to maintain the statutory discharge volumes at higher summer volumes (compensation flow). In dry years this can result in the reservoir effectively becoming empty. During these dry periods the company will take the opportunity to remove silt from critical structures such as the draindown valve and pump intakes. Note that we never drain the reservoir artificially for de-silting operations.
  • During the past 10 years, conditions dry enough to cause the reservoir to empty and allow some de-silting works took place occurred in 2003 2005 and 2011.
  • When Chew reservoir (vol. 20,000 Ml or 20,000,000 m3)was completed in the 1950’s, a pumping station was installed at the Western end of Chew Magna reservoir to capture the high winter inflow volumes available from the Winford Brook catchment. The capacity of this station is a maximum of 30 Ml/d ( 0.3m3/s) The pumps are usually only operated in winter when the main Chew reservoir is low and there are high inflows to Chew Magna reservoir.
  • We manage the reservoir volume in order to ensure that there is sufficient water to provide statutory discharges. This means we would not normally pump any water from the reservoir during summer and try to maintain the reservoir as full as natural conditions allow.
  • In winter we may pump water from Chew Magna until the water level is 2m below the level of the overflow weir. This will reduce volume by about 40 Ml and is sustainable on the assumptionthere will usually be sufficient water to refill the reservoir by the following spring). In a dry winter or a very wet winter we may take a decision not to transfer water to Chew (depending on the refill characteristic of the larger reservoir. We are not able to maintain a level of 2m below the spillway at all times due to the large variations in volumes.

Flood conditions

  • The reservoir volume is very small in relation to the volume of water that can flow from the catchment in wet conditions, or as a result of heavy rain. In these conditions the reservoir will overflow rapidly (even when half full). Simply put, the reservoir is no more than a slight widening of the Winford Brook.
  • During heavy rain and flood conditions, the flood peak moves directly through the reservoir with practically no delay, or a very short delay period. For example, in the floods of 2012, the inflowing volume to the reservoir was over 10m3/s for periods of 6 hours, with peaks possibly in excess of 15m/s (flows recorded have been off instrument scale).
  • At 15m3/s the volume entering the reservoir is over 50,000 m3/h. Over the period of a flood lasting 10 hours the volume passing through the reservoir will be in excess of 360,000 m3 This can be compared to the theoretical maximum detention volume of 70,000 m3 (assuming the reservoir could be maintained in a completely empty condition at all times). Note that the pumping capacity is similarly insignificant at 0.3 m3/s compared to the rate of flood inflow. The pumping capacity is defined by the hydraulics of the main between Chew Magna and Chew.

Past flooding

There has been a persistent record of flooding, with 10 or so significant events having occurred since the 1960’s. We have no evidence to indicate whether the frequency or severity of flooding has changed significantly in the last 50 years. We have not materially changed the operation of the reservoir during this period

Third Party access

The reservoir is leased as a leisure facility to Knowle Anglers for game fishing. While they have the right of riparian access, their requirements do not influence the operation of the reservoir in any way. The primary driver for operation is the need to maintain statutory conditions.