Exercising the dam

December 8th 2007. A night of rain swelled the Harbourne to a threatening level. According to hearsay, the sluice again had to be levered into defensive position by men in fluorescent jackets, supplementing the digital automation. A spokesman for the Environment Agency, Bryn Williams, explained that the automatic mechanism was alert but the water had not reached the level at which the gradual shutting of the sluice was programmed to begin. The manual operation was in response to a number of phone calls from anxious inhabitants of the lower ground in Harbertonford. Bryn has provided a full description (below) of the working of the sluice, and the water level which will cause it to close.

Upriver, the reservoir was well filled by mid afternoon, raising the problem of how to rescue the cows which are proposed to munch the over-abundant vegetation. Maybe some artificial islands could be thrown up, ideally by deepening the rapidly silting ponds.

For the Environment Agency's description of the dam and its purpose see:

http://library.coastweb.info/488/1/Harnessing_the_harbourne.pdf

(tim padfield 8 dec 2007, revised 15 December)

Bryn Williams writes:

HARBERTONFORD FLOOD SCHEME

As part of the scheme improvements, the watercourse through Harbertonford village has been improved to pass a flow of nearly 800 gallons per second; this is equal to the 1 in 10 year’s peak flow. The impoundment dam and gate control system upstream has been designed to cope with a flood approaching a 1 in 40 year’s return period. River levels downstream of the dam are measured with a pair of hydrostatic level recorders.

The river level: flow rate relationship is stored in a computer which calculates the flow rate in the river based on the river level. In practice when the downstream flow rate approaches the maximum limit, the lead flood gate will progressively close down so as to control downstream flows. When this gate is fully closed and flows continue to remain high, the second gate will start to close progressively. However the gate does not fully close but will close only to a preset limit allowing an opening (approx. 800mm or 32" high) through which the impounded water can pass safely downstream through the village. Flows through the village will be considerable but will stay within the design tolerances. All of these operations should happen automatically.

After the flood event and when the impounded water has passed downstream, the gates are manually reset to the initial fully open position. This manual resetting will only take place when water levels in the dam are at a safe level to avoid causing flooding downstream. This manual intervention also allows scheme checks to be made on site by Environment Agency staff.

The downstream water monitoring equipment. The red numbers are metres above ordnance datum (mean sea level at Newlyn, Cornwall)

Downstream view of the dam gates

This article is posted by Bryn Williams, Flood Risk Management , Exminster, Exeter, Devon EX6 8AS

Office phone 01392 316148