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The Overflow Spillway Weir & Conduits

The overflow weir or "spillway" is of a type known as a side channel spillway. It was built in concrete at the eastern end of the dam and was faced with stone from Jumbles Quarry.  

Overflow weir & channel leading to the overflow conduits

Overflow weir & channel leading to the overflow conduits under construction - temporary railway line in the channel

 

The overflow weir over which surplus water from the reservoir is discharged, is 300ft. long and empties into 459ft. of open channel cut in the hillside on the east side of the dam embankment. This open flood water channel originally fed into three 700ft. long discharge conduits, each 8ft. in diameter, which fall 110ft. down the 1 in 6.6 gradient embankment slope into the tailbay pool below the dam. 

 

The Overflow Conduits under construction - Timber centring in place

The Overflow Conduits under construction - Timber centring in place and stone facing being set

 

Withdrawing the wooden centres

Withdrawing the timber centres

 

The downstream ends of the overflow conduits

The downstream ends of the overflow conduits

Stocks Reservoir first overflowed on 3rd March, 1933. 

Following an infamous cloudburst over the fells between Slaidburn and Wray in August 1967 which caused flooding and massive damage to Water Board installations in the Forest of Bowland area, major modifications were made to the Stocks dam overflow channel and conduits.

Overflow spillway conduits during heavy rain - 12th December 1964

Photograph used by kind permission of Mrs. J. Lawson

 

There was a fear that debris, fallen trees etc. from a similar catastrophic flood might choke the spillway conduits and cause the dam to overflow the top of the earth embankment and wash it away.  As a means of reducing the risk, the level of the spillway weir was lowered 4 ft. by removing the top layers of masonry  thereby reducing the capacity of the reservoir.  

Spillway weir alterations as seen in 1974

Photograph used by kind permission of E. & M.C. Higham

 

These emergency alterations were eventually tidied up and the masonry of the overflow spillway weir was permanently fixed. Alterations to the overflow bywash conduits were also made at this time.

Altered spillway weir & overflow bywash

Altered spillway weir & overflow bywash

Photograph used by kind permission of  Ken Geddes 2004. All rights reserved.

 

An open concrete spillway chute was constructed on top of the existing overflow conduits to carry more surplus water if required. 

Altered  overflow bywash - open channel above the conduit pipes

Altered  overflow bywash - open channel above the conduit pipes

Photograph used by kind permission of  Ken Geddes 2004. All rights reserved.

 

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