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The Culvert A tunnel or culvert was cut through the hillside on the east side of the dam embankment to take the flow of the River Hodder during the construction of the works and to carry the permanent pipelines from the reservoir. 
Dam culvert - inlet end with steam crane in the foreground
There are three pipelines in the Stocks culvert: a scour pipe; a compensation pipe to supply water back into the river below the dam, and a supply pipe into the filter house for purification before being piped to Blackpool and the Fylde. Each pipeline is controlled by two valves in tandem at the reservoir end and the whole of the pipework on the downstream side is accessible for repairs if required. Fixing pipelines in the outlet end of the culvert
The tunnel is 712 ft. in length; circular in section; 17 ft. in internal diameter, and was driven from four faces by hand with pneumatic tools. Miners brought in from Cumberland were commonly engaged in this work. The tunnel was lined with cast-iron tubing externally sealed by pressure grouting with Portland cement.
The gangs of navvies driving the culvert tunnel worked in shifts around the clock from early 1924 until October, 1925. On the 31st October, 1925 there was a potato-pie supper in the cinema at Hollins village hosted by members of the Fylde Water Board to celebrate the completion of the culvert tunnel.
Potato pie supper in Hollins Cinema to celebrate the culvert completion - October 31st, 1925 Photograph from the late May Jackson collection, used by kind permission of Mrs. J. Cowking
Group photograph of the "navvies" after completion of the Dam Culvert - 28th November, 1925 Photograph used by kind permission of Mrs. E.D. Redmayne
The inlet end of the culvert tunnel and the front face of the dam embankment are protected by a concrete wall called the forebay which helped divert the river through the tunnel during construction.
Dam forebay wall, culvert & 0-4-0 Peckett locomotive "Hodder" Photograph used by kind permission of Mr. A. Walmsley
The first water from the diverted River Hodder is turned through the culvert tunnel - 27th August 1926. The forebay wall is still under construction with the dam main trench workings in the background.
Pack ice at the culvert entrance - forebay wall now completed
The River Hodder occasionally flooded during the dam construction; most notably on the 26th of October, 1927, when much of the scaffolding and woodwork was swept away and massive baulks of timber from the construction were found carried downstream as far as Dunsop Bridge.
The River Hodder in flood coming through the culvert tailbay -26th October 1927 - The dam embankment works in the background Photograph used by kind permission of Mr. A. Walmsley
The culvert is sealed at the upstream or reservoir end by a steel plate bulkhead - backed up by 35 ft. of concrete - through which the scour and compensation pipes extend into the reservoir. The supply pipe to the filter house and the Fylde Water Board supply passes through the valve tower.
Fixing the culvert bulkhead
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