The Construction of Block Eary Reservoir
Block Eary Reservoir was constructed during the Second World War and completed in 1945. It has a capacity of 50,000 cubic metres and a crest level of 186.06 OD. with a surface water area at top water level of 0.7 hectares.
The geological map shows the Block Eary valley to follow the outcrop of a "greenstone" (diabase, epidiorite and chlorite schist) dyke, intersected by micro-granite dykes, all intruded through the Cambro-Ordovician Manx Group "slates". A layer of peat overlies the ground surface and locally shallow clay, probably boulder clay, appears.
The catchment area is 412 hectares receiving an average annual rainfall of 1740mm. It is on the steeply sloping valley on the northern slopes of Snaefell (621m). At its eastern extremity is Clagh Ouyr (551m) and to the south is Slieau Managh (383m). The Block Eary stream flows west into the Sulby River.
The dam is a concrete gravity dam. The maximum height is 16.35m. The upstream face is vertical over the upper 7.39m, below this level the upstream face slopes outwards at a slope of 1 horizontal to 12 vertical. The downstream slope is stepped. The general slope is about 0.7 horizontal to 1 vertical but near the foot of the downstream face the slope slackens. The steps vary slightly in their dimensions but the going is about 380mm and riser about 533mm. Galleries on two levels pass through the dam, the upper being accessed from the left bank and the lower from the right and are interconnected internally by steps. The dam was constructed in monoliths using local slate as the coarse aggregate in the concrete. From the records it appears that all the monoliths were raised equally as construction proceeded.
The overflow is a 10.97m long ogee weir formed by a lowered section of the crest and discharging down a channelled section of the stepped downstream slope of the dam. The spillway section is bounded by concrete training walls 1219mm wide projecting 300mm above the face of the steps. Sections of the crest either side of the spillway are set at an elevation of 184.40m the remainder being at a slightly higher elevation of 185.62m. At the foot of the spillway slope there is a concrete bottomed stilling basin, from which overflows discharge along the rocky river course downstream.
The outlet pipework was modified in the1980's to provide the headrace for a hydropower station situated at the confluence of the Block Eary stream and the Sulby River. A 700mm ductile outlet pipe was connected to the reservoir through a hole stitch drilled through the right abutment of the dam by means of a downward facing bellmouth and a sluice valve operated from a cantilevered platform constructed off the crest of the dam above. The pipe is supported on pillars on the downstream face of the dam and passes high across the stilling basin to the left flank of the valley and then along the former construction access route for about 1 km to the hydropower station. The existing dam pipework was also connected into this main to the left of the stilling basin.
There is no instrumentation in or on the dam.
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