The Construction of Cringle Reservoir
Cringle Reservoir was constructed between 1938 and 1946 by direct labour, it has a capacity of 14,100 cubic meters with a surface water area at top water level of 1.8 hectares. The reservoir is 245m at its longest point with a crest level of 173.74 OD.
The dam is sited in the Cambro-Ordovician Manx Group close to the Manx Synform. Geological maps show "altered slates" with intrusions of Agneish Grits outcropping north-east, south-west, dipping south-east, and an olivine dolerite dyke outcropping south-east, north-west, nearby. The Foxdale Granite outcrops about 3 kms to the north-east. The site is coated with glacial drift to a depth of about 2m. There is some evidence of an infilled valley to the west of the of the present day valley in which the dam is sited.
The catchment area is 154 hectares receiving a standard average annual rainfall of 1242mm and lies to the south of Round Table and South Barrule and is largely forestry plantations of mixed conifers and broad-leafed trees.
The dam is an earth embankment with a puddle clay core and a concrete cut-off. The embankment height is approximately 21.3m with a freeboard of about 1.5m and a total length of 160m. The upstream slope is at 3 to 1 and the downstream slope at 2 to 1 near the top, reducing to 2.5 and 3 to 1 down the slope. The top of the core is about 3m wide and widens to 9m at the deepest point above the concrete cut-off trench. The concrete at the top of the cut-off widens to about 4.5m and protrudes above the rock surface. The puddle clay fits around the protrusion and into a trapezoidal trough about one metre wide in its upper surface.
On the upstream side of the crest there is a jointed masonry wave-wall about 0.75m high to a top level of 174.5 OD. Stone pitching, placed on edge protects the upper part of the upstream slope, down to about 7m below TWL, below this the slope is protected by shingle. The downstream face of the dam is grassed.
The overflow is a 10.67m long broad-crested trapezoidal weir at the right abutment. This discharges into a channel downstream of the weir, which then sweeps around to follow the mitre of the dam into the stilling basin downstream of the toe. A flood channel along the right shore of the reservoir discharges into the overflow channel through a culvert that enters the channel just downstream of the overflow weir. At the downstream toe there is an extensive stilling basin where overflowing water discharges over a metal rectangular weir to enter the river channel and pass under a highway bridge about 50m downstream. It then flows through Silverdale Glen and ultimately joins the Silver Burn just upstream of Ballasalla, about 5km downstream of the reservoir. The Silver Burn discharges into the sea at Castletown about 2km downstream of Ballasalla.
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