One of the best free shows in the Peak District National Park, in rainy seasons, is the bell-mouth spillway beside the A6013 road that skirts Ladybower Reservoir, the biggest of the three Derbyshire Derwent valley reservoirs.
The original Derwent Valley water scheme of 1899 envisaged six reservoirs but only two of these, Derwent and Howden, were built.
The engineer, Edward Sandeman, pointed out that repositioning the Derwent Dam slightly further upstream would dispense with the need for the top dam, Ronksley. Geological problems in the tributary Ashop valley led to the abandonment of the other three dams, Hagglee, Ashopton and Bamford, which were superseded by a single huge reservoir, contained by a dam at the next available nick-point, Yorkshire Bridge.
This great dam, named Ladybower after a local farm, was begun in 1935. It drowned two villages, Derwent and Ashopton, and was so badly needed that construction continued without interruption throughout the Second World War.
It was opened by King George VI and Queen Elizabeth on September 25th 1945. Designed by G H Hill & Sons of Manchester, and constructed of earth around a clay core by Richard Baillie & Sons, East Lothian, the dam is 416 yards across. Its trench and embankment required 100,000 tons of concrete, 1,000,000 tons of earth and 100,000 tons of puddled clay.
Unlike its predecessors at Derwent and Howden, which spill their excess water over the stone sill of the dam, at Ladybower the dam has a clay core and a grassed slope downstream.
The overflow water is directed into two bell-mouth spillways, which from above look for all the world like plugholes, but are actually shaped like ear-trumpets, 80 feet across at the rim, tapering to a 15-foot pipe that emerges at the foot of the embankment.
This footage brings the still picture to life: https://www.youtube.com/watch?v=cg-mjoLm1Jo. In-depth explorations can be found at https://www.youtube.com/watch?v=GqXGM_L7Zp0 and https://www.youtube.com/watch?v=q5BVsk9o9hw.