Drainage blankets were used to prevent the loss of groundwater beneath the lake.

It was the first example in the UK of an earth dam with horizontal drainage blankets.

The "clear stone" drainage blanket was constructed from uniform, coarse gravel (with a nominal 50 mm particle size) obtained by crushing dolomitic limestone.

To protect the membrane from puncture, it is normally embedded in a sand-bentonite overpack, which is itself covered with a geotextile (and often also, gravel) drainage blanket.