WATER SUPPLY

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from Thirlmere in the Lake District, and Liverpool and Birmingham
 their supplies from artificial lakes in Wales.
Reservoir SiTes—The site of a storage reservoir needs
careful geological survey to avoid loss by percolation and to
secure safe foundations for the dam. Sandstone and limestone
 are unsuitable, for their permeability may lead to serious
leakage. Loss of water through a thin porous bed can be
prevented by a puddle trench—a deep trench beside the dam
filled with packed clay or concrete. Faults may lead to loss
of water and render the foundation of the dam insecure.
The reservoir must also be provided with an adequate
overflow channel to discharge any sudden inflow of water as
from a cloudburst. Few catastrophies reek such complete
devastation as the rush of water through a broken dam.