Floods during Construction
When the dam is completed floods are designed to be discharged through the spillway without damaging the dam. Before floods can be directed through the spillway the dam wall must be at or approaching its final height and the spillway excavation must be at least almost complete. During the time the dam wall is being built there is a risk that a flood will occur that cannot be directed through the spillway and which might therefore overtop and wash away the partially completed dam wall.
Dam designers cope with this problem in 2 ways: the diversion of the river and construction of the dam in the lowest, river bed area of the dam site is always timed to be carried out during the driest time of the year (based on available rainfall records), when floods are least likely to occur; in the event that a flood does occur and the partially completed dam wall is overtopped the downstream face of the dam is covered in protective steel mesh which is designed to prevent flood waters that flow over the top of the dam from washing away the rock and soil which form the dam wall.
||Steel mesh being installed on downstream rock fill face of Windamere Dam as protection against overtopping during diversion. In the background the impermeable brown clay core of the dam can be seen under construction.
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