Optimum Spillway Breadth for Gravity Dams

The provision of overflow capacity in an impounding reservoir may occupy a large fraction of the total project cost. With a higher cost per unit volume than elsewhere in the dam, an optimum spillway flow breadth exists that minimizes total cost. This optimum is explored for simple gravity dams both under the restriction of negligible storage and with finite storage using level pool routing combined with a gradient search technique. It is found to be a sensitive product of cost ratio, peak inflow, and duration. A dimensionless treatment of the quantities involved makes possible the comparison of four widely different dams, three constructed in the United States and one in the United Kingdom. All are found to have spillway breadths close to the optimum. Since small variations from the optimum may result in cost increases that are large in absolute terms, the concept appears useful as a datum for economic assessments at feasibility stage.