Selection of design flood frequencies for hydraulic structures

The first step required in the design sequence for a hydraulic structure is selection of the design discharge. The size and cost of the structure will be directly related to the magnitude of this value. If the selected discharge is too large, the structure will be uneconomic; if it is too small, it will be unsafe. The optimum design corresponds to the point where the cost of the structure and the cost of exceeding the design discharge is a minimum. Ideally, the optimizing process should be carried out for each structure. For practical reasons it is not always possible to do this. In such cases, design flood frequencies are used which have apparently been satisfactory for similar structures in the past. Some typical flood frequency values for various structures are given in this paper, along with the rationale for differences in values for different kinds of structures. Key words: design floods, frequency, economics, hydraulic structures.