While for such engineering design problems as dykes, river training, bridges, and stormwater drainage systems peak discharge rates are sufficient, they are inadequate for the design of hydraulic structures where storage is involved. A spillway of a dam, for example, is hit not by the natural hydrograph with its peak discharge of the catchment but by a design discharge significantly attenuated by the retention effect of the reservoir: flood volume counts, and therefore, the design work is usually based on the hydrograph rather than the peak discharge rate.