EFFECTS OF VARIOUS UNCERTAIN FACTORS ON THE NECESSARY FLOOD CONTROL CAPACITY OF A DAM RESERVOIR

In Japan, the flood control capacity of a dam reservoir is usually determined to include a certain “marginal capacity for safety” plus the necessary capacity that is calculated so as to control design floods established for each dam by an optimum flood control method*. This “marginal capacity for safety” for flood control is needed because the necessary capacity is affected by various factors including uncertainty of flood scales and the shape of flood hydrographs predicted during planning, and other limitations that may occur during practical flood control operations. However, the effects of such uncertainties and limiting conditions on the necessary flood control capacity are not quantitatively understood. In this study, factors those possibly increase the capacity necessary for practical flood control exceeding the capacity calculated so as to control design floods were first identified. Then, case studies were conducted on several dams in Japan using flood control records to investigate the effects of the identified factors, such as uncertainty of flood hydrograph, on the capacity to be secured for flood control. The study showed that: 1) the difference between the design and practical flood hydrographs is the most important uncertain factor that must be considered at determination of the flood control capacity, and 2) adding a certain “marginal capacity for safety” to the net capacity necessary to control design floods is an appropriate measure to ensure safety of practical flood control operations. * In Japan, the optimum method for flood control of dam reservoir is selected from various methods considering the flood runoff properties, control efficiency and safety. “Constant rate and constant discharge method”, in which the fractions of flood inflow greater than the discharge at the beginning of flood control are stored at a fixed rate to the inflow and fixed discharge is released after the peak, “constant volume discharge method”, in which a fixed discharge is released while the flood inflow exceeds a certain discharge, “non-gate discharge method”, which requires no gate operation are typical examples of flood control methods in Japan.