To drain or to reserve? That is the question discussed in this paper. The capacities of drainage and storage facilities for flood control are determined to minimize the total cost under the condition that chance of flooding is less than a tolerable level. An analytical expression of the chance constraint for a flood control system with a storage facility is presented.
The capacity of a drainage facility is evaluated based on frequency analysis of “peak discharge” of floods. The capacity of a storage facility may be reasonably evaluated through frequency analysis of “total runoff” of floods. Thus, characteristics of the flood-chance constraint are analyzed in view of the joint distribution of peak discharge and total runoff. Consequently, simplified expressions of the constraint are presented for practical applications.