MATTERS TO BE ATTENDED TO FLOOD CONTROL PLAN THROUGH EXTREME ANALYSIS WITH PEAK DISCHARGE AND CATCHMENT AVERAGE RAINFALL

In hydrological frequency analysis, it is common to analyze the frequency of annual maxima of peak discharge or rainfall amount during the rainfall duration. The shape parameter of the generalized extreme value distribution used in the analysis of annual maxima is related to the thickness of the upper tail of the probability density function. The peak flow at a given point in a watershed is closely related to the average rainfall in the watershed upstream of that point. Although the quantile through extreme analysis of peak discharge and the peak value of discharge hydrograph obtained by runoff calculation with the quantile through extreme analysis of the average rainfall in the watershed for an appropriate rainfall duration are assumed to be equivalent, they are often quite different. This paper discusses the causes of this difference and points out some key considerations to keep in mind when using the return levels for flood control planning.