The distribution character of heavy rainfall in recent Japan sounds a warning to us that it is necessary to put into consideration of the record flood peak observed at a point in a river situated within a region in which the meteorological and hydrological conditions are almost simillar to each other, in determining the required spillway capacity to insure the safty of a dam. Unfortunately, however, the physical or statistical characteristics of the areal maximum of flood peak has not hitherto been studied enough to serve for the engineering practice in Japan.
This study has been carried out to cope with this situation from the view point that the flood peak discharge is a reflection of both characteristics of rainfall concentration in time and space and of runoff of the river basin.
The Hi and Gonokawa river basins have been chosen as the objective basin. The record of heavy rainfall observed at several points covered a river basin have been investigated to disclose the DAD characteristics of the basin. The DAD equation is then combined with the equation of the time of flood concentration to find a rational form of the equation for flood peaks. Finally, the equation for flood peaks is modified to express a curve enveloping record flood peaks in m^3/sec/<km>^^^2. The formula for enveloping curve has shown good fitness to record flood peaks in Sanin region.