The circumstance of occurence of record heavy rainstorms alarms that the spillway capacity to insure the safety of a dam must be based on an intelligent study of all the available data as to great floods and heavy rainfalls in not a specific river watershed but a wide area under the same meteorological conditions. The enveloping curve of flood peak discharges at many stations with different drainage areas under the same meteorological conditions would give a solution for this problem.
This study aims to develop the equation of the enveloping curve rationally in Sanin Region considering the DAD characteristics of heavy rainfalls and the concentration time of flood runoff.
The Gonokawa, Hi, Tenjin and Sendai 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.