A THEORY OF FLOOD FREQUENCY AT A CONFLUENCE

Improvements of a tributary for flood control may result in an increase of flood frequency of the mainstream. It is primarily because a portion of flood discharge which is equal to the increment of the drainage capacity of the tributary and overflows there before the improvement runs down into the mainstream. The frequency distribution of peak flood discharge at a confluence should be based on the distribution of the sum of discharges of the tributaries. The analytical expression of the flood risk at a confluence in terms of the drainage capacities of the tributaries is derived in view of overflow along the tributaries and the distribution of the sum.