由良川大野ダムの洪水調節効果について

Abstract

The regulation of the outflow for flood control by a reservoir located in the valleyfarther upstream must be carefully planned to determine the available method of thegate operation, on the basis of the flood flow expected to occur with any given frequencyat not the dam site but the area planned to decrease flood damage. In this determina-tion, it is necessary to find the behavior of runoff from every subdivision of a riverbasin under various meteorological circumstances. The dimension of drainage area ofsmall watershed from which the flood hydrograph can be reliably estimated by theuse of the average rainfall over its area is discussed by a new method of runoffanalysis based upon the method of characteristics and it is found, in YURA River, that it is less than 300~-400 km2 in area. Moreover, the effects of channel storage inthe reach from the ONO Dam to FUKUCHIYAMA, having about 42 km long, on naturaland controlled flood flows are evaluated by using authors' analog conputer for floodrouting and the amount of reduction of flood crest at the downstream point is found tohe less by about 100 m2/sec than that at the dam, 1000 m'/sec. Taking account ofthese results, it is disclosed that the successful way of flood control at the ONO Damis to regulate the outflow with a constant ratio to the inflow, but that the gates of thedam must be carefully operated to avoid a total flow downstream in excess of themaximum safe flow by adding inflows to the stream below the dam under specialrainfall condition.