洪水の伝播速度について

Abstract

The runoff phenomenon of a heavy rainfall in a river basin is divided briefly into thetransformation process from rainfall to outflow on mountain slope and the propagationprocess of water resulting from the above outflow in river channel. As the part of theresearch of such a runoff, this paper describes the physical background of the propagationspeed of flood waves in an actual river channel, and consists of two parts.In the first part, the change of propagation speed of a flood wave passing through alocation of river channel is discussed. As the results, it is found out that the flood wave ischaracterized remarkably as a uniformly progressive flow at the rising stage, and that thepropagation speed at a location of river channel holds nearly constant regardless of changeof discharge, because the relationship between the cross-section area of the flow and thedischarge shows approximately linear.In the second part, the change of propagation speed of a flood wave along a river courseis examined. The river bed configuration is divided into mountainous river bed, dune bedtype-I and dune bed type-II, and the formulae of flow resistance for each bed configurationis discussed in the light of open-channel hydraulics. It is obtained from the discussions thatthe mean velocity of a flood flow, corresponding the propagation speed, has a tendency toincrease from upstream to downstream in a mountainous region, that is, in the range ofmountainous river bed, and to decrease slightly form upstream to downstream in a alluvialregion, that is, in the range of dune bed type-I or type-II.