LONG-TERM EVOLUTIONS OF SHIRAKAWA RIVER DELTA DUE TO THE EXTREME EVENTS

The current state of understanding of long-term morphological changes of river mouth deltas is not sufficient to develop physically meaningful models of all processes involved. The objective of this paper is to predict the topography change in the mudflats of Shirakawa river delta due to an abnormal weather phenomenon. In this study, long-term evolutions of Shirakawa River Delta during 1976 to 2013 were investigated using bathymetry, river discharge, wave, and tide data. Moreover, the delta response due to flash flood caused by torrential rainfall on July 12, 2012 in Kumamoto examined using echo soundings. The geomorphological response of Shirakawa River delta indicated that foreset slope and critical bypass depth were advanced to off-shore due to flush flood over 1, 500 m3/hr. The critical bypass depth is controlled by tide and located below mean low water spring. The numerical model was found to be good agreement to explain the phenomena before and after the flood occurred with parameter fitting to the change of cross-shore profile.