A PHYSIOGRAPHIC FORMATION PROCESS GOVERNING FLOOD FLOW IN VALLEY-BOTTOM PLAIN

Abstract

Localized torrential downpours have recently caused flood damage on small and medium-sized rivers with relatively small design flood discharge in valley-bottom plains. One such event occurred on the Appetsu River in Hokkaido Prefecture in 2003 and another on the Ashiba River in Fukui Prefecture in 2004. In both cases, the water level rose in a very short period of time, the flow was not confined to the river channel, and many houses, roads and bridges were damaged by inundation. In addition to the flow in the channel, there was additional flow that cut off the meanders to form figure-eight patterns, and damage at points of such flow tended to be particularly severe. As a first step toward mitigating damage caused by such large-scale floods, this study investigated the Appetsu River, whose 2003 flood was a record. The goal was to clarify the possibility of predicting flood flow behavior by examining the relationship between riverbed morphology and valley shape during large-scale flooding.