Numerical assessment technique on damages of river flood and inundations caused by large floods

Tatsufumi Miyazaki, Shoji Fukuoka

Thursday 2 july 2015

17:30 - 17:45h at Amazon (level 1)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 13B. Engineering - River

Successive large floods which exceeded the designed discharge occurred in the Kagetsu River in July 3 and 14, 2012. The Kagetsu River with a steep slope and narrow width runs through dense housing areas and has serious problems of deficit in discharge capacity. The 2012 floods caused serious damages such as levee breaches and flood inundations. Especially, severe overflows from levees were caused by drift-wood loggings around a bridge and resulted in large inundations over dense-housing areas. Such a disaster often happens in many middle and small-scale rivers, so it is important to learn inundation lessons from drift-wood disasters. We investigated the mechanism of damage in river flood and inundations by measured data of inundation and numerical computation of the flooding. We attempted to explain both flooding within the river and in housing areas by the analyses of quasi-three dimensional flows and two-dimensional bed variations and two-dimensional inundation considering drift-wood accumulations at a river bridge. The results of numerical analysis provided valuable hydraulics information and bed variations in the Kagetsu River. Levee breaches occurred in curved reaches in the river. The floods caused levee breaches by severe bed scouring at the outer bank side. Numerical analysis of inundation showed that violent overflows occurred from the foot of the bridge, and inundation water flowed down at high velocity and large depth over housing areas. Many houses were suffered from damages by large flood forces. Numerical inundation analysis was compared with measured one and reproduced that flooding water attacked houses and flowed back to the Kagetsu River in consequence of arrangement of buildings and ground level. An important flood assessment technique was improved by a simultaneous flood analysis of both river itself and surrounding inundation areas. This analysis method was applied to the determination of new design flood and cross-sections of the Kagetsu River.