Hydrodynamic effects on refugia of cobble-bar vegetation established along river bend

In order to find mechanisms of maintaining cobble bars found along many Japanese rivers, we have conducted a field survey, two-dimensional flow analysis and LES simulation of flood flows of various discharges at a mid reach of Ibogawa River in Western Japan. The prominent feature of the river reach studied by the present work is the cobble bar on the inner bank of an acute bend after a steep sloped riffle but there are a number of other features such as large boulders and a sudden drop. The roles of these features are investigated by the field survey and the simulation. The simulation under various conditions indicate that it is the sharp bend and a relatively steep slope that create a bar on the inner bank where flow of slow speed occurs during medium to large floods.