Abstract
Aerial photographs of a cobble bar taken by an unmanned aerial vehicle (UAV) are analyzed for the purpose of measuring spatial distribution of cobble sizes before and after a flood occurred in the Ibo River in 2013. The photographs cover the bar area spanning about four hundred meters and the images were analyzed by using a software BASEGRAIN that can measure particle size distributions by a sophisticated image analysis method. A significant change of cobble size distributions due to the flood was made clear by comparing the mean diameter distributions. At the same time, surface flow measurements by an image analysis method, STIV, was conducted during the flood to estimate the shear forces acted on the river bed, which can well explained the movement of cobbles by the flood forces.
