FIELD INVESTIGATION OF TOPOGRAPHIC RESPONSE TO FLOODS AND WAVES AROUND TENRYU RIVER MOUTH

Abstract

Bathymetric changes around the mouth of the Tenryu River, one of the largest rivers in Japan and flowing into the Pacific Ocean, were analyzed using the narrow-multibeam survey data taken between 2006 and 2011. The formation/deformation of the river mouth terrace triggered by floods, the formation of bar and trough topography associated with the incidence of storm waves, and the gradual erosion by the imbalance of longshore sand transport were investigated. The sand volume in the study area decreased by 1.66×106 m3 between 2006 and 2011 at a rate of 3.1×105 m3/yr. Because fluvial sand is assumed to have been supplied at a rate of 1.5×105 m3/yr, the rate of decrease in sand volume transported away from the river mouth area by longshore sand transport reached 4.6×105 m3/yr. Urgent measures to mitigate beach erosion are required.