Observations of Bottom Turbidity Characteristics during Some Extreme Events at the Inner Shelf off the Tenryu River, Japan

Field measurements were conducted to measure bottom turbidity conditions during several extreme storm and flood events that occurred off the Tenryu River mouth in 2006 and 2007. River discharge, river turbidity conditions, sea bottom turbidity, and sea bottom hydrodynamic conditions (waves and currents) were measured. The site for turbidity measurements was located similar to 1.4 km from the river mouth and 18.2 in from mean sea level and was considered to be representative of the seafloor sediment inner shelf processes that occurred during the extreme conditions. A set of sensors (i.e., acoustic Doppler currents profiler (ADCP), turbidity meters, and wave gauges) was installed at this site. Five high wave events (S1 to S5) were recorded, during which the wave shear stresses correlated closely with the bottom turbidity data. The high bottom turbidity levels measured following the flood were likely evidence of the hyperpycnal condition of the Tenryu River. Bathymetric measurements were made before and after two storms and a flood event in 2007 to examine the morphological changes caused by these extreme events. The storms and flood created significant morphological changes around the river mouth. Based on bed profile data, bed material around the breaking zone was eroded and deposition occurred similar to 500 m from the coastline.