Mamoru Arita

Measurement of Critical Shear Stress for Erosion of Cohesive Riverbanks

Riverbank erosion near river mouth is an increasingly serious problem for social-economic activities and natural environment of Southeast Asia. There are many factors impact on erosion process such as bank materials, mangrove trees, aquatic animal burrows, meteorological and hydrodynamic conditions. In this paper, the authors investigated the critical shear stress for erosion of cohesive riverbanks in the Soai Rap River, southern Vietnam. The results showed that the riverbanks have been moderately eroded with the critical shear stresses range from 0 to 5.731 N/m2 and the erodibility coefficients range from 0.497 to 9.07 cm3/N-s. Simultaneously, the authors applied numerical models to verify the distribution of shear velocities in the study area. In condition with tidal currents, the significant shear velocities mainly appear in the upper part of the Soai Rap River and in the Dong Tranh River (near river mouth). In condition with waves, the significant shear velocities only appear at the river mouth. In conditions with both tidal currents and waves, the significant shear velocities appear in upper part of the Soai Rap River and at the river mouth. The simulation results indicate that the distribution of shear velocities agree quite closely with the field survey data.

Measurement of water surface using Stereo Image

It is important to grasp water surface profile when we discuss various phenomena in coastal zone. For example, sediment transport and rip current are determined mainly by the spatial distribution of waves in surf zone. Measurement of sea waves using Eulers method is a general. However it is difficult to determine spatial distribution of waves. We had measured wave profile using experimental water tank without tracers or targets. We generated a few kinds of regular waves and took the pictures by two digital cameras and analyzed the azimuth difference in digital images by stereo matching method.