Less than one year (2007/11 – 2008/08), Soai Rap riverbanks (southern Viet Nam) have been eroded severely. The erosion rates of some positions have been greater than 10 m/year. The wind waves and ship waves integrated with strong tidal currents have been considered as the dominant factors. The cohesive soil properties with dense dead mangrove roots and many holes of aquatic animals cause the banks to be weakened. The aim of this study is to investigate the erosion mechanisms of cohesive bed and bank materials based on in situ measurements, flume and wave basin experiments. The numerical models are also applied to calculate the distribution of shear velocities caused by waves and tidal currents. The results showed that erosion rates of cohesive bank materials are greater than those of cohesive bed materials from 2 to 10 times. The applied shear stresses are mainly distributed around river mouth and in the surfzone. These results indicate that local high waves approach to the banks are the main factor which causes bank erode severely. The mixing rates of sand, silt and clay contents, moisture contents are the factors effect on stability of the cohesive bed and bank strongly. Other factors such as dead roots and leaves, holes of aquatic animals and salinity can also cause bank and bed unstable.
River bank and beach erosion have been a serious problem and caused much damage to the environment and social-economic activities. In some parts of Soai Rap river banks, the erosion rates greater than 10 m/year have been observed. The erosion mechanisms of cohesive bank and bed materials around river mouth have not been well understood. Many factors influence on the cohesive properties of bank and bed materials. Therefore, it is necessary to investigate those mechanisms based on field investigation, scour test, flume experiments, wave-current basin experiments, and numerical study.
