Physcial properties and structures of river bank sediments in Bang Ban canal, Amphoe Bang Ban, Changwat Phra Nakhon Si Ayutthaya

Bang Ban Canal is a stream that has a lot of river bank failure reports. To find out the geological causes of river bank failure in the area, this study focuses on physical properties and structures of river bank sediments. There are 4 boreholes of soil which are BC1, BC2, BC3, and BC4. In every borehole, mean grain size of soil changes at the depth of 3 metres approximately. Upper part of river bank mainly consists of silt and clay while lower part of river bank consists of coarser materials (fine sand to very fine sand). It can be concluded that upper part of river bank is cohesive soil while lower part of river bank is cohesionless soil. According to GPR survey, the structure of river bank in Bang Ban Canal (at Wat Bang Ban) can be classified into two parts. First, lower part of river bank is the old point bar and palaeochannel of Bang Ban Canal, therefore, it was a meandering stream. Second, upper part of river bank is natural levee which overlain lower part as unconformity. These are corresponded to aerial photograph interpretation. Bang Ban Canal’s bank type is composite bank. The upper part is cohesive soil which is more resistant than the lower part, cohesionless soil. If river level lowers to the lower part, the erosion rate will increase significantly. When the lower part is eroded, river bank failures will occur easily. According to Bank Erosion Hazard Index (BEHI), river bank at Wat Bang Ban is classified as very high risk.