Soil properties and erosion mechanisms of composite riverbanks in Lower Jingjiang Reach

The Lower Jingjiang Reach (LJR) is experiencing problems with continuous channel degradations, and local riverbank erosion occurs frequently. In order to investigate quantitatively the soil properties and erosion mechanisms of composite riverbanks, a field survey is conducted at 6 sections of riverbank collapses along the LJR. An indoor soil experiment is also performed, and both physical and mechanical properties of soil samples are analyzed. The analysis reveals that the vertical profile of soil compositions in the LJR can be characterized as being a typical composite structure. The lower layer is thicker with non-cohesive soils, and the upper is thinner and loose with cohesive soils. A new method for calculating the safety parameter of overhanging block in a composite riverbank is then proposed. The method uses the principle of cantilever stability with the bank stability degree being closely related to the unit weight of the overhanging block and the corresponding tensile strength. Based on the calculated near-bank hydrodynamic conditions and the experimental results from the indoor soil tests, bank stability degrees and influencing factors of a typical composite riverbank in the LJR are investigated quantitatively. The following conclusions can be drawn from the study. The incipient velocity for the non-cohesive lower layer is obviously less than the mean bank-toe velocity, which easily lead to the occurrence of severe fluvial erosions. The cohesive upper layer before failure has sufficient strength to resist from direct fluvial scour. However, the failed soil blocks deposited in the near-bank zone would be broken down and transported downstream by the near-bank fluvial entrainment due to the soil properties of loose structure and low liquid-limit clay. The safety parameter measuring the degree of the bank stability at each stage during a hydrological year varies periodically and is calculated. The lowest value at the flood recession stage is obtained, which is consistent to the recent statistics from the field surveys on bank erosions.