Riparian Groundwater Level Variation and Its Impacts on Bank Erosion in the Middle Yangtze River

The interaction between groundwater flow and river flow plays an important role in the bank erosion process, but field measurements on riparian groundwater flows remain very limited, especially in large river systems. Three monitoring wells of groundwater flow were constructed at two typical sites of the Middle Yangtze River (MYR) in 2021, with the hydrographs of the riparian groundwater level and river stage being obtained. Results indicated that (a) the variation in the groundwater level generally followed with the river stage during the flood peak period, but decoupled from the river stage during the recession period; (b) the delayed response of the groundwater level to river stage variation was quantified by the ratio of their change rates, which linearly increased with the magnitude of the river stage, but decreased with the change rate of the river stage as a loose power function; and (c) the increasing ground water level resulted in a 61% loss of the cohesion of upper cohesive bank soil and a complete loss of the apparent cohesion of the lower noncohesive bank soil. The major mechanism underlying the influences of the groundwater flow on bank erosion in the MYR is probably the development of pore water pressure that changes the forces acting on soil block and reduces the soil shear strength, whereas seepage erosion is seemingly not intensive. The current study can provide a direct evidence for the response of the riparian groundwater level variation to the river stage change in a large river system.