Modeling seepage erosion and bank retreat in a composite river bank

In large alluvial river courses where bank failure is prominent, seepage erosion may be a significant contributor that is often overlooked. Seepage erosion typically occurs simultaneously with fluvial erosion and other mass failure processes during the recession period of hydrographs. In this study, a functional relationship between seepage erosion rate and its controlling variables was developed using lysimeter experiments. This experimental relationship was used to formulate a mathematical seepage model that predicts the daily seepage erosion rate for a composite river bank. Soils collected from the Brahmaputra River banks were repacked in lysimeters with horizontal stratification similar to the actual bank stratification. A total of 71 lysimeter experiments were conducted with various combinations of the controlling variables. The results indicate that the seepage gradient has a dominating effect on the time taken in developing undercuts that lead to the bank collapse. Increase in the number of silt layers in the bank profile increased the time to collapse significantly. In situ bank erosion measurements were taken to validate the performance of a bank erosion model for the middle reaches of the Brahmaputra River in India. The model predictions of seepage erosion rate showed a close agreement with the in situ bank retreat measurements.