Bank collapse predicted by asset profile and liquidity

Posted on December 16, 2014

Although the downstream movement of flowing water is the most obvious cause of riverbank erosion, seepage erosion is also significant. When the water level in a stream is high, water fills the spaces in the bank soil, and then returns to the channel again as the water level falls. This return movement can cause seepage erosion, especially in composite river banks, where layers of sand are washed out from between adjacent layers of relatively impervious silt. This leads to undercutting and the eventual collapse of the overhanging soil mass. A recent Indian study is the first attempt to develop a model to predict seepage erosion in composite banks. Because seepage erosion is difficult to measure in the field, laboratory experiments, using layers of soil from the banks of the Brahmaputra River packed behind a perspex sheet, were carried out to see how erosion was affected by the seepage gradient (i.e. the head of water in the soil divided by the width of the bank) and the pattern of soil stratification. The time to bank collapse described a downward curve as the seepage gradient increased, dropping by about half as the seepage gradient rose from 0.4 to 0.8. No collapse occurred below a threshold gradient of about 0.015. The time to collapse rose significantly as the number of silt layers increased. The seepage model was quite reliable, with 70% of erosion observations from the Bramaputra falling within the predicted range.

Reference: Karmaker, T. & Dutta, S. 2013. Modeling seepage erosion and bank retreat in a composite river bank.
Journal of Hydrology 476, 178–187.