Riverbank Stability Due to Hydropower Dam Operations

Several factors influence the water surface elevation in a river. Most are due to factors outside our control such as seasonal climate changes or unexpected heavy rainfall, but in some cases, human activity also affects these fluctuations. A hydropower dam is one such case, artificially dominating fluctuations in the river water elevation depending on its different operational modes based on considerations such as electricity generation, nature conservation, and/or natural hazard mitigation. Downstream water elevation inevitably fluctuates due to the different discharge rates of an upstream dam and, as a result, the water table in the riverbanks is also affected, ultimately affecting the stability of the riverbanks themselves. In this study, transient seepage and slope stability analyses are performed for representative riverbanks on the Lower Roanoke River under typical and extreme flow conditions experienced as a result of the operational and emergency modes of a hydropower dam. Variations in the riverbank stability due to different flow and soil conditions are explored taking into account unsaturated soil properties. The results illustrate that regular dam operations such as steady state, drawdown, and peaking do not affect the slope stability significantly. The factor of safety decreases as the WSE decreases. Drawdown and fluctuation also decrease the safety factor, though the rate of the decrease depends on the hydraulic conductivity of the soils rather than the discharge pattern. Geotechnical factors such as bank geometry, soil type, and hydraulic conductivity appear to make riverbanks more susceptible to stability issues rather than hydraulic characteristics such as discharge rate, frequency, and duration.