Stability of riverbanks under three unsteady river flow conditions is simulated. First is the case of constant water level in the river, second is the case of filling the river to the riverbank top and finally the case of drawdown. Two modes of drawdown are analyzed; the rapid drawdown and the slow drawdown. A finite element model of saturated unsaturated seepage flow was de-coupled with a plain strain elastic-plastic finite element model using strength reduction technique to calculate the stability factor of safety (FOS) of bank material. The influences of location of phreatic surface, pore-water pressure, drawdown rate and ratio on the riverbank stability are discussed in details for each case. Results showed that safety factor for saturated riverbank is nearly 83% of its dry value (for the studied case). The case of filling is most likely to stabilize the riverbank. For the case of slow drawdown, the minimum FOS occurs when the water depth is about 0.25~0.3 the bank height. The case of rapid drawdown is the most critical case.
Stability of hypothetical riverbank subjected to five different flood hydrographs is discussed using the results of numerical simulation. Three models of hydraulic fluvial erosion, seepage flow, and slope stability are coupled to discuss the effect of the seepage flow and river bed deformation on riverbank stability. The three models are based on the finite element method with moving boundaries. The response of riverbank to the oscillated water level in the river and the consequent groundwater table is analyzed. The trend of factor of safety through time is presented. The influence of relevant geometrical, internal and external forces are also discussed.
A new approach to simulate the mass failure process in riverbanks is developed and verified by experimental work. Both planer and circular failure modes can be expressed by this approach. The proposed approach is capable of distributing the collapsed bank material. Failure mechanism of riverbanks is described, failure plane is determined, shape of collapsed material is proposed, and travel slide distance is estimated. Eight experimental cases were conducted to investigate the processes of mass failure that follows the hydraulic erosion of an artificial riverbank with different heights and slopes. Deformations, developed cracks, plane of failure, and shape of deposited material were recorded. It is watched from experimental results that there exist many cracks are developed and thus many planes of failure are found, while in numerical simulation only one failure plane exists. The difference between simulated and experimental results may return to the large deformation of collapsed bank material which can't be simulated by simplified assumptions. Further improvements to the proposed approach are needed.
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