Investigation of River Bank Protection Using Riprap Under Drawdown Condition

Besides the severity of the problem of riverbank failure, the stability of the protection works is also a great challenge. Quite often, stream bank protection techniques do not last long due either to a lack of understanding of the failure mechanism or to inadequacies in the design methodology. The existing guidelines and formulas for river bank protection to combat erosion and failure are primarily based on the hydraulics of river flow. Thus, the formulas used in practice for the design of the weight and size of the riprap consider the tractive force of flow velocity and the wave action of the stream. In this study, the often-overlooked aspect of seepage failure in bank protections is examined. Varying sizes of riprap and three different types of filter layers were also tested in physical models under different drawdown conditions. The tests conducted suggest the most suitable filter layer and the optimum size and weight of the ripraps under these conditions. A mathematical formulation for the design of a river bank protection technique is also proposed, subjected to drawdown conditions. The equation considers the stability of the riprap against the seepage forces arising out of the pore water pressure difference during a quick lowering of the river water level. The formulation is validated by the results of the physical model studies. The proposed design methodology is suggested to be used as a supplement to the existing design standards for ripraps based on river hydraulics.