Maximum amplitude of river meanders

Abstract

The prediction of planform changes is important for the site selection of structure like bridges or intakes. In the design of bank protection works it is indispensable to know future planform developments for the safety of structures and lands. The meander development process consists of translation and expansion of the bends due to bank erosion. A characteristic feature of curved flow is the spiral flow. The effect of flow through a river bend is erosion along the outer banks, while deposition of material along the inner banks takes place. The curvature of meander bends changes continuously during the bank erosion implying a variation of the lateral bed slope and a continuous process of redistribution of the flow and sediment transport. Mathematical models are becoming increasingly important. They are used for the prediction of planform development of meandering rivers. The 2-D MIANDRAS model of Crosato (1990) used in this study, through via this improved bank erosion model considers, in determining of the erosion rate, to be proportional to the excess near-banks flow velocities to a critical value. Different from the earlier version of the model, the critical flow velocity was assumed to be constant and therefore different from the zero-order flow velocity. The existing bank erosion models consider an immediate reduction of the longitudinal bed slope, being true for the cases with fast bank erosion. In this study the real longitudinal bed slope is determined, accounting for the different time-scales of the two processes involved, notably the bank erosion (lateral growth of river bends due to the fact that sinuosity increases) and the longitudinal aggradation of the bed. The results are based on numerical simulations from the 1-D WENDY model developed by DELFT HYDRAULICS. The reduction in flow velocity due to the increasing sinuosity was studied as cause of reaching a maximum amplitude of meander bends. For this purpose numerical simulations were carried out with the MIANDRAS program developed by Crosato (1990). The model proposed was adapted the bank erosion module of the MIANDRAS program. In this study was found that many parameters influence the development of bends, notably geometry, discharge, channel width, the interaction parameter, the wave length (Lp)and the damping length (Ld) of the bed deformation. The translation of bends is dominant for high values of Lp, being an important result for determining the maximum amplitude of river bends.