Tidal influence on sediment transport and bed level in the river Merwede

Abstract

The area of the Merwedes is a transition zone between a tide-dominated area and a river-dominated area. With increasing river discharge, the influence of river flow dominates in this part of the Rhine-Meuse Delta. The composition of the river bed of the Merwedes is also a transitional area, because of the presence of both sand and mud. It is unknown how sediment transport and morphology in this area are influenced by the complex interaction of tidal flow and river flow. To be able to explain the morphological changes in the area of the Merwedes and to be able to anticipate on these changes, there is a need for better understanding of the hydraulic and mor-phological processes. This study contributes to a refinement of the system description of the Rhine-Meuse Delta by determining the influence of the tide on sediment transport and bed level in the river Merwedes. The aim of this graduation research is to gain insight into the contributions of tidal flow and river flow to sediment transport and bed level changes in the Merwedes, with a view to application of the obtained knowledge to Room for the River projects in this reach. The Room for the River project ‘floodplain excavation Avelingen’ has been chosen as case study. Three methods have been used to gain insight in the contribution of the tide to sediment transport and bed level changes in the Merwedes: (1) analysis of simulated flow, (2) analysis of simulated sediment transport, (3) analysis of simulated bed level changes. The following ranking has been determined with respect to the relevance for one-dimensional morphological modelling of the Merwedes: (1) including variations in river discharges, (2) the choice of a accurate sediment transport model, (3) including the tidal influence in the Waal and Merwedes, (4) including salt intrusion in the Rhine-Meuse Delta, (5) using a spring-neap cycle instead of a less detailed tidal cycle as sea boundary condition. These adjustments improve the simulation of the autonomous development of the bed level of the Merwedes. This ranking ap-plies to the yearly sediment transport. However, the ranking of modelling aspects varies between the Merwedes at specific river discharges. Both variations in river discharge and the tidal influence should be included in morphological studies of the Merwedes, because of interaction between river flow and tidal flow. The influence of the tide on sediment transport in the Merwedes can best be represented by a spring-neap cycle. However, a less detailed tidal cycle is a reasonable approximation of the tidal influence on sediment transport in the Merwedes.