One and two dimensional effects of floods on river morphology

Abstract

The importance of the Rhine and its branches is unquestionable. It has an important role as an attraction point to human activities and as a navigation channel from the North Sea into Europe. The present report is one of the numerous studies made for the Rhine branches, in this case the River Waal. The main "tool" used during this work is SOBEK, a software package developed by Delft Hydraulics and the Institute of Inland Water Management and Waste Water Treatment (RIZA) of the Dutch government. In this report, a one-dimensional model for the river Waal is presented, see Chapter 3. This model was used for several simulations that led to the following conclusions. By means of the 1D model SOBEK it was possible to conclude that the dominant discharge can be used to predict riverbed changes without losing accuracy, see Chapter 6. When using a variable discharge for river simulations a simplification can be made by averaging the peaks, see Chapter 6. With the results from the 1D model some parameters of the Waal were computed as relaxation, wave and damping lengths, see Chapter 6. In the present work an attempt was made to use the quasi two-dimensional option of SOBEK (Sedredge) to model the interaction between the floodplains and the main channel during periods of high water periods, see Chapter 4. A two-dimensional model was built with this propose. This model was based on two parallel and laterally coupled channels, one simulating the main channel and the other the flood plains. Unfortunately the simulations showed the impossibility of doing this. SOBEK didn't compute any sediment exchange between both channels probably due to the large difference of levels between them. Another incapacity of SOBEK is related with the fact that it doesn't work when one of the channels is not inundated, see Chapter 7. With a one-dimensional model and with a two-dimensional model (2 parallel channels) for the main channel, it was possible to make conclusions about the relative importance of the bends and of the floodplains in the Waal, see Chapter 8. Definitely the influence of the floodplains on the river morphology is minor when compared with the influence of the bends. The difference of levels between the two channels in the two-dimensional model is considerable. When neglecting them, this will have large morphological consequences. Hence it is suggested that any future study about this Rhine branch should include the bends. In Chapter 9 one simulation is presented to study river behaviour when there is a narrowing in a channel. For this simulation an academic model was built based on characteristics of the Waal. The simulations were made for a permanent constriction and for a temporary one. July/98 In the case of a permanent constriction the evolution of the riverbed to its equilibrium state can be seen. The local perturbation creates a sand wave, which propagates downstream before the river reaches the equilibrium. Upstream the river doesn't show any kind of perturbation. After removing the obstacle after 3 years, the perturbation starts migrating downstream, damping as it proceeds. The riverbed takes a long time to re-establish the equilibrium bed along 15 km. An important fact to remind is that the consequences of making the constriction are not only felt locally, but have a large influence downstream.