Morphological aspects of non-erodible reaches in rivers

The bed of some rivers partially consists of non-erodible layers. These non-erodible layers may become obstacles for navigation during the low-water period. To improve navigation development in the future, one can consider the removal of the non-erodible layers. To predict the morphological consequences due to these works, knowledge about the effects of non-erodible layers on the bed level of the river is necessary. The present research was concentrated at the following two main subjects: Sediment transport travelling over non-erodible layers. Bed level variations upstream and downstream of the non-erodible layer. The latter subject is subdivided into the cases of a short and a long non-erodible layer. Aspects which were studied with respect to the description of sediment transport over non-erodible layers were: Incipient motion of sediment particles on a fixed bed. Bedforms on a non-erodible layer. Roughness of non-erodible layers, with or without the influence of bedforms. In the theories concerning the alluvial bed and the above mentioned subjects, it was pointed out were these theories are not valid in the case of non-erodible layers. Suggestions for adaptations of these theories are given. The most important adaptation is the introduction of a reduction function which defines che sediment transport reduction in comparison with the alluvial situation. Next analytical solutions for the bed level variations upstream and downstream of the non-erodible layer were derived using Laplace transforms. To do this, the basic equations were linearized and boundary conditions were derived. Relatively much attention was payed to the boundary conditions at the non-erodible layer. Suggestions were made of how the research results concerning sediment transport over non-erodible layers could be used in the determination of these boundary conditions. Finally the analytical solutions were compared with numerical results which were obtained from calculations with an adapted version of the computer program ODIRMO (version 3.0). The calculations were made using data of the Mekong river (Thailand). The research led to the following results: Insight into the usefulness and shortcomings of the present sediment transport models of non-erodible layers is obtained. The analytical solutions of the bed level variations upstream and downstream the non-erodible layer agree well with the numerical results of ODIRMO. Now a computer program is available (ODIRMO, version 3.0) which can simulate non-erodible layers in rivers using the adaptations in sediment transport and roughness modeis.Hydraulic EngineeringCivil Engineering and Geoscience