DEVELOPMENT OF NUMERICAL MODEL FOR ALLUVIAL STREAMS WITH MULTI-CHANNEL CONFIGURATION

Abstract

The art of modeling an alluvial river of multi-channel configuration is still in a developing stage and a lot of ground yet remains to be covered. Unlike modeling approach for a single channel stream, the multi-channel model development entails dealing in complex problems of flow division around movable bars and islands, formation and deformation of islands and highly random nature of channel morphological changes. The dynamics of flow is further complicated in a natural stream due to wide differences in hydraulic properties and resistances of flow in the main channel and the subsidiary channels. A number of contributions have been made for simulating flood routing, overland flow and related time-dependent transient flow problems by solving the gradually varied unsteady flow equations numerically however, invariably in all the existing models the solutions are obtained based on certain simplifications. These are: (I) The geometric properties of all the irregular sections of a natural stream over the entire reach are averaged with respect to the flow depth to form a single representative cross-section. The cross-sectional areas and the top widths of the representative cross-section are then fitted by a polynomial. (ii) The change in the volume of sediment per unit length at every time-step after sediment routing is distributed uniformly over the entire cross-section of the natural stream. The above simplifications can work satisfactorily only when natural stream consists of single channel at all the stages of flow. However, in an alluvial stream of multi-channel configurations these simplifications can introduce serious error and the simulation results based on this cannot be relied upon. iii The purpose herein is to develop a numerical model for a multi-channel configuration of an alluvial river with capability to simulate flow of water as well as aggradation/degradation in an active sediment transport regime. This model will be different from the conventional one-dimensional treatment of unsteady flow in natural stream wherein the flow is averaged . across total cross-sectional area. This model treats the main flow channel and subsidiary channels as separate entity. The technique is based on a modified form of Saint Venant's one-dimensional equations of unsteady flow (1) the equation of continuity for sediment, (2) the equation of continuity for sediment laden water, and (3) the equation of momentum for sediment laden water. The three classic equations have been modified in such a way that all the geometric and hydraulic properties of all the channels at various stages of flow remain intact during the process of simulation. Also in the existing models, the bed erosion or deposition of sediment is uniformly distributed over the entire cross section. To improve upon the accuracy in the case of multi-channel configuration an endeavour has been made to formulate suitable functional relationships to represent aggradation and degradation phenomena in a natural stream realistically. In order to arrive at a solution a suitable numerical scheme with solution algorithm is also presented and a computer program has been developed. To facilitate the modeling endeavour with real life situation, the data of Brahmaputra River have been adopted for testing and simulation of numerical model. The large alluvial river like Brahmaputra is a perfect real life example of a stream with multi-channel configurations and it's complexities due to wide variations in the channel form and discharge poses many difficulties in modeling. To overcome these, certain steps have been taken which have been summarised in this work. Finally, the simulation results obtained from the model developed herein have been compared with the available observed data of Brahmaputra and discussion thereupon has been presented.