CONSIDERATION OF SEDIMENT NON - UNIFORMITY IN "RUMAR"

Abstract

Problems related to rivers are complex and analytical solutions are almost non-existent for most of them. Although physical models provide a close representation of natural processes, construction of the physical model of a long alluvial river reach is very difficult, if not impossible. More elaborate mathematical modelling of the river engineering processes is, therefore, necessary for understanding the behaviour of a river and also for prediction of its response when subjected to some kind of change in its controlling parameters Such an attempt was initiated by the Hydraulic Engineering Section of Department of Civil Engineering, University of Roorkee (India) in 1994 . The outcome of that was a one-dimensional, uncoupled, cipasi-steady mathematical model for alluvial rivers known as Roorkee University Model for Alluvial Rivers (RUMAR) The first version of RUMAR could only handle the transport of uniform sediment, However, natural streams are invariably characterized by the presence of non-uniform sediments and thus varying transport rates of the different fractions of the sediment. The nonuniformities of river bed material result in hydraulic sorting and hence in armouring which further affects the transport rate of sediment. For a better simulation of natural river processes the model RUMAR has been modified and extended to incorporate the process resulting due to the transport of non-uniform bed material. The transport rate of sediment is computed for each individual fraction of sediment and the interaction of transported load and the bed material is computed by sediment continuity equation solved for each size sraction in the mixing layer. Three relationships were used for computation of mixing layer thickness. The model response has been observed for varying values of the standard deviation of the size distribution of the bed material, the mixing layer thickness using two different sediment transport relationships. The new version of the model is found to produce physically realizable results for the situations studied herein