Density stratification, suspended-sediment transport, and downstream fining in large, low-slope, sand-bed rivers

Large sand-bed rivers typically display an upward concave longitudinal profile and a downstream decrease in median bed sediment grain diameter. This thesis consists of four chapters related to the study of these phenomena.; Chapter 1 focuses on the effects of density stratification on the vertical velocity and concentration profiles in open-channel flows. It is found that in general density stratification effects tend to be greater in large, low-slope rivers than in their smaller, steeper brethren. The effect on the total suspended load and size distribution of suspended-sediment can be significant, and thus density stratification should be included in sediment transport predictions for these conditions. Chapter 2 incorporates density stratification into new formulations for the prediction of the flow depth and grain-size specific suspended-sediment transport rate in sand-bed rivers. The new formulation also includes new predictive equations for near-bed entrainment rate into suspension and form drag.; Chapter 3 presents a numerical model for the simulation of the longitudinal profile and bed sediment distribution in sand-bed rivers over the past 5,000 years of relatively stable sea-level. In Chapter 4 the model is applied to a generic river reach typical of a large, sand-bed river flowing into the ocean in order to investigate the effects of sea-level rise and tectonic subsidence on the degree of profile concavity and downstream fining. Also, several other physical mechanisms that may affect fining are studied, including the relative importance of the suspended versus bed load, the effect of the loss of sediment overbank, and the influence of the delta bottom slope.