FLOW SIMULATION IN ALLUVIAL STREAMS

Abstract

The flow resistance characteristics of an alluvial stream are highly complex, which warrant considerable research efforts. 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. The position of the free surface is likely to change with respect to time and space and also by the fact that the depth of flow, the discharge and the slopes of the channel bottom and of the free surface are interdependent An integrated study in Fluvial Hydraulics and Sediments Transport involves the analysis of the capacity of the river or channel to carry water and sediment, and the corresponding morphological changes in both the main channel and floodplain. Sediment transport in concise, replicates the various aspects of the dynamics of solid particle movement, properties of the transported materials, and characteristics of the transporting medium, which in turn, may be affected by the solids transported. The morphological changes of rivers are deeply interrelated with bed deformation and bank erosion because of the mutual relationship between water flow and sediment transport. A better understanding of these processes is very important in river engineering to prevent disasters due to flooding, to design and manage hydraulic structures, like bridges and water intake towers, and to maintain river ecosystems and the landscape for environmental engineering purposes. Reliable and quantitative estimate of the bed aggradation or degradation are very important in river engineering and management projects as well as accurately predicting the water surface elevations during floods in estimating flood related damage. Thus, engineers are greatly interested in accurately predicting the behavior of river under various flows and sediment loads so that better information can be obtained for the planning and design of river control structures, flood protection measures and other water diversion structures. Present study is to investigate the flow propagation behavior at different flow stages with varying sediment transport capacity and sediment concentration and thereby predicting time variant bed profile using 1-D flow simulation model HEC-RAS Hydrological and Hydraulic data of Brahamputra River have been adopted for the purpose of the study to facilitate the modelling endeavor with real life situation. The study area stretch over the length of 114.750 km of the river Brahmaputra in the Assam valley of India. The study is based on the hydrographic data (river cross sections) collected over the span of 5 years from 1993 to 1997 comprising two discrete data years. The observed variations of the thalweg levels with the model predicted data base have quantified the aggradations and degradations in the reach between the cross sections and has shown that aggradation phenomena is the most pronounced trend in the study period. In the three segments of the total length, the middle reach is observed to be more susceptible to degradations due to relatively high stream power feedback and negative feed back from sediment inflow during low flow season of the river Brahmaputra in this reach. The slope of the longitudinal profile has been varying from flatter in lower reaches to steeper in upper reach. The results of the study also suggests that Manning roughness coefficient n is significantly varying temporally as well as spatially indicting dynamic, mobile persistently changing bed forms. With increasing discharges Manning's n reduced considerably. Present study also seems to suggest that sediment supply is most fundamental parameter controlling aggradations whereas unit stream power is more responsive to degradations.