The evolutionary changes of channel morphology in initial stage of water flow and sediment transport in shallow water

Abstract

When sediment laden flow enters the broad areas from a narrow river channel, the flow velocity declines abruptly, due to the enlarging width. As a result, the sediment silts in the region. This phenomenon occurs when the sediment laden flow enters a plain from a mountainous river, or when it flows into lakes and oceans from rivers. The former condition results in the development of an alluvial fan, while the latter leads to the formation of a delta, and then again gradually evolved into an alluvial river which by the developing process of water flow and sediment transport, and it appears various pattern along with its growth. The river pattern can divide into many types, the classics types are straight channel, meandering channel, branching channel and braided channel. Model test is one of the most important technical means researching water flow and sediment transport and predicting riverbed evolution. At present, many scholars, domestic and overseas, excavate straight river in the foundation bed to research channel morphology changes, and do less research on non-straight river in the foundation channel morphological changes. The formation and development process of alluvial river is a continuous variation, the essay will mainly discuss the evolution change of river channel from nothing on the surface of rivers into the lake delta, under the constant coming water and sediment and constant erosion base level. In this study, in order to study the evolutionary changes of channel morphology in initial stage of water flow and sediment transport in shallow water, non-uniform sediment was used, for a model generalization test, under the conditions of a constant inflow water and sediment, gradient, as well as an outlet base level. The sediment test results showed that the evolutionary changes of channel morphology experienced three stages. At the first stage, straight channel and branching channel were appeared alternately. At the second stage, a single micro-bend channel morphology swing development on the surface of delta deposition occurred with the branching channel of tributary shunt gradually decrease and die. At the third stage, there’s mainly meandering channel occurred with the coefficient of twists become larger with the concave bank continuously scouring regressive in the micro-bend channel. Through analysis and experiment, results show that the channel morphology of the second stage and the third stage present as sine curve characteristics: the upper channel located in the inlet of the sine curve, the middle and lower of the bend channel are located in the peaks and troughs of sine curve. With time going on, the amplitude and direction of sine curve become large gradually, and the coefficient of channel tends to be stable. Finally, the evolutionary changes of channel morphology is analyzed from the bed load and boundary conditions. This study is of great theoretical significance, and the achievements will enrich the theory of river dynamics and riverbed evolution.