Experimental Study On Hydrodynamic Characteristics And Sediment Motion At Mountain Confluent River

Channel confluences are common in natural rivers and much hydraulic engineering. However, the comprehensive description of 3D flow field, sediment transport and bed morphology within the confluences are limited although lots of hydrodynamics researches are preferable at channel confluences. The purpose of the paper is to present the evolutions of 3D flow field, the hydrodynamic characteristics and sediment motion at mountain confluent rivers based on a series of flume experiments. The experiments were conducted in a combined flume with 30o angle between main and tributary flume located at Sichuan University. Water depth measurements were made using a point gauge, the velocity and turbulence intensities measurements were taken using a three-component acoustic Doppler velocimeter (ADV) over a grid defined throughout the junction region. The average velocity turbulence intensity were calculated according to a series of instantaneous velocities that was recorded at each location for different discharge ratios q* of the branch channel flow Qb to combined tail-water flow Qt. In addition, 2D mapping of the water surface and velocity vectorgraph were painted throughout the junction region for different q*. This study presents the details of the experimental procedure and the general flow characteristics observed. According to those results, the distinctive characteristics of a sharp-edged , channel confluence flow may be divided into six main zones: a stagnation region immediately upstream of the junction, a flow deflection zone near the junction, a zone of separation immediately downstream of the junction branch channel, a maximum velocity region in the main channel due to the separation zone, a flow recovery in the main channel after the maximum velocity region, and a shear plane developed between the two combining flows. Based on flume experiments of the flow and sediment, this study discuss the evolutive process of sediment transport and bed morphology at confluent river by way of three cases, namely without branch channel flow, branch channel flow only with clear water, branch channel flow with both water and sediment. The bedload transport rate and its accumulative sediment transport amount with the increase of time were recorded and analyzed for different discharge ratios q* in each case. In addition, 2D mapping of bed morphology for each case were also presented. The results presented that the characteristics of bedload transport and local scoured bedform strongly depend on the flow and sediment conditions from tributary and the relationship between main and tributary rivers as far as hydraulic factors were concerned.