| The box culvert along the river is one of the important measures to control point source and non-point source pollution entering the river.On the one hand,the sedimentation of box culvert caused by solid material deposition in sewage would weaken the water conveyance capacity of box culvert;on the other hand,the pollutants carried by sediment could accumulate continuously,which may aggravate the overflow pollution in rainy days.However,at present,there were few studies on the sedimentary characteristics of sewage box culverts in urban river channels.Reasonable control of box culvert deposition is the key to ensure the normal and stable operation of box culvert.Based on the actual project,this study carried out research,analyzed and summarized the current situation and characteristics of sediment deposition of sewage box culverts along the river,and then analyzed the key factors affecting sediment deposition with the help of software simulation technology,and finally evaluated the control efficiency of sediment prevention measures to provide references for the reasonable settlement of sediment problems.This study firstly investigated the sediment problem of sewage interception box culvert in a river in shenzhen,and found out that the low sewage flow velocity in box culvert was the main cause of serious sediment deposition in dry season,and box culvert slope,sediment particle size and sediment concentration were the possible influencing factors.The analysis of the pollutants in the sediments of the box culm showed that the concentrations of the pollutants fluctuated in a small range and were low at the beginning of the dry season.At the end of the dry season,the concentration of pollutants in the sediment increased greatly,and the concentration peak moved lower than that in the dry season.As the rainy season progresses,the concentration of pollutants in the sediment decreases to the level at the beginning of the dry season,with less fluctuation along the line.On the basis of field investigation and analysis,the simulation results of box culvert sediments showed that the influence of different factors on sediment concentration was the largest,followed by velocity and slope was the least.After the formation of deposition,the turbulent kinetic energy of the flow in the box culvert would be weakened,the flow velocity would be reduced slightly,and the water conveyance capacity of the box culvert would be reduced.Within the range of deposition height not exceeding 30% of the water passing section,the percentage of deposition height in the water passing section will decrease by about 1.9% for each increase of 1%.Based on the results of sampling and simulation,this study further evaluated the control effect of inlet water pretreatment facilities(horizontal flow online separation facilities,swirling flow online separation facilities and sand settling ponds)and section shape optimization(t-shaped sections and egg-shaped sections)on sedimentation through simulation.The results showed that the water inlet pretreatment was mainly used to control the sediment concentration and particle size.When the velocity was lower than the critical non-silting velocity,the control effect of the horizontal flow online separation device was the best,followed by the sand settling pond,and the control ability of the swirling flow online separation device was relatively poor.The main function of box culvert section shape optimization was to reduce the sedimentation probability of the sediment in box culvert as much as possible.The control effect and stability of egg-shaped section were better than that of t-shaped section.The optimal reduction capacity of tshaped section in dry season and rainy season was stable at about 55%.The optimum reduction capacity of egg-shaped section was about 55% in dry season and 65～90% in rainy season. |
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