Two-dimensional Transport Model And Sensitivity Study On Riverbed Silt During Peak Runoff Period

The transportation situation of the water environment system of the basin changes significantly with the change of rainfall.During periods of heavy rainfall,due to the large river runoff and the increase in flow velocity,the silt deposited at the bottom of the river bed throughout the year will be sucked into the river along with the peak runoff and exported to the drainage basin.The output is much greater than that of the pollutants in the river channel.The amount of deposition during the period.In the non-rainfall period,due to the decrease of the river flow velocity,the organic matter input in the upper reaches of the river is deposited in the slow flow area of the river due to bioconcentration,and it continues to accumulate with time.Because the silt deposited at the bottom of the river has the function of storing and releasing pollutants,especially in the scouring and silting phase,the large amount of release will directly lead to the collapse of the aquatic ecosystem,and the harmful effects even exceed the direct input of pollutants.This topic analyzes the erosion and deposition laws of the Fuxi River during different rainfall periods and different time periods,and analyzes the time and rate of river erosion and deposition transportation during the peak runoff period.And establish a two-dimensional transport model,calculate the horizontal and vertical transport laws of the tracer TP released by silt in the study section under small-scale time conditions,determine the time and space distribution of silt,and prevent water pollution in urban river sections of small watersheds.And the scale and frequency of river dredging provide theoretical and technical support with scientific basis to prevent the erosion and silting process during the peak runoff period from seriously damaging the local water environment and ecosystem.(1)The peak TP index concentration of the water body of the Fuxi River exceeds the Class V water quality standard,most of the time it exceeds the IV standard,and the average is above the Class IV water quality indicator level.Among the five evaluation indicators,TP exceeds the standard accounted for the most serious,and it is the main source of pollution.There is a big gap between the TP concentration of the two monitoring sections.From 2015 to 2017,the TP concentration of the two sections of the Fuxi River showed a decreasing trend,because the input of TP mainly came from the urban industrial wastewater,residential sewage and agricultural pollution sources.The decrease in the average concentration of TP may be due to the replacement of traditional detergents by non-phosphorus detergents,and the significant reduction in phosphorus emissions from domestic sewage is related to the government’s start to control industrial sewage discharge indicators,resulting in a downward trend in the TP monitoring concentration in the cross-section.(2)The single rainfall at the Shuanghekou-Carbon Research Institute section is below 25 mm,the peak flow rate is less than 6.5m3/s,and the peak flow velocity is less than 0.3m/s.The single rainfall reaches more than 70 mm,the peak flow rate is higher than 18.2m3/s,and the peak flow velocity is higher than 0.46m/s.The section transportation time is 5.53～24.54 h.The peak flow in 2015 was 58.86m3/s in September,the peak flow in 2016 was 54.07m3/s in June,and the peak flow in 2017 was 33.63m3/s in August.s.Therefore,during the rainy season,the river flow velocity in the urban river section is very high,so that a large amount of silt at the bottom of the river bed is entrained,and the organic matter released in the silt can be transported to the lower reaches of the river with the water flow.(3)In the input and output of tracer TP in urban rivers,the input amount of tracer TP is 0.836kg/month～0.958kg/month,and the input amount increases with the increase of rainfall.The rainy season is 6-9 every year.The monthly input is larger than other months,and the output of tracer TP during the rainy season is much larger than that of the low rainfall period.Because the input and output of tracer TP are closely related to the peak runoff,the two-dimensional The transportation model conforms to the urban river transportation mode.(4)The tracer TP input at the bottom of the riverbed enters the river channel and continues to move forward and diffuse with the water flow to both banks and downstream.The concentration of tracer TP in each section continues to increase with the increase of transport time,but the rate of this concentration increase As time goes by,the concentration of tracer TP changes little.This is because the concentration of T tracer P in the river water gradually increases and approaches the concentration of tracer TP entrained by the silt at the bottom of the river bed.The internal water body is close to a stable state,and the concentration of tracer TP in the water is close to the maximum.Among the parameters of the tracer TP two-dimensional transport model,the disturbances of the lateral dispersion coefficient Dy,the silt vertical diffusion coefficient k,and the silt layer thickness δ have the greatest impact on the prediction results.