Influence Of Urban River Morphology Reconstruction On Pollutant Migration And Transformation

Urbanization not only breaks the ecological balance of river systems,but also accelerates the degradation of river morphology,causing serious damage to the structure and ecological functions of urban river channels,ultimately leading to ecological environmental problems such as black and foul water bodies.Nowadays,reshaping the morphology of river channels to create diversity in the spatial distribution of aquatic habitats in river channels is an important means of river ecological restoration.However,research mainly focuses on the response of changes in river terrain and landforms to river ecosystems,but there is a lack of exploration on the effectiveness of river morphology in improving water quality.Liangtan River Basin is a primary tributary on the right bank of Jialing River.Due to urbanization,its rivers are seriously channelized,which worsens the water quality.In recent years,the local government has invested a large amount of funds in the comprehensive treatment of the water environment in the Liangtan River basin.However,due to the difficulty in establishing a complete ecosystem of the repaired river,the water body in the river section still cannot stably reach Class IV water.This article takes a typical urban channelized river channel（Urban）in Liangtan River,Chongqing as an example.Based on the concept of natural restoration of river channels,digital river synthesis technology（River Builder）is used to design and transform the three geometric variables of river width(W_bf),water depth(D_bf),and meandering degree（M_d）of typical urban channelized river channels,generating five different forms of digital river terrain,The constructed two-dimensional hydrodynamic convection diffusion model was used to investigate the effects of river channel morphology reshaping on pollutant migration and diffusion indicators such as water turnover time（TOT）,pollutant concentration curve（CC）,pollutant maximum time（MT）,and pollutant downstream boundary time（AT）.Based on this,a two-dimensional hydrodynamic model was used to investigate the impact of different morphology reshaping on multiple ecological hydraulic indicators,By exploring the changes in the suitable habitat area of the biological community in the river system,the impact of ecological hydraulic indicators on pollutant transformation is further proposed.Finally,the numerical simulation results of two actual river channels are verified and corresponding river morphology improvement measures are proposed.The main research conclusions are as follows:（1）The experimental study of physical hydraulic model found that the removal rate of pollutants（COD,NH₃-N,NO₃-N and TP）of Urban watercourses was lower than that of other watercourses during dry season and average runoff,followed by the watercourses with changes in W_bfand M_d（U_Wand U_M）,and the watercourses with changes in D_bf（U_D）had better removal effect.The multi-factor variable composite channel(U_W+D+M)with the best pollutant removal effect.However,the removal efficiency of TN and TP in each channel is poor,with a removal rate of 4%-45%,while the removal efficiency of NH₃-N is the best,with a removal rate of more than 95%.（2）Through two-dimensional hydrodynamic simulation,the river flow field of each channel shape under the discharge in dry season is simulated and calculated.It is found that three geometric variables,namely W_bf,D_bfand M_d,are changed respectively to reshape the morphology of Urban channel.Although the uniformity of Urban channel flow field is changed to a certain extent,it is far less than the hydraulic diversity provided by multiple complex flow sites of U_W+D+Mchannel.At the same time,through the simulation of two-dimensional hydrodynamic and water quality model,it is found that U_W+D+Mhas the best effect of inhibiting the diffusion of pollutants,followed by U_Dwith depth variation.The inhibition effect of U_Wand U_Mis not significant,but it is still better than Urban river.（3）（3）For urban waterways with severe channelization,their hydraulic form diversity（HMID）values are relatively small at different flow scales,indicating poor hydraulic diversity of urban waterways and a lack of a good ecological environment.U_W+D+Mriver channel performs better than other rivers,with rich hydraulic characteristics,complex riverbed changes and higher spatial heterogeneity,which improves the diversity of biological communities in the river ecosystem,has the buffer capacity against the deterioration of micro ecological environment,promotes the degradation of pollutants,and plays a role in the self purification of water environment.Under different ecological replenishment conditions,each river channel can improve the velocity variability index to a certain extent.Among them,the velocity variability index of U_W+D+Mriver channel is most affected by the ecological replenishment amount,followed by U_Driver channel,which can improve the hydraulic performance of the river at a small replenishment amount.Establishing velocity gradients for four suitable target protected fish carp populations,it was found that as the flow rate increased,the suitable habitat area（WUA）for fish at 1-4 m³/s in the Urban River was lower,accounting for no more than 0.23%,respectively.However,the WUA values of rivers with changes in width,depth,and meandering degree decreased but were generally higher than those of the Urban River.The U_W+D+Mriver with the highest WUA value and complex changes is considered to be due to the fact that the complex morphology of the river can provide fish with richer habitats and good hydraulic characteristics.The increase in the suitable habitat area for fish habitat will promote the richness of fish communities,and fish can control the number of planktonic animal and plant populations through predation,improve water transparency,accelerate pollutant degradation,and enhance water quality,It also has the potential to promote the growth of aquatic plant,thereby promoting the self purification ability of water bodies.（4）According to the numerical simulation of two human-disturbed river channels in Liangtan River Basin,it is found that compared with the natural river channel,the Urban river channel’s TOT value,MT value,AT value and CC value are all inadequate for the migration and transformation of pollutants in the form of urban river.Natural watercourses can provide more diverse habitats for fish,and the effective habitat area is larger.