Research On The Numerical Simulation Of Urban Stormwater Runoff And Its Application In Typical Urban Flooding Problems

In the context of global climate change and rapid urbanization,the extreme pluvial flood hazards triggered by extreme rainstorms have continuously increased worldwide.As a crucial non-engineering measure for flood control,the accuracy and adaptability to calculation conditions of numerical models for urban flooding determine the scientific performance of urban flood risk assessment work.This study proposed and constructed a coupled hydrological-hydraulic numerical model based on the current problems in urban flooding.Taking the typical urban catchment areas and constructions as the study objects,the main work of this paper is as follows:(1)A nesting of Kinematic Wave Equations(KWEs)into Shallow Water Equations(SWEs)method and depth correction and redistribution mechanism under the condition of the extreme thin water depth have been proposed to solve the stability and water balance problems of the grid cell where the thin flow depth occurs under rainfall conditions.The model was validated by simplified urban flood laboratory experiments.The results suggest that the model can capture the complex phenomenon of water flow under the influence of urban buildings,including the water jump in a short time and spatial steps,backwater,and receding water.In addition,the relative error between the simulated and measured values is generally less than 20%,and the kinematic wave equation can ensure the continuity of the flow under the extremely small water depth.A coupled one-dimensional(1D)and two-dimensional(2D)hydrodynamic model for the surface flow was constructed based on source terms and numerical fluxes coupling.The test results show that the flux-based coupling approach is more suitable for supercritical flow,while both source term-based and flux-based coupling approaches under sub critical flow conditions are in good agreement with the results of the complete two-dimensional hydraulic model.(2)According to the runoff characteristics of the ramp pavement,a nesting Diffusion Wave Equations(DWEs)and Gravity Wave Equations(GWEs)hydraulic model with the Finite Volume Method(FVM)solution format has been proposed based on the simplified theory of local approximation for shallow water equations.Based on the characteristic wave theory and test cases,rationality of the local inertia approximation was demonstrated,and the errors caused by Local inertial approximation are discussed.The RMSE and NSE between the simulated and measured values of pavement depth are 0.3 mm and 0.92,respectively,and the maximum water depth difference is 0.8 mm.The model was applied to reveal the hydraulic characteristics of urban pavement.The orthogonal test results and extreme difference analysis show that the cross slope is the decisive factor of pavement water depth and ponding zone width under diffuse drainage conditions.The role of longitudinal slope(longitudinal slope difference)on ponding depth is enhanced under concentrated drainage conditions.Especially in the vertical curve pavement with variable longitudinal slope,the influence factors of water depth show noticeable longitudinal differences.(3)Considering the rainfall loss of the underlying surface,the unstructured grid and catchment response cell are used as the minimum unit for the runoff yield and hydrological concentration process,respectively,and the coupling hydrologicalhydrodynamic numerical model of urban flooding is constructed by combining the surface hydrodynamic and pipe network hydrodynamic models.The model was validated by the measured drainage discharge and surface waterlogging depth in actual rainfall events.The results show that the model can accurately reflect the ponding and drainage process with the change of rainfall amounts.The relative error of the peak flow at the outlet of the pipe network is about 10%,and the error between the simulated and measured values of surface water depth is less than 0.011 m,which indicate that the model can ensure accuracy in dealing with the simulations of surface runoff,flood routing and pipe network drainage during rainfall events.(4)According to the different runoff yield and concentration characteristics of the underpass bridge ramp pavement and the road network and community within the corresponding catchment area,a modular modeling approach was used to construct a coupled hydrological-hydrodynamic process based on a multi-scale rainfall loss calculation unit.Based on the hydraulic connections and water volume interactions between multiple modules,the characteristics of the ponding and drainage process of underpass bridges and the influence of pump station drainage on the operation and safety of the upstream community pipe network were revealed and analyzed.Under the scenarios of setting rainfall events,the drainage of underpass bridges and pumping stations can guarantee the drainage capacity of 5～10a,and the impact of the drainage of the underpass bridge pumping station on the operational safety of the pipeline network is observed in increasing pressure-bearing time and the maximum pressure of the pipeline.Under the design storm conditions of 5a～50a,with the distance to the pump station confluence node decreasing,the increment of full load time and the proportion of drainage time caused by pump station drainage are 11 min～46 min and 6.9%～18.1%,respectively,and the increment of pipe gauge head is 0.20 m～0.65 m and 4.81%～18.28%,respectively.The results indicate that those effects significantly increase with the increasing rainfall return period and drainage discharge but become less pronounced with the increase of the distance between the pipeline and the pump station.(5)A coupled hydrological-hydrodynamic numerical model was developed for the central city of Shijiazhuang,in which the runoff yield and concentration process were calculated in terms of the catchment response units.Based on the transformation scheme of the underlying surface,the differences in total runoff volume and highresolution distribution of surface hydrodynamic variables were used to reveal the effects of different rainfall patterns on the runoff control effectiveness of permeable pavement.The results indicate that the rainfall amount is the main factor affecting the runoff control of permeable pavements,and it is more sensitive to changes in rainfall amount during rainfall events with lower return periods.Under the same rainfall amount,the runoff control effectiveness decreases with the increase of rainfall peak coefficients(r),and it is more sensitive to the changes in peak coefficients during rainfall events with r<0.5.In addition,the runoff control effectiveness tends to decrease with the increase of rainfall durations during the short duration of the rainstorm.In terms of the waterlogging characteristics of the typical area,the reduction of the maximum depth,the total inundated area,and the inundated area corresponding to the water depth above 0.2m in the typical ponding area are 17～7%,38～10%,and 13-2%respectively for the rainfall return period of 5a～100a.The corresponding reduction is 16～12%,45～28%,and 12～2%under the rainfall peak coefficient 0.2～0.8 in the return period of 10a.Thus,the control effectiveness on waterlogging exhibits a general reduction trend with the increasing rainfall amount and rainfall peak coefficient,and the effect is more pronounced under the rainfall conditions with a lower return period and rainfall peak coefficient.