| The urbanization of population is close to 60%,which will sustainably increase in the future.And the forest,grass,and wetland are replaced by the impervious pavement,leading the interruption of the surface and underground runoff.Accordingly,the capacity of the urban underlying surface declines to regulate and store the flood and waterlogging,which makes it happen constantly.The sponge city construction aims to restore the nature hydrological cycle and the capacity of regulation and storage.But the hydrological and hydrodynamic processes are complicated and characterized by the spatio-temporal variability between the sponge city construction measures and the flood and waterlogging.The runoff generation and concentration law need to be excavated using suitable model in the processes.Based on this law,the scientific planning and construction has been the heated and important issue to avoid the deterioration of the flood and waterlogging lead by the urbanization.This paper built the distributed urban hydrological and hydrodynamic model,which was applied to analyze the effect and reason of the sponge city construction on the flood and waterlogging regulation and storage in Pingxiang city,Jiangxi Province.The main results of this study were shown as follows:(1)The functional requirement and principle of the model was analyzed to develop the distributed urban hydrological and hydrodynamic model.It was programmed to form the independent intellectual technology.Specially,the functional requirement of the model was set to simulate the runoff generation and concentration of the sponge facilities and the flood and waterlogging coupling processes in the sponge facilities-plot-region scales.According to the requirement,the model distributed urban hydrological and hydrodynamic model was built.The runoff generation module adopted the soil conservation service curve number(SCS-CN)model and excess infiltration model to characterize the relationship between the storm and flood.Moreover,the runoff concentration module adopted the hydrodynamic models to characterize spatio-temporal distribution of the flood and waterlogging.The overland,river,and underground flow concentration module adopted the kinematic wave equation of Saint-Venant equation,Muskingum method and recession curves,respectively.Then the overland flow concentration was implemented according to the orders of the accumulated flow calculated by D8 algorithm.Meanwhile,the overland and underground net precipitation were the water balance elements for the flow concentration in the 2-D computational grids.The sponge facilities influenced the runoff generation and concentration by altering the parameter CN,constant filtration rate,and surface roughness.Lastly,the model was interactively programmed by the WebGIS and IntelliJ IDEA to form the independent intellectual technology.(2)The sponge city construction derived land use and cover law was systematically analyzed to build the hydro-meteorological and land use and cover database for the model.It was evaluated in the Pingxiang city,Jiangxi Province.Specially,the Wufeng River basin was selected as the study area and its high-resolution image and field investigation data were collected between 2016 to 2018.Those two-type data were overlaid to build land use and cover database of the basin by ENVI5.3software.The dynamic degree,transition matric and probability matrix were calculated for the area of the land use and cover to prove the sponge facilities as the main land use and cover change.Its area increased by 19 km~2 in 2017 and 77 km~2 in 2018 with the dynamic degrees of 25441%and 401%,respectively.The sponge facilities were mainly converted from the square and road.Then the storm,flood,land use and cover,digital elevation model(DEM),flow velocity,flow direction,and accumulated flow databases were built.The temporal and spatial scales were confirmed to be 1 hour and 30 m to meet the accuracy and spatio-temporal variability,respectively.The storm,flood,CN,constant filtration rate,and surface roughness were interpolated to the 1-hour scale in the computational grids.The overland flow concentration was implemented according to the orders until 800 of the accumulated flow calculated by D8 algorithm.Lastly,the model was validated by the Nash-Sutcliffe efficiency coefficients based on 14 storms and floods.The mean coefficients were 0.68,which proved the practicability of the model.(3)The response and reason of the sponge city construction on the runoff generation and concentration was evaluated in the plot-region scale.In special,the 2016and 2017,2018 were selected as the base and benefit analysis periods according to the land use and cover change.The runoff generation and concentration were simulated in those periods by the model.The CN decreased by 31%after the sponge city construction in the Wufeng River basin and by 61%for the permeable pavements.It indicated the decrease in the runoff generation capacity.Actually,the runoff coefficient decreased by44%for the storms of 20-30 mm,30-40 mm,40-90 mm,and above 90 mm.In addition,the constant filtration rate increased by 53%after the sponge city construction in the Wufeng River basin.While the rates in the road and municipal land increased by 20times.The increasing constant filtration rater would lead the ratio of surface net precipitation to the total precipitation to decrease.In contrast,it would lead the ratio of underground net precipitation to total precipitation to increase.And the decreasing ratio of surface net precipitation triggered the overland excess infiltration weaken,followed by the decrease in the overland runoff generation capacity combined with decreasing runoff coefficient.Conversely,the increasing ratio of underground net precipitation triggered the underground runoff generation accelerated.But it could offset the influence of the decrease in the runoff coefficient to make underground runoff coefficient decrease and runoff generation capacity weaken.Moreover,the surface roughness increased by 27%after the sponge city construction in the Wufeng River basin and by 2 times for the permeable pavements.And the parameter K in the Muskingum method decreased by 91%in the same scenario.Those changes worked together to lead the unit discharge of the overland flow decrease by 37%,leading the runoff concentration slow.Lastly,the sponge facilities were characterized by source reduction,runoff concentration block,and point flood and waterlogging regulation and storage function.It distributed in the basin as the reservoir to enhance the runoff control capacity of the basin,weak the overland excess filtration runoff,and block the runoff concentration.But it strengthened the underground runoff generation.(4)The response of the sponge city construction on the flood was evaluated,followed by its sensibility to runoff generation and concentration coefficients.Then the peak discharge and total volume of the flood were assessed to decrease by 38%and 14%after the sponge city construction for the storm of 20-30 mm,30-40 mm,40-90 mm,and above 90 mm,respectively.And the flood calming was observed in the same period.The sensitivity analysis of the peak discharge and total volume of the flood were implemented for CN,constant filtration rate,and surface roughness at 5 levels.Which was formulated by the empirical orthogonal function.It indicated that the sponge city construction made the decrease in CN and increase in the constant filtration rate,and surface roughness,which was the reason of the decrease in the peak discharge and total volume of the flood.Additionally,the peak discharge and total volume were detected to be more sensitive to CN.Accordingly,the decrease in the peak discharge and total volume was mainly attributed to the decrease in CN.(5)The response of the sponge city construction on the waterlogging was evaluated.And the relationship was built between responses of the sponge city construction in the waterlogging and runoff generation and concentration.The corresponding reason was revealed.Specially,the temporally mean and maximum depth of the waterlogging were assessed to decrease by 38%and 47%respectively,in addition to the decrease in waterlogging area by 51%and waterlogging duration by62%.In addition,the source reduction function of the sponge facilities led to the decrease in the waterlogging depth,area,and duration.In contrast,the runoff concentration block function of the sponge facilities led to the increase in the waterlogging depth,area,and duration.And the observed decrease in the waterlogging depth,area,and duration was attributed to more significant influence of the source reduction function of the sponge facilities. |
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