Runoff Response To Urban LID Construction In Gully Region On The Loess Plateau

The loess plateau region was hindered by water shortage and drought.In recent years,climate change and strong interference of human activities have combined to cause the change of urban hydrological cycle.As a result,extreme rainfall events increase in the loess plateau region and urban waterlogging events occur sometimes,which not only bring adverse effects on people's production and life,but also restrict the urban development.In the process of urban construction,the natural infiltration capacity of urban surface is weakened,and the old urban drainage facilities cannot meet the needs of urban drainage requirement.As a result,the urban runoff keeps increasing,and the frequency of flood inundation in urban areas is increased.Urban rainstorm flood design is an important basis for the construction project of urban flood control and drainage.In recent,how to scientifically estimate the urban storm flood design result is to guide the construction of sponge city in the gully region of the Loess Plateau,and it play a key role to make the sponge city effectively.Taking Xi feng district of Qing yang city,Gansu province as an example,this paper discusses the variation characteristics of LID measures on runoff control during the construction of sponge city.By collecting and sorting out the historical rainfall data of Qing yang city,the formula of rainstorm intensity in Qing yang city was deduced,and the rainfall pattern of short duration in different recurrence periods was designed.Through the calibration of simulated discharge from the hydrological model with the observation discharge,to use the storm flood management model(SWMM)analysis and research urban rain flood relations and existing problems under the current condition,and select the applicable measures of low impact development(LID)to quantitatively analysis runoff regulation function under LID measures with different design rainfall return period(2 a,10 a,20 a,50 a)for,the most reasonable low impact development measures are put forward.The main conclusions are as follows:(1)the precipitation data of Xi feng station of Gansu province from 1960 to 2003 and from 2005 to 2014 were collected from the national meteorological administration,and the extreme value sequence of rainstorm was established.By using the Gumbel distribution curve and Pearson-? distribution curve to fit analysis of rainfall data frequency,optimal choice is the Gumbel distribution curve for the optimal fitting of linear extremum sequence through the error compared analysis,and by using the least square method the design storm formula of Qing yang city.Through comparative analysis of different rain types,the Chicago rain type was selected as the design rain type,and the peak coefficient of historical rainfall data was statistically analyzed,and the comprehensive peak coefficient r=0.38 of Qing yang city was obtained.Finally,the designed rainfall processes under different recurrence periods were obtained.(2)ArcGIS software was used for preliminary processing of underlying surface data,pipe network data and other data in Qing yang city,and the rainstorm flood management model(SWMM)in the study area was constructed.The model parameters were calibrated and the simulation results were verified by two measured flood events in Qing yang cities at the discharge outlet.The calibration and validation results show that the Nash efficiency coefficients of the model are 0.89 and 0.85,and the relative errors of peak flow are 5% and 7% respectively.The simulation results are in good agreement with the measured data,and the model is reliable.(3)By analyzing the calculation results of rainstorm flood in different recurrence periods,the main problems of urban surface runoff and pipe network load were investigated and analyzed.For example,when the return period of more than 2 a,due to low elevation of Dong zhi yuan avenue and low drainage pipe network design standard,it caused the high surface runoff,and surface runoff is increasing with the increase of return period.With the runoff coming from Qin zhi dong Road and Dong zhi yuan avenue,serious waterlogging situation is occurred in this road cross.The overflow time and depth of J95 node is the most serious node.(4)The SWMM model was constructed on the basis of the data of special topography,land form and urban construction of Qing yang city in gully region of loess plateau.The applicability of the six technical measures of "infiltration","stagnation","storage","net","use" and "discharge" in urban construction of sponge city construction in gully region of loess plateau is compared and analyzed.Combined with the characteristics of regional soil infiltration,permeable pavement and rainwater garden are selected as two LID measures adopted in this simulation study.By comparing the total runoff control rate,runoff peak reduction rate,runoff coefficient reduction rate and peak lag time of these two measures with those of the current situation,the change characteristics of urban rain-flood process under different LID measures are analyzed.The results show that rainwater garden is more effective than permeable pavement in runoff control.Permeable pavement has poor effect on the delay of peak value,while rainwater garden has obvious effect on the delay of peak value.LID measures' control effect on runoff decreases with the increase of rainfall recurrence period.