Impact Of Urbanization On The Rainfall-runoff Processes

Since the Economic Reform and Opening Up,urbanization in China has entered a development stage.In 2019,the proportion of the urban population had reached up to 60.6%.With the development of urbanization,the frequency of urban flood disasters has also increased.In recent years,urban flooding has become the major bottleneck affecting the safety and sustainable development of cities in China.To achieve early warning of urban flooding,it is particularly important to understand the changes in urban rainfall and runoff processes,which has become a key direction of urban hydrology research.Consequently,this study takes the Wenyu River Basin in Beijing as an example and uses remote sensing technology to obtain the basins land use data in 1990,1995,2000,2005,2010 and 2015.Based on the HEC-HMS hydrological model,a hydrological simulation system for Wenyu River Basin was established based on the watershed digital elevation model,soil type data,land use change data,and hydrometeorological data to analyze the rainfal-runoff processes in an urbanizing watershed.THen hydrological simulation techniques and scenario analysis methods were used to comprehensively evaluate the impacts of urbanization on rainfall-runoff processes based on historical rainstorm and flood events and design rainstorm scenarios.The main research contents and conclusions of this paper are as follows:(1)Land use change in Wenyu River Basin under the background of urbanizationThe change of underlying surface is an important index to evaluate the development of urbanization.With the understanding of the change characteristics of land use types in Wenyu River Basin,is beneficial to explore the influence of urbanization degree on land use change in the basin.Based on the land use data of six periods(1990,1995,2000,2005,2010 and 2015)in the Wenyu River Basin,this paper comprehensively analyzes the land use change pattern of the basin by analyzing the change characteristics of land use types and impervious area changes of sub-basins.The results show that,during 1990 to 2015,the Wenyu River Basin was dominated by arable land and woodland before 2005,and it was dominated by arable land,woodland and construction land after 2005;During the 25 years,the main characteristics of the land use change are the decrease of cultivated land area,woodland area,and the increase of construction land area.The impervious area of the underlying surface of the sub-basin increased rapidly,and the urbanization process was mainly concentrated in the southern part of the basin.(2)Construction and application of Wenyu River Basin hydrological simulation system based on HEC-HMS platformBased on the hydrometeorological data and topographic data of the Wenyu River Basin,Arc GIS and HEC-Geo HMS module are used to complete the feature extraction of the basin water system and the division of calculation units.Based on the land use data and soil type data of the study area in different periods,the SCS curve method and the SCS unit line method are used.As well as the calculation method of the Muskingum to calculate the river basin runoff yield,establish a hydrological simulation system of the Wenyu River basin suitable for the characteristics of the underlying surface in different periods,and based on the 6 typical storm flood processes("198408","199407","199807",“201207”,“201507” and “201607”)to carry out model application research.The research results show that the established hydrological model system of the Wenyu River Basin simulates the storm flood process in the basin better.After parameter optimization,both the peak discharge error and water volume error of the typical field are less than 20%,and the Nash efficiency coefficient is above 0.6.The test shows that this paper: The established model is suitable for the study area and can be used for rainfall-runoff process simulation and flood forecasting in the Wenyu River Basin.(3)Impact of urbanization on rainfall-runoff processesBased on six historical rainstorm and flood data of different size(" 198408 ","199407","199807","201207","201507" and "201607")and daily design rainstorm data of different return periods,the established hydrological simulation system for Wenyu River Basin was used to analyze the variation law of rainfall-runoff process under different land use scenarios(1990,1995,2000,2010 and 2015)under the background of urbanization,and quantitatively evaluate the influence degree of underlying surface change on flood process.It provides scientific basis for flood control and drainage design under changing environment.The results show that,based on the simulation results under the land use background in 1990,the peak flood discharge(2.4-39.2%)and flood volume(2.4-36.1%)of the six rainstorm and flood events increased to varying degrees under other land use scenarios,and the peak occurrence time was earlier(1-3h).Under the design rainstorm condition,the research results are similar to the historical flood analysis results,that is,urbanization causes land use change,and the flood process line of the basin becomes higher and narrower as a whole.In addition,the simulation results under the design rainstorm condition show that,compared with the design rainstorm condition with the high return period,the land use change has a greater impact on the peak flood discharge and flood volume under the low return period.has a bigger impact on the peak discharge and flood volume.