Study On Surface Runoff Response To Urban Sprawl And Its Regulation Mechanism

Rapid urbanization in China has led to severe urban expansion.Large amounts of natural land around the city converted into construction land,causing various ecological and environmental problems.Expansion of construction land leads to the increase of impermeable surface area,which exacerbated the generation of surface runoff in urban areas,and increases the risk of urban waterlogging.There are many deficiencies in studies on the influence of urban expansion on surface runoff at present,especially the lack of quantitative study on urban scale.In addition,green infrastructure（GI）development is the main way treat the surface runoff problems in urban areas,which is been applied in spongy city construction in China.However,as the domestic researches on the theories and methods related to sponge city construction are still in the initial stage,the evaluation research on the effectiveness of green infrastructure development on the urban scale is basically blank.Understanding the surface runoff response mechanism of urban expansion and exploring the methods and strategies of urban surface runoff control are of great value for quantitative understanding of ecological effects of urban expansion and guiding urban land use,stormwater management and other decision-making departments to formulate control strategies.Thus,taking the Nanjing city for example,surface runoff response of urban expansion and the evaluation on the implementation effectiveness of GI development were conducted in the study.First,the domestic and foreign research progress was systematically reviewed and the deficiency of existing researches was discussed.Secondly,research on urban expansion trend and driving mechanism based on impermeable surface,research on surface runoff response of urban expansion,research on the mechanism of controlling surface runoff by GI development,and case study on implementation effectiveness of GI development were conducted one by one.Research contents,methods and results are as follows:There exist two difficulties in study of the trend and driving mechanism of urban expansion:（1）Maxed pixels of traditional landscape data affect the accuracy of urban expansion trend research.（2）The collinearity of the driving factors affects the reliability of the driving mechanism.To fix these problems,taking Nanjing City for example,the impermeable surface was first extracted using the linear spectral analysis technique with the ENVI 5.1 based on the Landsat TM/ETM dataset.Of which,the Normalized Difference Build-up Index（NDBI）and Normalized Difference Vegetation Index（NDVI）were used for calibration.Then,the weighted average barycenter analysis,linear direction mean analysis and standard deviation elliptic analysis based on impermeable surface were conducted to study the urban growth characteristics.Thirdly,14 indicators including population variable,economic variable,living standard variable and policy variable were selected as the main potential driving factors affecting urban expansion,and build-up area as the dependent variable,to study the main driving mechanism of the urban growth with the Partial least squares regression（PLS）method.Results showed that imperious surface in Nanjing showed multicentric and inconsonant expansion.Gravity center transfer distance of impervious surface increased over the three periods（2000 to 2005,2005 to 2010,2010 to 2017）,which annual gravity center transfer distances were 303m,379m and 611m respectively.In general,the gravity center transferred 2564m to the direction that 259.12 degrees to the north clockwise.The exterior morphological characteristics of urban growth had obvious directivity,which the oblateness of the standard deviation elliptic of the impervious surface were all larger than 2.23 in 2000,2005,2010 and 2017.The external spatial characteristics of the administrative region,distribution of rivers and nature reserves,and the land use plan were the main factors that constrain and limit the direction of urban expansion.The number of employees in the secondary industry,population density,fixed assets investment in the tertiary industry,regional GDP,and per capita disposable income in urban and rural areas were the main factors that influenced the scale of urban expansion.In general,land use change influences the generation of surface runoff by changing the surface properties which affect soil permeability,changing the topography which affects the runoff catchment path,and changing the lag time of surface runoff and the runoff peak.At the same time,increase of surface runoff increases its scour effect on the surface,aggravates the deterioration of water quality,pollutes surface water and causes non-point source pollution.The Long-term hydrology impact assessment（L-THIA）model was used to evaluate the impact of laud use change and rainfall change on surface runoff.First,in model construction,the hydrology soil group data was conducted based on the relationship between the Chinese and US soil texture classification system,and the relationship between US soil texture classification system and classification criteria of hydrology soil group types.Second,multiple scenario simulation of surface runoff response on land use change and rainfall change was conducted（Multi-scenario simulation of surface runoff response to land use change and rainfall change;Multi-scenario simulation of the response of surface runoff to rainfall changes;Multi-scenario simulation study on the impact of land use change（construction land expansion）on runoff）.Results showed that,with the change of land use and rainfall,the annual average surface runoff depth in the study area shows an increasing trend in fluctuation,with an average annual runoff depth increase of 50mm.However,the annual increase of surface runoff in each municipal district varies greatly due to the different surface natural characteristics;Since 2000,with the land use of each year unchanged,rainfall change could cause an average annual increase of 7.75mm to 7.89mm;Compared with rainfall change,land use change has a much smaller impact on surface runoff in macro scale.With the rainfall data unchanged,land use change in every 5 years can result in increase in surface runoff depth of 2.7mm to 4.5mm;However,construction land growth played an important role on surface runoff generation,conversion of agricultural land（or other natural land）to construction land around the city could result in 124mm to 297mm annual runoff increment.With traditional urban land use and rain-flood control mode,ecological environment problems such as urban waterlogging,urban heat island,insufficient green space,and deterioration of space and environmental quality are prominent.Accordingly,GI development could have a variety of ecological and environmental benefits,such as reduce the risk of urban waterlogging,improve the environment and air quality,reduce surface non-point source pollution,add the underground water,increase and improve habitat and human entertainment.However,there are also some limiting factors in implementation of GI practices,for example,GI practices have certain requirements on site conditions,construct of GI practices in the old city is difficult.In this study,taking the Jianye district as an example,feasibility analysis of GI practices was conducted.Wet pond,dry pond,wetland,green roof,rain barrel/cistern,grassed swale,bioretention and impervious surface were suitable GI practices,of which,wet pond,dry pond,wetland were only implemented in development areas.Studies on the assessment of green infrastructure（GI）practice implementation effect and cost efficiency on an urban watershed scale helps the GI practice selection and investment decisions for sponge city construction in China.However,few studies have been conducted for these topics at present.In this study,the Long-Term Hydrologic Impact Assessment—Low Impact Development（L-THIA-LID）2.1 model was applied to assess the effectiveness and cost efficiency of GI practices on surface runoff volume reduction in an urban watershed—the Jianye district,Nanjing City,China.Grassed swales,bioretentions,green roofs,rain cisterns,permeable pavements,wet ponds,dry ponds,and wetlands were chosen as potential GI practices for sponge city con-struction based on feasibility analysis.Results showed that grassed swales were the most cost-effective practice（0.7 CNY/m3/yr）,but the total implementation effect of grassed swales was not obvious due to the small area of suitable locations.Permeable pavements performed best on runoff reduction,but the cost efficiency was much lower.Correspondingly,bioretentions were compromise practices.Green roofs were the least cost-effective practices,with the cost efficiency at 122.3CNY/m³/yr,but it was much lower for rain cisterns,which were 3.2 CNY/m³/yr.Wet ponds,dry ponds,and wetlands were potential practices implemented in development areas,of which dry ponds were the most cost-effective（2.7 CNY/m³/yr）,followed by wet ponds（10.9CNY/m³/yr）.The annual runoff volume of the total area could be reduced by up to 47.01%by implementing GI practices in buildup areas.Rain cisterns（RC）and permeable pavements（PP）were the best combination for this area,and bioretentions（BR）and green roofs（GR）followed.Grassed swales（GS₁）,dry ponds（DP）,wet ponds（WP）,and wetlands（WL）were not wise choices due to the small suitable location areas.This study also demonstrated the feasibility of the L-THIA-LID 2.1 model for the evaluation of GI practice implementation effects and cost efficiency on urban runoff in sponge city construction in China.