Spatial Pattern Changes Of Urbanization And Its Flood Response For Qinhuai River Basin

Increases in the impervious surfaces associated with urbanization has dramatic impacts on catchment hydrology cycle,resulting in increased runoff peaks,total runoff volume,magnitude and frequency of flood flows,and decreased time for flood flows to peak.Thus,a better understanding of spatial-temporal change characteristics of urban landscape configuration and its flood responses against the background of urbanization has important significance on flood control planning and hazard prevention in urban watershed.Based on RS and GIS technology,nine-year land cover maps of Qinhuai River Basin from 1988 to 2015 was obtained using multi-temporal Landsat images and rotation forest algorithm,which belongs to the category of ensemble learning.Landscape metrics analysis was used to quantify the spatial and temporal changes of impervious surface pattern in Qinhuai River Basin during the last 30 years.Distributed hydrological model HEC-HMS was calibrated and verified using 14 flood events with different magnitudes.Most parameters of the models for each subbasin were determined by geographic and hydrologic features.The effect of impervious spatial configuration to flood response was investigated by simulating different magnitudes of flood event for the various land use.The main contents and conclusions are as follows:(1)Spatial-temporal change characteristics of urban landscape configuration.Based on multi-temporal Landsat images,the land use images of Qinhuai River Basin including five classes:impervious surface,bare area,farm land,water and forestland was obtained using rotation forest algorithm.Impervious surface coverage dynamic analysis?land cover change trajectory analysis and landscape metrics analysis was used to quantify the spatial and temporal changes of impervious surface pattern during the past 30 years.(2)Construction of distributed hydrological model HEC-HMS.According to the characteristics of Qinhuai River Basin and historical flood data,rational precipitation model,Loss model and transform model was selected.Most parameters of the models for each subbasin were determined by geographic and hydrologic features.Reasonable model parameter setting plan was investigated which lay a foundation of flood events simulation with various land use scenarios.(3)Flood responses of urbanization configuration.Based on flood events simulation with various land use scenarios,responses of different magnitude flood events to impervious surface expansion and various urbanization configuration was researched.responses of flood peak,flood volume and the time from flood flow to peak to impervious surface expansion and the resonses of various magnitude flood events to impervious surface expansion are analysed.The different effects of the impervious surface which distributed in different located are investigated.The research shows:over the past 30 years,the spatial pattern of Qinhuai River Basin has changed remarkably.The impervious surface coverage has increased four times.Impervious surface in the turning point 2001-2003 had the highest spatial heterogeneity,which then decreased significantly and has turned from dispersed distribution to higher connectivity from that point up to 2015.The impervious surface expansion rate of Nanjing city has greatly decreased after 2003.At the same time,there is a sharp rise in the expansion rate of Lishui district and Jurong.The increase of impervious surface posed significant influence on the flood hydrograph which become sharper,thinner and taller.In addition,the impervious surface distributes in different locations has different contribution on the flood peak.Jurong is the main source of the flood peak in the basin,where the changes of peak discharge by impervious surface expansion accounts for a proportion of 54%among the whole basin.