Changes Of River Network Structure And Connectivity Of Taihu Lake Basin And Its Influence On Floods

The study of hydrology and water resources in a changing environment is one of the hotspots of the current hydrological research. As a main factor in the short-time scale, human activities play more and more significant role in river structure evolution and changinghydrological processes and hydrology laws, with the social and economic development and the strengthening of man’s ability to transform nature.As one of the most urbanized regions and the most developing areas, Taihu Lake Basin suffers flood disaster frequently because of the terrain factors and hydrometeorological conditions. And the dramatic disruption of the river structure and its connectivity under the background of urbanization has exacerbated this phenomenon. Therefore, the study of the pattern of river system, connectivity of river network and their hydrological response under the background of urbannization is urgent in the field of current hydrological research. In this paper, river system changes, hydrological process changes, hydrological connectivity, river system storage, drainage water production capacity and regional variation and the relationship between them ware in-depth researched in Wuchengxiyu and Yangchengdianmao region based on the techniques and methods of GIS spatial analysis, hydrological time series analysis, water characteristic parameter index analysis and hydrological modeling techniques and methods. The main contents and results ware showed as follows:(1) Land use and cover changeBased on remote sensing images of landuse in1991,2001and2009, this paper analyzed the spatial and temporal changes in landuse comparatively from the perspective of total change, structural change and urban land expansion intensity index. The results showed that the type of land use changed greatly during1991to 2009. The urban construction land in2009was more than three times than its in1991; the decreasing rate of water and paddy field was36.88%and42.86%. From the spatial distribution of land use change, urban expansion areas ware mainly distributed in the Taihu Lake area, and the constructions of emerging high-tech industrial parks, residential areas, roads, squares, external transport land for public buildings and other large urban areas ware the main reasons.(2) River system characteristic and river network pattern changeThe water system characteristics and patterns of change were analyzed and comparatively studied in sub are near50a with the parameters of drainage density, water area rate, development coefficient one-line river, ration area-length of two-line River, etc. The results showed that the drainage density and water area rate ware decreased nearly50a, and the stream pattern showed a trend of attenuation and this trend was intensified. From the district perspective, the river network disrupted dramatically in Jiangyin and Suzhou because of urbanization; the drainage density and surface rate showed a slight upward trend in Zhangjiagang because of the construction of the river into the sea.The hydrological process change and drving factors ware researched based on M-K test, R/S analysis in Wuchengxiyu. The results showed that the average annual water level occured around from low to high mutation point in the1990s, and the level of water would increase in the future. The contribution ate of human activities on highest, average, and the lowest water level changes ware54.17%,76%and92.31%, respectively. It indicated that human activity was the most important factor to water level change.The change of hydrological connectivity was calculated based on water level difference of stations. The results showed that the overall situation of hydrological connectivity was better.(3) Storage capacity influenced by the change of river systemThe storage capacity was researched on C, AC, SR and ASR in different periods and different river grades. The results showed that the river and lake storage capacity showed a continued downward trend, and the magnitude of reduction increased. The second and third level river capacity decreased the most obviously, and the first and fouth river capacity has a litter increase. The maximum storage capacity reduced was in Wujiang, the rate was more than30%; then Suzhou, Kunshan, and Wuxi was following, the rate was more than17%; the reduction in Zhangjiagang was the smallest, the rate was nearly5%.Using IHA model to calculate the rainfall runoff of the diffient units in1991, and let it to compare with the reginal flood storage capacity. The results showed that the storage capacity could not meet the needs of the reginal water production, it would become more severe during the process of urbanization.(4) Discharge capacity influenced by the change of river systemContinuous drainage modulus calculation model was used to calculate the discharge capacity of river network. The results showed that the maximum discharge capacity reduction happened in Wujiang, the rate was about33.62%; then Suzhou, Kunshan, and Wuxi were following, the rate was more than17%; the reduction in Zhangjiagang was the smallest, the rate was nearly4.96%, the contradiction between discharge capacity and water production was becoming more severe.