Study On River Network Pattern And Connectivity Of Hangzhou-Jiaxing-Huzhou Plain Region Based On River Health

Hangzhou-Jiaxing-Huzhou plain region of Taihu Basin is located in the heart of the Yangtze River Delta. There are many rivers and numerous lakes in this area, so it’s a typical plain water network area, and it’s also one of the areas with the highest level of urbanization in our country. Since1960s, great changes have taken place in river network pattern and connectivity under the double effects of climate change and human activities. A large number of natural river systems were destroyed. The quantity, length and area of the river systems decreased greatly. So many end rivers have passed away, causing the backbone of the rivers. Rivers are silting serious, and the storage capacity of the rivers and lakes is reducing. River systems have been divided by criss-crossing roads, so sleepy rivers appears frequently, and the river internal access is sluggish. Some relatively closed polder drainage units are formed by a large number of dyke, seawall and various sizes, blocking the regional water cycle path, causing poor river connectivity. To a certain extent, the disruption of river network pattern and connectivity caused many problems such as the decrease of water resources carrying capacity, deterioration of water environment, high frequency of flood disasters and the increase risk of water security. The problems have been important expressions of the discord relations of water and human, and have become key factors of sustainable development and river health.Take Hangzhou-Jiaxing-Huzhou plain region as research object, the article analyzed the characteristics of river network pattern and connectivity under urbanization, and the influence of river system changes on river health. According to the theories of fluvial geomorphology, geohydrology, graph theory, and landscape ecology, the integrated techniques and methods were used, which are GIS spatial analysis, statistics, time series analysis, landscape network and landscape pattern analysis, hydrological variation analysis and entropy analysis. Depend the study, put forward the improvement measures of river network pattern and connectivity, providing a theoretical reference and methodological support for river management and restoration, water ecological planning and river connectivity improvement based on river health. The main research contents and results are as follows:(1) The evolution of river network pattern and connectivity under urbanization. Three periods of digital river network maps, which were taken as the basis for analyzing the water system changes, were extracted from topographic maps and remote sensing images in the1960s,1980s and2009in Hangzhou-Jiaxing-Huzhou plain region. Based on the basic river pattern parameters, the evaluation system for river network pattern was constructed, including drainage density, water area rate, river frequency, river development coefficient, river area-length ratio and average branching ratio etc. Based on Landscape Ecology and Hydrology, the river connectivity evaluation system was builded from static index and dynamic index. The static index included β,γ and C index, and the dynamic index was hydrologic connectivity. Through the analysis of the river network pattern and connectivity evaluation systems, the river network pattern shows the attenuation and backbone trend, and the system is simplified; the river network connectivity shows the down then up trend, and the hydrological connectivity of the trunk river is higher than that of the low-level channel.(2) Construction and quantified of river health assessment system. Supporting by GIS and Fragstats3.3, and comprehensively using the theories and methods of River Geomorphology, hydrology variation analysis, entropy analysis and landscape pattern index, the article took the ecohydrological process,which directly affected the river health, as the breakthrough point to constructed the river health assessment system. The system took into account the river’s natural and social attributes. And it was built from five aspects, which were the river’s natural shape, flow pattern element, water quality, river riparian habitat environment as well as the river’s function of economic and social, and was built by seven indicators, which were river network structure stability, river structure naturalness, the variation coefficient of the average annual runoff, water quality comprehensive evaluation index, the landscape pattern index, the ten thousand yuan GDP water consumption and water quality compliance rate of function areas.(3) The law of the influence on river health under river system changes. Take Huzhou city as the typical area in Hangzhou-Jiaxing-Huzhou, to analyze the response of river health to river network changes. The results show that:With the recession and backbone of the river network, the river network structure stability and naturalness were significantly reduced; The variation coefficient of the average annual runoff was negatively correlated with hydrological connectivity, ie, the higher hydrological connectivity, the lower variation coefficient of the average annual runoff, and the more conductive to the use of water resources; The higher hydrological connectivity, the stronger river’s self-purification capacity, the greater water quality degradation coefficient, and the better water quality; With declining of water patch dominance and connectivity, the artificial landscape dominance, connectivity, and fragmentation show an upward trend, and the landscape diversity and heterogeneity increased, that prompting the riparian landscape patches artificialized and simplified, so human intrusion and disturbance of the natural landscape has gradually increased, the habitat of riparian degraded; The river’s social and economic attributes weakened as the recession of river network pattern and connectivity, however, the demand for water resources continues to increase with urbanization, so the impact of river connectivity on water resource utilization is more significant; In addition, changes of river network pattern and connectivity also seriously affected the river ecosystem composition.(4) Finally, from the perspective of river health and water environment comprehensive management, and according to the status of river network pattern and connectivity changes in Hangzhou-Jiaxing-Huzhou plain region, put forward the engineering and non-engineering measures to improve river network pattern and connectivity, in order to provide reference for regional river regulation and water system renovation.