Changes Of River Network Structure And Connectivity Of The Southeast Plain River Network Area Of The Yinzhou And Theirimpacts On Storage Capacity

The change of regional underlying surface and river system and its impact on hydrological processes is one of the hot issues of the current hydrological science research especially in plain river network region. Urbanization is one of the most intensive processes of human activities which have changed the earth’s surface and river network structure and it also altered regional water cycle greatly. The southeast plain of Yinzhou is experiencing a rapid urbanization process, at the meanwhile, affected by topography and other natural factors; it often suffers flood disaster, while it is also reinforced by the increasing of impervious surface area, dramatic changes of river network structure, block of the river connectivity and the descent of river flood storage function under the urbanization. Therefore, it’s of great important theoretical significance and application value for regional flood control and mitigation as well as urban sustainable development to study the changes of underlying surface, river network structure and connectivity and their impacts on storage capacity under rapid urbanization process.The research study chose the southeast plain of Yinzhou as the study area. Based on the data of land use and water network and the characteristic water level of representative water level station during three periods, the study applied various methods to explore and analyze the change characteristics of the underlying surface, river network and river network storage capacity during different development stage of urbanization. In addition, the study put forward two kinds of river connected evaluation methods suitable for the plain river network area and analyzed the change characteristics of river connectivity, and then explored the impacts of changes of underlying surface, river network structure and connectivity on river storage capacity. The result can provide beneficial reference for river network protection and administration and the flood storage capacity improvements. The main research contents and conclusions are as follows:1. The urbanization process and the change of underlying surfaceThe urbanization process in the southeast plain of Yinzhou showed periodical as a whole. From the onwards of 2000, urbanization process has experienced a slow-to-rapid development phase. Among the impacts of urbanization on the underlying surface, the change of land use was the most conspicuous. According to the analyzed results of 1990-2010, the main characteristics of land use change were the increasing of urban construction land and the decreasing of farmland and water area. The proportion of urban area increased nearly 2.6 times during 1990-2010. The urban expansion intensity shows weak-strong-weak spatial characteristics from Ningbo city center to the countryside. During 1990-2003, it was given priority to the scale enlargement of Ningbo city center and then, the most significant strength change was suburban zone by the limitation of available land.2. The change of river networkThe research analyzed the general characteristics, the structure characteristics and the comprehensive characteristics of the river network of the study area in 1990,2003 and 2010 with the parameters of the density of river network, water surface ratio, the coefficient of river network development, the area length ratio and the box dimension. The results showed that the river network showed the trend of decay in recent 20 years, and the density of river network and the rate of water surface decreased 19.39% and 33.32%, respectively. The coefficient of river network development, the area length ratio and the box dimension also showed a trend of declining on the whole. With the development of urbanization, the river network appeared an obvious tendency of simpler and attenuation, and the intensity varied for different grades. The result showed that the reduction in low-grade river was more obvious, and urbanization first influenced the low-grade river and then the high-grade river. Pearson correlation analysis and spatial overlay analysis showed that urbanization has closed relation with the changes of water characteristics, which also showed urbanization have profound impacts on the river network structure.3. The connectivity change of river networkDue to increased urbanization effects, river systems produced a variety of social functions, for which, the study proposed river network connectivity evaluation method of plain river network area based on natural and social functions of river systems, and analyzed river connectivity level of the study area. The analyzed results showed river network connectivity was from 0.3154 in 1990 to 0.2281 in 2010, and it decreased 27.68% during 1990-2010. Average annual decrease rate was bigger during 1990-2003, which is the result that it has carried out a series of main rivers remediation projects in the study area and river network protection with standard of prevention floods improved, and social functions of main rivers is becoming more and more important. The method is feasible and can evaluate connectivity of different river systems.According to the characteristics of plain river network flow and considering regional runoff process and differences of different river water conveyance capacity, this research proposed the plain river network connectivity evaluation method based on water flow resistance and hydrological processes, evaluated the effect of the changes of the river network on river network connectivity. The study results showed:Lots of low-grade rivers were buried, even disappeared; river system structure was destroyed; water cycle process was prevented. So that the river network connectivity of the study area during 1990-2010 followed a decreasing trend from 0.6176 in 1990 to 0.4413 in 2010, with a total drop of 28.55%. The method was more effective and feasible.4. Impacts of water system changes on storage capacityFour factors included river channel storage capacity, available storage capacity, storage capacity and available storage capacity of per unit area are applied to analyze the storage capacity in different periods. The results indicated that with river network weakened the storage capacity showed a tendency of declining during the recent 20 years, and its river channel storage capacity and storage capacity have declined 32.26% and 31.22%, respectively. Rivers of different grades have different river storage capacity change intensity. With the grade of river decreased, river’s storage capacity changes intensity is increased, which showed that decrease of low-grade rivers is the main reason for storage capacity weak. The channel storage capacity and available storage capacity of per unit area in study area were both declined. Regional differences of space storage capacity were obvious. During the three periods, Dongqian Lake Development Zone and Wuxiang Dongwu Zone obtained the strongest channel storage capacity and available storage capacity, while Ningbo Center was the lowest in all the conservancy districts.Taking advantage of water production model, the study calculated its regional runoff in 1985 (average water year), and compared it with regional available storage capacity, which concluded that change of the underlying surface and the river network have changed the balance of "production-storage relationships" with influence of urbanization human activity. Regional water production increased and river storage capacity decreased, which exacerbated flooding.River network structure characteristics largely determine its flood and storage function. By the correlation analysis of flood storage capacity, river network structure and connectivity indicators in 10 water conservancies, it showed that storage capacity is closely related to river network structure and connectivity, storage capacity is affected by both water area(water surface ratio), river network structure and the degree of river network connectivity. The degree of river network connectivity is the higher if the rate of water surface is bigger, then river storage capacity is stronger. Therefore, it’s possible and feasible to improve river network storage capacity and thus to achieve regional water system structure as well as function protection and restoration by maintaining the present river system, dredging and widening the river, rebuilding water systems natural pattern, optimizing water system connectivity, speeding up the construction of the river and so on.