Identification, Assessment And Optimization Of Greenway Networks In Urban Area

To cope with urban environment and development problems caused under the background of rapid urbanization, the urban greenway network undergoes functional localization, identification, assessment and optimization. Thus, the following conclusions are reached.(1) Among the greenway network identification methods, for the identification of ecologically functional greenway, different from the past minimum resistance model and network analysis model, the improved minimum resistance cost model based on the ecological process is raised in this research by applying the ecological service value and biodiversity index to improve the composition of resistance aspects, thereby covering the shortage in land use type and landform characteristics frequently concerned by resistance aspects in past greenway identification and enabling the identified green corridor network with ecological function to have stronger authenticity and reliability in manifesting the urban natural features and ecological flow movement areas. For the identification of green corridor network with disaster prevention function and recreational function, different from the past network analysis model, a multi-scene identification method for green corridor network which is based on network analysis model is used in this research. This has covered the shortage of the past network analysis model in the overall impact considerations of such factors as road corridor class, road corridor safety and population density near the road corridor, and good results have been obtained from the identification of green corridor network with different population and different travel modes.The greenway networks with ecological function, disaster prevention function and recreational function are finally identified through the above two models. Among them, the greenway network with ecological function manifests the feature that the urban center is sparse and the urban suburbs are quite dense. The greenbelt along Han River, the horizontal green corridor along Guishan Mountain, Sheshan Mountain, Ma’an Mountain and Jiufeng Mountain as well as Green Park along Yangtze River, and the two major longitudinal greenway networks along three urban lakes, namely Yanxi Lake, East Lake and South Lake, constitute the main framework of green corridor network in the main urban area of Wuhan City. The overall distribution of the greenway networks respectively with disaster prevention function and recreational function manifests the feature that the urban center is sparse and the urban suburbs are quite dense, while the local distribution manifests the feature that the distribution is quite dense around the public service facilities and housing estates. Meanwhile, as the green corridor network with disaster prevention function undergoes safety assessment, its distribution is slightly decreased in the central urban area.(2) Among the greenway network assessment methods, the landscape index method was frequently used for the assessment of greenway network corridor in the past research to evaluate the spatial structure connection characteristics of corridor in the overall network. The qualitative characteristics not taken into consideration in the overall network, while such features as cost of the nearest distance, green land size, type and quality are only taken into consideration in the node assessment. The network flow measurement method based on the modified gravity model of cost distance is applied in this research to evaluate the importance of corridor in the greenway network under different functions, and at the same time the landscape function centrality of ecological function nodes as well as the supply and demand relationship in disaster prevention function and recreational function are evaluated by applying the results based on network flow measurement, the shortage in the past research is effectively covered by using the flow concept and thus good results are obtained in the greenway network corridor and node assessment.The assessment results show that:the overall structure of the greenway networks with all functions is featured by less loop number in corridor network, few node connections, main intermittent connection, and prominent node isolation, spotting and planarization problems.In the corridor assessment, the high-function flow corridor of the greenway network with ecological function mainly approaches the ecological land as it is relatively concentrated inside the city due to the population influence. But the distribution of high-function flow corridor is quite scattered as the ecological conditions are good in the suburbs. The corridor flow in the greenway network with disaster prevention function manifests the scattered distribution feature with the improvement of disaster relief and refuge attainability, and otherwise it manifests the locally-centralized distribution feature. The corridor in the greenway network with recreational function manifests the differences between south of and north of the Yangtze River in the travel modes respectively at ordinary times and at weekends. The corridor in the north of the Yangtze River is mainly concentrated at the confluence of Yangtze River and Han River, while the corridor in the south of the Yangtze River is mainly concentrated in the area around East Lake. Meanwhile, the aggregated distribution of corridor flow at weekends is more obvious than that at ordinary times.In the node assessment, if seen from the spatial distribution features, the high-centrality node is mainly concentrated in Ma’an Mountain and Jiufeng Mountain areas on the southeast of East Lake in the suburbs among the ecological function nodes. But the ecological function nodes of core greenbelt in the central urban area appear in local areas like the marshland greenbelt in the central urban area. Among the disaster prevention function nodes, the supply and demand ratio of refuge for the greenbelt nodes in the suburbs is better than that in the urban center. With the increase of distance from urban center, the supply and demand ratio accordingly increases, but the supply and demand ratio of refuge for old people manifests the feature of local optimum. The disaster relief nodes have the characteristics that are opposite to the distribution characteristics of refuge nodes, mainly showing that the supply and demand ratio of disaster relief for urban internal nodes is better than the greenbelt nodes in the suburbs. For the supply and demand ratio of disaster relief with different travel modes, the better the attainability is, the lower the supply and demand ratio is, and the average overall discrete distribution characteristics are manifested. When the attainability is poor, the feature that the high supply and demand ratio is close to local agglomeration appears. Among the recreational function nodes, the nodes that have a high supply and demand ratio manifest the feature that they are distributed along the rivers in the north of the Yangtze River and around the lakes in the south of the Yangtze River. This feature has similar distribution characteristics as the high-centrality nodes of ecological function.If seen from the quantitative features, the mean value of refuge node in the main urban area is more than1, showing a better refuge effect, but the supply and demand ratio of subarea nodes shows that the supply and demand ratio in Jianghan District, Wuchang District and Qingshan District is less than1, failing to meet the refuge demand. Among the disaster relief nodes, the overall and subarea supply and demand ratio of disaster relief with fire fighting and material functions is less than1, indicating that the supply is short of demand through these two disaster relief methods. But the supply and demand ratio of disaster relief with medical treatment and public security functions is more than1and especially the supply and demand ratio of disaster relief with medical treatment function is more than4, manifesting the feature that the supply and demand ability of disaster prevention with medical treatment function is higher and the supply and demand ability of disaster relief with public security function is relatively god. The supply and demand ratio of recreational nodes is more than1at ordinary times, showing a better supply and demand relationship, while the supply and demand ratio of recreational nodes is less than1at weekends, manifesting that the pressure is large for recreational nodes at weekends and the greenway network nodes cannot meet the requirements.(3) In the optimization of greenway network, the newly-built greenways are difficult to be executed due to relatively tight urban construction funds and land resources. In the past optimization research of greenway network, the attention was only paid to the density of network spatial structure, but the dense feature of network flow was neglected. In this paper, a weighted kernel density model based on spatial density clustering thought was applied to extract the greenway network that has a higher flow level and a compact spatial structure among the existing ideal networks by using the priority protection principle. Therefore, the overall greenway network is extracted through functional and hierarchical network optimization and integration. Meanwhile, practical strategies are provided for the short-term, middle-term and long-term objectives of the future greenway network construction.Through optimization, a greenway network with three single functions as well as a comprehensive network is identified in this research, comprised of seven greenway network plans and corresponding to three levels of demand and construction time. The comprehensive greenway integrates the basic demand with ecological, disaster prevention and recreational functions and constitutes the greenway infrastructure in the main urban area of Wuhan City. Its overall shape is like a flying kite. And the identified comprehensive greenway also can be divided into water greenway, mountain greenway and road greenway. The water greenway mainly refers to the greenways distributed along the Yangtze River and Han River and is the most important greenway in this research area. These water greenways connect the whole greenway into a complete and connected greenway system as the greenway structure foundation in this research area. The greenway which connects Yanxi Lake, East Lake, Shahu Lake and South Lake not only connects the main landscape greenbelt but also takes an important position in connecting the disaster prevention greenbelt nodes and the important animals and plants habitat. A vegetation buffer zone for the lakes built among these greenways not only can prevent the lakes from being further eroded by urban development, but also plays an active role in reducing the non-point source pollution of lakes. The mountain greenway is mainly comprised of east-west forest patches in the main urban area, forming a strong green wedge that is inserted into the city. The road greenway contains the main ring roads and the east-west arteries. These road greenways connect the main disaster prevention and recreational greenbelt nodes.Based on the feature that the greenways are distributed along the rivers and near the lakes and mountains as the final comprehensive greenway optimization results show, an urban greenway planning which is combined with green kernel, green heart and greenway and has the mountain and water features applicable to Wuhan City is put forward in this paper. In combination with the urban waterfront space, slow-moving traffic as well as people-oriented implementation strategy for urban greenway network planning, the superiority of original natural conditions and the extensibility of existing roads’ linear space can be applied to increase the urban green open space shown by greenways, thereby improving the urban environment, alleviating the conflicts between population and land and enhancing the urban sustainable development capacity.