城市用地扩展对长沙市水系变化的影响

为了探讨水系变化与城市用地扩展之间的联系,为未来水生态修复及城市与生态环境协调发展提供依据及经验借鉴,基于长沙市1950s、1970s、1990s、2010s和2016年五期地形图及同期城市建设用地数据,对长沙市中心城区近60年来的五期水系及城市用地扩张情况进行统计,选取河网密度、水面率、干流河流曲度、河网发育系数指标定量描述水系变化特征。同时,运用城市用地扩张特征分析方法分析同期城市扩张的强弱与快慢,并叠加各流域范围内同期水系变化指标与城市拓展强度指标,分析长沙各阶段水系变迁与城市用地扩张强度之间的关系。研究表明:(1)城市建设用地扩展对城市水系数量及形态变化有直接影响;(2)各时期城市拓展强度均与水系特征指标值衰减速度呈正相关关系;(3)在城市扩展过程中破坏水系特征将加大水系生态、自然灾害风险,而有效的水生态保护政策和保护措施不仅可以使片区水系缩减趋势放缓,还可以加快周边用地扩展速度。

The relationship between the change of water system and the urban land expansion in Changsha

In order to examine the connection between water system changes and the expansion of urban land use, and provide evidence and experience for the future coordinated development of water ecological restoration and urban and ecological environment, statistical methods were used to analyze the five-phase water system and urban land expansion in Changsha in the past 60 years. We selected river net density, water surface rate, mainstream river curvature indexes and river development coefficient to quantitatively describe changes in water system characteristics and used urban intensity expansion index to measure the strength and speed of expansion of urban construction land in the same period based on the topographic maps and urban construction land data of Changsha city in the 1950s, 1970s, 1990s, 2010s and 2016. After superimposing the water system change index and urban expansion intensity index in each basin, we analyzed the relationship between the water system changes in various stages of Changsha and the urban land expansion intensity. The results show that: (1) The expansion of urban construction land has a direct impact on the quantity and shape of urban water coefficients. In the past 60 years in Changsha, the main characteristic values of the water system experienced a large attenuation. In general, the amount and area of the water surface were greatly reduced, and both the river structure and the water system tended to be simple. Meanwhile, the river network density and water surface rate reflecting the characteristics of water coefficient decreased by 3.74% and 26.26%, respectively. (2) The intensity of urban expansion in each period was positively related to the decay rate of water quality index values. From the 1950s to the 1990s, the urban expansion of Changsha was concentrated in the Xiangjiang River system, the Liuyang River system and the Guitang River system east of the Xiangjiang River. During this period, the water surface rates in the three water systems decreased by 6.7%, 6.38%, and 6.75%, respectively. After 1990, Changsha entered the stage of rapid urbanization, but the urban expansion intensity in areas west of the Xiangjiang River was stronger than that in areas east of the Xiangjiang River. At the same time, the density of river network west of the Xiangjiang River was faster than that east of the Xiangjiang River, down by 1.13% and 6.19%, respectively. (3) Destruction of water system in urban expansion will increase the risk of ecological and natural disasters, and effective water ecological protection policies and protection measures can not only slow down the shrinking trend of the water system in the area, but also accelerate the expansion of surrounding land use.