公路网系对喀斯特流域水系结构的影响

Impact of highway networks on stream structure in a karst watershed

为了定量评价公路对地表水文过程的影响,选择典型喀斯特流域,在GIS水文分析功能支持下探讨了公路网系对流域水系结构的改变。研究表明:公路代表的人类强烈扰动集中分布在水系两侧1 km范围内。叠加路网之后流域内三级以下的低级水系数量和总长度显著增加,主河道分枝能力和流域平均分枝比增大,说明路网将增大河道行洪压力,同时减弱喀斯特流域的溶蚀作用而增强流水侵蚀作用。对子流域单元的统计分析表明,路面产流强度与子流域水系长度增加量之间存在较好的线性关系。在空间上,路网增加的水系在流域出口附近的集中程度较高。公路影响抑制喀斯特地下水系的径流调蓄作用,可能导致洪水期河流洪峰提前并增大洪峰流量,从而加剧洪灾风险。

Highway networks spatially interact with stream networks in mountmnous watersheds. The impervious highway surface may increase the runoff generation of underlying surface. Therefore, local hydrological processes and watershed stream network structure may possibly be changed. This research was therefore conducted to evaluate the degree of highway impact on watershed hydrological processes. The Qijiang watershed, located in the conjunction zone of Chongqing city and Guizhou province, was chosen as the research area. GIS hydrological analysis method was used to study how the highway network affects the structure of stream network. The results showed that highway networks were mostly distributed in the area within 1 km along both sides of stream networks. Provincial highway, national highway and expressway, which account for more than 90% of the roads lengths, were gathered in the above mentioned area. The existence of highway network led to remarkably increases in both number and total length of stream networks lower than the 3rd strahler order. Meanwhile, the main stream furcation ability was increased, which implied that highway networks may exacerbate watershed flood discharge pressure. On the other hand, the value of mean furcation ratio meant that highway networks can weaken underground dissolution and strengthened surface runoff erosion of karst watershed. Regression analysis illustrated that there was a good linear relationship between highway surface runoff generation ability and sub-watershed stream length increases. Further spatial analysis showed that the increase of stream length and density was mainly observed in areas close to watershed outlet, suggesting the potential flood risk as a result of the acceleration and enlargement of peak discharge during rainy seasons.