城市扩张对董志塬沟道侵蚀演化的影响

EFFECTS OF URBAN EXPANSION ON GULLY LANDFORM EVOLUTION IN THE DONGZHI TABLELAND OF CHINESE LOESS PLATEAU

溯源侵蚀作为黄土高原最为活跃的沟谷侵蚀方式,是导致塬面萎缩和破碎化的主要原因,对黄土塬区的国土安全、民生安全和生态安全构成了严重威胁。董志塬是黄土高原面积最大、最为连续的塬面,但随着人类活动的不断增强尤其是城市化的快速发展,董志塬溯源侵蚀剧烈,完整的塬面被不断切割。本文以董志塬西峰城区火巷沟(35°44'24"N,107°39'03"E)为例,采用GIS与RS技术,通过对1964~2017年的7期高精度遥感影像进行解译分析,定量研究了1964年以来火巷沟沟头前进、后退和沟岸扩张的动态特征及其与城市发展和降雨因素的关系。研究结果表明:1)1964~1969年,火巷沟沟头平均每年以3.1 m的速度前进;1970~2002年,由于城区的发展和扩张,沟头因人工回填后退了359.8 m;2003~2017年,随着城市建设用地的进一步扩大,15年间沟头因回填后退了178.3 m;2)1964~2017年,火巷沟沟岸不断向两侧扩张。1964~1966年平均每年向外扩张1.1 m,1967~1969年每年扩张1.4 m,1970~2002年每年扩张1.1 m,2003~2008年每年扩张2.9 m,2009~2014年每年扩张8.0 m,2015~2017年每年扩张2.7 m;3)1964~2017年,西峰城区面积由2.1 km2扩大至19.8 km2,城区面积的不断扩大与火巷沟沟岸的扩张呈显著正相关关系;西峰城区与火巷沟沟头在早期接触面积较小,而现今随着城市的扩张,火巷沟沟头已被城区包围,致使沟道周围汇水能力增强冲刷沟岸,导致沟岸不断向外扩张;4)因受沟岸扩张和降雨数据时间分辨率的影响,本研究没有发现降雨与沟岸扩张之间存在明显的相关关系。本研究对深入认识黄土塬区城市化影响下沟头侵蚀演变特征和固沟保塬具有重要的参考意义。

Gully erosion has been recognized as one of the most impactful processes in sediment production and land degradation in a wide range of environments.To quantitatively characterize the dynamics of gully erosion and elucidate the impacts of urban expansion and rainfall,a typical eroded gully near the urban area of Qingyang City(35°44'24"N,107°39'03"E)in the Dongzhi tableland of the Chinese Loess Plateau was selected.Seven aerial and satellite imageries between 1964 and 2017 were obtained and the techniques of GIS and RS were applied to analyze the geometry of the gully landform based on these images.Moreover,the relationships between gully erosion,urban expansion and rainfall were analyzed.Results showed that:(1)The studied gully(Huoxiang gully)advanced 3.1 m annually during the period of 1964~1969,and that change was mostly caused by agricultural activities.During the period of 1970~2002,the gully head retreated 359.8 m,which caused by backfilling the gully channel.In 2003~2017,the gully channel was further backfilled due to urban development,which led to the gully head retreated 178.3 m;(2)Although gully headcut has been prevented through human intervention,the gully banks have continuously expanded on both sides.In the period of 1964~1966,the average rate of gully bank expansion was 1.1 m/a,and it was 1.4 m/a in 1967~1969.In 1970~2002,the gully bank expansion rate was 1.1 m/a and then increased to 2.9 m/a in 2003~2008.In 2009~2014,the expansion rate increased to 8.0 m/a,and it was 2.7 m/a in 2015~2017;(3)There was a significant positive relationship between gully bank expansion and urban area development.In 1964,the urban area of the Xifeng District was only 2.1 km2 and increased to 19.8 km2 in 2017,which was a 9-fold increase during the 54-year period.Moreover,the contact area between the gully head and the urban was small in 1964,while the gully was totally surrounded by the urban area in 2017,which indicates that the increase of impervious land surface in the urban area would profoundly increase the surface runoff and thus increase the erosion of the gully banks;(4)Based on the current data,we did not find significant relationship between rainfall and gully bank erosion,while acquiring high-resolution time series of gully bank erosion and rainfall data is crucially important in the future research.Results of this study have improved our understanding of gully landform evolution and the effects of urbanization on the tableland area of the CLP.