基于“3S”技术的南流江采砂点时空分布特征

利用Landsat系列遥感数据，通过目视解译的方式分析了南流江近30年的采砂点的时空分布特征及其环境效应，并探讨其驱动因素。结果表明：南流江采砂点数量在1988—2008年快速上升，2018年的采砂点数量出现大幅下降；各时期均为下游采砂点最多，中游次之，上游最少。采砂点时空分布的主要驱动因素是流域的经济快速发展带来的城市化快速扩张，2018年采砂点数量下降的主要原因是相关部门加强对非法采砂活动的监管。采砂活动是南流江含沙量下降的重要原因之一，同时也是南流江水质下降的重要原因；采砂活动使得南流江采砂区域面积在1988—2018年总共减少了7.24 km²；目前河口三角洲岸线并没有因为采砂活动的加剧输沙量减少而发生明显的蚀退。

Temporal and Spatial Distribution Characteristics of Sand Mining Points in Nanliu River Based on "3S" Technology

With the rapid progression of urbanization worldwide, the demand for sand and gravel is growing exponentially, resulting in a natural growth rate of sand and gravel far behind consumption. Unsustainable sand mining activities produce a series of environmental hazards, such as the formation of scattered sand pits in riverbeds, changes in channel shape, and altered physical and chemical properties of river water, leading to water quality degradation. Nanliu River is the largest river flowing into the sea in southwest China. The intense exploitation of river sand has had a considerable impact on the stability of the basin's ecological environment. To understand the historical change process of sand mining activities in Nanliu River and the dynamic factors influencing these activities, we analyzed their environmental impacts and provided a basis for the comprehensive management of the basin. Landsat images were used to extract the sand extraction points of Nanliu River through visual interpretation and field verification according to the image characteristics of the sand extraction points. The temporal and spatial variation characteristics of the sand extraction points of Nanliu River over the past 30 years were analyzed. The factors driving changes in the sand mining sites were explored by extracting the area of construction land in the basin, combined with economic index data and regulatory information from three aspects: economic development, changes in the area of construction land, and regulatory measures. Vectorization of sand mining areas and estuarine delta shorelines combined with water and sediment content and river water quality data were used to analyze the environmental effects caused by long-term sand mining activities from four aspects: area change of sand mining area, delta shoreline change, characteristics of water and sediment changes, and water quality change. The results show that the temporal and spatial changes of sand mining sites in Nanliu River have been remarkable in the past 30 years. The number of sand mining points increased continuously from 1988 to 2008. After reaching a peak in 2008, the number of sand mining points decreased rapidly in 2018. The changes in sand mining points were similar in different river basins and counties, with the largest number of sand mining points being in the lower reaches, followed by the middle reaches, and the smallest number being in the upper reaches. The factors driving temporal and spatial changes in sand mining sites are diverse. Rapid urbanization caused by economic development is the main reason for the increase in the number of sand mining sites. The number of sand mining sites decreased in 2018, mainly due to the continuous strengthening of sand mining supervision in Nanliu River since 2015. Sand mining activities in Nanliu River have had a great impact on the river environment. First, sand mining activities continuously decrease both the area of sand mining and the sediment load on the bed, thereby leading to a continuous decrease in the sediment load of the river. Second, sand mining is an important reason for the decline of water quality in Nanliu River. In addition, although sediment transport has decreased due to human-related factors such as sand mining activities, no remarkable erosion of the coastline in the delta area has been found to date. To contribute to this research, we first obtained research results on the temporal and spatial variation characteristics of sand mining sites in Nanliu River, and then revealed their driving factors and environmental effects, which can provide a reference for the planning of sand mining activities in Nanliu River and the comprehensive management of environmental protection. Secondly, with Landsat image as the data source, the method of extracting river sand extraction points in combination with river bed morphology and other characteristics provides a long-awaited new method and approach for studying the temporal and spatial changes of river sand extraction activities.