Remote-sensing monitoring for spatio-temporal dynamics of sand dredging activities at Poyang Lake in China

Abstract

As the largest freshwater lake in China, Poyang Lake is suffering from declining water quality related to the excessive dredging of sand. Field supervision is difficult due to the size of the lake (>3000 km2, wet season) and limited human resources. In this study, an approach is proposed to monitor sand-dredging activities using medium-resolution optical remote-sensing imagery, including 45 Landsat TM/ETM+ images from 2002 to 2012 and 140 HJ1A/B CCD images from 2009 to 2012. The procedure for detecting dredging vessels involves three steps. (1) The entire image is segmented into different homogeneous partitions to overcome water body heterogeneity, and ships in each partition with different levels of water clarity are detected using three types of contrast box architecture. (2) Dredging vessels are then identified based on a spatial overlay analysis of ships and dredging plumes, which are extracted from remote-sensing imagery. (3) False alarms (FAs) of dredging vessels are screened according to the distribution of the sandy lake bed. The results showed significant spatio-temporal variation in dredging activities; sand-dredging activities were concentrated at the northern part of Poyang Lake prior to 2008 and have expanded southwards since 2009. The northern part of the lake experienced persistent dredging operations throughout the year, whereas dredging was observed only during the wet seasons in the southern portion of the lake. A high intensity of illegal dredging was discovered based on two lines of evidence: dredging vessels were detected during the sand-dredging ban, and the estimated quantities of sand dredging were much higher than those planned by the authority. The sediment balance in Poyang Lake has continued to be disrupted, and the lake has become a sediment-exporting system. This study provides an effective solution for monitoring sand-dredging dynamics as well as useful information for managing sand dredging in fresh water environments and assessing its potential impacts on aquatic ecosystems.