Abstract
A river island is a shaped sediment accumulation body with its top above the water’s surface in crooked or branching streams. In this paper, four river islands in Yangzhong City in the lower reaches of the Yangtze River were studied. The spatio-temporal evolution information of the islands was quantitatively extracted using the threshold value method, binarization model, and cluster analysis, based on Thematic Mapper (TM) and Enhanced Thematic Mapper+ (ETM+) images of the Landsat satellite series from 1985 to 2015. The variation mechanism and influencing factors were analyzed using an unstructured-grid, Finite-Volume Coastal Ocean Model (FVCOM) hydrodynamic numerical simulation, as well as the water-sediment data measured by hydrological stations. The annual average total area of these islands was 251,224.46 m2 during 1985–2015, and the total area first increased during 1985–2000 and decreased later during 2000–2015. Generally, the total area increased during these 30 years. Taipingzhou island had the largest area and the biggest changing rate, Xishadao island had the smallest area, and Zhongxinsha island had the smallest changing rate. The river islands’ area change was influenced by river runoff, sediment discharge, and precipitation, and sediment discharge proved to be the most significant natural factor in island evolution. River island evolution was also found to be affected by both runoff and oceanic tide. The difference in flow-field caused silting up in the Leigongdao Island and the head of Taipingzhou Island, and a serious reduction in the middle and tail of Taipingzhou Island. The method used in this paper has good applicability to river islands in other rivers around the world.
Highlights
Formed river islands can be divided into lithoid islands and alluvial islands, according to their nature
The ETM+ sensor of the Landsat 7 satellite malfunctioned on 31 May 2003 and black stripes appeared over 25% on the captured images
By establishing a binarization model and clustering the remote sensing images, the area of the four islands could be extracted during different periods (Figure 6)
Summary
Formed river islands can be divided into lithoid islands and alluvial islands, according to their nature. Most of the islands in the lower reaches of the Yangtze River (hereinafter referred to as LRYR) are alluvial islands, which are formed by the silting up of flood plains. The formation of river islands usually takes a very long time, such as Jiangxinzhou Island in the Maanshan section of the Yangtze River, where porcelains from the Tang Dynasty were excavated. This shows that some islands formed over a period of one thousand years. Research on river islands in Yangzhong City has positive practical significance, and provides references for the management and protection of other islands in LRYR
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