Abstract
The downstream reach of the Three Gorges Dam (TGD) along the Yangtze River (1560 km) hosts numerous mid-channel bars (MCBs). MCBs dynamics are crucial to the river’s hydrological processes and local ecological function. However, a systematic understanding of such dynamics and their linkage to TGD remains largely unknown. Using Landsat-image-extracted MCBs and several spatial-temporal analysis methods, this study presents a comprehensive understanding of MCB dynamics in terms of number, area, and shape, over downstream of TGD during the period 1985–2018. On average, a total of 140 MCBs were detected and grouped into four types representing small (<2 km2), middle (2 km2 – 7 km2), large (7 km2 – 33 km2) and extra-large size (>33 km2) MCBs, respectively. MCBs number decreased after TGD closure but most of these happened in the lower reach. The area of total MCBs experienced an increasing trend (2.77 km2/yr, p-value < 0.01) over the last three decades. The extra-large MCBs gained the largest area increasing rate than the other sizes of MCBs. Small MCBs tended to become relatively round, whereas the others became elongate in shape after TGD operation. Impacts of TGD operation generally diminished in the longitudinal direction from TGD to Hankou and from TGD to Jiujiang for shape and area dynamics, respectively. The quantified longitudinal and temporal dynamics of MCBs across the entire Yangtze River downstream of TGD provides a crucial monitoring basis for continuous investigation of the changing mechanisms affecting the morphology of the Yangtze River system.
Highlights
Most large rivers in the world are strongly regulated and engineered by artificial facilities [1]
Kiss and Balogh [33] showed that mid-channel bars (MCBs) developed quickly in the upstream and laterally in the whole section of the Dráva River because of the coarse sediment supply and the decreasing stream energy due to the dam. These findings suggest that the downstream geomorphic adjustments to dam operations could result in different types of MCB dynamics, which depends upon the new hydraulic regime, sediment supply, and the type of structures employed [1,34]
This study provides the first opportunity for examining the general pattern of MCBs downstream of Three Gorges Dam (TGD)
Summary
Most large rivers in the world are strongly regulated and engineered by artificial facilities [1]. Since the 1980s, research on MCB dynamics in the context of large scale and long-term temporal span, received increasingly wider attention [10,15], thanks to the rapid development of Remote Sensing (RS) technology and the sophisticated requirements for river regulation [29,30] Among these studies, the common observations are that the area and number of MCBs tend to decrease due to the dam-induced sediment supply reduction [1,23,31]. This work, made the attempted to comprehensively analyze the MCBs dynamics over the entire Yangtze River downstream of TGD throughout the last three decades (covering pre-TGD and post-TGD periods), by (1) inventorying MCBs extracted from time-series satellite imagery and (2) understanding the spatial (or longitudinal) and temporal patterns of the MCBs under the local damming and other anthropogenic contexts with three morphology-related metrics: MCB number, area, and shape
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