Abstract
Channel bars are a major depositional feature in channels, and are considered as an important part of the morphodynamics of an alluvial river. The long-term morphodynamics of bars have been intensively investigated. However, relatively little is known about the response of channel bars to a major river flood, which is considered to be the predominant force in shaping bar morphology. This is especially the case for the monsoon-affected Yangtze River, where fluvial geomorphic work is largely carried out during monsoon floods. In this study, multi-temporal satellite images and river stage data were used to examine the morphodynamics of four large channel bars in the middle Yangtze River in response to a major monsoon flood in 2002. Based on bar surface areas estimated with Landsat images at different river stages, a rating curve was developed for each of the four bars, which was used to estimate bar volume through an integral process. Our study shows that two of the bars tended to be stable, while the other two experienced severe erosion during the flood. The results reveal that the flood caused a total bar surface area decrease of 1,655,100 m2 (or 8.30%), and a total bar volume decline of 5.89 × 106 m3 (or 6.10%) between the river stages of 20.81 m and 25.75 m. The volume decrease is equivalent to a sediment loss of approximately 8.25 × 106 metric tons, based on an average bulk density of 1.4 metric tons per cubic meter. The results imply that channel bars in the middle Yangtze River can also be large sediment sources rather than depositional areas during the flood. The decrease of sediment load in the middle of Yangtze River was found to be responsible for the dramatic morphodynamics of channel bars, which could last for a long period of time, depending on the operation of the Three Gorges Dam, which opened in 2003. Hence, we suggest making management efforts to protect the bars from further erosion.
Highlights
Bars are elevated region in rivers that form by fluvial deposition of sediments within the channel
Utilizing multi-temporal satellite images and river stage records, this study aimed to investigate the morphodynamics of four channel bars in the middle Yangtze River (YR) in response to a major monsoon flood
Unlike the YR, river engineering projects are more likely to dominate over natural processes in governing channel bar evolution along the Mississippi River in the long run
Summary
Bars are elevated region in rivers that form by fluvial deposition of sediments within the channel. They can provide information about active processes and the sediment regime at the reach scale, and are employed for geomorphic and sediment assessments for rivers worldwide [1]. Bars are an important part of the morphodynamics of a river, and are widely used to investigate the dynamics of flow regime [2]. The form and development of channel bars are important for understanding fluvial processes and their controlling factors, and have been the focus of a significant body of work. Jackson [3] ascribed the occurrence and characteristics of bars in rivers to the overall flow, sediment regime, and geological controls on morphology. Little work has been reported regarding the dynamics of channel bars in response to a single flood event
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