Abstract
The formation of channel bars has been recognized as the most significant sediment response to the highly trained Mississippi River (MR). However, no quantitative study exists on the dynamics of emerged channel bars and associated sediment accumulation in the last 500-kilometer reach of the MR from the Gulf of Mexico outlet, also known as the lowermost Mississippi River. Such knowledge is especially critical for riverine sediment management to impede coastal land loss in the Mississippi River Delta. In this study, we utilized a series of satellite images taken from August 2010 to January 2012 to assess the changes in surface area and volume of three large emerged channel bars in the lowermost MR following an unprecedented spring flood in 2011. River stage data were collected to develop a rating curve of surface areas detected by satellite images with flow conditions for each of the three bars. A uniform geometry associated with the areal change was assumed to estimate the bar volume changes. Our study reveals that the 2011 spring flood increased the surface area of the bars by 3.5% to 11.1%, resulting in a total surface increase of 7.3%, or 424,000 m2. Based on the surface area change, we estimated a total bar volume increase of 4.4%, or 1,219,900 m3. This volume increase would be equivalent to a sediment trapping of approximately 1.0 million metric tons, assuming a sediment bulk density of 1.2 metric tons per cubic meter. This large quantity of sediment is likely an underestimation because of the neglect of subaqueous bar area change and the assumption of a uniform geometry in volume estimation. Nonetheless, the results imply that channel bars in the lowermost MR are capable of capturing a substantial amount of sediment during floods, and that a thorough assessment of their long-term change can provide important insights into sediment trapping in the lowermost MR as well as the feasibility of proposed river sediment diversions.
Highlights
The Mississippi River Delta (MRD), a 25,000 km2 dynamic region on the southeastern coast of Louisiana in the USA, has been experiencing rapid land loss since the early 20th century [1,2,3,4].The loss rate varied from 17 km2/year in 1913 to 102 km2/year in 1980 and averaged about 43 km2/year during 1985–2010 [5,6]
After the flood when the river stage dropped to 7.17 m, which was nearly the same river stage like that before the flood, sediment accumulation could be seen along the bars
Our findings presented here indicate that if the sediment diversions open during these periods, the channel bars in the lowermost Mississippi River can trap a considerable amount of sediment, which may impair the capacity of diverting sediment to the river surrounding wetland
Summary
The Mississippi River Delta (MRD), a 25,000 km dynamic region on the southeastern coast of Louisiana in the USA, has been experiencing rapid land loss since the early 20th century [1,2,3,4].The loss rate varied from 17 km2/year in 1913 to 102 km2/year in 1980 and averaged about 43 km2/year during 1985–2010 [5,6]. A number of factors have been attributed to the rapid land loss, including riverine sediment reduction due to upstream dam construction and river engineering, subsidence, and sea level rise [7]. It has been projected that, if no actions were taken, at least another 2118 km land of Louisiana’s coast would be lost over the 50 years [8,9]. This possesses a serious threat to the energy industry, river transportation, and commercial fisheries in this region, all of which have the level of national importance
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