Abstract
Abstract. Many river deltas in the world are vibrant economic regions, serving as transportation hubs, population centres, and commercial hotspots. However, today, many of these deltaic areas face a tremendous challenge with land loss due to a number of factors, such as reduced riverine sediment supply, coastal land erosion, subsidence, and sea level rise. The development of the Mississippi River Deltaic Plain (MRDP) in southeast Louisiana, USA, over the past century is a good example. Since 1932, approximately 4877 km2 of the coastal land of MRDP has become submerged. The lower Mississippi River main channel entering the Gulf of Mexico has become an isolated waterway with both sides losing land. In contrast, large open water areas in the Mississippi River’s distributary basin, the Atchafalaya River basin, have been silted up over the past century, and the river mouth has developed a prograding delta feature at its two outlets to the Gulf of Mexico. The retrospective analysis of this paper makes it clear that the main cause of the land loss in the MRDP is not the decline of riverine sediment, but the disconnection of the sediment sources from the natural flood plains. Future sediment management efforts in the MRDP should focus on restoring the natural connection of riverine sediment supplies with flood plains, rather than solely using channelized river diversion. This could be achieved through controlled overbank flooding (COF) and artificial floods in conjunction with the use of a hydrograph-based sediment availability assessment.
Highlights
The Mississippi River in the present day enters the northern Gulf of Mexico through two distributary channels: the lower Mississippi River main channel southeast of New Orleans, and the Atchafalaya River located to the west on the Louisiana central coast (Fig. 1)
Since 1927, no over-levee flood has occurred in the lower Mississippi River, while the engineered Mississippi-Atchafalaya River system has played its role as a critical transportation route for industry, agriculture and commerce
Knowing that discharge during the rising limbs of Intermediate Flow Stage and High Flow Stage provides the greatest amount of sediment, managers can effectively operate diversions based on predictions of the timing of a flood pulse moving down the Mississippi River
Summary
The Mississippi River in the present day enters the northern Gulf of Mexico through two distributary channels: the lower Mississippi River main channel southeast of New Orleans, and the Atchafalaya River located to the west on the Louisiana central coast (Fig. 1). Since 1927, no over-levee flood has occurred in the lower Mississippi River, while the engineered Mississippi-Atchafalaya River system has played its role as a critical transportation route for industry, agriculture and commerce These have been managed at the cost of disconnecting the sediment supply to the previous flood plain (Fig. 3). In contrast to the rapid land loss in the Mississippi River main channel delta, the Atchafalaya River Basin and the river mouth have been gaining land steadily in the past century. In the 1950s, with the open water areas sediment-filled, formation of subaqueous deltas at the two outlets of the Atchafalaya River, Morgan City main channel (ARMC) and Wax Lake Outlet (WLO), became noticeable (Shlemon, 1975). Over the past 30 years, the ARDC continued to grow with a net prograding rate of approximately 3 km year-1 (Xu, 2010; Rosen & Xu, 2013), while the basin area continued to be silted up at a sedimentation rate of 44 mm year-1 (Rosen & Xu, 2014)
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