Abstract
The Mississippi River Delta Plain has undergone substantial land loss caused by subsidence, relative sea-level rise, and loss of connectivity to the Mississippi River. Many restoration projects rely on diversions from the Mississippi River, but uncertainty exists about the timing and the amount of actually available sediment. This study examined long-term (1980–2010) suspended sediment yield as affected by different hydrologic regimes to determine actual suspended sediment availability and how this may affect diversion management. A stage hydrograph-based approach was employed to quantify total suspended sediment load (SSL) of the lower Mississippi River at Tarbert Landing during three river flow conditions: Peak Flow Stage (stage = 16.8 m, discharge >32,000 m3 s−1), High Flow Stage (stage = 14.6 m, discharge = 25,000–32,000 m3 s−1), and Intermediate Flow Stage (Stage = 12.1 m, discharge = 18,000–25,000 m3 s−1). Suspended sediment concentration (SSC) and SSL were maximized during High Flow and Intermediate Flow Stages, accounting for approximately 50% of the total annual sediment yield, even though duration of the stages accounted for only one-third of a year. Peak Flow Stage had the highest discharge, but significantly lower SSC (p < 0.05), indicating that diversion of the river at this stage would be less effective for sediment capture. The lower Mississippi River showed significantly higher SSC (p < 0.0001) and SSL (p < 0.0001) during the rising than the receding limb. When the flood pulse was rising, Intermediate Flow and High Flow Stages showed greater SSC and SSL than Peak Flow Stage. Together, Intermediate Flow and High Flow Stages on the rising limb annually discharged 28 megatonnes over approximately 42 days, identifying this to be the best period for sediment capture and diversion.
Highlights
The Mississippi River Delta Plain in Louisiana, USA has undergone extensive land loss during the past century [1]
Atchafalaya River (AR); Tarbert Landing (TBL) where long-term discharge and suspended sediment records are collected for this study; Red River Landing (RRL) and Carrolton, New Orleans (CAR) where river stage data are used for this study; and Davis Pond
Three important conclusions arise from this long-term data assessment: (1) A hydrograph-based analysis of sediment is critical for determining the actual availability of riverine sediment due to the variability of the quantity and timing of total suspended sediment during Mississippi River flood pulses
Summary
The Mississippi River Delta Plain in Louisiana, USA has undergone extensive land loss during the past century [1]. Couvillion et al [1] have estimated that a total of 4877 km of coastal land in Louisiana has been submerged since 1932. The rapid land loss has been attributed to both natural and anthropogenically exacerbated reasons [5,6,7]. These include subsidence, erosion, relative sea level rise, and loss of connectivity to the Mississippi River. With land loss endangering coastal communities, various restoration projects have been developed in the past two decades [8,9]. In recent years much discussion has centered on diverting Mississippi River water and sediment to build and maintain coastal wetlands
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