Efficient retention of mud drives land building on the Mississippi Delta plain

Abstract

Abstract. Many of the world's deltas -home to major population centers -are rapidly degrading due to reduced sediment supply, making these systems less resilient to increasing rates of relative sea-level rise. The Mississippi Delta faces some of the highest rates of wetland loss in the world. As a result, multibillion dollar plans for coastal restoration by means of river diversions are currently nearing implementation. River diversions aim to bring sediment back to the presently sediment-starved delta plain. Within this context, sediment retention efficiency (SRE) is a critical parameter because it dictates the effectiveness of river diversions. Several recent studies have focused on land building along the open coast, showing SREs ranging from 5 to 30 %. Here we measure the SRE of a large relict crevasse splay in an inland, vegetated setting that serves as an appropriate model for river diversions. By comparing the mass fraction of sand in the splay deposit to the estimated sand fraction that entered it during its life cycle, we find that this mud-dominated sediment body has an SRE of ≥ 75 %, i.e., dramatically higher than its counterparts on the open coast. Our results show that transport pathways for mud are critical for delta evolution and that SRE is highly variable across a delta. We conclude that sediment diversions located in settings that are currently still vegetated are likely to be the most effective in mitigating land loss and providing long-term sustainability.

Crevasse Splays Versus Avulsions: A Recipe for Land Building With Levee Breaches

“…Nonvegetated splay morphology is smoother, and nonvegetated floodplains tend to favor avulsions. Strongly vegetated floodplains develop more intricate channel patterns and are more efficient in retaining sediment (Figure 4c), in agreement with earlier studies that inferred that vegetation promotes sediment trapping on floodplains and reduces erosion (e.g., Esposito et al, 2017;Schumm & Lichty, 1963). From our morphodynamic simulations we find the highest rate of land growth for intermediate vegetation strengths, similar to the findings from the hydrodynamic simulations of Nardin and Edmonds (2014).…”

“…The percentage of imported sediment retained in the splay ranges from 62 to 100%. Esposito et al (2017) estimated an imported sand fraction of 6.6% and a likely range of sediment retention efficiency between 75 and 100%.…”

“…The splay studied by Shen et al (2015) and Esposito et al (2017; Figure 1) has a volume of 1.6 . 10 8 m 3 and was formed by a roughly 4 m water level head during floods.…”

“…Inspired by recent, detailed studies of crevasse splays in the Mississippi River Delta (Esposito et al, 2017;Shen et al, 2015;Törnqvist et al, 2008; Figure S1 in the supporting information), we developed MATLAB soil consolidation (note that we use the term "soil" here broadly, that is, in a geotechnical rather than a pedogenic sense) and vegetation routines that feed back into Delft3D to study their effects on floodplain morphology. From our model experiments we develop an understanding of floodplain response to levee breaching and derive a simple predictive metric of erosional versus depositional floodplain response.…”

“…This knowledge gap is critical because crevasse splays are an efficient tool for land building and they are therefore considered a natural analog for sediment diversions currently planned in the Mississippi River Delta (Coastal Protection and Restoration Authority, 2017;Gagliano & Van Beek, 1975). For example, one such crevasse splay in the Mississippi River Delta covers~60 km 2 and was able to retain >75% of incoming sediment for land building (Esposito et al, 2017; Figure 1). However, effective diversion design is challenging if breach evolution is poorly understood.…”