Field‐Based Estimate of the Sediment Deficit in Coastal Louisiana

Abstract

Coastal and deltaic sediment balances are crucial for a region's sustainability. However, such balances remain difficult to quantify accurately, particularly for large regions. We calculate organic and mineral sediment mass and volume balances using field measurements from 273 Coastwide Reference Monitoring System sites across the Louisiana Coast between 2006 and 2015. The rapid relative sea level rise rate (average 13.4 mm/year) is offset by the small dry bulk densities observed (average 0.3 g/cm3) to produce a 16.2 ± 41.1% mass deficit and 24.1 ± 14.0% volume deficit, significantly smaller than recent predictions for 2000–2100 (73–79% mass deficit). Geostatisical estimates show that this deficit is primarily located in areas not directly nourished by major rivers, yet these regions still accumulate ~24 MT/year of mineral sediment. A fluvial sediment discharge of 113.8 MT/year suggests a coast‐wide trapping efficiency of 31.5 ± 15.8% of the riverine sediment, excluding subaqueous deposition. Organic accumulation accounts for 25% of all mass accumulation during our study period, and total organic mass accumulation per unit area is relatively constant in both directly and indirectly nourished regions. Sediment characteristics in the modern coastal wetlands differ from the Holocene deposit, suggesting secular changes within the system that will likely continue to influence coastal dynamics over the coming decades. Our results suggest that the gap between accommodation and accumulation (mass or volume) during this decade was not as large as the previously predicted century average.

Coastal Wetland Resilience, Accelerated Sea‐Level Rise, and the Importance of Timescale

“…This latter area includes coastal Louisiana, where the Coastwide Reference Monitoring System (CRMS) (Steyer et al., 2003) has been operational since 2005. This system consists of nearly 400 monitoring sites, the vast majority equipped with a SET‐MH station, and has enabled considerable advances in understanding the relationship between wetland health, accretion patterns, shallow subsidence, and sediment budgets (Jankowski et al., 2017; Sanks et al., 2020). The CRMS data extend far beyond SET‐MH measurements and include systematic monitoring of a wide range of other environmental parameters (mainly associated with vegetation, soils, and hydrology) and they are publicly available (https://www.lacoast.gov/crms/Home.aspx).…”

Influences on Discharge Partitioning on a Large River Delta: Case Study of the Mississippi‐Atchafalaya Diversion, 1926–1950

“…River avulsions control sedimentary basin filling, and the Mississippi‐Atchafalaya system is considered an important modern analog (Bhattacharya et al., 2019). Furthermore, sustainable management of modern river deltas subjected to rapid relative sea level rise requires a clear understanding of how natural and anthropogenic processes influence water, sediment, and nutrient transport pathways (Knights et al., 2020; Sanks et al., 2020). The USACE analyses (Fisk, 1952; Latimer & Schweitzer, 1951) were based on empirical analyses of extensive datasets.…”

Engineered Continental‐Scale Rivers Can Drive Changes in the Carbon Cycle

“…In the LMR, the change in the suspended sediment load related to the increase of soil erosion is not well quantified, but the load has been substantially reduced by damming. The construction of dams and reservoirs along tributaries of the LMR is mainly responsible for the decrease of its suspended sediment load from ∼400–500 × 10 6 t/yr before the 1950s to ∼150 × 10 6 t/yr today (Allison et al., 2000; Blum & Roberts, 2009; Meade & Moody, 2010; Sanks et al., 2020). Despite variability in sediment and soil delivery to the river, the LMR largely migrated naturally, exchanging sediments between the channel and floodplain during the early 20th century (Hudson & Kesel, 2000).…”

“…Crosses indicate core locations used to establish sedimentary architecture ( Figure S1). & Roberts, 2009;Meade & Moody, 2010;Sanks et al, 2020). Despite variability in sediment and soil delivery to the river, the LMR largely migrated naturally, exchanging sediments between the channel and floodplain during the early 20th century (Hudson & Kesel, 2000).…”