Influence of Floods, Tides, and Vegetation on Sediment Retention in Wax Lake Delta, Louisiana, USA

Abstract

Sediment is the most valuable natural resource for deltaic environments because it is required to build new land. For land building to occur, sediment must be retained in the delta instead of being transported offshore. Despite this, we do not know what controls sediment retention within a delta. Here we use a calibrated numerical model of Wax Lake Delta, Louisiana, USA to analyze sediment retention for different riverine flood magnitudes, tidal amplitudes, and vegetation extents. Our results show that as riverine flood magnitude increases, areally averaged vertical accretion increases from 0.33 to 2 cm per 60-day flood, but sediment retention decreases from 72% to 34%. For the uniform vegetation characteristics considered, the buffering effect, defined as the reduction of sediment flux onto the islands in the presence of vegetation, reduces the sediment flux onto the islands 14 to 22% on a fully vegetated delta. When sediment is transported onto the islands, vegetation enhances retention, which we refer to as the trapping effect, bỹ 10%. But, this does not offset the buffering effect, and vegetation decreases vertical accretion and retention in the delta up to 6% (or~0.5 cm per 60-day flood). We suggest that vegetation will increase sedimentation only when trapping compensates for buffering. Finally, greater tidal amplitude at higher discharges enhances vertical accretion by~0.5 cm per 60-day flood compared to smaller tidal amplitudes. These results provide insight on the mechanisms behind coastal systems growth, and suggest how sediment diversions might be operated more efficiently in deltas with reduced sediment supply.

A new algorithm for estimating ground elevation and vegetation characteristics in coastal salt marshes from high‐resolution UAV‐based LiDAR point clouds

“…The future evolution of these ecosystems could be tracked by using a coupled hydrodynamic, ecological and morphological model numerical model, in which initial vegetation and ground morphology can be initialized with data from UAV LiDAR surveys. This is a major improvement with respect to typical modelling approaches, in which roughness is either represented by a calibrated time‐invariant value, uniform over the domain (Ashall et al ., 2016; Mariotti and Canestrelli, 2017; Bennett et al ., 2020; Olliver et al ., 2020), or is made time dependent by including vegetation height, density and submergence rate, but it is still spatially uniform over the marsh (Nardin and Edmonds, 2014; Nardin et al ., 2018). Spatial variations in vegetation height and density modulate frictional resistance, favouring flows over less vegetated regions (Pinton et al ., 2020).…”

“…Modelling studies of marsh hydrodynamics and morphodynamics typically represent vegetation uniformly over the domain (Ashall et al ., 2016; Mariotti and Canestrelli, 2017; Bennett et al ., 2020; Olliver et al ., 2020). However, spatial variations in vegetation height and density modulate frictional resistance, favouring flows over less vegetated regions (Pinton et al ., 2020).…”

The Relationship Between Delta Form and Nitrate Retention Revealed by Numerical Modeling Experiments

“…Contrary to our hypothesis, removal decreases with delta top area across the six morphologically different deltas (Figure 5b). To examine the relationship between delta area and nitrate removal independently from the confounding effects of elevation-dependent removal kinetics, we repeated our simulations using uniform removal kinetics (6.39 mm hr −1 , representative of the average V f measured in Wax Lake Delta by Knights et al (2020)) across each delta top, which was defined by the 0 m elevation contour (Olliver et al, 2020) and prescribed no reactions outside the delta top. For these simulations, nitrate removal does increase with delta area as expected-the larger the delta top wetland, the more nitrate it can remove (Figure 5h).…”

A Functional Form for Fine Sediment Interception in Vegetated Environments

“…Sediment-flow-vegetation interactions play crucial roles in governing geomorphological and biological processes of low-gradient aquatic landscapes such as coastal marshes and floodplains [1][2][3]. Sediment transport governs land building and is critical to marsh restoration efforts and sea-level rise models [2,4,5]. The transport of fine sediment, the fraction with the greatest organic matter concentration, is especially important for biological processes such as nutrient provisioning and carbon cycling [6][7][8], representing a key linkage between geomorphology and other disciplines.…”

“…Vegetation can also enhance turbulence [7,19,[27][28][29], decreasing effective settling by promoting re-entrainment of particles from the bed. This can lead to scour and channel formation around discrete vegetation patches [4,30]. Patches of vegetation can also promote settling within their wakes due to reduced velocity [15,31]; interactions between multiple patch wakes can similarly promote settling and thus land building [32].…”