Comparing the Cohesive Effects of Mud and Vegetation on Delta Evolution

Abstract

Cohesive sediment exerts a significant influence on delta evolution, increasing shoreline rugosity and decreasing channel mobility. Vegetation has been assumed to play a similar role in delta evolution, but its cohesive effects have not been explicitly studied. We use the model DeltaRCM to directly explore two effects of vegetation: decreasing lateral transport of sediment and increasing flow resistance. We find that vegetation and cohesive sediment do alter delta morphology and channel dynamics in similar ways (e.g., more rugose shorelines and deeper, narrower, less mobile channels) but that vegetation may have additional implications for deltaic sediment retention and stratigraphy, by confining flow and sand in channels. Our results suggest that sediment composition is a first-order control on delta morphology, but vegetation has a stronger influence on channel mobility time scales. To fully understand the cohesive influences acting on a delta, vegetation influence should be considered in addition to fine sediment.Plain Language Summary Human use of deltaic land relies on the existence of stable wetland habitat. Both vegetation and mud influence delta stability by making sediment more difficult to erode. Both introduce cohesion-mud because individual grains stick together, and vegetation because roots hold the sediment together. Vegetation also introduces friction, which has a similar effect to cohesion (decreasing erosion). When it is more difficult to erode channel banks, channels remain in the same location for longer, influencing where water and sediment are transported on the delta. Most studies consider only mud, but we compare the effects of these two sources of cohesion using a simple model that routes water and sediment on a delta. We find that mud and vegetation can affect deltas in similar ways. However, vegetation also changes the patterns in which sand and mud are deposited on deltas and can cause more sediment to be transported to the ocean instead of deposited on the delta. Additionally, we find that cohesive sediment is more important in determining delta shape but that vegetation is more important in controlling how much channels move. To fully understand how cohesion influences deltas, we should consider the effects of both mud and vegetation.

Linking the Surface and Subsurface in River Deltas—Part 1: Relating Surface and Subsurface Geometries

“…In addition, the modeling conducted in this study does not account for the effects of vegetation, permafrost, or ice on deltaic evolution. We note that the DeltaRCM model has been modified to simulate these effects (Lauzon & Murray, 2018; Lauzon et al., 2019; Piliouras et al., 2021), however, these modifications were kept out of this study for the sake of simplicity.…”

“…Simplified models have been developed to understand, for example, the lobate growth of delta landforms (Moodie et al., 2019; Seybold et al., 2007). Established models have been modified to explore the influence that multiple variables such as waves and SLR (Ratliff et al., 2018), mud and vegetation (Lauzon & Murray, 2018), and ice and permafrost (Lauzon et al., 2019) have on delta morphology and dynamics. In contrast to physical experiments, numerical models allow more experiments to be conducted and therefore additional conditions and forcings to be tested.…”

Buffered, Incomplete, and Shredded: The Challenges of Reading an Imperfect Stratigraphic Record

“…For example, Toby et al () recognized that the scale of autogenic volume fluctuations that defines transfer thresholds likely varies as a function of the mean and stochastic components of a system's forcing. Work to define the magnitude of autogenic fluctuations as functions of forcing conditions is starting to accelerate, with studies quantifying autogenic scales as functions of the ratio of sediment to water supply (Powell et al, ; Straub & Wang, ), sediment grain size (Caldwell & Edmonds, ) and cohesion (Edmonds & Slingerland, ; Hoyal & Sheets, ; Li et al, ), vegetation (Lauzon & Murray, ; Piliouras et al, ), flashiness of system hydrographs (Esposito et al, ; Ganti et al, ; Miller et al, ), basin water depth (Carlson et al, ), and wave (Ratliff et al, ) and tidal climate (Kleinhans et al, ; Lentsch et al, ).…”

When does faulting-induced subsidence drive distributary network reorganization?

“…Simulations used the DeltaRCM numerical delta model , implemented in Python as pyDeltaRCM (Moodie, Hariharan, Barefoot, & Passalacqua, in review). DeltaRCM has been robustly validated (Liang, Ge-leynse, Edmonds, & Passalacqua, 2015b; and used to examine delta morphology and evolution under various external forcings, including sea-level rise , and presence of vegetation (Lauzon & Murray, 2018), and ice and permafrost (Lauzon, Piliouras, & Rowland, 2019;Piliouras, Lauzon, & Rowland, 2021). The perturbation was modeled as instantaneous vertical land movement (i.e., displacement without hanging-wall rotation or translation), with channel evolution and geometry before and following displacement emerging based on model boundary conditions (Supporting Information).…”