The effects of sediment properties on deltaic processes and morphologies: A numerical modeling study

Abstract

There is a pressing need to understand how different delta morphologies arise because morphology determines a delta's ecologic structure, resilience to relative sea-level rise, and stratigraphic architecture. We use numerical modeling (Delft3D) to explain how deltaic processes and morphology are controlled by the incoming sediment properties. We conducted 36 experiments of river-dominated delta formation varying the following sediment properties of the incoming grain-size distribution: the median, standard deviation, skewness, and percent cohesive sediment, which is a function of the first three properties. Changing standard deviation and skewness produces minimal morphological variation, whereas an increase in dominant grain size (D 84 ) and decrease in percent cohesive sediment produce a transition from elongate deltas with few channels to semicircular deltas with many channels. This transition occurs because critical shear stresses for erosion and settling velocities of grains set the number of channel mouths and the dominant delta-building process. Together, the number of channel mouths and the dominant process-channel avulsion, mouth bar growth, or levee growth-set the delta morphology. Coarse-grained, noncohesive deltas have many channels dominated by avulsion, creating semicircular planforms with relatively smooth delta fronts. Intermediate-grained deltas have many channels dominated by mouth bar growth, creating semicircular planforms with rugose delta fronts. Fine-grained, cohesive deltas have a few channels, the majority of which are dominated by levee growth, creating elongate planforms with smooth delta fronts. The process-based model presented here provides a previously lacking mechanistic understanding of the effects of sediment properties on delta channel network and planform morphology.

Effects of mud supply on large-scale estuary morphology and development over centuries to millennia

“…Long-term morphologic calculations are rare due to computer limitations and lack of spatially and temporally dense data of mud in the bed. For deltas, on the other hand, long-term morphologic development by numerical modelling (Edmonds and Slingerland, 2009;Caldwell and Edmonds, 2014;Burpee et al, 2015) showed large effects of mud on plan shapes, patterns and dynamics with fairly simplistic sediment transport processes. In particular, cohesion reduces the ability to re-erode, resulting in more stable bars and levees and longer and deeper channels.…”

Plan-view evolution of wave-dominated deltas

“…In the absence of waves and tides, channel leveeing and bifurcation lead to the development of crenulated shorelines typical of fluvial dominance, with plan-view morphologies determined by sediment size [Orton and Reading, 1993;Caldwell and Edmonds, 2014] and cohesion [Edmonds and Slingerland, 2009]. Wind waves discourage accumulation of finegrained sediment at the delta mouth, and tend to sculpt delta shorelines into a cuspate shape consisting of sandy shorelines composed of shoreline-parallel beach ridges .…”

Measuring Subaqueous Progradation of the Wax Lake Delta with a Model of Flow Direction Divergence

“…Although dimensions vary, this general morphology has been observed on the Wax Lake Delta (Shaw et al, 2016b;Shaw and Mohrig, 2014), Brant's Pass crevasse on the birds-foot delta of the Mississippi River (Esposito et al, 2013), the Mobile and Apalachicola river deltas (Edmonds et al, 2011b) and the St. Clair River Delta (Figure 1b; NOAA, 2017). Additionally, numerical models often produce this morphology (Caldwell and Edmonds, 2014;Geleynse et al, 2010;5 Liang et al, 2016). These deltas can be qualitatively classified as river-dominated (Galloway, 1975), both by their large fluvial sources and relatively small winds and tides.…”

“…In order to achieve some validation independent of the Wax Lake Delta, the FD2C model was also evaluated on four numerical river deltas originally presented by Caldwell and Edmonds (2014). These deltas were modelled using Delft3D on a 25 x 25 m 2 grid.…”

“…They differed in incoming median grain diameter between 0.01 and 0.1 mm, the sorting of the sediment distribution, and the fraction of the sediment that was cohesive. Full descriptions of the runs are found in Caldwell and Edmonds (2014). Measurements began at time step 500 to allow a significant deposit to develop and then every 5 time steps thereafter.…”