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Marsh vegetation, a definitive component of delta ecosystems, has a strong effect on sediment retention and land-building, controlling both how much sediment can be delivered to and how much is retained by the marsh. An understanding of how vegetation influences these processes would improve the restoration and management of marshes. We use a random displacement model to simulate sediment transport, deposition, and resuspension within a marsh. As vegetation density increases, velocity declines, which reduces sediment supply to the marsh, but also reduces resuspension, which enhances sediment retention within the marsh. The competing trends of supply and retention produce a nonlinear relationship between sedimentation and vegetation density, such that an intermediate density yields the maximum sedimentation. Two patterns of sedimentation spatial distribution emerge in the simulation, and the exponential distribution only occurs when resuspension is absent. With resuspension, sediment is delivered farther into the marsh and in a uniform distribution. The model was validated with field observations of sedimentation response to seasonal variation in vegetation density observed in a marsh within the Mississippi River Delta.

Vegetation provides habitat and nature‐based solutions to coastal flooding and erosion, drawing significant interest in its restoration, which requires an understanding of sediment transport and retention. Laboratory experiments examined the influence of stem diameter and arrangement on bedload sediment transport by considering arrays of different stem diameter and mixed diameters. Bedload transport rate was observed to depend on turbulent kinetic energy, with no dependence on stem diameter, which was shown to be consistent with the impulse model for sediment entrainment. Existing predictors of bedload transport for bare beds, based on bed shear stress, were recast in terms of turbulence. The new turbulence‐based model predicted sediment transport measured in model canopies across a range of conditions drawn from several previous studies. A prediction of turbulence based on biomass and velocity was also described, providing an important step toward predicting turbulence and bedload transport in canopies of real vegetation morphology.

Velocity and forces on individual plants were measured within an emergent canopy with real plant morphology and used to develop predictions for the vertical profiles of velocity and turbulent kinetic energy (TKE). Two common plant species,Typha latifoliaandRotala indica, with distinctive morphology, were considered.Typhahas leaves bundled at the base, andRotalahas leaves distributed over the length of the central stem. Compared to conditions with a bare bed and the same velocity, theTKEwithin both canopies was enhanced. For theTyphacanopy, for which the frontal area increased with distance from the bed, the velocity, integral length‐scale, andTKEall decreased with distance from the bed. For theRotala, which had a vertically uniform distribution of biomass, the velocity, integral length‐scale, andTKEwere also vertically uniform. A turbulence model previously developed for random arrays of rigid cylinders was modified to predict both the vertical distribution and the channel‐average ofTKEby defining the relationship between the integral length‐scale and plant morphology. The velocity profile can also be predicted from the plant morphology. Combining with the new turbulence model, theTKEprofile was predicted from the channel‐average velocity and plant frontal area.