Vegetation wakes and wake interaction shaping aquatic landscape evolution

Abstract

Vegetation found in marshes and tidal systems influences the flow of water through these systems. Vegetation blocks the flow of water, leading to areas of low water velocity immediately downstream of a vegetation patch and areas of high water velocity at the sides of a patch. Areas of lower velocity provide good habitat for young plants and seedlings, and thus encourage plant growth such that the patch expands in the downstream direction. The addition of more vegetation near the original patch further reduces the local velocity that further promotes growth, creating a system of feedbacks. Recently, studies have found that when several patches of vegetation are near each other, they can change the shape and extent of the low‐velocity regions downstream. Based on these findings, the present study develops a model to evaluate how a field of vegetation might evolve over many years and the final distribution patterns that result. The model initially evaluates the water velocity based on a simple theoretical model and then modifies the velocity field to incorporate reduction of flow downstream of the patches, based on laboratory observations. Once the velocity at every point is calculated, vegetation is added probabilistically, with areas of high velocity having a low probability of growth and areas of low velocity having a high probability of growth. The number of initial vegetation patches and the limiting velocity (the velocity above which no plants can grow) were varied, and three different types of landscapes were produced: full vegetation coverage, channeled, and sparse. The effect of vegetation on the velocity field in the wake (i.e., downstream) of the vegetation is shown to accelerate landscape development. Recent field and experimental studies show that the wakes behind individual patches of aquatic vegetation, as well as the interaction and merger of neighboring wakes, produce zones of diminished velocity that may enhance deposition and encourage patch growth and patch merger. In the present study, these patch‐scale biogeomorphic interactions are incorporated into a simple model for vegetated landscape evolution. The initial flow field is solved by using a porous media formulation for hydraulic resistance. The velocity in wake regions is then adjusted to match the wake structure measured in laboratory studies with individual and pairs of vegetation patches. Vegetation is added based on a probabilistic function linked to the velocity field. The simulations explore the influence of initial plant density ( ID ) and limiting velocity ( LV , the velocity above which no plants can grow) on landscape evolution. Three types of stable landforms can occur: full vegetation coverage, channeled, and sparse. By including the influence of wakes, full vegetation coverage can be achieved from initial plant densities as low as 5%. In contrast, simulations that exclude the influence of wakes rarely reach full vegetation coverage, reinforcing the idea that growth within wakes is an important component in vegetated landscape evolution. The model also highlights the role of flow diversion into bare regions (channels) in the promotion of growth within vegetated regions. Finally, sparse landscapes result when the initial plant density is sufficiently low that no wake interactions can occur, so that patch merger cannot occur, emphasizing the importance of the patch interaction length scale.