Vegetation wakes and wake interaction shaping aquatic landscape evolution

Abstract

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.