Mass and Momentum Flux Prediction at the Channel‐Floodplain Interface Associated With Riparian Vegetation

ABSTRACT The mass and momentum exchange between the main channel and floodplains impacts the river systems' hydraulic, geomorphic, and ecologic functions. Floodplain vegetation introduces additional flow resistance, significantly affecting mass and momentum fluxes. The objectives of this study were to (1) quantify the mass and momentum fluxes between the floodplains and the main channel in the presence of vegetation using a two‐dimensional (2D) hydrodynamic model; (2) investigate the impacts of varying degrees of floodplain vegetation density on mass and momentum exchange; and (3) advance the conceptual understanding of the processes that control the connections between vegetation structure, hydrodynamic processes, and mass and momentum exchange. The Sedimentation and River Hydraulics 2D hydrodynamic model, coupled with a vegetation hydraulic roughness solver, was used to simulate steady‐state conditions for the San Joaquin River, California, USA. Mass and momentum fluxes were analyzed for varying vegetation density (± 50% from baseline leaf area index). A hypothetical prismatic channel with features resembling the San Joaquin River was also modeled. Our findings suggest that mass and momentum fluxes in irregular channels were insensitive to vegetation density changes beyond a certain density, and exchanges were mostly driven by the geometry of the river (e.g., sinuosity). However, vegetation density changes had minor effects on prismatic channels. Further study is recommended to analyze unsteady flow conditions, adding more insights regarding the spatial and temporal characteristics of mass and momentum fluxes. The preliminary findings of our study provide new insights into the complex interactions between vegetation, hydrodynamics, and momentum exchanges with implications for river ecosystem dynamics.