Using Small Unoccupied Aerial System (sUAS) Multispectral Imagery to Compare Hydrodynamics Around Different Aged Patches of Marsh Vegetation

ABSTRACT Wetland vegetation influences water and sediment transport by altering flow resistance. Vegetation presence, stem density, leaf and stem morphology, and flexibility, among other characteristics, control the degree to which vegetation impacts transport patterns. While well‐predicted by laboratory and numerical modelling studies, in‐situ observations of flow interaction with wetland vegetation remain scarce largely due to instrument limitations. Using a small Unoccupied Aerial System (sUAS) equipped with a multi‐spectral camera, we performed dye surveys and tracked the intensity of dye propagating through and around patches of Phragmites australis in the lower Mississippi River Delta (MRD). High‐temporal resolution images were used to develop a new methodology to calculate the time‐averaged streamwise velocity along multiple parallel transects around and through patches. These methods were performed on an established, high stem density patch of Phragmites and compared to a restoration planting of low stem density. The established patch had limited dye transmission through the patch and preferentially deflected flow around the patch relative to the restoration planting. Additionally, there is a greater water velocity variation along the marsh platform adjacent to the established patch. Flow patterns influence sedimentation rates on the marsh platform; the established patch provides evidence of the sediment buffering effect, while the restoration patch exhibits characteristics of the trapping effect. By directly measuring in the field phenomena predicted in laboratory and numerical models, we provide field‐based observational support for our understanding of marsh vegetation hydrodynamics.