The influence of vegetation and wind waves on the distribution of bed shear stress on estuarine mudflats.

The effects of vegetation and small amplitude wind waves on the distribution of bed shear stress are investigated in a tidally dominant, shallow estuary in the Gulf of Maine (Great Bay estuary, New Hampshire). In this work, a three-dimensional coupled hydrodynamic-wave-sediment transport numerical model (COAWST) was implemented in the Great Bay estuary, and used in conjunction with available observational datasets to predict the influence of subaquatic vegetation (eelgrass meadows) and wind waves on the distribution of shear stress from waves, currents, and combined flows. Observational datasets include the spatial distribution of eelgrass (Zostera marina; SeagrassNet 2007-2016), several field campaigns that measured currents and water levels (2011, 2016, 2018), and the deployment of four Spoondrift Spotter buoys in the summer of 2018. The model grid at 10 and 30 m resolution uses bathymetric data collected in 2009 and 2015. The hydrodynamic model has been verified (Cook, et al., Ocean Mod., 2019) with tidal analysis (tidal amplitude decay and phase changes) and current observational datasets (vertical current structure). Model estimates of bed shear stress including the presence of subaquatic vegetation are shown to compare well with field estimates from observed near-bed velocity profiles at two different locations on the mud flats. Modeled wind wave heights and periods compare reasonably well to the observed wave climate in the Great Bay; however, the effects of typical wave motions on bed stress are much less than from the tidal currents except near the fringes of the estuary and a lower water levels. Basin wide estimates of combined wave-current bed shear stresses are used to estimate total nutrient flux from sediments over month-long periods as well as over a single average, spring and neap tidal cycle, both with and without the presence of eelgrass meadows and waves.