Hydrodynamics and Sediment Dynamics in Macrotidal Drainage Channels and Salt Marshes

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Ryan Mulligan, Logan Ashall, Danika van Proosdij, Emma Poirier

Thursday 2 july 2015

12:15 - 12:30h at Mississippi (level 1)

Themes: (ST) Sediment transport mechanisms and modelling, (T) Sediment management and morphodynamics

Parallel session: 11A. Sediment - Transport

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The Bay of Fundy in Nova Scotia, Canada, has nearly resonant semi-diurnal tides that produce a tidal range of up to 16 m and tidal currents up to 5 m/s making this region a site for future power generation using in-stream turbines. In this study we use a high-resolution numerical model to examine the tidal flooding and draining of muddy channels and vegetated salt marsh flats in Minas Basin by comparison with observations. Field observations were collected in spring/summer 2013 using acoustic Doppler current profilers, acoustic velocimeters, pressure sensors, optical backscatter sensors, sediment traps and Lidar topographic surveys. The Delft3D model is used to simulate the hydrodynamics, waves, sediment transport and morphology change. The model uses 18 vertical layers and the horizontal domain is composed of three grids connected using domain decomposition achieve the highest resolution in the salt marsh. A spatially varying bottom drag map was developed from satellite imagery for four different roughness types including low marsh (Spartina alterniflora), high marsh (Spartina patens), intertidal mud, and intertidal sand. The hydrodynamic model also includes a vegetation sub-model that parameterizes plants based on a force balance according to the stem diameter, stem height and plant density and is implemented to characterize intertidal flows over different marsh grasses. The results indicate that strong shallow flows occur in the drainage channels at the onset of flooding and at the end of ebb that transport high concentrations of suspended sediment up to 300 g/m3. To determine the influence of the marsh grasses, the model was run with and without the vegetation sub-model. Without vegetation, tidal drainage flowed freely marsh surfaces at all elevations. With vegetation, flow over the marsh was significantly dampened, re-directed into channels, and resulted in higher correlation with observations especially at higher marsh elevations. This work indicates the importance vegetation in controlling the hydrodynamics and sediment exchanges between salt marshes and tidal flats via tidal channel networks.