Response of Suspended-Sediment Fluxes to Storms in a Back-Barrier Estuary, Chincoteague Bay, MD/VA, USA

Recent major storms have generated increased interest in understanding and predicting the geomorphic response and resiliency of estuaries and coastal wetlands to these events. To that end, flow velocity, wave characteristics, and suspended-sediment concentration (SSC) were measured for one year at eight locations in Chincoteague Bay, MD/VA, USA, a shallow East-coast back-barrier estuary. The 377 km² microtidal lagoon receives little freshwater input and is connected to the Atlantic Ocean via inlets at its north and south ends. Daily sea breezes and episodic storms generated sediment-resuspending waves and modified the flow velocity at all sites, which occupied channel, shoal, and inlet environments with different bed-sediment characteristics. Despite comparable SSC during calm periods, SSC at the channel locations was considerably greater than at the shoal sites during windy periods because of larger wind waves and a relative lack of sediment-sheltering vegetation in the channels. Sediment fluxes were strongly wind modulated: within the bay’s main channel, depth-integrated unit-width sediment flux increased from calm values of 2–6 g m^-1 s^-1 by about 5 g m^-1 s^-1 per 1 m s^-1 increase of wind speed. When averaged over all sites, about 35% of the instantaneous sediment flux occurred during windy periods (wind speed greater than 6 m s^-1), which represented just 15% of the deployment time. The residual sediment flux was highly episodic, with net transport overwhelmingly occurring during wind events. At channel sites, the net sediment flux was opposite to the direction of the wind forcing, while at shoal sites, the flux generally was aligned with the wind, implying complex channel–shoal material exchange. The field observations are compared to a realistic numerical model of Chincoteague Bay simulating coupled effects of flow, waves, and vegetation on sediment transport. Modeled scenarios represent postulated effects of climate change including changes in storm magnitude, frequency, and character.