Spatial differences in wind-driven sediment resuspension in a shallow, coastal estuary

Abstract

Two locations approximately 11 km apart along the axis of the New River Estuary near Jacksonville, NC USA were continuously monitored for eight years. Included in the observations are vertical profiles of turbidity, temperature, salinity, chl-a, dissolved oxygen, pH and water velocity as well as local wind velocity. Differences between the two sites result from a number of factors, including bathymetry, wind strength, direction and fetch, estuarine morphology, tidal currents and sediment properties. The site near the head of the estuary, Morgan Bay, is deeper, experiences generally weaker winds and has less fetch in most directions. Stones Bay, the down-estuary site, is shallower, experiences stronger winds and has longer fetch, particularly in the prevailing wind directions. Current speeds also differ along the estuary with the down-estuary Stones Bay site being more tidal.The observations were used together with a simple wave model to analyze the estuarine turbidity response to different forcing mechanisms. Results suggest that sediments are resuspended primarily by wind-wave generated bottom stress at both locations. While turbidity is generally higher in Stones Bay than in Morgan Bay, turbidity as a function of the local wave-induced bottom stress (including forcing from all directions) is similar at both locations at low stress but diverges at higher stresses. At higher bottom stresses, turbidity in Stones Bay responds primarily to winds from the NE, S and NW while turbidity in Morgan Bay responds primarily to winds from the NW and S. Accounting for sediment resuspension within an approximate spatial advection scale around each of the observation sites, yields a similar turbidity vs bottom stress response curve for the three primary directions in Stones Bay and the S direction in Morgan Bay but a greater turbidity response for winds from the NW in Morgan Bay. In the latter case, waves are crossing the section of the New River Estuary just downstream of the confluence with the New River and are presumably encountering sediments that are more easily resuspended.Average sediment export is down-river with more sediment leaving Stones Bay than Morgan Bay.