Modeling deposition of suspensate in Great Sound, New Jersey

Abstract

A sediment deposition model is developed for application to Great Sound, New Jersey. A determination of the average annual accumulation rate is of primary interest. The settling tank concept is used for the model, employing a plug flow approach to model the tidal hydrodynamics. Assumptions inherent in this modeling technique include no mixing between plugs, a uniform vertical velocity profile and simplified geometry. Model inputs were based on hydrodynamic and suspended sediment data obtained for Great Sound during other investigations, including initial volume in the sound at mean low water, the inflow hydrograph and tidal range, the sediment sizes, concentrations and settling velocities, and a frequency versus concentration relationship. The model simulates a single tidal cycle in Great Sound for spring, neap or mean tidal conditions for a specified sediment concentration. Three tests were run to define the sediment deposition characteristics of the sound. The first test defined the relative impact of spring, neap and mean tidal ranges on the deposition. Deposition during mean tide was found to be the average of the spring and neap tide deposition. Concentration hydrographs for ebb flow were determined. The second test determined the average annual sediment accumulation rate in Great Sound to be 8.9 mm/yr by running multiple tidal cycles for fair, pre- and post-storm, and storm conditions. Model predictions compare favorably with predictions of other researchers. The distribution of the average annual accumulation across Great Sound is also defined. In the third test, the relative influence of storm conditions versus predominant fair-weather conditions was established. Only 8 storm days are required to match a year of fair-weather deposition.