Field Study of the Effects of Storms on the Stability and Fate of Dredged Material in Subaqueous Disposal Areas: final report

Long Island Sound is a large estuary. Dredged sediment placed on the bottom of the Sound is subject to dispersion by the tidal stream, estuarine circulation, waves, and disturbances of the hydraulic flow field by storms. The tidal stream is the dominant source of energy for the resuspension and transport of sediments; waves do not contribute significantly to dispersion in water depths greater than 60 ft. Random fluctuations in the water velocity are detected at all depths. During a storm fluctuations in velocity increase in intensity and are important agents of sediment resuspension. Direct, wind-driven flow over the bottom is weak, but storm winds cause water level increases up to 3 ft above the usual tidal level. The energy available for sediment transport is then greatly increased. Water level records are sued to evaluate seasonal and long-term changes in storm energy release in the Sound. During extreme storm conditions resuspension of the bottom in deep water is limited to a layer of sediment with an average thickness of less than 1 cm. Despite this activity, silt is accumulating in the central sound at rates as high as 10x3 gm/ (m2 yr). Repeated bathymetric surveys of a deposit of dredged material at the New Haven disposal site show that after initial self-consolidation of the mound, no significant changes in pile configuration occurred over a three-year period; erosion of the deposit is not detected. The data obtained show that to best contain silt-clay dredged material, the disposal site should be on a naturally accreting mud bottom, the disposal operation should emplace a large volume of material on the site expeditiously, and the deposit should be built to an optimum configuration. The capacity of the disposal site is limited by the maximum height of the disposal mound and the maximum slope of the pile slides which present a minimum disturbance of the natural hydraulic regime. The capacity of the New Haven site is estimated to the up to 1.7 x 10 x6 yd3 of unarmored, silty, dredged material. Larger volumes may be contained if the surface of the deposit is armored with coarser material.