Assessment of Sediment Transport of a Dredged Material Sediment Cap, New York Bight

A sediment cap approximately 3m thick consisting of 2m of fine sediment and 1m of sand was dumped by hopper dredge over several meters of fine dredged material at the New York dredged material dumpsite in 1980. The dumpsite is located in the open Atlantic Ocean approximately 15km south of Ambrose Light Tower. It is a topographically high area with about 11m of relief above the Hudson Shelf Valley immediately to the east, and is thus exposed to wave action from an open fetch to the east, the source of infrequent but intense storm winds. The possibility of erosion of the sediment cap and subsequent exposure of possibly contaminated dredged material prompted our study of the cap to determine its erodability and an estimated life of the cap. Initial work was done in November 1980. Grab samples on a 200m grid, 115% coverage by sidescan sonar, box cores, and SEAFLUME deployments were made. SEAELUKE records photographically the threshold of sediment transport resulting from an artificially-induced current, through an open-bottomed flume. At the end of the cruise, two concentration-velocity (CV) probes were deployed, one each at the center and the northern edge of the cap. Each probe consists of an electromagnetic current meter located 1m off the bottom, and a transmissometer mounted just below it. Readings are taken from both instruments once a second for 8 minutes each hour for 60 days. A maintenance cruise in February 1981 serviced the instruments and added a third probe near the eastern edge of the cap. Servicing will be repeated in April. A final cruise in June will repeat all the November work and retrieve the probes. Preliminary results from bottom samples and sonographs indicate a relatively clean sand cap with relatively rough microtopography from dumping. Previous transport studies in the New York Bight indicate that sand is easily transported during ´northeaster´ storms, but that fine sediment, particularly if it is partially compacted, resists transport. Ther- fore, in the case of this cap, it may be possible that the sand portion of the cap will be easily eroded, but that it will have compacted the underlying fine sediment sufficiently to provide long-term stability.