Observations of bottom currents and estimates of resuspended sediment transport at the New York Bight 12‐mile dumpsite

Abstract

To document storm events that may induce a redistribution of sediment in the vicinity of the New York Bight 12‐mile sewage sludge dumpsite, current meter moorings were deployed in water depths from 20 m (near the mouth of New York Harbor) to 53 m (within the Hudson Shelf Valley) from July 1986 through June 1989. Ten usable instrument records ranging from one month to one year in duration were obtained; eight of them near‐bottom records. Seasonal and geographic variability of wind‐induced flow were examined. The wind is most efficient in driving the subtidal currents in the 2–10 day frequency band during winter when the water column is well mixed and when the eastward component of the wind often induces and sustains an up‐valley (northward) bottom flow. Maximum efficiency occurs for wind from 300° (WNW) and at sites located within the Hudson Shelf Valley. A continental shelf bottom boundary layer model (Glenn and Grant, 1987) was used to estimate resuspended sediment transport. Model inputs include bottom currents (observed), orbital wave velocities (estimated), and sediment grain size (from the literature). Model output indicates that sediment resuspension at the current meter sites occurs approximately 5% of the time, primarily during winter months. The difference in along‐valley flux between two moorings provides a rough estimate (6‐month time series) of deposition and erosion. The net deposition (+.02 mm) was no greater than the deposition and erosion resulting from individual storms. A three‐dimensional circulation model (You et al., 1991) is applied to increase the spatial resolution of the near‐bottom current field (4 km grid) for a storm event in May of 1987. Given these velocities that vary in space and time, the redistribution of sediment was modeled for different surface wave conditions. Areas of deposition aligned with the Hudson Shelf Valley due to less wave‐induced resuspension in deeper waters. Given all the uncertainties in the input variables (grain size, surface waves) and the simplistic assumptions made in modeling the deposition and erosion, it is still uncertain how much sludge is permanently removed from the area, but episodic redistribution of surficial sediment evidently occurs throughout the Inner New York Bight.

Observations and a Model of the Mean Circulation over the Middle Atlantic Bight Continental Shelf

“…The exception to this is the flow within Hudson shelf valley, the only canyon that extends across the entire MAB shelf. Hudson shelf valley has a dramatic influence on the local circulation, with mean along-valley flows that are onshore at 5-10 cm s Ϫ1 (Nelsen et al 1978;Mayer 1982;Manning et al 1994;Harris et al 2003). The subsequent analysis does not include the current observations from the two sites within Hudson shelf valley.…”

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“…Manning et al (1994) proposed a similar scheme for the generation of deep layer currents in the New York Bight. Imposed wind stress induced surface-layer flow to the right, while compensating return flow developed in the bottom boundary layer.…”

Recovery of benthic macrofauna from sewage sludge disposal in the New York Bight

“…All 5 species showed a significant sludge cessation effect based on data from only 1 reference site. Physical oceanographic factors near the 12MDS (Manning et al 1994, Davis et al 1995, Mountain & Arlen 1995, Packer et al 1995 may have intermittently altered the presence of sludge and its effect on the biota. For example, in the summer months, higher water temperatures resulted in water mass stratification; in addition to reduced wave and current activity, this increased the possibility of accumulation of sludge contaminants.…”

“…5b-d, Table 1) may be due not only to the elimination of contaminant input from sewage sludge, but also to rapid degradation of organic sludge components (Nedwell & Lawson 1990), the nature of bottom currents in the vicinity of NY6 (Manning et al 1994, Packer et al 1995, Zdanowicz et al 1995, and the relief associated with bottom topographic features such as the Hudson Shelf Valley (Fig. 1).…”

“…Oceanographic events may have also masked the effects of cessation of sludge disposal at our intermediate site, R2. Episodic up-valley flow events (Manning et al 1994) may have transported contaminated resuspended material from the vicinity of NY6 and the upper Hudson Shelf Valley towards R2 at the edge of the Christiaensen Basin after cessation. It is also likely that R2 was influenced by contaminant input from the outflow of the Hudson-Raritan estuary (EPD 1988, Zdanowicz et al 1995.…”