Hydraulic and Sediment Characteristics at the North Channel Bridge, Jamaica Bay, New York

Staubitz, Ward; Wolcott, Stephen

Data were collected during the spring of 1984 in the vicinity of North Channel bridge in Jamaica Bay to define the hydraulic regime and the physical characteristics and chemical quality of bottom sediments. The data were used in a semiquantitative analysis to predict the effects of bridge replacement on flow patterns, chemical quality of bay water, and distribution of bottom sediments within Jamaica Bay. delineated, and continuous tidal stage and tidal velocity data were collected for about 1 month. In addition , eight bottom-sediment samples were collected near the bridge and analyzed for particle size, settling velocity , biochemical oxygen demand, concentration of selected metals and organic compounds, and exchange of metals and nutrients to the water column. Jamaica Bay is roughly circular with scattered landmasses in i t s center; the circulation patterns that form during tidal cycles can be evaluated from fluxes measured at the bridge. Results of the hydraulic analysis show that the present rate and direction of water movement at the bridge vary, but a net eastward flux of about 0.67 x 10^8 cubic feet occurs during each tidal cycle. The proposed bridge should have no measurable effect on the net water-volume transport at the bridge crosssection. Scouring of bottom sediment beneath the proposed bridge is expected to be less than beneath the present one because fewer pilings will be used. The sediment data indicate that bottom sediments are relatively unpolluted in the bridge vicinity and that sand- and silt-sized aggregated particles predominate. About 75 percent of the resuspended bottom sediment will settle within 610 f t of the new bridge during an average ebb tide. A simple analysis of sediment transport and deposition indicates that the depth of sedimentation resulting from bridge replacement is not likely to exceed 0.12 inches at any location within the bay. Also, the large volume of water passing the bridge site during each tide (up to 2 x 10^8 cubic feet) will dilute any metals and nutrients released from the sediments to several orders of magnitude below their detection limit. The extra oxygen demand exerted by the resuspended bottom sediments is also expected to be several orders of magnitude lower than ambient biochemical oxygen demand of the water column.