Resuspension

Resuspension rates may be determined directly by testing undisturbed sediment samples in a flume or by monitoring conditions at the sea floor closely over time. Flume tests have not been done on Hudson sediment but the importance of resuspension has been calculated from measurements of changes in the near-bottom suspended sediment concentration (Geyer, Woodruff, and Traykouski, 2001) at locations in the lower estuary along the Manhattan shoreline. Within the turbidity maximum, concentrations of suspended sediment were observed to decrease to low levels during slack tides from levels of several hundred milligrams per liter (Geyer, 1995).Although this tidallymodu-lated deposition suggests that subsequent tidal resuspension is necessary to maintain the turbidity maximum, Geyer (1995) did not find a correlation between the suspended load and water velocity, suggesting that advection was the predominant control of concentrations.

Alternatively, resuspension rates may be estimated by assessing the vertical flux of settling sediment particles. The downward vertical flux of particles to the seafloor is often found to be much larger than the net, long-term deposition rate. As a result, the vertical flux to the seafloor is balanced to a first approximation by resuspension. One way to determine the vertical flux is from near-bottom sediment traps. These devices are designed to intercept the flux of sediment to the seafloor. Measurements of the vertical particle flux in the vicinity of the turbidity maximum ranged from 106 g cm-2 y-1 to 586 g cm-2 y-1 (Achman, Brownawell, and Zhang, 1996), which are three orders of magnitude greater than the long-term accumulation rate. Assuming that this is the rate at which sediment reaches the seafloor, this is also the resuspension rate.

Few measurements are available in the estuary and these were not taken for the purposes of determining the flux at the seafloor. Estimates can also be made from a combination of settling velocity and concentration. If 0.04 cm s-1 is taken as the settling velocity (Arnold, 1982) and 30 mg L-1 taken as a typical concentration away from the turbidity maximum, the vertical settling fluxbecomes

I.2 x 10-6 or 38 g cm-2 s-1. This is lower than the sediment trap results but still much higher than the long term accumulation rate. As a result, it maybe considered an estimate of resuspension.

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