FY 1989

An attempt to evaluate the effects of an anti-turbidity system on sediment dispersion from a hopper dredge

Nakata, K., K. Tsurusaki, Y. Okayama, and J.W. Lavelle

NOAA Tech. Memo. ERL PMEL-85, NTIS: PB89-139786, 30 pp (1989)

Measurements were made during six hopper dredge operations to investigate the differences in plumes of overspilled particulates when the dredger was and was not using an anti-turbidity system. Observations for discharge rates of suspended solids were taken aboard the dredge ship while concentration samples of suspended solids were taken by survey boats in the plume and currents were metered by instruments on moorings. Measurements were given a common framework by the use of a dispersion model for the plume. Modeled and observational profiles match well when the rate of discharge is reserved as a fitting parameter. However, differences in results of the use and non-use of the anti-turbidity system are not discernible with the field data. Consequently, the model was used under identical advection and diffusion conditions to study the differences theoretically. Those numerical experiments suggest that there is an increase of about 25% in the amount of deposition in the immediate area of dredging with the anti-turbidity system, though the fractional amount of redeposition in both cases is small. The differences in results for the two systems calculated with the model depend on the assigned initial vertical distributions. Because these are poorly known at present, better definition of the differences with and without the anti-turbidity system await better measurements of the vertical distributions of suspended solids in the ocean immediately following discharge.