The Fate of a Fine-grained Dredge Spoils Deposit in a Tidal Channel of Puget Sound, Washington

Abstract

Fifteen thousand cubic meters of fine-grained sediment dredged from Olympia Harbor, Washington, were disposed at Dana Passage, a large tidal channel in Puget Sound, Washington. Water depth at the disposal site is 30 m; maximum tidal currents range from 110 cm/sec at the surface, to 50 cm/sec at the bottom (measured 100 cm above the bed). Gravity coring, bottom photography, and diver measurements allowed delineation of the dimensions and nature of the spoils deposit. Four months after completion of disposal, 16% of the volume of sediment remained at the disposal site in a flat conical deposit (maximum thickness 70 cm, radius 60 m). The 84% loss represents sediment placed in suspension during disposal of the spoils, sediment eroded soon after disposal, and water lost during consolidation of the deposit. Selective removal of fine-grained sediment and organic material caused the spoils deposit to be coarser (mean grain size 0.027 ram), and to have a lower total volatile content (4.1%) than the sediment originally dumped (mean grain size 0.014 mm; total volatile content 8.9%). After four months, loosely consolidated sediment had been removed from the deposit, and the consolidation of the remaining spoils provided sufficient cohesion to resist further erosion by the relatively strong bottom currents. The factors which control the stability of a fine-grained dredge spoils deposit are: (a) the amount of spoils disturbance (dilution) caused by the dredging and (disposal techniques, (b) the water depth at the disposal site, and (c) the energy considerations of the bottom environment at the disposal site, e.g. , strength of bottom currents. Dredging and disposal by techniques which minimize sediment disturbance, and disposal of spoils in shallow water and low energy environments are processes which maximize stabilization.