Module 4: Cutterhead Dredge and Turbidity Copy

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A cutterhead dredge is the most used dredge in the United States, the cutterhead is versatile and uses a rotating cutter located at the end of a ladder which excavates the bottom sediment and guides it into the suction. For conventional cutterhead dredges, the diameter of the cutter is almost four times the diameter of the suction pipe.

Most sediment being resuspended by a cutterhead dredging operation (exclusive of placement) is usually found in the vicinity of the cutter as determined by the Barnard study in 1978. The levels of sediment are directly related to the type and quantity of material cut but not picked up by the suction.

The amount of material supplied to the suction is controlled primarily by the rate of cutter rotation, the vertical thickness of the dredge cut, and the horizontal velocity of the cutter moving across the cut.

In addition to the dredging equipment used and its mode of operation, sediment resuspension can also be caused by sloughing of material from the sides of vertical cuts and inefficient operational techniques.

Field data is available for sediment in suspension in the vicinity of cutterhead dredges at various places. The 1978 Barnard study also determined that the limited data collected under low-current conditions showed elevated levels of suspended material which appeared to be localized to the immediate vicinity of the cutter as the dredge swings back and forth across the dredging site. Within 10 feet of the cutter, suspended solids concentrations are highly variable, and the concentrations decrease exponentially from the cutter to the water surface.

Near-bottom suspended solids concentration was found to be on the order of a few hundred milligrams per liter (mg/L) at distances of a few hundred meters from the cutter.

In 1975, a study done by Yagi et al. concluded from that “in the case of a steady dredging of a thin sedimented mud layer, the effect of dredging on turbidity was almost found to be imperceptible at locations several tens of meters distance from the cutter.”

A properly designed cutter efficiently cuts and guides the bottom material toward the suction, and unfortunately, between the cutting action and the turbulence associated with the rotation of the cutter. A portion of the bottom material being dredged is resuspended.

Operational conditions exert considerable influence on sediment resuspension in the vicinity of a cutterhead dredge depending primarily on the type and amount of material that is excavated but not drawn into the dredge suction.

This “residual” material may remain in suspension or may settle into the existing cut, where it again becomes susceptible to resuspension by ambient currents and turbulence generated during subsequent cuts.

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