Hopper dredges lose some material at the trailing drag head, but a significant source of fines is the overflow at or near the surface. With a cutter suction dredge the sediment losses occur predominantly at the cutter head and may not appear in the surface waters. The different dredges have been placed on this schematic to indicate the amount of turbidity generated by each dredge type.

Natural conditions refer to the minimum disturbance that can occur while storm conditions refers to the maximum turbidity that can be expected. Figure 1.12 schematically presents the two extremes of turbidity encountered in a water body. In unconsolidated fine-grained sediments, clamshell dredges generate turbidity levels comparable to overflowing hopper dredges. Hopper dredges are able to discharge their load by dumping through hopper doors, using the split hull version (Figure 1.8), or by pumping into a confined shoreline disposal facility.Īs illustrated in Figure 1.11, sediment re-suspension is lowest with cutter suction dredges or suction hopper dredges operated without overflow. The distance to the disposal site is limited only by the economics of vessel turnaround time, not by physical constraints or weather considerations. Modern dredges are equipped with swell compensators on the suction pipe which allow operation even inģ5 knot winds. Typically, these vessels are self- contained and combine the dredging and transport functions.

(Additional discussion on 'overflow' can be found in Chapter 2.0). In normal operation, the hoppers are surcharged until an economic load is obtained (perhaps 50% solids) with excess water and fines being discharged overboard.

These self-contained ships pump the dredged material aboard through a trailing suction pipe (or pipes) equipped with a draghead which loosens the material and directs it into the suction inlet as the vessel moves ahead. The trailing suction hopper dredge is particularly useful for the removal of unconsolidated sand or mud in channels (Figure 1.7).

Small scale dredging habitat project