DREDGING-RELATED SUSPENDED-SEDIMENT PLUMES

ERDC TN-DOER-E9

May 2000

lethal concentrations for adult bivalves exposed for as long as 3 weeks were in the realm of fluid

mud, i.e., around 10,000 mg/L. Data concerning responses to longer term exposures were not

available.

Crustaceans: The suspended-sediment tolerances of crustaceans have not been the focus of many

laboratory studies (Table 2). Most studies recorded the concentration of suspended sediments

required to induce mortality over durations of days to weeks. In experiments that lasted up to

2 weeks, nearly all mortality was caused by concentrations of suspended sediments that exceeded

10,000 mg/L. The majority of these mortality levels were less than 25 percent, even at very high

concentrations. None of the crustaceans tested exhibited negative responses at dosages within the

realm of suspended-sediment conditions associated with dredging projects.

Turbid estuarine waters may provide refuge from predation for juvenile shrimp, which are heavily

preyed upon by estuarine fishes. Brown (Peneaus aztecus) and white (P. setiferus) shrimp are

common over a wide range of turbidities, whereas pink shrimp (P. duorarum) are most abundant

in areas with low turbidity levels. Juvenile penaeid shrimp burrow in sandy sediments during the

day and emerge at night to forage. In experimental aquaria, predation by Atlantic croaker and pinfish

on juvenile shrimp was reduced in turbid water, whereas the predation efficiency of the southern

flounder (Paralichthys lethostigma), an ambush predator, was increased (Minello, Zimmerman, and

Martinez 1987).

DREDGING-RELATED SUSPENDED-SEDIMENT PLUMES

Spatial Scales of Exposure. Spatio-temporal dynamics and concentration gradients within

suspended-sediment plumes are dependent on numerous factors, including specific dredge plant, in

situ sediment characteristics, and environmental conditions at the time of dredging (Havis 1988;

McLellan et al. 1989; Herbich and Brahme 1991; Collins 1995). While predictions of suspended-

sediment conditions for dredging projects in locations other than where field data have been

collected yield variable results (Johnson and Parchure 1999), the relative impacts of different types

of dredges and the concentrations of suspended sediments associated with each can be generalized.

For instance, mechanical dredges (e.g., bucket or clamshell) are associated with higher suspended-

sediment concentrations than hydraulic (hopper and cutterhead) methods. Mechanical dredges

generate suspended-sediments through the impact of the bucket on the bottom and withdrawal from

the bottom, washing of material out of the bucket as it moves through the water column and above

the water surface, and additional loss when the barge is loaded (LaSalle 1990). Mechanical dredges

are commonly used for small projects near docks and piers or where rocky deposits are present

(Morton 1977) and can be used more easily at greater depths than hydraulic methods. A suspended-

sediment plume associated with clamshell dredging at its maximum concentration (1,100 mg/L)

may extend up to 1,000 m on the bottom (Havis 1988; LaSalle 1990; Collins 1995).

Temporal Scales of Exposure. Although many factors such as weather and hydrodynamic

conditions, sediment type, and dredge operator skill interact to affect the progress of a dredging

operation, the average advance rate of a given dredge can be estimated based on existing information.

For example, an average advance rate of 12-18 m/hr for clamshell dredging projects in the Mobile