Assessment of Potential Impacts of Dredging Operations Due to Sediment Resuspension

ERDC TN-DOER-E9

May 2000

Assessment of Potential Impacts

of Dredging Operations Due

to Sediment Resuspension

PURPOSE: This technical note summarizes the known biological responses of estuarine and

coastal fish and shellfish to suspended sediments and relates these findings to suspended-sediment

conditions associated with dredging projects. An objective approach toward evaluation of sediment

resuspension impacts is proposed that requires full consideration of (a) the existing state of

knowledge concerning the effects of suspended sediments on fish and shellfish, including recogni-

tion of the gaps therein, and (b) concentration-exposure duration combinations likely to be

encountered by organisms in the vicinity of dredging operations. This note focuses upon the

exposure of organisms to sediments in the water column. Effects of deposited sediments will be

treated separately.

INTRODUCTION: Estuarine and coastal organisms are exposed to suspended sediments from

storms, tidal flows, and currents and therefore have behavioral and physiological mechanisms for

dealing with this feature of their habitat. Because dredging-related suspended-sediment plumes

may differ in scope, timing, duration, and intensity from natural conditions, dredging may create

conditions not typically experienced by resident or transient species. Assessing how dredging-

related suspended-sediment conditions may affect organisms requires knowledge of (a) the thresh-

olds at which relevant life history stages of organisms respond negatively to suspended

sediments, (b) reliable estimates of dredging-induced suspended-sediment plume temporal and

spatial dynamics, and (c) the probability that organisms encountering a dredging-related suspended-

sediment plume will exceed a concentration and/or exposure duration tolerance threshold.

The literature concerning suspended-sediment effects on aquatic fauna has been periodically

reviewed (e.g., Morton 1977; Moore 1978; Priest 1981; Kerr 1995). However, until recently, there

has been little attempt to assess environmental impacts with models that can be easily used by

resource managers (Newcombe and MacDonald 1991). Simplified models have been developed to

predict suspended-sediment impacts on freshwater fishes (Newcombe and MacDonald 1991;

Gregory, Servizi, and Martens 1993; MacDonald and Newcombe 1993) and were expanded to

include estuarine fishes (Newcombe and Jensen 1996). Historically, suspended-sediment impacts

were considered to be a function of concentration; i.e., most experimental studies used concentration

alone as the variable of interest, and exposure durations were not varied and in many cases not

reported (Sherk and Cronin 1970; LaSalle et al. 1991). Regulatory controls on suspended-sediment

impacts also focused on concentration thresholds. More recently the duration of exposure to

suspended sediments has been emphasized as another important factor. Concentration alone is

poorly correlated with the responses of salmonid fish to suspended sediments, whereas dosage

(measured as mg h l-1) is more strongly associated with fish responses (Newcombe and MacDonald

1991).

Newcombe and Jensen (1996) recognized the utility of "look-up tables" that can be used in the field

or as an easy reference to predict suspended-sediment impacts on biological resources. This