Rates and Effects of Sedimentation in the Context of Dredging and Dredged Material Placement

ERDC TN-DOER-E19

March 2005

Rates and Effects of Sedimentation

in the Context of Dredging

and Dredged Material Placement

PURPOSE: Dredging and disposal of dredged material in aquatic environments can expose

animals and plants to episodic pulses of suspended sediment. Resuspended material can be

deposited in thin layers adjacent to the dredging or disposal areas in some cases as much as

several thousand meters distant (LaSalle et al. 1991). While our understanding of the potential

effects of such far-field deposition is limited, some estuarine organisms may be highly sensitive

to suspended sediments and certain life stages (eggs, juveniles) may be particularly affected by

resuspension and deposition. In this report potential impacts of sedimentation (bedded materials)

are reviewed with emphasis on those habitats believed to be most sensitive.

BACKGROUND: Ambient conditions for estuarine organisms are rarely static, and most

organisms are adapted to varying concentrations of suspended sediment (see bibliography in

Kerr (1995)). The intensity and duration of resuspension from dredging and disposal operations

is highly dependent on the type of equipment, operator, character of sediment, and local

hydrodynamic conditions (Collins 1995; Clarke and Wilber 2000). While the effects of elevated

concentrations of total suspended solids (TSS) and thin layers of sediment on estuarine

organisms are poorly understood (Wilber and Clarke 2001, Wilber et al. in preparation), we do

know that some of the defining characteristics of an estuarine environment are highly variable

conditions in the water column for temperature, salinity, and particulate flux.

The direct measurement of TSS is straightforward and can be complemented with indirect

optical and acoustic measurements (optical backscatter sensors, transmissivity, acoustic doppler

profiling) to achieve rapid characterization of large volumes of water over relevant spatial scales

(Lohrman and Huhta 1994, Tubman et al. 1994, Land and Bray 1998, Puckette 1998, Tubman

and Corson 2000, Reine et al. 2002). Understanding of the effects of TSS on salmonids and a

limited number of non-salmonid estuarine fish and shellfish is sufficient to provide quantitative

guidance on acceptable levels of TSS under well-studied circumstances for a few animals.

Although further research is required on dose-response curves of estuarine fish and early life

stages, the technology to conduct these studies is well established (Wilber and Clarke 2001;

Berry et al. 2003); dredging-induced TSS and their associated effects have been studied since the

Army Corps' DMRP initiative in the 1970's. There are also well-established protocols for

bioeffects testing for TSS impacts (U.S. Environmental Protection Agency and U.S. Army Corps

of Engineers 1991, Caux et al. 1997); unfortunately, no such protocols exist for assessing

sedimentation effects.

The measurement and assessment of effects of thick layers of sediment deposition (>1 cm) is

advanced and well within the capabilities of existing technologies (e.g., Sediment Profile

Imagery). It is far more difficult to reliably measure thin layers of sediment deposition from

episodic events, although some promising techniques have been developed (Thomas and Ridd