SCENARIO IMPLEMENTATION

ERDC TN-DOER-R2

December 2001

(far-field) models to estimate suspended sediment concentrations at specified water column loca-

tions. It then uses a linear equilibrium partitioning model to convert initial contaminant concentra-

tions on in situ sediment and downstream suspended sediment concentrations to downstream water

column particulate and dissolved contaminant concentrations. All calculations made by DREDGE

assume steady-state time-invariant conditions. DREDGE predicts the short-term contaminant

concentration distribution in the water column for determination of the acute effects from exposure

to dredging and the spatial extent of the acute effects.

DREDGE uses empirical formulations developed from field studies to estimate the rate of sediment

resuspension that results from a dredging operation (near-field source strength). DREDGE allows

user-selected source strength values to be entered for any dredge type. Additionally, correlation

models for source strengths are available only for cutterhead and bucket dredges. A number of

limitations are associated with the models used in DREDGE. The sediment resuspension models

are applicable only to dredging operations similar to those used in the development of the empirical

equations. The models generally produce reasonable estimates for normal operating characteristics,

but unusual operating parameters may yield unreasonable results.

The far-field transport models used assume a dominant, unidirectional current that exists sufficiently

long for suspended sediment concentrations to reach steady state, assuming a steady source from a

specific location and settling by Stokes' law. Although the dredge is moving continuously, the

movement is usually slow compared to transport in the water column. Transport models solved

analytically for plume geometries characteristic of cutterhead and bucket dredges are used to

estimate downstream (far-field) transport of suspended sediments under steady-state conditions.

Considerable simplifications are necessary to solve the fundamental transport equation analytically.

While these simplifications limit the applicability of the resulting models, the analytical solutions

allow for rapid calculation of suspended sediment concentrations with an accuracy compatible with

the source strength models.

ERED. The USACE/USEPA ERED is a compilation of data taken from the literature where

biological effects (e.g., reduced survival, growth, etc.) and tissue contaminant concentrations were

simultaneously measured in the organism. Currently, the Web-based database is limited to those

instances where biological effects observed in an organism are linked to a specific contaminant

within its tissues (Bridges and Lutz 1999).

Currently, the system contains data from 736 studies published between 1964 and 2000. From these

studies 3,463 distinct observations have been included online. The ERED includes data on 222

contaminants, 188 species, 13 effect classes, and 126 end points. Updates to the central database

will occur periodically as new data sources and citations are discovered. Most papers involving

mixtures of contaminants were excluded from the database because these effects could not be linked

to a specific contaminant.

SCENARIO IMPLEMENTATION: The development of template dredging scenarios for the

evaluation of both dredging and no-action alternatives in ARAMS provides the user a starting point

for conducting a risk assessment. Implementation templates for these scenarios use existing

dredging models to characterize exposure for the evaluation of potential human health and

ecological risk. The exposure characterization in the scenarios is analogous to that currently