Requirements and Limitations of Predictive Models

ERDC TN-DOER-E19

March 2005

likely to be confounded by natural resuspension and settlement events, but might detect more

subtle impacts if paired with adequate controls. Longer-term experiments might require an entire

recruitment to grow out with a cycle of months to years.

Appropriate in situ volume scales for measuring impact of sedimentation on

fish/shellfish/SAV. Evaluate the initial mixing zone and extent of any density flows adjacent to

the dredging or disposal event(s). Spatial extent of measurable sedimentation could range from

200-1,000 m away from source, but strongest effects will occur less than 300 m from the source.

Requirements and Limitations of Predictive Models

Resuspension or sedimentation input data needed for validation of models. The

characteristics of resuspension are explicitly provided to all currently operational models (both

the loss rates as well as the production rates are input variables), but there are no models of

resuspension per se unless one considers empirical algorithms as models. However, additional

field data providing a direct measure of suspended material concentrations at various points over

the vertical dimension and at various distances downstream of the operating dredge are always of

value. The increasing use of acoustic techniques should improve three-dimensional (3-D)

understanding of plume structure; if the detail over the vertical is considered, an agreement on

the character of the plume resulting from both hydraulic and mechanical dredging operations

may finally be reached. The achievement of this goal requires close collaboration between

scientists/engineers and dredge operators.

The variety of models detailing sedimentation of materials placed in suspension by dredging

would benefit from direct measures of the resulting deposit characteristics (3-D measurements)

following completion of dredging. This might be best realized by high-resolution measurements

of short-lived radionuclides (e.g., 7Be) from diver-placed cores obtained before and after

dredging at selected points along and across mapped plume trajectory. In some cases, these

measurements could be complemented by sediment profile camera data. These latter

observations might only work if placement of the camera was very accurately controlled (staked

locations, diver deployed) and the thickness of deposition was in excess of ~0.5 cm. The latter

criteria suggest that measurements would be best used in the initial mixing zone the area

characterized by an exponential decrease in suspended material concentration with distance.

Limiting factors in existing models. All models are less than perfect in handling settling

velocities. Because gravitational settling is the primary process driving the resuspended materials

to the bottom, this parameter must be accurately defined. Most of the models struggle with this to

a greater or lesser degree. Work is in progress but more needs to be done.

All of the existing models considered (TASS, SSFATE, STFATE, and DREDGE) are

fundamentally advection/diffusion formulations dealing with the dispersion of sediments

emanating from a source. The sediments are carried by the local flows and settle to the bed. Once

on the bottom they stay in place. There is no consideration of subsequent resuspension and

transport and/or mixing with ambient sediments. This is a major deficiency. Freshly deposited

sediments are subject to nearly immediate resuspension and mixing with the ambient suspended

material field. Such mixing has the potential to significantly reduce the effect of the newly