Construction and Monitoring of a Mixed-Sediment Mound Offshore of Mobile Bay, Alabama

ERDC TN-DOER-N6

March 2000

Construction and Monitoring

of a Mixed-Sediment Mound

Offshore of Mobile Bay, Alabama

PURPOSE: This technical note describes the mixed-sediment dredged material mound offshore

of the entrance to Mobile Bay, Alabama, and presents the results of monitoring.

BACKGROUND: In nearshore environments, the only dredged material usually allowed for

placement is clean sand (with a small percentage of fines) because of concerns that a mixture of

sands and a substantial amount of fine sediments (silts and clays) will create excessive turbidity.

The turbidity may in turn affect the use of the area by marine organisms for feeding or spawning,

and may directly impair the growth of such organisms as sea grasses or corals. The fine materials

might also find their way onto beaches, lessening their appeal to the public. Placing mixed

sediments nearshore could be beneficial, though, in that it is a way of replenishing often scarce

supplies of nearshore sand and may in fact create habitat. Nearshore placement can also lower

the cost of dredging if the transport distance is shorter than to other placement areas.

The actual turbidity that might be generated from a nearshore mixed-sediment placement area and

its possible adverse environmental effects are unknown. The ability to simulate and predict

mixed-sediment erosion and transport numerically would be helpful for this problem in that a variety

of climatic scenarios could be studied and the risks and benefits of a given nearshore placement

evaluated. A good description of the processes of erosion and transport of mixed sediments is

necessary to develop a model for simulations. To add to the existing understanding of mixed-sedi-

ment transport processes, the U.S. Army Engineer Research and Development Center (ERDC) and

the U.S. Army Engineer District, Mobile, are studying the fate of a mixed-sediment mound placed

on the Mobile Bay entrance ebb shoal (Figure 1). Field data are being collected to advance

understanding of the geotechnical properties of dredged material before, during, and after dredging,

and to provide a quality data set for verification of numerical models under development by the

Corps of Engineers.

The Corps of Engineers numerical models for predicting dredged material fate include the Short

Term Fate (STFATE) (Johnson 1990), Multiple Dump Fate (MDFATE) (Moritz 1994), and Long

Term Fate (LTFATE) (Scheffner et al. 1995) models. These models can be used to predict the fate

of dredged material (i.e., where the material moves) during placement and over a given period (up

to years) after placement. This study is aimed primarily at providing data for improving the LTFATE

model, and to a lesser degree, MDFATE. The LTFATE model is reasonably well verified for sandy

placement projects, but it has almost no verification for mixed-sediment materials, or more

specifically, cohesive materials. The data set collected through this study provides a unique

opportunity to advance understanding of mixed-sediment processes with the ultimate goal being

the ability to simulate the fate of nearshore placements more accurately.

PROJECT: Beginning in late October 1998, 267,750 cu m (350,000 cu yd) of cohesive dredged

material from the Mobile River in the upper reaches of Mobile Bay near St. Louis Point were placed