Planning Considerations for Nearshore Placement of Mixed Dredged Sediments

Technical Note DOER-N3

March 1998

Planning Considerations for Nearshore

Placement of Mixed Dredged Sediments

PURPOSE: Dredging planning and management decisions are based on a combination of

engineering and economic factors tempered by environmental considerations. The purpose of this

technical note is to address primary considerations (although not all-inclusive) for planning and

managing nearshore placement of mixed sediment from dredging projects.

BACKGROUND: As part of its ongoing navigation channel maintenance mission, the U.S. Army

Corps of Engineers (USACE) dredges large amounts of noncontaminated sand/silt mixtures, which

are then available for beneficial nearshore placement. Often this material is excluded from

nearshore placement consideration because of unknowns related to regulatory concerns about the

ultimate fate of fines: will the fines move onto the beach; is water quality negatively impacted; and

what, if any, impact is there to environmental and economic resources like fisheries and oyster

beds?

Typically, USACE alternatives for disposing of mixed dredged sediments are limited by State and

local regulatory requirements that restrict the percentage of fines (usually less than 20 per- cent) allowed

to be placed directly on the beach. Beach placement of material containing large quantities of fines

is undesirable because of the environmental impact that the smaller grain size (may cause the beach

face to harden) and/or darker color (increases beach temperature, thus affecting sea turtle nesting

and/or aesthetic appeal) may have on the beach and beach habitat.

Little is known about the impact of placing sediments with larger relative percentages of fines in

the nearshore. Because knowledge of the behavior of mixed sediments placed in the nearshore is

limited, USACE has as its only alternatives offshore disposal or upland placement for mixed

dredged sediments. This activity not only keeps much-needed sand resources from the local littoral

zone, but it can also be more expensive and time-consuming than nearby placement (i.e., nearshore).

Research on nearshore placement of mixed sediments is currently being conducted under the

Dredging Operations and Environmental Research (DOER) Program at the U.S. Army Engineer

Waterways Experiment Station (WES). One of the DOER focus areas, Dredged Material Nearshore

and Offshore Placement, is investigating how fine- and coarse-grained sediments behave together

and independently when placed in the nearshore as a mixed sediment. Results of this research will

lead to tools that will assist planning, engineering, and operations personnel in (a) predicting the

ability of a mixed-sediment placement to enhance the nearshore region, (b) assessing physical and

environmental benefits/impacts, and (c) justifying USACE projects, promoting cost-sharing, and

addressing regulatory agency concerns.

NEARSHORE PLACEMENT PURPOSE: The primary purpose for nearshore placement of

dredged material is to reduce overall navigation maintenance costs. Navigation channel