Predict water column mixing and dispersion effects of contaminated sediment during placement (8)

consideration, and cost. The primary concern with compatibility relates to

geotechnical considerations and the ability of the contaminated material to

support the cap, considering the material characteristics and dredging and

placement techniques.

Guidance on the compatibility of various dredging and placement tech-

niques for differing material types has been developed based on field expe-

rience and knowledge of the resulting dispersion and spreading behavior

and physical stability of the materials. If the various site, sediment, and

selected equipment components are compatible, additional and more de-

tailed design requirements can be addressed. If there is a lack of compati-

bility at this point, a different capping site (3), a different capping

sediment (4), or different dredging and placement equipment and tech-

niques (5,6) must be considered. A close examination of the project de-

sign components at this decision point is essential before performing the

more detailed and costly evaluations that come later in the design process.

Guidance on evaluation of sediment, site, and equipment compatibility is

found in Chapter 5.

Predict water column mixing and dispersion effects of

contaminated sediment during placement (8)

If water column effects during placement of the contaminated material

are of concern, an evaluation of the suitability of the material from the

standpoint of water column effects must be performed. This evaluation

involves the comparison of predicted water column contaminant concentra-

tions with water quality criteria and predicted water column dredged

material concentrations with bioassay test results. Use of available mathe-

matical models and/or case study field-monitoring results to predict the

water column dispersion and concentrations is an integral part of such

evaluations. In addition, the prediction indicates what portion of the con-

taminated material is released during placement and thus is not capped.

Evaluation of initial deposition and spread of material is used in determin-

ing the mounding characteristics for the entire contaminated material

volume to be placed. If water column release is unacceptable, control

measures need to be considered to reduce the potential for water column

effects, or other dredging equipment and placement techniques (5) or use

of another capping site (3) must be considered. Guidance on prediction

of water column effects during placement is found in Chapter 6 and

Appendix D.

Determine cap design (9)

The cap must be designed to adequately isolate the contaminated mate-

rial from the aquatic environment and achieve the intended cap functions.

The composition and dimensions (thickness) of the components of a cap

can be referred to as the cap design. The composition of caps for dredged

material projects is typically a single layer of clean sediments because

relatively large volumes of cap material are involved; clean sediments

from other dredging projects are often available as cap materials; and

dredged material capping sites with low potential for erosion can be

Chapter 2 Design/Management Sequence for Capping