Summary of Design Sequence for Capping

Summary of Design Sequence for Capping

The flowchart shown in Figure 2 illustrates the major design require-

ments for a capping project and the sequence in which the design require-

ments should be considered. There is a strong interdependence between

all components of design for a capping project. For example, the initial

consideration of a capping site and placement techniques for both the con-

taminated and capping materials strongly influence all subsequent evalu-

ations, and these initial choices must also be compatible for a successful

project (Shields and Montgomery 1984). Each step in the process must be

clearly identified and documented before a decision can be made to proceed.

When an efficient sequence of activities for the design of a capping

project is followed, unnecessary data collection and evaluations can be

avoided. General descriptions of the various design requirements are

given below corresponding to the recommended design sequence (Palermo

1991a). Each block in the flowchart (Figure 2) is numbered, and a descrip-

tion of each block is referenced by the number in parentheses in this chapter.

More detailed guidance on various aspects of the design is provided in

Chapters 3 through 9 and Appendixes B through I of this report. Chapter 10

describes capping case studies and field experience for major capping projects

under a range of project conditions. Chapter 11 summarizes the guidance

provided in this document.

Gather project data and select design criteria (1)

The first step in any capping project design is to gather and evaluate

the existing project data, which normally include surveys of the dredging

area, physical and chemical characteristics of the contaminated sediment,

equipment used for dredging and placement, and characteristics of potential

placement sites (i.e., area erosion trends, wind-wave resuspension, wave-

current interaction effects). Since capping is under consideration, data on

the suitability of the material to be dredged for open-water placement may

exist. These data may include results of physical, chemical, and biological

tests required under Section 404 of the CWA or Section 103 of the MPRSA.

Data on potential placement sites may vary. Bathymetry, currents, storm

frequencies, wave heights, and bottom-sediment characterization are nor-

mally available for open-water sites under consideration.

Once the existing data have been gathered, the design functions of the

cap can be determined and design criteria selected. Specific design crite-

ria will depend on the selected design functions for the cap, i.e., physical

isolation, stabilization, or reduction of contaminant flux. Design criteria

may be developed in a number of ways: providing cap thickness for isola-

tion of benthic organisms to a given bioturbation depth; reducing contami-

nant flux rates to achieve specific sediment, pore water, or water column

target concentrations; specific storm or flood flow return periods for cap

stability; limits on mound elevation to meet navigation or erosion constraints;

placement of all material within given site boundaries, etc. Such criteria

should be defined prior to starting design of the capping project. Three

main aspects of capping design must be examined: aspects related to

Chapter 2 Design/Management Sequence for Capping