Frequency of erosion studies

comprehensive evaluation of the potential mound stability from a single

storm. A more comprehensive approach, however, is to evaluate the long-

term physical stability by computing the frequency of occurrence of ero-

sion over much longer periods. This procedure is described in the

following section.

Frequency of erosion studies

While it is desirable to site capping projects in low-energy areas with

little or no potential for erosion, these sites are not always available. At

higher energy sites, the potential for erosion has to be estimated and taken

into account when designing the cap. Stated simply, an additional layer is

added to the overall cap thickness to account for expected erosion over a

finite time period. Knowledge of the frequency of occurrence of vertical

erosion (i.e., how often a given amount of vertical erosion will occur) is a

critical component of a probabilistic cap design. Too thin an erosion layer

may compromise the cap, potentially allowing the contaminants to be dis-

persed over the site and surrounding area. Conversely, too thick cap will

have an unnecessarily high cost and also reduce the capacity of the site to

contain additional dredged material. This section describes a rational

method to determine the erosion layer thickness for sites where erosion is

expected to be a problem. A detailed explanation of the frequency of ero-

sion procedure and background information is provided in Appendix G.

The amount of expected erosion will be a function of the depth of the

capped mound, mound geometry, the material used for the cap, and envi-

ronmental forcing functions at the site, waves and currents, and their dura-

tion. The designer/project manager can influence the depth of the capped

mound and the type of cap material. Therefore, most frequency of erosion

studies of capped mounds require an investigation of a range of mound ele-

vations (and thus water depths) and several different types of cap material,

e.g., sand of various grain sizes and typical fine-grained (silt and clay)

maintenance material.

Among existing procedures for computing frequency of erosion due to

tropical and extratropical storms (e.g., worst case "design storms" or the

joint probability method(JPM)), the empirical simulation technique (EST)

is the best. EST is a statistical procedure for simulating nondeterministic

multiparameter systems such as tropical and extratropical storms. The

EST, which is an extension of the "bootstrap" statistical procedure (Efron

1982; Efron 1990), overcomes the JPM limitations by automatically incor-

porating the joint probability of the historical record. The bootstrap

method on which EST is based incorporates resampling with replacement,

interpolation based on a random walk nearest neighbor techniques with

subsequent smoothing. More detailed descriptions of EST can be found in

Scheffner, Borgman, and Mark (1993) and Borgman et al. (1992).

In EST, the various geometric and intensity parameters from storms are

used to create a large artificial population (several centuries) of future

storm activity (Borgman et al. 1992). The only assumption required for

EST is that future storms will be statistically similar to past storms. Thus,

the future storms generated during EST simulations resemble the past

Chapter 8 Long-Term Cap Stability