Preliminary Ranking Approach

Preliminary Ranking Approach

Ranking of Uncertainty Sources

Uncertainty analysis is a critical step in improving dredged material decisions,

although it is preceded by several important activities. First, data and information

need to be collected, including historic data from the dredged material permitting

program and information regarding parameter and model inputs required for

estimating impacts. The quality of the collected data must be evaluated to

determine whether the statistical distribution of input parameters can be

described. In some cases, only a range (i.e., minimum and maximum values) of

possible values will be available for use in a quantitative analysis of uncertainty.

The preliminary ranking of uncertainty sources described in this report does

not ignore uncertainty sources that are hard to quantify, but it evaluates them

only qualitatively. If available data do not support a distribution, or even a

minimum and maximum value for a given input, one usually cannot quantify the

contribution of the input to uncertainty in the assessment of possible impacts. For

example, one cannot quantify the uncertainty associated with selecting a subset

of sensitive receptors to represent a larger ecological community. Still,

interpretation of predicted impacts and risks strongly depends (qualitatively, at

least) on this selection, so it should not be ignored simply because one does not

know how to quantify uncertainty.

To estimate the potential for environmental impacts associated with dredged

material disposal, USACE uses scientific judgment and predictive models within

the tiered evaluation procedure and, at times, within a full human health and

ecological risk assessment. As the first step in ranking sources of uncertainty,

data and information are gathered regarding these procedures. Relying on this

information, major categories of uncertainty in the USACE tiered approach to

dredged material evaluation are listed in Table 1. These categories are described

as "aggregated" because they encompass more than one source of uncertainty.

USEPA and others suggest that too much aggregation makes it difficult to

evaluate and compare technical areas (Hattis and Goble 1994; Morgan 1994). For

example, it might be important to separate "bioaccumulation of metals" from

"bioaccumulation of nonpolar organic compounds" because there might be much

greater uncertainty about bioaccumulation of metals. The aggregation in Table 1

is designed to simplify the ranking process without sacrificing important details.

Chapter 2 Preliminary Ranking Approach