Example 2

ERDC TN-DOER-C27

July 2002

Example 2

The second example assumes a 30-year period of analysis and available CDF storage capacity of

1,800,000 yd3 at the inception of the period of analysis. Capacity of the CDF constructed in the out

years of the project is assumed to accommodate the material produced from the time of filling of

the old CDF to the end of the period of analysis. Rate of capacity consumption is based on solids

volume at the end of each disposal operation, neglecting consolidation. Capping material volume

is assumed to equal freeboard and ponding volume. (Neglecting material consolidation results in

a conservative overestimate of storage capacity required and an underestimate of capping material

required. Programs are available to estimate time-dependent consolidation of dredged material and

fill, but these simplifying assumptions facilitate planning level cost comparisons. A more rigorous

evaluation of consolidation and effect on the CDF life cycle would be required for detailed cost

estimating.) In situ sediment volume for the construction phase was assumed to be 775,000 yd3

with 40 percent fines by weight. Maintenance dredging was assumed to take place every 3 years,

with an in situ volume of 75,000 yd3. A 16-in. (0.4-m) hydraulic dredge is assumed for hydraulic

dredging scenarios, and calculation of sediment storage requirements using SETTLE. A bulking

factor of 1.1 (applied to in situ volume) was used to estimate sediment storage requirements for

mechanical dredging alternatives. An annual cost to maintain the CDF (facility maintenance) of

$20,000 was assumed for all alternatives. The following alternatives were considered:

1. Hydraulic dredging, CDF disposal (Tables C14-C16).

2. Hydraulic dredging, sand separation, CDF disposal of fine residuals (Tables C17-C19).

3. Hydraulic dredging, sand separation, CDF disposal of dewatered fine residuals

(Tables C20-C22).

As for the first example, no cost was assumed to be incurred for disposal or transportation of sand

produced for beneficial use. No real estate acquisition costs are considered.

Comparison of average annual costs for the three alternatives reveals that Alternative 1 (Table C23)

has the lowest average annual cost ($1,785,100), with the CDF being replaced after two maintenance

dredging cycles. Alternative 2 (Table C24) is next at $1,898,300, with the CDF being replaced after

14 maintenance dredging cycles (beyond the end of the period of analysis). Alternative 3

(Table C25) has the highest annual cost ($2,776,000), but the CDF does not need to be replaced for

49 maintenance dredging cycles (beyond the end of the period of analysis). The base alternative

may therefore be different (Alternative 2 or 3, rather than Alternative 1) for a longer period of

analysis that captures the life extension of the CDF (Table C26). As the period of analysis increases,

however, the effect of consolidation on rate of capacity consumption becomes more important and

should be factored into the analysis.