Life-Cycle Cost Analysis: Step 3 - Estimate Future Damage Severity

HISTORIC APPROACH

The severity of future damage is established by evaluating the damage from the current event and damage from historical records. This approach is best when past damage is similar to current damage.

• When past and present damage is similar, it may be reasonable to project the same damage frequency and repair costs into the future.
• If there is a significant difference in past and present damage, it may not be reasonable to combine or average them together; rather, it may be best to keep them separate and assign different frequencies to the damages when calculating the life cycle costs.

HEC-17 APPROACH

Using the damage caused by an event with a known probability, the damage caused by an array of events (25-year, 50-year, 100-year, and 200-year) is estimated. Transportation facilities should be designed to at least a 10-year event so storms with a recurrence interval of 2-years to 10-years are expected to result in damage that is considered heavy maintenance, which is not ER eligible.

• If the current storm was a 50-year event and the cost to make repairs is $100,000, it is reasonable to forecast that a 100-year event will have more damage and a 25-year event will have less damage.
• If hydro-geomorphologic processes are resulting in channel migration, channel aggradation or degradation, then future damage from less-frequent events may cause significant damage.
• Current damage to bridges with scour critical abutments may be minor; however, damage may be significantly greater after a more severe future event.

Hydraulic Circular -17 (HEC-17) provides information that can be used to extrapolate the damage from an event with a known probability to other events. Starting with the recurrent interval of the current event, the severity of the damage, and depth of flow, a family of hydrographs can be used to estimate scour based on depth and duration of flow.

Write a short narrative, describing the severity of damage expected from likely future eligible events (i.e., 25-year, 50-year, 100-year, and 200-year).

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