Documentation from two previous events should be available to establish an on-going susceptibility of future damage. This approach generally assumes that future events and the cost to repair resulting damage will be similar to what has occurred in the past. If detailed records are unavailable, news reports and other sources may be used. Only events and damage that would qualify for ER funding should be used to establish the frequency and severity of future events.
Determine the event frequency (annual probability) of the current event (e.g., 50-year event) and preferably one previous event.
| Recurrence Interval in years | Probability of Occurrence in any given year | Percent Chance of Occurrence in any given year | Annual Exceedance Probability |
|---|---|---|---|
| 5 | 1 in 5 | 20% | 0.2 |
| 25 | 1 in 25 | 4% | 0.04 |
| 50 | 1 in 50 | 2% | 0.02 |
| 100 | 1 in 100 | 1% | 0.01 |
The majority of events are typically flood events and the event frequency should be determined from a hydraulic analysis. It should be noted that rainfall events do not necessarily correlate well with flood or runoff events due to factors such as storm duration, time of concentration, antecedent ground moisture or reservoir impoundment.
