COMPARISON OF ALASKAN FLOOD STAGES: RECURRENCE INTERVAL VS. IMPACT BASED
Possible correlations between AEP and flood stage were examined for all gages with available recurrence interval data, rating curves not older than 3 years, and flood stages based on observed impacts. Potential outliers were analyzed to determine if larger differences in these groups may have been influenced by environmental variables such as ice effect, mean annual discharge, population, or participation in the National Flood Insurance Program, although no relationship was found.
Analysis showed some discrepancies in the traditional pairing of recurrence intervals and flood stages, especially on the upper end, a trend that may be linked to the rarity of these events and limited scope of gage data in Alaska. Major flood stage appears to be most similar to the .2-1% AEP (100-500 year RI) while moderate flood stage best fits the 4% AEO (25 year interval). While there does appear to be a strong relationship between the two parameters, there are some unexplained outliers and this method should be used cautiously when establishing flood stages based on AEP.