COMPARISON OF ALASKAN FLOOD STAGES: RECURRENCE INTERVAL VS. IMPACT BASED

The Alaska River Forecasting Center issues water level forecasts that are used in conjunction with established flood stages to provide flood warning and advisory information to the public. Flood stages are typically established based on observed impacts; however, where this is not possible due to data sparsity, flood stages are determined according to calculated discharge at annual exceedance probabilities (AEP) (Curran et al, 2016). For forecasting purposes, bankfull stage has corresponded to the 50% AEP (2 year Recurrence Interval), minor to 10-20% AEP (15-40 year RI), moderate to 2.5- ̴7% AEP (25-50 year RI), and major to 1-2% AEP (50-100 year RI). The objective of this project was to validate the relationship between flood stages and discharge exceedance probability and to provide recommendations for improvement.

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.