Validating Derived Flood Stages from Streamflow Recurrence Interval

This project is designed to help the Alaska-Pacific River Forecast Center (APRFC) identify which hydrological parameters (precipitation, precipitation recurrence interval (RI), discharge RI, and gauge height) are the best indicators of flood occurrence and severity in Seward, Alaska. Seward is a mountainous city on the Kenai Peninsula. Seward has had over 26 state-declared flood events since 1981. The causes of the floods are mostly due to high rainfall events and gravel. Gravel buildup in rivers restricts flow, raises the water level, and causes a flood. Preventative measures could be taken by the Seward/Bear Creek Flood Service Area and the National Weather Service before damaging life or property. Raw data was analyzed to find a relationship between high precipitation, discharge, and gauge height events and declared flood events. The False Alarm Ratio (FAR) was calculated as a measure of reliability. A FAR ≤ 0.3 is an acceptable threshold for this project. The results were discharge RI and precipitation RI were more successful at predicting a flood, with 2/3 Discharge RI gauges having a FAR ≤ 0.3 and 3/5 Precipitation RI gauges having a FAR ≤ 0.3. Gauge height and precipitation gauges did not successfully predict historical flood events. Due to Seward’s unique flooding patterns, no hydrological parameters were strongly correlated with observed floods. Therefore, National Weather Service hydrologists will need to study other indicators for flood severity awareness in the Seward area.