Missouri River Flow Frequency Study: Hec-Wat Monte Carlo Analyses

The purpose of the Missouri River Flow Frequency Study was to update both unregulated and regulated flow frequency at ten locations along the Missouri River between Gavins Point Dam near Yankton, SD to Hermann, MO. Unregulated flow frequency analyses were conducted according to the methodology advanced in USGS Guidelines for Determining Flood Flow Frequency - Bulletin 17C published in May 2019. An estimate of regulated flow frequency was completed using a Monte Carlo approach that simulated 250,000 events split among 5,000, 50-year continuous simulations. A 500-year synthetic period-of-record was developed by combining flows from four different regions of the Missouri River Basin and three seasons. The 90-year observed record was also included so a total of 590-years were used as the “historic” record in the analysis. The 590-year historic record was also supplemented with scaled floods to help define the regulated flow frequency curves for events less frequent than the 1% AEP event. For each group of 2,500 events, a 90-year period-of-record was sampled from all available events to estimate knowledge uncertainty. A weighted adjustment was completed at each gage, which represented an adjustment to the Monte Carlo sampling. This allowed the unregulated flow frequencies to match the unregulated flow frequencies produced by the Bulletin 17C analyses at each gage. The weights corresponded to the event and not the plotting position to ensure the regulated flow was adjusted with the same weight as its corresponding unregulated flow. Compared to the traditional transform method, the Monte Carlo approach produced better estimates of regulated flow frequency by capturing thousands of combinations of reservoir pool elevations, inflows, forecasts, and operations.