Methods for Assessing Expected Flood Potential and Variability: Southern Rocky Mountains Region

Abstract

Enhanced understanding of flood hazards, and how they vary across regions and continents, is needed to help protect lives and develop more resilient communities. Using the greater Southern Rocky Mountains region as a study area, a novel methodology was developed to predict, rank, and communicate expected flood magnitudes across similar responding areas (zones). Using 463 streamgages, up to 93% of the variance was explained by regression models developed for 11 derived zones. These regressions define the expected flood potential of each zone, a term introduced to assist practitioners, policy makers, and the public in understanding what flood magnitudes can be expected given the maximum recorded streamgage floods in nearby watersheds. Discharges above the 90% prediction limit, the maximum likely flood potential, are considered extreme; departure above this limit denotes the degree of extremity. The seasonality of the largest 5% floods varied substantially between zones, with the greatest frequency in July, August, and September in some zones (due to the North American monsoon) and May and June in other zones (due to snowmelt and rainfall). Using the lowest flood potential zone as an index area, flood potential and hazard indices were developed for comparing flood hazards across broad regions. The largest floods occur in the southern portion of the eastern slopes of the Southern Rocky Mountains and the adjacent Great Plains, with these events being 15 times larger than floods experienced in central Colorado and New Mexico mountain valleys, on average for a given watershed area.