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

摘要

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.Plain Language Summary Increased understanding of flood hazards, and how they vary, 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 flood hazards. Specifically, it is assumed that floods recorded in the last 135 years across similar areas referred to as zones can be used to predict what can be expected in the future. The expected flood potential was developed to communicate these predictions, with about half of the experienced floods in each of eleven zones being greater that this line, and the other half being less. The largest floods were identified and ranked using the maximum likely flood potential. Floods greater than this are unlikely but still possible and extreme. In contrast, the expected flood potential provides the flood magnitudes that are generally expected, given the historic record of floods. This method offers complimentary terminology to the commonly used but confusing term 100-year flood. Indices were also developed to compare how large and hazardous floods are in different zones, with floods in some areas being up to 15 times greater than other areas for streams with the same watershed size.