Twenty-First-Century Tools for Extreme Rainfall and Flood Prediction in Colorado

In 2018 Colorado Dam Safety and New Mexico Dam Safety agencies, along with their meteorological consultants and scientific review board completed a 2-year, $1.65M regional extreme precipitation study (REPS) to estimate probable maximum precipitation (PMP) and extreme precipitation frequency for the two state study region. At the same time Colorado Dam Safety co-sponsored a research study with Colorado State University (CSU) to examine controlling extreme flood runoff-production mechanisms and associated modeling techniques for more accurate flood modeling in Rocky Mountain drainage basins. Finally, as part of the REPS study, NOAA and the Western Water Assessment authored a report with recommendations on how climate change could be incorporated into spillway design criteria for dam safety. These studies combined to provide new insight into safe dam design in the region. The REPS PMP study found that local convective thunderstorm PMP may be larger in magnitude than previous supposed in NOAA’s Hydrometeorological Reports (HMRs), and conversely synoptic-scale General Storm PMP may be smaller in magnitude than estimated in the HMRs. CSU’s mountain hydrology research study found that different runoff-production mechanisms can control during different types of extreme storms (ex. thunderstorms vs. synoptic-scale storms) and for different basins. Although REPS General Storm PMP may be smaller than HMR General Storm PMP, CSU found REPS General Storm PMP produces the worst case flood hydrograph when the saturation-excess runoff production mechanism is correctly modeled. Further, REPS extreme precipitation frequency provides a likelihood check on PMP and suggests PMP estimates for the Rocky Mountains are extremely rare in terms of annual exceedance probability (AEP<1e-7). The CSU mountain hydrology study found the controlling runoff mechanism can change with storm AEP. Finally, flooding in the Rocky Mountains needs to be considered within the context of climate change. Predicted increases in moisture availability and decreases in freezing levels have the potential to change the nature and magnitude of rainfall and thus flood production. Colorado Dam Safety now has tools to make informed regulatory decisions to address these concerns for safe design of dams and spillways.