Identifying Runoff Production Mechanisms for Dam Safety Applications in the Colorado Front Range

Hydrologic analyses are used in dam safety evaluations to determine the flow a dam must pass without failing. Many current guidelines model flood runoff solely by an infiltration-excess mechanism. Saturation-excess runoff and subsurface stormflow mechanisms are known to be important for common events in forested regions, but few studies have analyzed their role in extreme events. The objectives of this study are to determine the active runoff mechanisms for large historical storms, design storms in the Colorado Front Range, and propose methods to model these mechanisms that dam safety consultants can use. Hydrologic models are developed for five basins to simulate historical flood events in 1976 and 2013 as well as various design storms. The model results (and available in-situ soil moisture observations) show that the entire soil layer approached saturation during the 2013 storm, which had a long duration and low rainfall intensities. Thus, saturation-excess runoff was likely the dominant mechanism. In contrast, the modeled soil layer does not approach saturation for the 1976 storm, which had a short duration and high rainfall intensities, so infiltration-excess runoff was likely the dominant mechanism. Similarly, infiltration-excess runoff dominates for short duration (2-h) design storms, while saturation-excess dominates for longer (6-h and above) design storms in the Front Range basins. (c) 2020 American Society of Civil Engineers.