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7 When it Rains Does it Pour? Design Precipitation Depths for Dam Safety Introduction If a dam and its spillway are not sized appropriately to pass the required inflow, a precipitation event can lead to dam overtopping and failure. Selecting the design precipitation is the first step in the hydrologic analysis used to size the dam and spillway. The design precipitation is typically based on either a selected precipitation frequency (i.e. 100-year event) or Probable Maximum Precipitation (PMP) event. This article looks at the references available for estimating the design precipitation for small dams in Colorado, Montana, Utah, and Wyoming. The recent extreme precipitation event in Colorado is also examined in relationship to frequency estimates and discussed in the context of dam safety. Colorado’s 2013 Precipitation Event The September 9-16, 2013, precipitation event was caused by a slow-moving cold front stalled over Colorado, clashing with warm humid monsoonal air from the south. The precipitation resulted in catastrophic flooding along Colorado’s Front Range from Colorado Springs, north to Fort Collins. Numerous low hazard dams that were designed to withstand a 100-year precipitation event overtopped, with nine earthen dams breaching. According to the Colorado Division of Water Resources, the high hazard dams within the affected area performed well, with many conveying spillway flows for the first time since they were built. The Hydrometeorological Design Studies Center (HDSC) developed maps for the September event showing the annual exceedance probabilities of the worst case precipitation in relation to published frequency data presented in National Oceanic and Atmospheric Administration (NOAA) Atlas 14. Figure 1 shows the map for the full seven day storm duration. Maps for 24-hour and 48-hour durations are also available. Figure 1: Worst Case 7-day Rainfall Annual Exceedance Probabilities As shown in Figure 1, exceedance probabilities were estimated to be greater than the 0.1% (1/1000) for areas including Estes Park, Boulder, and Aurora. Figure 2: Maximum observed rainfall amounts in relationship to NOAA 14 estimates Figure 2 shows the observed rainfall amounts for the Justice Center rain gauge located in Boulder, in relationship to the NOAA Atlas 14 precipitation frequency estimates. For the seven day duration, the observed precipitation was greater than the upper