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8 bound of 90% confidence interval for the 1,000-year precipitation event. The September event is a reminder of the importance of designing a dam for the appropriate precipitation event and hazard classification. Flooding did result from the low hazard dam failures; however, there was little flooding from the state-classified high hazard dams, where failure would likely result in widespread damage and loss of human life, because these dams were designed appropriately for the PMP event. State Criteria for Design Precipitation The state criterion for determination of the dam design precipitation is based upon dam size and hazard classification. The hazard classification typically accounts for dam height, storage capacity, likelihood of failure (e.g. a dam located within a series of dams), and potential for loss of life and property, should a failure occur. The following discussion summarizes the hazard classification system and methods used to identify the dam design precipitation for Colorado, Utah, Montana, and Wyoming. For Colorado, design precipitation is selected based upon dam size and hazard classification as presented in Table 1. Additional guidelines are available for altitude adjustments in the Colorado Rules and Regulations for Dam Safety and Dam Construction. Table 1: Colorado Inflow Design Flood Requirements Note: NPH = No Public Hazard Dam. This table was taken from Table 5.2 of the Office of the State Engineer Dam Safety Branch’s “Rules and Regulations for Dam Safety and Dam Construction,” dated January 1, 2007. For Utah, design precipitation is selected based upon hazard classification as determined by the State Engineer. Design precipitation for all low hazard dams is the 100-year event, whereas significant and high hazard dams must use the Spillway Evaluation Flood (SEF). The SEF is defined as the most critical flood of either the 100-year event applied to a saturated watershed or one of the PMP events. For Montana, all dams with a potential for loss of life due to failure are classified as high hazard and the minimum design precipitation considered for any impoundment greater than 50 acre-feet is the 500- year event. Design precipitation for all high hazard dams is determined following a loss of life analysis using the requirements summarized in Table 2. Table 2: Montana Design Flood Requirements This table was taken from Montana’s Rules and Regulations, Rule 36.14.502 entitled, “Hydrologic Standard for Emergency and Principal Spillways”. For Wyoming, determination of design precipitation and dam hazard classification is at the discretion of the State Engineer. Additionally, all reservoirs with a dam height greater than 20 feet, storage capacity greater than 50 acre-feet, and/or a reservoir located in an area where extensive property damage or loss of life might result, are required to have a minimum design precipitation of the 100-year event. State rules and regulations typically prescribe the minimum criteria and not necessarily the method for satisfying the criteria. For example, a common requirement for low hazard dams is the 100-year event. This design criterion is typically the 100-year, 24-hour rainfall with a specific temporal distribution of hourly rainfall. Hydrological guidelines are then followed to determine the Design Flood. Alternately, the 100-year flood can be derived from actual stream gauge data collected within the drainage area or a similar nearby drainage area. The reader is cautioned to work with each state’s dam regulatory agency to High Significant Low NPH Large 0.90 PMP 0.68 PMP 100 YR 50 YR Small 0.90 PMP 0.45 PMP 100 YR 25 YR Minor 0.45 PMP 100 YR 50 YR 25 YR INFLOW DESIGN FLOOD REQUIREMENTS FOR COLORADO USING HYDROMETEOROLOGICAL REPORTS (HMR) DAM SIZE HAZARD CLASSIFICATION Loss of Life (LOL)Design Flood Less than or Equal to 0.5 500 YR 0.5 to 5 LOL x 1000 5 to 1000 P s = P5,000 (10rd) Where: r = -0.304 + .435 log10 (lol) d = log10 (PMP) - log10 (P5,000) lol = estimated loss of life PMP = probable maximum precipitation P 5,000 = 5,000-year recurrence interval precipitation P s = design precipitation to meet spillway standard Greater Than 1000 Probable Maximum Precipitation (PMP)