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engineering design and emergency preparedness criteria. Minor -sized Low hazard dams are <br />inspected every six years and have more basic engineering design and no emergency preparedness <br />criteria. These differences with respect to the design of spillways for dams are described below. <br />Hundreds of dams of all size and hazard classification lie within the area impacted by the <br />September 2013 rain event. Being a flooding (hydrologic) event, the primary concern for safety of <br />the dams was with respect to their spillway capacity. Spillways are structures constructed on <br />dams to safely route flood flows around the embankments to minimize the potential for <br />overtopping. The vast majority of dams in Colorado are earth dams which are susceptible to <br />erosion damage and failure if the earth embankments are overtopped by flooding flows. <br />The design and size requirements for spillways at dams and reservoirs in Colorado are dictated by <br />the Rules. Spillway size requirements are related to the magnitude and duration of an assumed <br />storm that will fall on a watershed above a dam. The size of the assigned or assumed storm is a <br />function of how often that depth or volume of rainfall is expected to be seen. For the sake of <br />design, this is reduced to a statistical probability of occurrence. For example, a 1 -year storm has a <br />100% probability of occurring every year. A 2 -year storm has a 50% probability of happening at <br />some time in the course of a year, and a 100 -year storm has a 1% chance of happening at any time <br />in the same year. The Federal Emergency Management Agency (FEMA) flood -plain mapping also <br />provides for the 500 -year storm which has a 0.2% chance of happening in any given time in a year. <br />A term used in the dam engineering community is the "Probable Maximum Precipitation" (PMP), <br />which is used to describe the largest conceivable rain event that can be calculated at a given <br />location through the application of the science of meteorology. A PMP rainfall event is difficult to <br />calculate, and even more difficult is providing a statistical explanation for this amount of rain. <br />Although it is difficult to assign a probability of a PMP rain event happening, it is thought to be <br />between a 10,000 -year event and a 1,000,000 -year event, or between a 0.01% and 0.0001% <br />chance of happening at anytime in a given year. <br />Table 1 below is from the Rules and shows the variation in design criteria for spillways on dams in <br />Colorado by hazard classification and dam size. The Inflow Design Flood is the flood created by <br />runoff of the required rainfall event that must be safely passed through the spillway without <br />causing damage to the dam. <br />A variation in the requirements described above for spillways is based in the concept of <br />incremental damages. Incremental Damage Assessment (IDA) for dam safety evaluation <br />determines whether or not a significant increase in flooding will result from dam failure beyond <br />that which would occur if the dam were not in place. The reduced requirements for spillways for <br />smaller and lower consequence dams are based on analysis of incremental damage effects. Past <br />analysis and experience have shown no incremental increases on flooding due to failure of these <br />categories of dams during storm events greater than or equal to those for which their spillways are <br />designed. <br />Report of the September 2013 Little Thompson River Flooding COLORADO <br />and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) A&V Divisionof Water Resources <br />Page 4 of 48 <br />DAM SAFETY BRANCH <br />