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19 Dam Overtopping Failures – Lessons Learned from the September 2013 Colorado Flood Event Introduction During the September 2013 flooding in Colorado, nine low hazard dam failures were documented. Forensic investigations for several of these dams identified possible causes of failure. This article shares those possible causes of failure and the lessons learned. It is the intent of this article for engineers and dam owners alike to apply the lessons learned to low hazard dams in an attempt to preserve these vulnerable but valuable structures from failure during events larger than those for which they were designed. The potential for an earthen embankment dam to survive an overtopping event depends on the condition of the dam at the time of the event, the materials used to construct the embankment, the duration and depth of overtopping, and the duration and intensity of the storm event. Small, low hazard embankment dams are typically designed for rain events with return periods between 25 and 100 years. Unlike high and significant hazard dams, most low hazard dams are not designed to safely pass large, infrequent storms and are therefore susceptible to damage and failure during inflow events greater than a 100-year type of event. Higher hazard dams have more stringent spillway design standards due to the consequences associated with their failures. Although consequences to the downstream public due to failure of low hazard dams are less severe, the value of these structures to their owners can be considerable. In this article, case histories of dam overtopping failures that occurred during the September 2013 storm event are presented. Each dam’s condition and performance during the event are reviewed and lessons learned following these dam failures are discussed. The dam failures presented in this article include: x The Upper and Lower Emerald Valley Dams located in the Pikes Peak Region of El Paso County x A series of five dams on the Little Thompson River in the Big Elk Meadows Subdivision within Larimer and Boulder Counties x Carriage Hills No. 2 Dam near Estes Park The lessons learned presented in this article may be old hat for some readers, but it is beneficial for the less experienced dam engineer and owner to understand the sensitivity of dam performance to some of these more common-knowledge issues. September 2013 Storm Event in Colorado The historic rainfall event that occurred between September 8th and 18th, 2013 was responsible for extensive flooding along the Colorado Front Range, extending from El Paso County in the south to Larimer County in the north over an area of approximately 11,000 square miles. During this event, several individual storms contributed to the overall precipitation totals; however, a large storm event that occurred on September 12 and ended September 13, 2013 was the most significant contributor to the dam failures, setting a one-day rainfall record for Colorado (Doesken 2014). The September 2013 event was one of the top three extreme storms documented in Colorado (Doesken 2014). The maximum measured rainfall for the September 2013 storm was recorded near the town of Boulder and resulted in 20 inches of precipitation over a 10-day period. Emerald Valley Dams The Upper and Lower Emerald Valley Dams (UEV and LEV) are located on Little Fountain Creek on U.S. Forest Service property in the Emerald Valley Ranch Resort, about 6.5 miles southeast of the Pikes Peak summit. An aerial image of the dams is shown in Photo 1. Both dams are classified as low hazard. Due to their small sizes and remote locations, the dams were not previously regulated and no information about the dams was available. The existing condition of the dams was inferred based on conditions observed post-failure and from information obtained as part of the design and reconstruction of these two dams.