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Table 1- Spillway Sizing Criteria from the Colorado Rules and Regulations for Dam Safety and Dam Construction, 2007 INFLOW DESIGN FLOOD REQUIREMENTS USING HYDROMETEOROLOGICAL REPORTS (HMR) DAM SIZE HAZARD CLASSIFICATION High Significant Low NPH Large 0.90 PMP 0.90 PMP 100 YR 50 YR Small 0.90 PMP 0.90 PMP 100 YR 25 YR Minor 0.45 PMP 0.45 PMP 50 YR 25 YR 4 PERFORMANCE OF DAMS DURING THE SEPTEMBER 2013 EVENT Although the September 2013 rainfall is considered extreme, it was well below what would be termed a PMP event. High and Significant hazard dams in the affected area have spillways designed for some percentage of the PMP. In the affected area the full PMP rainfall is on the order of 30 to 40 inches over a period of 72 hours. The maximum measured rainfall for the September 2013 storm was in Boulder at near 20 inches of rain over a period of 10 days. As a result, the High and Significant hazard dams within the affected area performed as designed and none of those dams were considered in danger of failing due to overtopping through the course of the event. The 100 -year 24 -hour rainfall typically used for design of spillway for small Low hazard dams in the affected area is in the range of 4 -6- inches. During the course of the storm, the maximum 24 -hour rainfall over much of the affected area was in the 6 -10 -inch range. It is therefore not surprising or unexpected that small Low hazard dams overtopped and failed as the rainfall and runoff exceeded the designed capacity of their spillways. The official count of failed dams during the September 2013 event stands at nine. Five of these, one minor -size Low hazard and four non - jurisdictional size dams were located within the Big Elk Meadows Subdivision on the West Fork of the Little Thompson River. Through post - flooding analysis of before and after Google Earth and Digital Globe" aerial images, the failure of additional non - jurisdictional dams within the area of the September 2013 flooding have been identified. Members of the Dam Safety Branch have also received second hand reports of NJ dam failures. Investigation of all NJ dam failures with no corresponding reports of downstream damage is beyond the current resources of the Dam Safety Branch. 5 PURPOSE AND SCOPE OF THE INVESTIGATION The remainder of this report focuses on the study of flooding that occurred during the September 2013 event, including the impact of the Big Elk Meadows dam failures on a portion of the Little Thompson River Basin. The section of the study includes from the headwaters downstream Report of the September 2013 Little Thompson River Flooding COLORADO and Big Elk Meadows Dam Failures, June 2014 (Revised, Dec 2014) A&V Division of Water Resources Page 5 of 48 . , , - - ■ 9go rm:1ovilU41:1