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24 <br />equivalent to dam failure; however, this is not <br />necessarily true. Dams may overtop in the event <br />of a flood, but failure occurs only when <br />overtopping forms a dam breach. <br />x The duration of rainfall during a storm event <br />impacts the severity of damaging flows. Longer <br />duration, lower intensity storms have a slower rate <br />of rainfall runoff within the watershed, allowing for <br />more infiltration and lower peak discharges. <br />Shorter duration, more intense storms have a <br />more rapid rate of rainfall runoff, leading to higher <br />peak discharges and more damaging flows. <br />Carriage Hills No. 2 Dam <br />Carriage Hills No. 1 and No. 2 Dams (CH1 and CH2) are <br />located in a residential development about 2.25 miles <br />southeast of downtown Estes Park. CH1 Dam is <br />located just upstream of CH2 Dam. An aerial image of <br />the two dams is shown in Photo 11. Both dams are <br />classified as low hazard. <br />Photo 11: Aerial image of CH1 and CH2 Dams. <br />Both dams were overtopped during the September <br />2013 event. CH1 Dam survived the overtopping event <br />with some erosion damage, while CH2 Dam failed. <br />There is little information available for CH1 Dam. CH2 <br />Dam was designed as a zoned earthen embankment <br />dam with 2H:1V (horizontal to vertical) upstream and <br />downstream slopes, a height of approximately 20 feet, <br />and crest width of 10 feet. Both dams have/had a low- <br />level CMP outlet conduit and uncontrolled overflow <br />spillway. There is no information available pertaining <br />to the size and capacity of the outlet works and <br />spillway at CH1 Dam. At CH2 Dam, the outlet works <br />consisted of a 24-inch-diameter, 12 gauge CMP with an <br />upstream slide gate and reported capacity of 35 cubic <br />feet per second (cfs). The spillway at CH2 Dam was <br />designed to have a 10-foot bottom width, 5 feet of <br />freeboard below the dam crest, and capacity of 377 <br />cfs. <br />Inspection reports for CH2 Dam dated 1983, 1985, <br />1986, 1991, 2002, and 2008 describe conditions with <br />trees and brush growing on the dam and obstructing <br />spillway flow. In 2008, the spillway was reportedly <br />rehabilitated; however, it was not restored to the <br />design size and capacity. <br />Results from a post-failure survey indicated the CH2 <br />dam and spillway were actually smaller in size than <br />designed. The dam had a height of approximately 10 <br />feet and crest width of 6.5 feet. The spillway had a 4- <br />foot bottom width, 3.4 feet of freeboard, and a <br />capacity of 213 cfs during the time of the September <br />2013 flood. The maximum storage capacity was not <br />reported, but was estimated for CH2 Dam to be 12.7 <br />acre-feet at the dam crest based on post-failure <br />analyses. <br />Prior to failure of CH2 Dam, the dam crest was <br />overgrown with trees and shrubs, which can be seen in <br />Photo 12. The spillway was also overgrown with <br />shrubs and small willow trees, as shown in Photo 13. <br />Previous inspection reports noted low areas along the <br />dam crest, specifically in the area of the outlet works <br />conduit. <br />Photo 12: CH2 Dam post-failure looking downstream <br />from reservoir. Note heavy vegetation on dam crest. <br />Carriage Hills No. 2 Dam <br />Carriage Hills No. 1 Dam <br />Dam Breach