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 Page 25 Upper and Lower Emerald Valley Dam Failure Report January 23, 2015 9.2 Damage caused by Failure As noted previously, it is important to remember that the City of Fountain representatives were actively trying to remove significant debris plugged against the upstream side of the bridge with a mini-excavator at the time of an increase in water level in Little Fountain Creek, that sent water over the right bridge abutment. The presence of the debris would greatly reduce the capacity that could be carried beneath the RR Tie bridge chord. Prior to the failure of Lower Emerald Valley Dam around 1130 AM on Sept 12, the HMS model shown herein calculated a peak flow of about 228 cfs at about 1115 AM would have been flowing through the Keeton Spillway control section. This would account for about ½ of the capacity of the channel beneath the RR Tie bridge chord in UNOBSTRUCTED conditions. The amount of debris that was damming against the bridge chord, railing, and deck would support the idea that there was enough flow to carry the debris, yet pass mostly beneath the bridge. Eyewitness accounts indicate the debris removal was going well and the flow appeared to be going down for a time, however a sharp rise in the creek was observed shortly after noon. This sharp rise correlates well to the peak caused by the second storm intensity peak and failure of Lower Emerald Valley Dam. The routed outflow of the LEV peak failure on TOP of the storm intensity peak from Keeton Spillway had a modeled peak of about 639 cfs. The peak was very short in duration and rose and fell very quickly, due to the relatively small volume of water contained in LEV Dam. It is reported that this spike in creek flows caused the creek to erode the right abutment of the RR Tie bridge and divert Little Fountain Creek flows into Keeton Ranch Road. A short time later in the early afternoon, a second spike was likely seen due to the failure of UEV Dam. Repairs may have been possible after these short spikes to divert the creek back into its normal channel, however, the September 2013 rain event continued, resulting in another distinct and prolonged peak of flow through the Keeton Spillway, in excess of about 500 cfs. While the peak failure from LEV (combined with damming of debris at bridge) may have contributed to the failure of the right bridge abutment, the longer duration storm volume peaks contributed to the extended damage of the access road. From an engineering standpoint, the bridge itself is considered a temporary structure. The construction methods utilized within a significant basin such as this make it likely that the bridge would wash out or become impassable during even more frequent storm events than observed during this storm. Based on analysis presented herein, it is the conclusion of this office that the damage caused by failure of these dams is commensurate with what would be expected from a low hazard dam failure during a significant rain event.