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25 <br />Photo 13: CH2 Dam spillway looking downstream. <br />Note heavy vegetation in spillway. <br />The overtopping of CH2 resulted in a breach of the <br />embankment, whereas CH1 survived the overtopping <br />event with some scour erosion on the downstream <br />slope, primarily around trees. Overtopping durations <br />and depths are not known. However, significant <br />overtopping of CH1 Dam and at least minor <br />overtopping of CH2 Dam was observed, as shown in <br />Photos 14 and 15. The larger overtopping depth of CH1 <br />Dam was attributed to the smaller spillway capacity at <br />CH1 Dam compared to CH2 Dam, based on visual <br />inspection and an overgrowth of small willows in the <br />spillway. During the flood, the willows matted <br />together, forming a 2-foot-wide notch through which <br />flow was concentrated (see Photo 16). <br />Photo 14: CH1 Dam overtopping. <br />Photo 15: CH2 Dam overtopping. <br />Photo 16: 2-foot-wide notch formed in CH1 Dam <br />spillway. <br />A post-flood survey indicated the dam crest at CH1 was <br />relatively level and the embankment was observed to <br />have a well-maintained vegetation cover. This allowed <br />for overtopping flows to be distributed relatively <br />evenly along the embankment, thus contributing to <br />the dam’s survival. <br />Conversely, based on the post-flood survey it appeared <br />that CH2 Dam failed due to concentrated flow over a <br />low area on the dam crest. Headcutting damage was <br />observed at the downstream slope of the right <br />abutment and to the left of the dam breach. Existing <br />vegetation and debris that had collected in the spillway <br />may have partially restricted flood flow and resulted in <br />greater overtopping flows over the embankment. The <br />inadequate capacity of the spillway at CH2 Dam likely <br />also resulted in greater overtopping flows. The <br />reservoir inflow was estimated to be between 237 and <br />508 cfs compared to the spillway capacity, which was <br />estimated to be 212 cfs at the time of flooding. It is <br />possible that had the spillway been cleared of <br />vegetation with a backhoe before or during the flood, <br />the spillway may have passed enough flow to allow the <br />CH2 Dam Crest