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CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 13 February 27, 2015 In November and December 1991, the DSB performed a dam breach study using NWS Breach 87. Four dam failure cases were investigated: (1) piping failure of the upper dam only, (2) piping failure of the lower dam only, (3) simultaneous piping failure of both dams, and (4) overtopping failure of the lower dam caused by a piping failure of the upper dam. Peak breach flows for cases (2) and (4), considered most relevant here, were 1162 cfs and 5409 cfs. No formal review of hazard classification was documented, but the Hazard is denoted as “3” in the analysis, where 3 in the previous 1‐3 hazard rating system corresponds to the current Low Hazard rating. The 2002 DSB EIR includes a photo of a 4‐ft diameter hole filled with water at the downstream toe of the dam, which was assumed to be related to the abandoned outlet. The 2002 EIR required the Town to restore the spillway to the size shown on the C‐1195 construction plans or else hire an engineer to do an alternative design. The 2008 DSB EIR reported that work was done to the spillway to restore it to the design width of 10‐ft as well as adding riprap protection to the approach and downstream channel. The 2008 EIR cited the old CMP outlet as a potential problem if it was not properly abandoned and required plans & specifications for outlet rehabilitation. A tabulated log of the SEO File is provided in Appendix A. 7. ANALYSIS 7.1 Rainfall Analysis: Applied Weather Associates, Inc. provided the SEO DSB with quality‐controlled spatial and temporal rain data for the September 2013 storm processed using their SPAS program. 7.1.1 Spatial Rain Data: Figure 6 maps rain totals for various subasins above the Carriage Hills dams and on Fish Creek. Subbasins 1‐3 are for Fish Creek downstream of the Carriage Hills dams. Subbasins 4, 5, 6 & 8 drain directly to the Carriage Hills dams. Subbasin 9 corresponds to the USBR’s Marys Lake. Marys Lake did not fill and reportedly did not discharge into the Carriage Hills drainage area during the Sept. 2013 flood. Finally Subbasin 7 is for the mainstem Fish Creek upstream of the Carriage Hills dams. Figure 7 maps Subbasins 4, 5, 6 & 8 (those that directly affected the Carriage Hills dams) on the USGS topographic map. Figure 6 shows the spatial distribution of rainfall. Maximum rain totals of 16‐17 inches occurred at the south end of the Fish Creek basin (SB7) on the upslopes of the Twin Sisters peaks. It is worth noting that large rain amounts fell as high as 10,000 ft elevation, in contrast to the often cited belief that rain events are not significant drivers of flooding above 7,500 ft along Colorado’s Front Range. Lower rain totals of 9‐10 inches fell at lower elevations (SB 1 & 2). The spatial average total rainfall for the Carriage Hills subbasins (SB 4, 5, 6 & 8) was 10.7 inches. Table 3 shows total rain amounts by depth (inches) and volume (acre‐feet) for each subbasin and for the aggregate.