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CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 22 February 27, 2015 Photo 19: Flood photo from Lakeshore Drive resident, reportedly taken on the morning of Thursday, Sept. 12, 2013, showing inflows to Carriage Hills No. 1 Dam (dam overtopping in foreground). Matrix Design Group (August 2014) estimated a natural (i.e. absent the Carriage Hills No. 2 dam breach flood) peak flow during the Sept. 2013 flood of 2,400 cfs over the 15.9 sq.mi. drainage area for Fish Creek. The resulting yield is approximately 152 cfs/sq.mi. Our estimate for the Sept. 2013 peak flow into the Carriage Hills No. 2 reservoir yields 192 cfs/sq. mi. We believe our estimate is reasonable but probably on the high side. 7.3 Spillway Discharge Rating: The existing spillway at the time of the Sept. 2013 flood did not match the spillway on the approved C‐1195 construction plans. The plans showed a spillway with a 10‐foot bottom width and 5 feet of freeboard; the estimated discharge capacity on the plans was 377 cfs. The existing spillway control section, as surveyed on Sept. 26, 2013, was found to have a 4‐foot bottom width and approximately 3.4 feet of freeboard to the low point of the dam. The spillway was also found overgrown with brush. Figure 16 shows a cross‐section of the spillway drawn from our Sept. 26th survey. We modeled the existing spillway in USACE’s HEC‐RAS open channel hydraulics program to estimate the spillway discharge capacity at the time of the flood. We used a roughness coefficient of 0.1 to account for brush in the spillway. We used the following four cross‐sections: River Section (RS) 0: Downstream end of spillway control section and dam crest RS 6.5: Upstream end of spillway control section and dam crest RS 20: Spillway approach section, with rock weir about 1‐ft higher than the spillway crest RS 40: Reservoir section to determine the full static head for spillway discharge rating