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DWR_2717593
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DWR_2717593
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Last modified
7/11/2017 11:09:13 AM
Creation date
3/3/2015 11:31:22 AM
Metadata
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Template:
Dam Safety
Document Date
2/27/2015
Document Type - Dam Safety
Report
Division
5
Dam ID
040110
Subject
CARRIAGE HILLS #2 (LOWER) DAM - FAILURE FORENSIC REPORT REVISED
DWR Send/Recipient
DSB
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CARRIAGE HILLS NO. 2 DAM, Dam Failure Forensic Investigation Report Page 36 <br />February 27, 2015 <br />Creek was caused by the Carriage Hills No. 2 Dam failure. He reports the surge receded after about 30 minutes. <br />Country Club Road was reportedly still intact at the time of the surge, but washed out sometime later that night. <br /> <br />The Jacobs (August 2014) hydrology model suggests that rain‐driven (i.e. natural) flow on Fish Creek peaked at <br />2000 cfs around 14:00 hrs on Sept. 12th (Figure 26) in response to the second spike in rainfall, with a slightly <br />lower rain‐driven peak of 1900 cfs around 02:00 hrs on Sept. 13th in response to the third rainfall spike and after <br />the surge flow reported by Mr. Bonza. The Jacobs model does not account for dam breaks or other surge flows. <br />Videos and photos confirm that significant flooding occurred during the daylight hours of Sept. 12th (Photo 20). <br />Mr. Bonza’s videos show that flood waters had already been up to his driveway by 14:46 hrs on Sept. 12th, but <br />had not yet flooded his house until later that night. The Carriage Hills No. 2 Dam breach flood could explain the <br />timing of the surge. We estimated a dam breach hydrograph duration of 36 minutes, corresponding well to Mr. <br />Bonza’s account of the duration of the surge. <br /> <br /> <br />Photo 20. Source: Remembering September 2013, Images of a Community’s Determination, Estes Park Trail Gazette, Sept. <br />2014. <br /> <br />Next we attempted to create a hydraulic model of the reported 2‐ft stage increase during the surge to see if the <br />incremental rise could be explained by our estimated peak dam breach flow of 677–801 cfs. Our model, based <br />on our survey of the post‐flood Fish Creek channel, required much larger flows to arrive at the reported stage <br />increase. The problem is that Country Club Road was reportedly still intact at the time of the surge and likely <br />created backwater hydraulic conditions at the Bonza house. We reviewed the pre‐flood FEMA HEC‐2 Fish Creek <br />hydraulic model, but it did not include the Country Club Road crossing. After the extensive flood damage we
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