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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
Fields
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 21 <br />February 27, 2015 <br /> <br />Figure 15: FHWA HY8 summary results for Cross‐Section 1, bike path crossing over the CH North Trib. <br />7.2.3 Cross‐Sections 2 & 3: We performed redundant measurements on the North and South Carriage Hills <br />tributaries and on the main channel, knowing it can be difficult to get good estimates using indirect methods. <br />After significant analysis we decided to discard our indirect flow measurements at Cross‐sections 2 and 3. After <br />numerous checks we felt our Cross‐section 2 estimate was too low and our Cross‐section 3 estimate was <br />unreasonably high. In the end our reasons for discarding these measurements were: we were uncertain in <br />determining the high water line associated with overtopping flows at Carriage Drive (Cross‐section 2), and on <br />further analysis Cross‐section 3 was determined to have non‐uniform flow in a series of drops and pools, which <br />prevented us from estimating discharge using Critical Depth or Normal Depth methods. <br />7.2.4 Estimated Inflow to Carriage Hills No. 2 Reservoir: Of our various indirect flow measurements we feel <br />most confident in the 175 cfs estimate at the Highway 7 culvert for the CH North Trib because: (1) we have <br />eyewitness corroboration of the high water line, and (2) it is a standard hydraulic structure for hydraulic analysis <br />purposes. Also we obtained reasonable agreement from the less certain estimate at Cross‐section 1 (151 cfs), <br />~14% error. Therefore we decided to estimate the peak flow into the Carriage Hills reservoirs by scaling up the <br />unit discharge from the CH North Trib (0.91 sq. mi.) to the total basin area (2.63 sq. mi.): <br />Estimate of Peak Inflow to Carriage Hills No. 2 Reservoir = (175 cfs/0.91 sq.mi.) x 2.63 sq.mi. = 506 cfs <br /> <br />Photo 19 provides a visual of the flood flows into the Carriage Hills reservoirs. Photos show that both dams <br />were overtopping. Reportedly Photo 19 was taken on Thursday morning, Sept. 12th, which was after the <br />longest, most intense period of rain occurred in the basin, but not necessarily the peak streamflow. Hydrologic <br />modeling (Jacobs/CDOT, August 2014) suggests stream flow on Fish Creek peaked at around 14:00 hrs on Sept. <br />12th.
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