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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
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 27 <br />February 27, 2015 <br />In this case the Froehlich method predicts breach dimensions almost identical to the surveyed breach bottom <br />width and average width as shown in Figure 19. Parameter checks suggest that a full breach would be expected <br />to form and the estimated erosion rates are considered reasonable for the Froehlich method breach formation <br />time of 0.24 hr. <br /> <br />In order to estimate the Carriage Hills No. 2 dam breach hydrograph we modeled the overtopping breach in <br />USACE’s HEC‐HMS hydrologic modeling program, using the surveyed breach dimensions and the breach <br />formation time from the Froehlich Method. We used a base inflow of 213 cfs, corresponding to our estimated <br />emergency spillway capacity, to represent the natural flood at the time of the dam failure. The actual inflow at <br />the time of dam failure unknown, but we are primarily interested in the peak and duration of the breach <br />hydrograph above the natural flood. Figure 20 shows HEC‐HMS results for the Carriage Hills No. 2 dam breach. <br />The estimated peak is 677 cfs above the assumed base flow and the breach hydrograph duration is <br />approximately 36 minutes. The volume under the breach hydrograph above the base flood is approximately 10 <br />acre‐feet; the model shows the 12.1 acre‐feet reservoir volume would not have fully drained during the dam <br />failure due to reservoir inflows. <br /> <br /> <br />Figure 20: HEC‐HMS results for the Carriage Hills No. 2 overtopping dam breach model <br /> <br />7.5 Fish Creek Flooding: This section documents our review of other studies and our analysis of Fish Creek <br />flooding for the purpose of trying to determine the timing of the Carriage Hills No. 2 Dam breach floodwave and <br />to help determine possible downstream damage that resulted. As a qualifier, it was very difficult to reconstruct <br />the Sept. 2013 flood events on Fish Creek. For one, the channel was virtually destroyed by erosion and crossings <br />were washed out. Much of the flooding occurred at night and some residents had already evacuated, so we <br />found few eyewitness accounts or photo documentation of the worst flooding. Last but not least, there were no
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