12000000053 Highland No. 2 Dams Inundation Report

• Contract/PO #: 12000000053
• Contractor Name: Highland Ditch Company
• Contract Type: Grant
• Water District: 5
• County: Boulder
• Loan Projects - Doc Type: Report

equations (collectively referred to as the "empirical equations").1 The empirical equations predict
the average breach width and failure time as a function of depth of water, dam height, and volume
of water stored. For the outlet structure failure scenario the same three empirical methods were used
to calculate failure time, but the average breach width was fixed at 52 feet for the North Dam, and
54 feet for the South Dam. Comparisons of the breach parameters calculated by each of the
empirical equations for both breach scenarios are shown below in Table 1 and Table 2 for the
North and South Dams, respectively.
Table 1. Highland Reservoir No. 2 North Dam Failure
Comparison of Dam Breach Parameters
Empirical Method Average Breach Width Time of Failure
(feet) (minutes)
GROUND-LEVEL BREACH
Froehlich 122 82
Von Thun and Gillette 113 80
USBR 63 13
BREACH INTO OUTLET CHANNEL
Froehlich 52 64
Von Thun and Gillette 52 24
USBR 52 10
Table 2. Highland Reservoir No. 2 South Dam Failure
Comparison of D am Breach Parameters
Empirical Method Average Breach Width Time of Failure
(feet) (minutes)
GROUND-LEVEL BREACH
Froehlich 89 113
Von Thun and Gillette 79 157
USBR 23 5
BREACH INTO HIGHLAND DITCH
Froehlich 54 86
Von Thun and Gillette 54 42
USBR 54 11
2.2 Dam Breach Simulation in HEC-HMS
The U.S. Army Corps of Engineers' (USACE)hydrologic modeling computer program HEC-HMS
was used to simulate clear-day piping failures of both of the Highland No. 2 Dams using the breach
parameters estimated by each of the empirical equations. As a conservative measure,the simulation
which produced the greatest peak outflow for each dam was selected for use in the downstream
hydraulic modeling and inundation mapping.
For each HEC-HMS breach simulation the water surface was set at the inlet ditch overflow crest
(gage height 32) at the onset of the breach, and all water above the breach invert was assumed to
drain. For the ground-level failure alternative the assumption was made that the breach would
A fourth empirical method known as the MacDonald&Langridge-Monopolis/Washington State(2007)
method was not considered based on recommendations found in the Guidelines for Dam Breach Analysis.
HIGHLAND NO.2 NORTH AND SOUTH DAMS - 2 - June 15,2012
INUNDATION MAPPING REPORT