12000000053 McIntosh Dam Inundation Report

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

1.0 INTRODUCTION
Deere&Ault Consultants, Inc. (D&A)has prepared this Inundation Mapping Report at the request
of the Highland Ditch Company. This report documents the procedure of modeling a clear-day dam
breach of McIntosh Dam, and the development of the resulting inundation maps downstream of the
dam. The inundation maps will be used to update the existing Emergency Action Plan(EAP) and to
assist those involved with emergency preparedness planning and response. The hydraulic models
and inundation maps contained in this report were prepared using the NAVD 88 vertical datum. The
report and all associated models have been prepared in accordance with the regulations and
guidelines set forth by the Colorado Department of Natural Resources, Office of the State Engineer
(SEO).
1.1 Location and Background
McIntosh Reservoir is located at the northwestern edge of Longmont, Colorado in Section 30 of
Township 3 North, Range 69 West of the 6th Principal Meridian. McIntosh Dam is classified as a
Small Significant Hazard Dam, per the Rules and Regulations for Dam Safety and Dam
Construction of the State of Colorado. It is an earthen compacted dam with a height of 17 feet and a
normal storage of approximately 2,460 acre-feet.
2.0 DAM BREACH ANALYSIS
2.1 Breach Parameter Estimation
McIntosh Dam was visually inspected on-site and with aerial photographs, and a ground survey was
conducted by D&A to determine the spillway crest elevation and the dimensions of the dam at the
maximum section.
Three empirical methods for predicting the average breach width and failure time were compared.
The methods, which are included in Appendix A, are the Froehlich (2008) equations, the Von Thun
and Gillette (1990) equations, and the USBR(1988) 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. A comparison of the
breach parameters calculated by each of the empirical equations is shown below in Table 1.
Table 1. McIntosh Reservoir Dam Failure
Comparison of Dam Breach Parameters
Empirical Method Average Breach Width Time of Failure
(feet) (minutes)
Froehlich 114 109
Von Thun and Gillette 95 102
USBR 42 8
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.
MCINTOSH DAM - 1 - June 15,2012
INUNDATION MAPPING REPORT