12000000053 Highland N. 3 Dam Inundation Report

• Contract/PO #: 12000000053
• Contractor Name: Highland Ditch Company
• Contract Type: Grant
• Water District: 5
• County: Weld
• 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 Highland No. 3 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
Highland Reservoir No. 3, also known as Foster Reservoir, is located approximately four miles east
of Longmont, Colorado in Section 27 of Township 3 North, Range 68 West of the 6th Principal
Meridian. It sits a third of a mile south of Highway 66 just west of I-25. The Highland Reservoir
No. 3 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 18 feet above the outlet channel invert and a normal storage of approximately 1,670 acre-feet. A
concrete wall which is meant to increase freeboard extends across the entire length of the dam crest.
This wall was ignored in the breach analysis contained in this report because the spillway crest
elevation is below the top of the earthen embankment.
2.0 DAM BREACH ANALYSIS
2.1 Breach Parameter Estimation
Highland No. 3 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, the dimensions of the dam
at the maximum section, and the dimensions of the outlet channel.
Two failure scenarios were evaluated for the Highland No. 3 Dam: 1) A ground-level breach
occurring at the dam's maximum section, and 2) a breach along the outlet structure and into the
outlet channel. For the second alternative the breach was assumed to extend to the outlet channel
invert,but the width of the breach was constrained to the dimensions of the channel.
For the ground-level failure scenario 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"),I 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
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.3 DAM - 1 - June 15,2012
INUNDATION MAPPING REPORT