12000000053 Highland No. 1 Dam Inundation Report

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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. 1 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. 1, also known as Mulligan Reservoir, is located approximately four miles northeast of Longmont, Colorado in Section 22 of Township 3 North, Range 68 West of the 6th Principal Meridian. It sits just west of 1-25 about a third of a mile north of Highway 66. Highland Reservoir No. 1 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 16.5 feet and a normal storage of approximately 1,033 acre-feet. 2.0 DAM BREACH ANALYSIS 2.1 Breach Parameter Estimation Highland No. 1 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"),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. A comparison of the breach parameters calculated by each of the empirical equations is shown below in Table 1. Table 1. Highland Reservoir No. 1 Dam Failure Comparison of Dam Breach Parameters Empirical Method Average Breach Width Time of Failure (feet) (minutes) Froehlich 58 125 Von Thun and Gillette 28 128 USBR 10 2 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. 1 DAM - 1 - June 15,2012 INUNDATION MAPPING REPORT