12000000053 Foothills 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 Foothills 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 Foothills Reservoir is located approximately two miles west of the town of Hygiene, Colorado in Section 34 of Township 3 North, Range 70 West of the 6th Principal Meridian. It is bordered to the north by Hygiene Road. Foothills Dam is classified as a Large High 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 52 feet and a normal storage of approximately 4,139 acre-feet. 2.0 DAM BREACH ANALYSIS 2.1 Breach Parameter Estimation Foothills Darn 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. Four 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,the USBR (1988) equations, and the MacDonald& Langridge— Monopolis/Washington State (MLM-WA, 2007) equations (collectively referred to as the "empirical equations"). 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. The MLM-WA method also takes into consideration the estimated volume of embankment eroded. A comparison of the breach parameters calculated by each of the empirical equations is shown below in Table 1. Table 1. Foothills Reservoir Dam Failure Comparison of Dam Breach Parameters Empirical Method Average Breach Width Time of Failure (feet) (minutes) Froehlich 139 49 Von Thun and Gillette 177 57 USBR 140 28 MLM-WA 140 97 FOOTHILLS DAM - 1 - June 15,2012 INUNDATION MAPPING REPORT