12000000053 Highland No. 2 Dams 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 clear-day dam breaches of Highland No. 2 North and South Dams, and the development of the resulting inundation maps downstream of the dams. The inundation maps will be used to update the existing Emergency Action Plans (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. 2 is located approximately 3.5 miles north of Longmont, Colorado in Section 5 of Township 3 North, Range 69 West of the 6th Principal Meridian. Highland Reservoir No. 2 North and South Dams are classified as Small Significant Hazard Dams,per the Rules and Regulations for Dam Safety and Dam Construction of the State of Colorado. They are earthen compacted dams that impound a normal storage of approximately 3,713 acre-feet. The North Dam crest is 37 feet above the north outlet channel invert, and the South Dam crest is 24 feet above the Highland Ditch invert. 2.0 DAM BREACH ANALYSIS 2.1 Breach Parameter Estimation The Highland No. 2 North and South Dams were visually inspected on-site and with aerial photographs, and a ground survey was conducted by D&A to determine the dimensions of the dams at the maximum sections. The dimensions of the north outlet channel and of the Highland Ditch near the south outlet works were also surveyed. The spillway for Highland Reservoir No. 2 is a portion of the inlet ditch where the south bank is intended to overtop and spill south towards the Highland Ditch. Highland Ditch Company officials confirmed that when the reservoir is overfilled it backs up into the inlet ditch and overtops the south bank of the ditch. Overtopping of the inlet ditch occurs when the reservoir reaches gage height 32 feet. Two failure scenarios were evaluated for each of the Highland No. 2 Dams: 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 North Dam, or into the Highland Ditch for the South Dam. For the second alternative for each dam the breach was assumed to extend to the 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) HIGHLAND NO.2 NORTH AND SOUTH DAMS - 1 - June 15. 2012 INUNDATION MAPPING REPORT