Empire Reservoir: East Embankment Dam Rehabilitation Case-Study Report

Empire Reservoir is located in the northeastern plains of Colorado in close proximity to the South Platte River, and approximately 3 miles west of Wiggins, Colorado. The reservoir is an off channel irrigation and recreation storage facility that is owned and operated by the Bijou Irrigation District. The reservoir is impounded by an array of compacted-sand earthen dams, the majority of which were originally constructed in approximately 1906. Over the years, the dams have been modified and enlarged, resulting in the current normal reservoir capacity of 37,710 ac-ft, at a gauge height of 30 feet. In the spring of 2007, the reservoir experienced a severe windstorm that resulted in significant wave erosion along sections of its east embankment dam. In particular, two sections of an existing parapet wall failed and significant erosion occurred near existing cottonwood trees located along the upstream toe of the east embankment. Following the annual inspection of the dam by the Colorado State Engineer’s Office (SEO), a fill restriction was placed on the reservoir until such time as the damaged sections of the embankment were repaired. In the fall of 2007, Applegate Group, Inc. was retained to prepare a rehabilitation plan for the repair of the damaged embankment sections identified during the dam safety inspection. Following a cost-weighted analysis of design alternatives, a three-phase rehabilitation plan was formulated to repair the existing embankment by removing the trees and damaged parapet wall sections from the upstream face of the dam, widening the dam topwidth with the placement of additional certified fill material borrowed from the reservoir bottom, and adding designed slope stabilization. A standard riprap-aggregate filter approach was selected for the slope stabilization based on its proven track record of use in similar dams, and in conjunction with the results of the cost analysis. Construction began in November of 2007, but by December was halted due to prolonged spans of cold winter weather. The weather setback jeopardized the 2008 irrigation season if the fill restriction could not be removed. A temporary embankment repair plan was then formulated and submitted to the SEO, who approved the revised approach with the contingency that it be removed following the 2008 water year, and replaced per original design. In September of 2008, construction began on the first 2 phases of the rehabilitation, which were completed in January 2009. 39 pp., 27 images.