Site Specific PMP and Inflow Design Flood for Lynx Lake Dam, near Prescott, Arizona

Abstract

Lynx Lake Dam is on Lynx Creek in Yavapai County, near Prescott, Arizona. The dam is owned and operated by the Arizona Game and Fish Department for wildlife and recreational purposes. The dam and lake are contained within the Prescott National Forest and therefore within the resource management of the U.S. Forest Service. As part of the National Dam Safety Inspection Program, the dam was determined to be inadequate to safely pass the spillway inflow design flood without overtopping the earthen and rock embankment. The dam is under the jurisdiction of the Arizona Department of Water Resources which classified the dam as unsafe, non-emergency. Accordingly, the Arizona Game and Fish Department was required to bring the dam into compliance with Arizona rules and regulations for jurisdictional dams. The initial hydrologic studies, using traditional methods for estimating the probable maximum precipitation (PMP) and the probable maximum flood (PMF) indicated that the spillway could only pass about 20 percent of the regulatory inflow design flood (0.5 PMF). However, spillway enlargement and/or dam raising require review and approval by the U.S. Forest Service along with environmental compliance. Additionally, the cost of such spillway enlargement and dam modifications could be significant. Therefore, due to regulatory, environmental and economic factors, there was interest by the Arizona Game and Fish Department in cooperation with the U.S. Forest Service and the Arizona Department of Water Resources to enter into a reappraisal of the inflow design flood and assessment of dam modifications. A three level analytic procedure was developed for the reanalysis of the dam. The first level was the estimation of the inflow design flood based on PMP estimates from the Hydrometeorological Reports. That level included the development and calibration of the watershed hydrologic model. Model calibration was accomplished by reconstitution of the 1993 storm over the watershed and use of the hydrologic model to reproduce historic high water marks at the dam. The second level included the development of a site-specific PMP by state-of-the- art hydrometeorologic methods. This resulted in a reduction of the PMF. The third level was an assessment of dam modifications. The development of the site-specific PMP, and the development of the watershed model is presented. Also, the procedure that was used to systematicall y develop these hydrologic and meteorologic analyses and to concurrently achieve technical review and approval by two state and one federal agency is presented.