2022-11-28-minutes-public-works-study-session

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Finding Of No Signaf leant impact Introduction In compliance with the National Environmental Policy Act of 1969, the Bureau of Reclamation (Reclamation) prepared this Kachess Safety of Dams (SOD) Modification Project Environmental Assessment (EA) to assess the potential consequences of a proposed modification to Kachess Dam and its appurtenant spillways in the Yakima River basin in west -central Washington. Kachess Dam, located about 14 miles northwest of Cle Elum, Washington, was constructed between 1910 and 1912. The dam was completed in 1912 to increase the storage capacity in an existing natural lake. Along with Keechelus Dam to the west and Cle Elum Dam to the east, Kachess Dam forms one of the upper basin storage reservoirs of the Yakima Project.' The 115 -foot - high, 1,400 -foot -long, earth -filled Kachess Dam created a reservoir with an actively managed capacity of 239,000 acre-feet.z The reservoir typically fills in the winter and spring. It is used for irrigation and fisheries -enhancement purposes in the summer and fall. Reclamation owns and operates the dam; the Yakima Field Office of the Columbia -Cascades Area Office is the entity responsible for operations. Reclamation has identified seepage and internal erosion issues through the dam embankment along the outlet works conduit, which conveys water from the reservoir to the Kachess River downstream. As reservoir water levels rise, water begins to seep in the downstream end of the conduit. The seeping water begins to scour and erode the outlet works, creating voids or holes within the dam. The eroded materials leave the conduit and are deposited into the toe drain—or the "horseshoe"' drain—surrounding the downstream end of the conduit or into a large repository formed by continuous existing voids in the conduit. As erosion intensifies, water continues to seep, and sinkholes appear in the downstream base of the dam. Combined with the pressure of the water in the reservoir behind the dam, these existing voids can crack or expand, further impacting the dam's integrity. In other words, water seeping through the dam embankment and the soils surrounding the conduit carries soil materials with it and leaves behind voids, which impact the dam's stability. This internal erosion creates a risk of potential dam failure. I In 2013, the Washington legislature authorized funding for the initial development phase of the Yakima Basin Integrated Plan, a consensus -based effort to assure sustainable water supplies for families, farms, and fish in the Yakima River basin over the next 30 years. Projects and activities outlined in the plan's fust phase are designed to quickly improve streamflows, habitat, and fish passage, and secure water for farms, cities, and industry, especially during times of drought and in response to climate change. 2 A reservoir is a managed surface water system, consistent with terminology used by Reclamation. Specifically, Reclamation defines "reservoir" as "[al body of water impounded by a dam and in which water can be stored. Artificially impounded body of water. Any natural or artificial holding area used to store, regulate, or control water. Body of water, such as a natural or constructed lake, in which water is collected and stored for use." Accordingly, Reclamation refers to the waterbody behind the dam as Kachess Reservoir. 3 "Horseshoe" is a design term based on the shape and configuration of the drain. Kachess Safety of Dams Final Environmental Assessment