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Scope/Content: Abstract: The City of St. Helena (City) and the U.S. Army Corps of Engineers (Corps) are in the planning and permitting stages of the Upper York Creek Ecosystem Restoration Project, which involves removal of the Upper York Creek Dam and Reservoir (UYCD).1 One concern is that removal of the dam may exacerbate flooding in York Creek within the City limits. During the 2005-06 New Year's Eve (NYE) storm, York Creek overtopped its banks at several locations in the lower reach, indicating that flood conveyance capacity may currently be well below the 100-year flow level. This report addresses two questions related to potential increased flooding as a result of removing UYCD. First: Will dam removal increase flood peaks? Dams often function as flood control structures, dampening flood heights downstream in sensitive urban areas. UYCD is not currently operated in this manner. It is unknown whether the reservoir has storage capacity to influence peak flows. Second: How will the renewed sediment deliveries downstream of the dam affect flood conveyance capacity of the lower York Creek channel? UYCD traps all the coarse sediment delivered from the watershed upstream of the dam -- 63% of the watershed's sediment source area. Removal of UYCD will allow sediment produced in the upper watershed to be transported through the lower reaches to the Napa River. The gradient of the channel upstream of the City is relatively steep (3-5% slope) and appears to have sufficient competence to transport the annual coarse sediment load. However, the reach through the City is less steep (1-2% slope) and may not be capable of transporting the delivered sediment size and load. If sediment drops out in lower York Creek, and the channel aggrades significantly over time, the flood capacity of lower York Creek will be reduced. Scope/Content: Height: 50 Length: 140 Scope/Content: Date constructed: 1900.
Engineering Dams Dam retirement
Napa River, CA York Creek, CA Upper York Creek Dam United States