SCH Number 2020090032
- NID Hemphill Diversion Structure
- Located in Placer County, just east of the City of Lincoln, the Hemphill Diversion Structure is located within Auburn Ravine. The structure diverts water from Auburn Ravine into the Hemphill Canal, located south of the ravine, for delivery to Nevada Irrigation District (NID) raw-water customers. The diversion structure is an approximately eight-foot-high concrete structure, with an approximately 40-foot-long concrete apron extending downstream, and has been operated by NID since its purchase in 1933. The existing diversion has been subject to damage during high flow events, has required ongoing maintenance, and is currently in need of additional repairs. The Hemphill Diversion Structure is located in Section 13, Township 12 North, and Range 6 West (Mount Diablo Base and Meridian) of the “Lincoln” 7.5-minute quadrangle. The structure is located at latitude 38.896731° and longitude -121.251885°. Hemphill Diversion has historically presented an impediment to the passage of migrating anadromous fish species that spawn in Auburn Ravine upstream of the diversion. NID is considering three alternatives to eliminate this impediment while still maintaining water deliveries to customers served by Hemphill Canal. These three alternatives include: - Alternative 1 - Riverbank Infiltration Gallery Alternative: Includes the removal of the diversion structure, site stabilization, and construction of a subterranean riverbank infiltration structure and pipeline connection to Hemphill Canal. - Alternative 2 - Fish Passage Alternative: Includes the removal of the diversion structure, site stabilization, construction of a nature-like roughen rock ramp instream fish passage, installation of a fish screen and improvements to a portion of the Hemphill Canal. - Alternative 3 - Pipeline Alternative: Includes the removal of the diversion structure, site stabilization, and installation of the majority of the pipeline within roadway right-of-way (ROW) from the NID Placer Yard facility to the Hemphill Canal near the existing diversion structure. Each alternative is designed to allow anadromous fish to migrate past the Hemphill Diversion Structure site. NID has not yet identified a preferred alternative. While varied in their approaches to enhancing fish passage at the Hemphill Diversion site, each Project alternative would require removal of the existing diversion structure. The area in which the three alternatives are located is relatively flat, with elevations ranging from 196-450 feet above mean sea level (AMSL). Auburn Ravine at this location is a perennial stream with a cobbly/rocky/sandy bottom in an incised channel that averages approximately 100 feet in width. When the Hemphill Diversion flashboards are in place during spring and summer, the stream is impounded to form a slack pond behind the diversion structure. The stream supports a band of riparian vegetation dominated by narrow-leaved willow (Salix exigua var. exigua) and red alder (Alnus rubra) below the ordinary high-water mark. Incising of the channel has resulted in the stream being mostly isolated from its historic floodplain.
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