Reconnaissance Report Yolo Bypass, California: Fid-Bau Portal

Reconnaissance Report Yolo Bypass, California

The purpose of this study was to investigate flooding and related water resources problems associated with the Yolo Bypass and determine the Federal interest in proceeding into feasibility phase studies. The study area included lands on the west side of the Yolo Bypass from the Fremont Weir in the north to the vicinity of Liberty Island in the south. The Knights Landing area, west of the bypass, was included because it experiences potential flood impacts from the Yolo Bypass. At the request of Yolo County, the study area also included the Elkhorn Slough area, east of the bypass. During the flood of 1986, the Yolo Bypass flows approached or exceeded design flows. Although no levee failures occurred in the study area, wave action in the bypass required emergency actions to prevent levee overtopping and continued loss of levee embankment material. Future floods of a greater magnitude or duration could result in levee failures and flooding. Preliminary alternative plans were developed for five areas: Knights Landing, Elkhorn Slough, Willow Slough Bypass, the unleveed area south of Putah Creek, and the area west of Liberty Island and north of Cache and Haas Sloughs. The objective was to provide the areas with increased levels of flood protection. The plans consisted primarily of raising and strengthening existing levees, providing new cross levees, and constructing a new levee in one area. However, all preliminary alternative plans were eliminated from consideration due to economic reasons, except for plans for the Knights Landing area which appeared more promising. First costs for the eliminated plans ranged from $4,330,000 to $13,740,000, with benefit-to-cost ratios (BCRs) ranging from 0.1 to 0.4.