Feasibility of Fish Passage at Alameda Creek Diversion Dam

Feasibility of Fish Passage at Alameda Creek Diversion Dam■■Installing a screen system in the sediment channel behind the existing trash rack.Constructing a new screen structure outside of the sediment channel in front of the existingstructure, and reusing the foundation of the existing trash rack or replacing it altogether.The feasibility of each location listed above is evaluated based upon corresponding physical andtechnical constraints and fish screen design criteria.Each configuration has its own advantages and disadvantages, both in terms of cost ofimplementation and maximum allowable diversion rates. All of the design configurations will requirea fish bypass flow estimated at 5 cfs, which will return flow to Alameda Creek downstream of thefacility, and a power source for operation of the required automatic cleaning mechanism.SCREEN SYSTEM IN THE SEDIMENT CHANNELThis configuration involves the placement of a fish screen in the sediment channel as shown inFigure 4-13. The maximum amount of allowable diversion is a function of approach velocity andscreen area. A minimum screen area of 1,600 square feet would be required to maintain the existingdiversion capacity of the ACDT. Installation of the screen behind the existing trash rack, dependingon the angle of incline or angle of orientation with the diversion channel (skew) at which the screen isplaced, would likely limit screen area to approximately 1,000 square feet because much of the surfacearea is composed of beams and the existing walks. At 1,000 square feet, diversion capacity wouldlikely be limited to a maximum of 260 cfs. The effective screen area may be increased by placing thescreens at an incline; a greater exaggeration of the skew and incline results in greater effective surfacearea of the screen. However, it may decrease the uniformity of the flow velocity passing through thescreen, and flow uniformity is one of the screen criteria regulated by NMFS (2008). Thisconfiguration would require a new steel support structure, a walkway on top of the screen, baffleguides and baffle assembly, flat-plate screen panes, and installation of a fish screen bypass.A fish screen bypass via a notch in the crest of the dam was evaluated, but is rejected from furtherconsideration. In order to pass water at all levels of diversion, the notch would have to extend fromthe dam crest to below the invert of the worker access tunnel within ACDD, which is a verticaldistance of approximately 15 feet. This was determined to be an unacceptable solution because (1) alarge hollow gallery in the dam that provides access to the left side of the dam would be blocked bysuch a notch, (2) the engineering challenges associated with preventing water from bypassing thediversion through the notch at high flows would be significant, and (3) the structural integrity of thedam could be compromised by such a notch. A fish screen bypass through the gravity wall of thesediment channel was determined to be a better solution.The fish screen bypass could be configured as a tunnel through the gravity wall. Open channel flowis typically required to protect fish in the bypass from injury. It may be possible to modify thepreviously designed CDRP-proposed stream flow bypass tunnel for use as the fish screen bypasstunnel. In order to limit the fish screen bypass flows to 5 cfs, and to maintain an open channel in thebypass that would provide safe passage for fish, a concrete control structure could be installed at thescreen exit to accommodate a range of water elevations. The control structure could potentially bedesigned to accommodate water and fish from several openings at different elevations, therebyfunctioning for a variety of water surface elevations. Further analysis of the fish bypass would needto be developed upon design.ACDD Passage June 2009 Page 4-22