Feasibility of Fish Passage at Alameda Creek Diversion Dam

Feasibility of Fish Passage at Alameda Creek Diversion DamA vertical slot, weir, and orifice, or a hybrid type of fish ladder could be feasible on the right side ofACDD. The fish ladder would likely involve an initial vertical slot configuration at its upper end due tothe variation of forebay elevations during operational and fish migration periods. Two potential fishwaypaths were considered during the analysis of fish ladders at ACDD. In either case, the fishway wouldextend from below the bottom of the existing concrete apron to a location above ACDD (Figure 4-2).In general, the fishway entrance is most successful when located near the downstream face of abarrier. At this location, it may be most effective to provide a barrier at the downstream edge of theexisting concrete apron approximately 100 feet downstream of the spill crest. At this location, aphysical drop structure or velocity barrier could be constructed to inhibit passage farther upstream.With appropriate attraction flow to the fishway and a barrier to movement towards the face of ACDD,fish migrating upstream would have greater success finding and entering the fishway.Due to the wide range of unimpaired flows in Alameda Creek, the upper end of either fishway wouldrequire incorporation of a control structure to regulate the flow of water into the ladder. The controlstructure may require multiple fish exits (water entrances) to accommodate the various water surfaceelevations that would be expected to occur at the top of the fishway. The control structure wouldlikely incorporate vertical slots with a width of 1 to 1.5 feet.Since flows and channel conditions are uncontrolled upstream of ACDD, a large amount of debrisand bedload is flushed downstream, which accumulates in the area around ACDD. It would thereforebe necessary to implement a maintenance program to ensure that the fish ladder remained operable.Two potential fishway paths have been evaluated: a Short Fishway and a Long Fishway (Figure 4-2).A Short Fishway would involve construction of a fishway around ACDD on the right bank. Thefishway exit would be just upstream of the ogee crest spillway structure. This fishway would likelybe a vertical slot design, or a hybrid vertical slot with weir and orifice at the lower end. If theuppermost pool in the fish ladder (fish exit) was configured near the spillway, the structure wallswould be in the range of 10 to 20 feet high depending on the relative operational diversions andcorresponding variation of forebay elevations. Based on the elevation of the ACDD ogee crestspillway and the invert of the ACDT (Table 2-2), when the creek is flowing the forebay elevation isexpected to fluctuate a minimum of 12 feet, prior to spilling over the crest. The flow range of thefishway is dictated by the hydraulic control at the water entrance. A control structure with multiplefish exits would allow the fish ladder to function over a range of forebay elevations. Water flow intothe fishway is proportional to water surface height. In most cases, 5 cfs or more would be requiredfor operation of a vertical slot fishway. It is estimated that operational flows in the fishway wouldrange from 10 to 40 cfs 5 when stream flow is greater than 5 to 10 cfs. It may be possible to operate apool and weir fishway at lower flows.A Long Fishway would also involve construction of a fishway around ACDD on the right bank, butthe fishway exit would be located farther upstream, above the hydraulic influence of the ACDDforebay. This ladder would likely be a hybrid type consisting of a vertical slot and possibly weir andorifice for the lower portion, and a roughened channel for the upper section. If the upstream pool wasconfigured above the forebay, the variance in hydraulic water surface elevations during anticipatedfish migration flows would dictate structure height, in this case assumed to range between 8 and12 feet. The flow range of a Long Fishway would also be dictated by the hydraulic control at thewater entrance, and water flow into the fishway is proportional to water surface height. In this case,5 No flow duration analysis or a storm peaking analysis has been performed as part of the estimate of fish ladderoperational flows. These analyses, combined with ladder design work and coordination with NMFS and CDFG,would be used to determine the actual operational flows through a fish ladder at ACDD.ACDD Passage June 2009 Page 4-6