Feasibility of Fish Passage at Alameda Creek Diversion Dam

4.0 Design Components and Preliminary Analysisneed for NMFS approval. Several installations located in the Pacific Northwest are currently beingreviewed and evaluated by NMFS to determine whether they have the potential to harm emigratingjuvenile and post-spawn adult steelhead that may travel across the screen during low flow conditions.As with all of the other screen types, a fish bypass would be required at the end of the screen to returndebris and emigrating steelhead downstream. A schematic diagram of a downward-sloping horizontalflat-plate screen is depicted in Figure 4-12.Figure 4-12Horizontal Downward-Sloping Flat Plate ScreenAs shown in Figure 4-12, a horizontal flat-plate screen operates by allowing a percentage of the flowover the screen to pass through it. The flow that passes through the screen falls into a canal below thescreen where it is routed as required. Operation of this screen is very sensitive to the minimum depthof surface water bypassing the diversion. Normally a minimum depth of 12 inches is required(WDFW, 2000a).An advantage of the downward-sloping horizontal screens is that, if designed properly, they areinherently self-cleaning as debris is removed by the normal stream flow. Disadvantages include thepotential for non-uniform approach velocities across the screen, sensitivity to fluctuating forebayelevations, and the “experimental” nature as defined by NMFS. For these reasons, this type of screenis not analyzed further for use at ACDD in this report.4.4.2 POTENTIAL SCREEN CONFIGURATIONSAs described in Section 4.4.1, vertical/inclined flat-plate screens and vertical traveling screens mayboth be suitable for use at ACDD. For the purposes of this analysis, only vertical or inclined flatplatescreens have been carried forward. These types are used widely and typically cost less thantraveling screens. Vertical flat-plate screens could be installed in several configurations at ACDD.For each type, it is necessary to carefully consider how sweeping velocity and approach velocity canbe kept uniform across the face of the screen and how effective operation of the fish bypass facilitycan be maintained throughout the range of forebay fluctuations. In this section, two conceptual-levelscreen configurations have been identified to protect steelhead from entrainment at the ACDT intake.Each screen configuration will require modification of the existing diversion structure in order to beeffective. For purposes of this discussion, the area behind the existing trash racks and fish screen isreferred to as the sediment channel. Two potential configurations are shown in Figures 4-13and 4-14:ACDD Passage June 2009 Page 4-21