Feasibility of Fish Passage at Alameda Creek Diversion Dam

Feasibility of Fish Passage at Alameda Creek Diversion Damvelocities. A support structure and walkway across the diversion could provide guideslots for thescreens and velocity baffles for ease of installation and for performing routine maintenance.Advantages of the vertical traveling screen include its ability to be self cleaning; flexibility in heightso that variances in forebay can be addressed; and ability to operate without electricity (using apaddle wheel). Disadvantages include its requirement for routine maintenance due to the presence ofseveral moving parts; and reduction of the effective screen area and associated diversion rate due tothe presence of gear mechanisms at top and bottom as well as rollers and frames along the sides. Itappears that this type of screen may be suitable for use at ACDD, but this type of screen is not studiedfurther in this analysis because it has typically higher maintenance costs (due to more moving parts)than the other types of screens.VERTICAL AND INCLINED FLAT-PLATE SCREENSFlat-plate screens can be oriented vertically or at an incline. The application of these two configurations isgenerally the same, with the caveat that due to a recent acceptance by NMFS in appropriate applications,the effective screen area of incline screens can be calculated using the actual screen area rather than thevertical projection of the screen alone. In such cases, inclined screens may provide additional effectivescreen area over vertical screens when compared over the same screen length, because by tilting the screenaway from the horizontal plane, more of the screen can be submerged in the same depth of water.Vertical and inclined flat-plate screening systems are among the most common screen types in use.They are used for irrigation, domestic, and industrial intakes at both pump and gravity diversions.Vertical flat-plate screens are relatively easy to seal because the screen mesh is fixed directly to thestructural frame. The absence of moving parts between the mesh and screen frame also reducesmaintenance. The disadvantage of a vertical flat-plate screen is that cleaning systems must operate atregular intervals and debris management systems must have automatic systems that trigger when debrisloads increase. Automatic cleaning is triggered by a high-level water differential across the screens, anelapsed time period, or manual activation. Potential cleaning systems vary and include air burst, waterjet, and mechanical brush mechanisms. These devices are normally powered by electric motors andrequire a power supply. Baffles located behind the screens are used to create uniform approachvelocities across the screens. A schematic diagram of a vertical flat-plate screen installation and typicalcross section are shown in Figures 4-8 and 4-9, respectively. Figure 4-10 illustrates a typical inclinedflat-plate fish screen, and Figure 4-11 shows a photo of a working vertical flat-plate fish screen.The vertical flat-plate fish screen is potentially suitable for use at ACDD. Like the vertical travelingscreen, a series of structural supports, a walkway, and several screen panels would be required. Someof the advantages of the flat-plate screen are that there are no moving parts on the screen itself (onlypotentially the cleaning system) so annual maintenance may be less than other screen types, thescreen can be configured in a wide variety of lengths and heights to remain effective over the range offorebay fluctuations, there is less structural blinding and more effective area on average than otherscreening systems, and the cost associated with fabrication of flat-plate screens is less than for otherscreen systems. (Structural blinding is the reduction of flow through portions of the screen that areblocked by screen support structures.) Additionally, one potential configuration of the screen wouldfit within the existing diversion structure at ACDD (i.e., behind the existing trash rack), minimizinginstallation and construction costs while fully using existing structures. The disadvantage of thesescreen systems, as mentioned previously, is that an automated cleaning system is required to controldebris accumulation at the face of the screen and maintain uniform approach velocities throughout thesubmerged portion of the screen. The flat-plate screen is retained for further analysis in this technicalmemorandum because it is used widely for screening diversions, is likely to work at ACDD, andtypically has lower capital and maintenance costs than the vertical traveling screen.ACDD Passage June 2009 Page 4-18