Feasibility of Fish Passage at Alameda Creek Diversion Dam

5.0 Capital and Operating and Maintenance Costs5.2.1 REDUCTION IN DIVERSION CAPACITY DUE TOSCREENINGThe capacity of the ACDT to transfer water to Calaveras Reservoir would likely be reduced if screeningwas implemented according to the criteria described by NMFS (1997 and 2008) and CDFG (2009). Ingeneral, fish screening criteria include a combination of (1) low approach velocity, and (2) adequatesweeping velocity. Approach velocity is defined as the velocity of water that passes through a screeningdevice perpendicular to the screen openings. Sweeping velocity is the velocity of water that runsparallel to the screen openings. At ACDD, the sweeping velocity requirement would be met byproviding a fish screen bypass at the downstream end of the screens. Water costs associated with thefish screen bypass are addressed in Section 5.2.2. The purpose of this section is to quantify reductionsin the amount of water that could be diverted to Calaveras Reservoir due to potential reductions in themaximum rate at which water could be diverted through the ACDT, if screening was implemented.The current maximum diversion rate through the ACDT is estimated to be approximately 650 cfs. Whenscreens are used, an approach velocity of 0.33 fps 9 is typically required by CDFG (2009) to protect fishfrom impingement. NMFS (1997 and 2008) also has approach velocity criteria, which vary dependingupon site specifics. At ACDD, NMFS criteria would be less restrictive than CDFG’s (2009), so theCDFG criterion is used for the purposes of analysis in this memorandum. Approach velocity anddiversion rate are related to screen area; for a given rate of diversion, a larger screen area will result in alower approach velocity. Alternatively stated, for a given approach velocity, increasing the screen areaincreases the maximum potential diversion rate, as shown on Figure 5-1. Therefore, a primary constraintduring screen design is the size of screen that a site can accommodate. Because of the site constraints and2500Screen Area in square feet20001500100050000 100 200 300 400 500 600 700 800Flow in CFSFigure 5-1Flow versus Required Screen Area9 The screen approach velocity at ACDD would be determined in coordination with NMFS and CDFG.ACDD Passage June 2009 Page 5-3