Feasibility of Fish Passage at Alameda Creek Diversion Dam

Feasibility of Fish Passage at Alameda Creek Diversion Damthe need to maintain the specified approach velocity, the installation and use of fish screens at ACDDwould result in a reduction in the amount of water available for diversion to Calaveras Reservoir for use asmunicipal water supply.It is estimated that a maximum screen area of 1,400 square feet could be accommodated outside of thesediment channel at the ACDD. With any screening installed at ACDD structural supports, baffles, andother infrastructure would cause some structural blinding. Structural blinding typically reduces theeffective screen area by 20 or 30 percent. To account for anticipated structural blinding, a reduction ofapproximately 20 percent was applied to the estimated maximum screen area for a reduced screen area of1,120 square feet. Based on the relationship shown in Figure 5-1, after accounting for structural blinding,the estimated maximum flow rate of the diversion would be limited to approximately 370 cfs. Bycomparing daily average flows in Alameda Creek with the estimated maximum diversion flow rate of370 cfs, the potential reductions in annual water diversions with screening at the ACDT can be estimated.At the time estimates of the cost of this lost water diversion opportunity were first developed, thehydrologic record for upper Alameda Creek (USGS 11172945) extended only from 1995 to 2004. Thisis a relatively short period of record and may not accurately characterize the temporal distribution ofunimpaired flows that could potentially occur above ACDD. To more accurately predict the frequencyand magnitude of unimpaired flows that could potentially be available during the diversion period(November through April), a synthetic hydrology was produced based on a correlation with theunimpaired daily average flows recorded at the Arroyo Hondo gage (USGS 11173200) from 1969 to1981 and 1995 to 2004 (no flow data are available from Arroyo Hondo for the period from 1982through 1994). This analysis was completed to estimate potential water yields above ACDD over abroader range of hydrologic conditions by extending the period of record from 10 years to 24 years.Appendix B contains a description of the model selection process and detailed flow data.In support of the limited record of measured flow data for upper Alameda Creek, predicted dailyaverage flows from the model described in Appendix B were used to estimate an unimpaired flow foreach day of each year in the simulated period. These daily unimpaired flows were then used tocalculate the difference in potential diversion volumes at ACDD between screened and unscreeneddiversions during the typical ACDD operational period of November through April.Table 5-2 is a comparison of the total annual (water year) diversion with and without screening, aspredicted by the model for simulated flows for the time periods 1969 through 1981 and 1995 through2004. The following list describes each column in the table.■■■Total Flow represents the total simulated flow in upper Alameda Creek for the period of recordduring the November-through-April water diversion time period.Unimpaired Diversion (without screens) represents the estimated amount of water that could bediverted under simulated flows if all flows up to 650 cfs were diverted through ACDT during theNovember-through-April time period.Unscreened Diversion with Schedule B Bypass Flows represents the estimated amount of waterthat could have been diverted under simulated flows if bypass flows consistent with the SFPUCproposednormal water year instream flow schedule 10 (Section 2.3.2.2) were bypasseddownstream at ACDD, while other flows up to 650 cfs were diverted during the Novemberthrough-Apriltime period.10 As referred to here and subsequently in this memorandum, the term “SFPUC-proposed instream flow schedule” alsoincludes the WSIP Final PEIR mitigation flows detailed in Section 2.3.2.2.ACDD Passage June 2009 Page 5-4