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(figure 2). At the district's request, the Corps agreed to keep the grade control at the best <br />location for the Rillito Recharge Project if the district would pay for any additional <br />modifications needed due to the project. <br />Design considerations included bank stabilization elevation, need for additional toe -down <br />due to scour, and stability due to presence of the fuse plugs and inflatable dam. <br />Project Activities <br />Feasibility Studies <br />In 1987, the district, ADWR, and the city of Tucson entered into an agreement for jointly <br />financing the feasibility studies. Feasibility studies were initiated before developing the <br />cooperative agreement with Reclamation. Three sources of water were studied: CAP <br />water, reclaimed water, and surface water from Rillito Creek. <br />A University of Arizona study of a natural recharge event found that most flow events in <br />the project area have peak discharges less than 240 cubic feet per second and infiltrate <br />into the streambed before reaching the Santa Cruz River. Stormwater is likely to be most <br />available in winter. Reclaimed water from Tucson Water Department's Sweetwater <br />recharge plant could be conveyed via the existing reclaimed water distribution system. <br />The availability of reclaimed water is constrained by other uses, with the least use <br />occurring during winter days. Treated CAP water was assumed available for recharge as <br />only a portion of the total CAP allotment was scheduled for use. An existing pipeline <br />could convey CAP water to the site. Availability would be highest during winter nights. <br />The interagency team eliminated the Alvernon to Swan reach from consideration because <br />an early assessment indicated that a recharge facility located in the reach would have a <br />high potential to impact landfills thought to exist near Columbus Road. Subsequent <br />landfill assessments found that most of the landfill was either removed by bank erosion <br />or appears to have received green waste and construction debris instead of municipal <br />waste. Eliminating the Swan -to- Alvernon study reach due to the uncertainties regarding <br />landfills resulted in significantly fewer recharge opportunities. The Swan- to- Alvernon <br />channel reach has greater depth -to -water than the Craycroft -to -Swan reach and thus has <br />more potential to store recharged water. <br />A wide variety of in- and off - channel recharge techniques were evaluated. Based on cost, <br />site constraints, and case studies, the concept of retrofitting bridges with gates or an <br />inflatable dam was eliminated, and an inflatable dam located on top of an invert <br />stabilizer approximately 1/2 mile upstream of Swan Road was proposed. In 1992, two <br />alternatives were evaluated. The first would use all potentially available land and water <br />sources and would demonstrate the widest range of recharge techniques. The second, <br />preferred alternative would use surface water and existing county land only. <br />Near - surface sediments were tested using ring infiltrometers and were found to have <br />highly variable but favorable infiltration rates. Mounding analyses were conducted for <br />the proposed alternatives using fall 1990 and May 1992 depths -to- water. Based on these <br />results, it appears that the project area would not provide much capacity for CAP <br />recharge, particularly if the inflatable dam were operated to capture and detain winter <br />surface flows for recharge. <br />Program Summary Report Part 111 — Demonstration Project Summary Reports 5 <br />