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Sediment Analysis <br />The sediment characteristics of the Rillito Recharge Project source waters, including the <br />sediment volume and variability, were estimated from empirical methods, USGS <br />streamflow records, and sediment samples from the Rillito Creek watershed taken by <br />USGS and by others. Sediment volume was estimated for the "typical" event as well as <br />for average annual conditions and key recurrence interval floods. Sediment delivery was <br />found to vary with season and the runoff source watershed, as well as with discharge <br />(Simons, Li and Associates, 1994). Even without the dam, average deposition was <br />estimated at 0.17 foot. This is the "background" rate of deposition. <br />Sediment maintenance would be required to remove background sedimentation, <br />additional bed - material deposition, and suspended sediment deposition. Taking into <br />consideration annual "background" bed - material deposition, plus bed -load deposition and <br />suspended- sediment deposition caused by operation of the inflatable dam during the <br />winter season, the estimated annual sediment deposition in the recharge basin, over the <br />long term, is predicted to be 2.3 to 3.3 acre -feet (Simons, Li and Associates, 1994). <br />Sedimentation would have a significant effect on the operation of the Rillito Recharge <br />Project. Sediment deposited in the in- channel recharge area or conveyed to off - channel <br />basins could substantially reduce or prevent effective recharge rates. Deposition of course <br />sediments would require regular inspection, maintenance, and removal to restore the <br />in- channel storage volume. Removal of sediments would be costly. At $3 per cubic yard, <br />the estimated average annual cost to excavate sediment could be as much as $16,000, not <br />including personnel, fees, or materials testing. The estimate does not account for <br />occasional large flows that would either "flush" the channel of sediments or deposit <br />larger volumes of sediment. <br />Project Termination <br />In April 1995, the district decided not to construct the recharge project due to technical, <br />economic, and institutional considerations. These considerations are discussed below: <br />• Limited aquifer storage capacity.— Sediments at the site are highly transmissive, <br />but total underground storage is limited by lower permeability sediments south <br />and west of the site. <br />• No economic benefits. —The district does not have statutory authority to receive <br />recharge credits for recharged water, and ADWR has never issued credits for <br />stormwater recharge. <br />• Inadequate hydrologic data base. —The dam operation system demands a higher <br />level of performance and greater sensitivity to low flows than does a flood <br />warning system. Much more system development is needed to operate and <br />develop the dam operation system, which would take additional time and money <br />during both design and operational phases. <br />• Increased staff needs. —The ALERT system is currently understaffed relative to <br />other flood warning systems in the country. <br />Program Summary Report Part 111 — Demonstration Project Summary Reports > > <br />