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Microgravity methods were used to estimate groundwater storage change from early <br />December 1992 to late August 1993. The distribution of gravity changes south of the <br />flood plain were consistent with geology and indicated preferential southward flow of <br />groundwater in the west part of the study area. However, in the area east of Swan Road, <br />flow away from highly transmissive stream deposits appeared to be inhibited by lower <br />permeability sediments to the south, resulting in groundwater mounding to the surface. <br />In all, some 6,000 acre -feet were recharged in the reach between Craycroft Road and <br />Dodge Boulevard; however, it appeared that much of the recharge occurred downstream <br />of Swan Road. <br />The ALERT System <br />The Rillito Recharge Project enabled the district to expand the existing Automated Local <br />Evaluation in Real Time (ALERT) system, a network of stream and precipitation gauges <br />which supplies data to the district on a real -time basis. This expansion of stream and <br />precipitation gauges in the Rillito watershed improves the National Weather Service's <br />ability to issue special weather statements and allows the Pima County Emergency <br />Services to direct field personnel in times of flooding. <br />Twenty -two precipitation gauges and four stream flow gauges were added to the ALERT <br />system during 1991 -95 for this project; however, no additional staff were hired. To <br />develop the full -scale Rillito Recharge Project, district staffing would have to increase to <br />5.5 full time equivalents to improve data base analysis and management, address higher <br />system maintenance needs, and oversee the dam operation system's development. <br />Hydraulic Analysis <br />The hydraulic analysis of the project reach was updated to provide detailed evaluation of <br />the fuse plugs and inflatable dam. First, new topographic mapping was provided by the <br />district due to bank erosion from the 1993 flood. The Corps' model, Hydrologic <br />Engineering Center (HEC) -2 hydraulic analysis, was modified using new soil- cement <br />bank stabilization plans from the Corps. <br />The inflatable dam and fuse plugs have intrinsic features to control the potential for <br />increasing flood risks. If water flowing over the inflatable dam were to exceed a certain <br />height, the dam would begin to deflate, regardless of whether the automated dam <br />operation system is active. If the reservoir level were to rise still higher, the fuse plugs <br />would begin to erode, thus providing additional conveyance in the channel to pass the <br />flows. <br />A dam failure scenario that assumed the fuse plugs were breached was evaluated. Based <br />on this evaluation, the preliminary design of the fuse plugs was modified to include <br />backslope erosion protection, sediment gradation, and pilot notches. In addition, a small <br />levee or vertical extension of the south embankment was recommended to prevent <br />backwater from entering residential areas if the inflatable dam failed to deflate and the <br />fuse plugs remained in place. <br />Program Summary Report Part /// — Demonstration Project Summary Reports <br />