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SECTION 3 Flood Control Euaination <br />public drywells which includes that they serve an area no greater than 40 acres and infiltrate all <br />of the runoff up to a 5 -year, 24 -hour event. In the model, when the capacity of the storage node <br />was reached, the areas draining to drywells would begin to contribute additional runoff to the <br />piped and surface water drainage system. The drywell storage nodes were sized using an <br />iterative trial and error process until the 5 -year, 24 -hour event filled the storage volume but did <br />not contribute runoff flows to the piped and surface stormwater drainage system. <br />As a result, two hydrologic modeling scenarios were developed for the River Road Santa Clara <br />Basin: <br />1) One model scenario was developed that did not account for the infiltration associated with <br />existing drywells. <br />2) The second model scenario was developed to account for the infiltration associated with <br />existing drywells. <br />The purpose of developing both model scenarios was to evaluate the impacts that the drywells <br />were having on the capacity of the piped and surface drainage system during the various design <br />events. Decommissioning of the public drywells in this basin is ultimately required (see Section <br />4.0 for a summary of relevant requirements). Therefore, comparing the results from the two <br />model scenarios provided useful information in order to better understand how <br />decommissioning will impact the system and planned capital projects in terms of capacity and <br />sizing. <br />3.1.2 River Road Santa Clara Basin Hydraulic Data <br />The primary purpose of the modeling was to evaluate the capacity of the existing storm drainage <br />system. The evaluation of the storm drainage system included a hydraulic analysis of the major <br />storm pipes, culverts, and open channels, which convey stormwater discharges. The original <br />River Road Santa Clara Basin Plan, created in 2002, identified a number of hydraulic data gaps <br />due to the multi jurisdictional ownership of the drainage system and the lack of a comprehensive <br />data set for the overall drainage system, a result of the multi jurisdictional ownership. Data gaps <br />have since been addressed by the City of Eugene and Lane County. Specifically, survey data <br />collected by Lane County between October and December 2005 allowed for refinement of the <br />piped and open channel segments of the drainage system (described further in this section and in <br />Appendix G). These refinements have been applied to the original model, and the following <br />parameters (i.e., model input data) were compiled for each pipe, culvert or open channel section: <br />1. Conduit name. <br />2. Upstream node number. <br />3. Downstream node number. <br />4. Conduit size (diameter for pipes and culverts; cross - section dimensions for open channels) <br />5. Conduit length. <br />6. Conduit material for pipes and culverts. <br />7. Upstream and downstream invert elevations. <br />8. Upstream and downstream ground surface elevations. <br />0:\25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-4 <br />