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