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SECTION 3 Flood Control Euaination Because the drywells were not shown to provide significant benefits with respect to resolving capacity deficiencies for the larger storms; and given that the public drywells will eventually need to be decommissioned; and given that the City and County do not have authority over the private drywells, a decision was made to continue with the flood control evaluation and the identification of capital projects using the model that did not include the infiltration of runoff associated with existing drywells. The hydraulic model results are summarized by conduit in Table 3 -2 for the system design storm, and full model results are provided in Appendix B. 3.4 Flooding Problems Identified by the Model This section provides a general description of model - identified flooding problems. The model results are summarized in Table 3 -2 and include both peak flows and water surface elevations for the relevant design storm under both existing and buildout conditions. The last columns in the table indicate the design event and land use condition when certain conduits are expected to be deficient and the associated capital project that addresses the deficiency (discussed in more detail in Section 3.5). For pipe segments and roadway crossings, surcharging was considered to be acceptable, and flooding problems were only identified if the models predicted water getting out of the system and into the streets. For open waterways, deficiencies were identified when the depth of the design flow was predicted to exceed the tops of the channel banks. In general, very few flooding problems were identified in the River Road Santa Clara basin. Specifically, one flooding problem is expected to occur in the Flat Creek drainage system during existing land use conditions. Nineteen open channel and 17 pipe segments were identified as deficient for their respective design storms in the remaining three drainage systems (i.e., A -1 Channel, Spring Creek, and Willamette Overflow). Eighteen of the 19 open channel segments and eleven of the 17 pipe segments are expected to be deficient under existing land use conditions. Additionally, one open channel and six pipe segments are expected to be deficient under buildout conditions. Each of these problems is listed in Section 3.5 in association with the proposed capital project to address the problem. In addition to flooding problems associated with predicted capacity deficiencies, decommissioning of drywells would result in the need for an alternative drainage system to handle or convey the 5- year flows that are currently discharging to drywells. Management strategies to address this issue are described in Section 3.5 and 3.6 as well. 3.5 Development of the Flood Management Strategy As shown in the stormwater basin master planning process flow chart in Figure 1 -1, Step 1 included a compilation of basin characteristics. These basin characteristics are summarized in Section 2.0 of this document. Step 2 in the process includes problem identification under both existing and future land use conditions, focusing on the major components of the public drainage system. These results are provided in Section 3.4 above. The next step includes the development of potential stormwater management tools (i.e., capital projects or development standards) to address the identified problems. This section describes the capital project and development standard alternatives that were considered to address the identified flooding problems. 0:\25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-10