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SECTION 3 Flood Control Euaination Based on the above information, the City and County agreed to move ahead with the #2 model adjustments from Table 3 -4, which included reducing mapped impervious percentage areas using Equation #1. These model adjustments were applied basin -wide to the A -1 Channel, Spring Creek, Flat Creek, and the Willamette Overflow major subbasins. 3.3 Model Results As described in Section 3.1 of Volume I (City -wide Study Methodology and Summary), models were run for the selected design storms, and model output was produced for peak flows and water surface elevations for both existing and future conditions. These results were used to identify capacity deficiencies in the system. Surcharging was considered to be acceptable and problems were only identified if the models indicated that water was exiting the system and onto the streets. For this basin, model results were produced for existing and future conditions for two scenarios as described in subsection 3.1.1: 1) the model scenario did not account for infiltration from the existing drywells in the model simulation; and 2) the model scenario did account for existing drywells in the model simulation. Given new rules related to stormwater discharges to drywells (under the Safe Drinking Water Act), decommissioning of the public drywells in this basin will ultimately be required (with the possibility of some exceptions depending on confirmed groundwater levels). Therefore, the model scenario without the incorporation of drywells was used to evaluate the capacity of the drainage system when public drywells are ultimately decommissioned. It should be noted that private drywells are under the authority of the Oregon Department of Environmental Quality (ODEQ) and any decommissioning associated with private drywells (if required) would be directed by ODEQ. Of the 785 drywells in the basin, 634 (81 %) are privately owned, 79 (10 %) are owned by Lane County, and 72 (9 %) are owned by the City of Eugene. Section 4.0 of this plan provides more detail regarding the Safe Drinking Water Act and associated DEQ requirements for stormwater discharges to drywells. As mentioned in Section 3. 1, the model simulation that accounted for drywells was based on an incomplete dataset at the time and included a portion (approximately 759) of the total 785 drywells. It was anticipated that the modeling results would show that the existing drywells are providing some relief with respect to capacity deficiencies. However, the comparison of model results between both scenarios (with and without drywells) for the 10 -year and 25 -year design events did not show significant differences with respect to identified flooding problems. Based on a more detailed review of the results, it was assumed that this occurred for the following two reasons: 1. The drywells were only designed to infiltrate runoff from up to the 5 -year storm event and the design events modeled to identify flooding issues were the 10 -year and 25 -year events. The accommodation of the flows from the 5 -year storm had minimal impacts with respect to flows from the larger storms when comparing the two model scenarios. 2. Only 22% of the total modeled drainage area was estimated to be draining to drywells (following this analysis, the number was updated to 25 %). Therefore, the majority of the drainage area was already accommodated via the pipe and surface storm drainage system and not highly impacted by infiltration associated with drywells for the larger storms. 0:\25695978 Eugene RR -SC Final Basin P1an\Nlaster P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-9