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SECTION 4 water Quality Eualuatiou system size and pipe size (using Manning's equation) based on the calculated flow rate. In some cases, more detailed information was desired regarding the proposed pipe size and length. In these cases, a new pipe was actually included in the XP -SWMM model and iterative model runs were conducted to size the pipe as opposed to using the Rationale Method and Manning's equation. See the design assumptions section of each CP fact sheet in Appendix A for specific sizing methods for each individual CP. The second method was used if routing of the drywell drainage area to the closest piped system would require a re- delineation of the existing subbasin boundaries. In other words, including a new pipe in the system would result in redirecting some flows into a different subbasin. In these cases, the drywell drainage area was delineated and subtracted from the existing subbasin and moved to the new subbasin that would incorporate the drainage. The model was then run to ensure that capacity would be available to handle the new drainage. Then, either the Rational Method/Manning's Equation, or if more detail was desired, an XP -SWMM model simulation was conducted to size the pretreatment system and pipe. The CP fact sheets for each of these projects are provided in Appendix A. Construction and retrofit of the piped system provides an opportunity to provide for water quality treatment. Costs of the proposed pretreatment systems (i.e., underground structural devices to provide water quality treatment) are also included along with costs of the proposed pipe systems. 2) Surface Infiltration/Rain Garden Option — Given the flat topography and long distance to the closest piped system for some of the drywell clusters, alternative options were needed to address the decommissioning of drywell clusters in areas north of Horn Lane, where street improvements were not likely to occur in the near future. For these drywell clusters, the proposed option was to route the flows to an area where a neighborhood -scale vegetated infiltration /rain garden type facility could be constructed to handle flows. It should be noted that infiltration of municipal stormwater runoff that occurs through the surface of the ground as opposed to the subsurface is not considered to be a UIC and is therefore not regulated under the Safe Drinking Water Act. For the modeling conducted and described in Section 3.0, drainage areas were initially delineated for the drywells that were included in the system at the time. The system included the 79 County wells, 46 of the 72 City wells, and 634 private wells. The system (drywell database) was under development and thus 26 of the existing City drywells were not included in this drywell drainage area delineation. These drywell drainage areas that were delineated represented areas draining to multiple drywells (i.e., clusters). Individual drywell drainage areas were not delineated. Based on this information, an average drainage area per drywell was estimated to be 2.70 acres. As drainage areas were only calculated for some of the drywell clusters, use of the average drainage area per drywell allowed for conceptual sizing, design and cost estimating for neighborhood - scale infiltration /rain garden facilities or CPs for all clusters on a normalized basis. Detail regarding the steps conducted to size the rain garden facilities for each cluster is provided as follows: 0:\25695978 Eugene RR -SC Final Basin P1an\Nlaster P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 4-10