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SECTION 3 Flood Control Euaination 3.5.2 Selected Projects to Address Flows Associated With Drywell Decommissioning As stated previously, DEQ is expected to require decommissioning of all the public drywells in the basin (more detail is provided in Section 4.0). As a result, alternative systems will be necessary to handle the flows (up to the 5 -year, 24 hour design event) that were previously handled through drywells. The drywell drainage areas were reviewed, and three project options were developed to handle the flows from these areas as follows: 1) Piped Option — If the drywell is located in close proximity to an existing storm drainage pipe and the pipe has the capacity to handle the flow, a new piped system would be constructed as necessary to route the drywell flows to the existing piped system. 2) Surface Infiltration/Rain Garden Option — If the drywell is located in an area where flow is not able to be routed to an existing piped system, flows would be routed to an area where a vegetated infiltration /rain garden type facility would be constructed to handle flows. Infiltration of municipal stormwater runoff that occurs through the ground surface as opposed to the subsurface is not regulated under the Safe Drinking Water Act. 3) On- Street Rain Garden Option — In areas where street improvements are planned, right -of- way plans /cross - sections that include street side rain gardens for the storage and infiltration of runoff could be used to handle flows from the right -of -way (ROW). For this option, properties adjacent to the R.O.W. would be required to deal with their individual drainage on -site in accordance with requirements for stormwater in the City of Eugene Code (Chapter 9, Section 9.6791(3)). Many of the drywells are concentrated in various portions of the basin. Therefore, prior to selecting an option for the individual drywells, drywells located in close proximity to each other where flows were proposed to be managed in accordance with the same option as defined above, were grouped into drywell "clusters ". This grouping of drywells was conducted because some of the management options could be applied and constructed in a manner to address a "cluster" of drywells. A total of 39 drywell clusters were delineated, as illustrated on the Stormwater Management Strategy Development map in Appendix H and listed in Table 3 -6 below. Table 3- 6 also lists the CPs associated with each drywell cluster to address decommissioning of the drywells. A capital project fact sheet including a map, estimated costs, and conceptual design assumptions is provided for each of these projects in Appendix A, and the location of each of these projects is also shown in Figures 3 -2 through 3 -8. The project options were selected for each cluster to maximize water quality benefits while addressing the flows associated with decommissioning. See Section 4.0 — Water Quality Evaluation for more detail regarding the development of each of these projects. 0:A25695978 Eugene RR -SC Final Basin P1an\Master P1anTINAL 2- 2010\Master_Plan 3- 11- 10_FINAL_ Word _Version.doc 3-13