Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />3.3 WATER QUALITY TREATMENT <br />As noted previously, this report is intended to address the stormwater created by the <br />improvements to the public roadway on Capital Drive, and the new private road circulating <br />through the site. <br />The primary stormwater management tools for the private roadway runoff will be underground <br />storage and mechanical treatment due in large part to the lack of available space for filtration <br />facilities. The northern and southern basins will be too small to justify the cost of mechanical <br />treatment and will therefore have to pay the higher rate of SDC's for the impervious area created. <br />The central basin will be directed through a combination of detention vaults as well as a <br />mechanical treatment device such as the Contech CDS manhole. <br />The basin area draining to Discharge location #2 likely would have a much larger peak flow rate if <br />not for the layout of the multiple underground storage tanks located along the roadway. These <br />detention facilities act as a pre-treatment, by collecting some solids and oils before releasing <br />downstream. The peak discharge rate during the water quality storm, located just upstream of <br />the final underground storage tank, is 0.08 cfs. It is at that location, just upstream of the 51'' <br />detention tank, that the mechanical treatment structure will be located. A treatment manhole or <br />some other proprietary treatment technology may be selected and approved by the City of Eugene <br />when applying for the building permit for the roadway and associated utility installation. <br />A majority of the public improvements will consist of replacing existing asphalt that doesn't <br />currently have any water quality treatment. The new impervious area created by the public <br />improvements, similar to the private roadway improvements, will be too little to justify the cost <br />of mechanical treatment, but will be directed through catch basins and underground detention <br />helping to filter out sediment before being released. The northern basin when released into the <br />existing ditch will in some ways unintentionally act as a swale, treating runoff before being <br />entering into Hendrick's Park. <br />As mentioned previously, this report is primarily concerned with the public and private roadway <br />improvements and how they will address stormwater with the intent that the private dwellings <br />will address flow control and water quality when applying for building permits on a case by case <br />basis. Possible water quality mechanisms for the private lots would be the use of mechanical <br />treatment or perhaps lined planters, rain gardens, sand filters, or swales. <br />3.4 FLOW CONTROL AND DESTINATION <br />Flow control devices and numerous underground storage facilities were applied to the runoff <br />directed to the Discharge locations 1, 2, 3, & 4 in the HydroCAD analysis. Flow control will be met <br />using orifices in underground storage facilities. The following table displays the summary of the <br />various detention facilities proposed as part of the Capital Hill PUD development. For a schematic <br />detail of the orifice layout, see sheet C2 of the plans. For additional information on the orifice <br />size and elevation information, please refer back to the HydroCAD Analysis in Appendix C. <br />Branch Engineering, Inc. <br />7 <br />