Capital Hill PUD, Tentative (14-020) <br />March 3, 2017 <br />acres. For that reason, the post developed condition will need to match pre-development discharge rates up <br />through the 10-year design storm. <br />3.0 PROPOSED DEVELOPMENT & STORMWATER MANAGEMENT <br />3.1 STORMWATER SUMMARY <br />This report is for the storm treatment and management resulting from new Capital Hill PUD development. The <br />development includes 33 total residential lots as well as a private road. <br />Stormwater management for the Capital Hill PUD can essentially be separated into three components; (1) Capital <br />Drive Public Improvements runoff, (2) Private Roadway Runoff, and (3) the individual lots. Due to the <br />developments elevation (greater than 500 feet) the site is in a headwater designated area, which triggers the <br />need for stormwater management to address flow control requirements in addition to water quality standards. <br />(1) For the improvements and roadway widening on Capital Drive, all new paving will be pervious <br />pavement (likely asphalt), with an open graded base to match the existing conditions of landscaped <br />and gravel shoulders. This pervious pavement will adequately address treatment and flow control <br />requirements for the public road paving improvements. <br />(2) Runoff created from the impervious pavement of the private roadway will be treated by mechanical <br />treatment devices in 3 separate outfall locations. A majority of the runoff will be detained in <br />underground storage tanks in order to meet the existing discharge rates offsite, thereby meeting flow <br />control requirements. The runoff discharged to the existing forested area to the east will be released <br />into level spreaders in order to disperse the flow across a wider path <br />(3) The individual lots will need to meet water quality and flow control requirements when submitting <br />for building permits and are not addressed by the improvements proposed above. <br />Due to the sensitivity of development on a hillside, infiltration facilities are not typically recommended and that <br />is why, as noted in (2) above, mechanical treatment devices will be used to treat and filter out stormwater <br />collected from the private road. There will be 3 discharge locations created by the private roadway construction. <br />(a) The first will be the southern portion that drains down from the first (southern) high point of the roadway. <br />This basin will be collected by catch basins or a trench drain and will go through a mechanical treatment and <br />then enter an underground detention tank. From there the tank will include a flow control structure that will <br />direct runoff to a bubbler on Capital Drive, where it will be released on the roadway where the existing drainage <br />pattern directs runoff. (b) The second discharge location from the private road will be at the bottom of the <br />greenway between lots 14 and 15. There will be various underground storage tanks reducing the peak discharge <br />rate and then releasing the runoff into level spreaders or a series of level spreaders in the eastern greenway west <br />of the Ribbon Trail. (c) The third and last discharge location will be the northern basin that collects runoff west <br />of the second (northern) high point. The drainage will be collected in a catch basin and treated with a mechanical <br />treatment device and be released via bubbler into Capital Drive and released onto the pervious pavement, again <br />matching the pre-developed drainage conditions. <br />3.2 STORMWATER ANALYSIS <br />All stormwater calculations were determined using HydroCAD 10.0 modeling software. This program utilizes <br />the Santa Barbara Unit Hydrograph hydraulic modeling method. All results of the analysis are included in <br />Appendix C. 24-Hour Rainfall depths are based on City of Eugene Stormwater Management Manual, Appendix <br />C. <br />Branch Engineering, Inc. <br />2 <br />