Capital Hill PUD, Tentative (14-020) <br />June 19, 2017 <br />City of Eugene's Stormwater Management Manual defines the flow control storm as the 10-year <br />design storm for a basin area less than 40 acres. For that reason, the post developed condition will <br />need to match pre-development discharge rates up 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 the new Capital Hill PUD <br />development. The 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 <br />components; (1) Capital Drive Public Improvements runoff, (2) Private Roadway Runoff, and (3) the <br />individual lots. Due to the development's elevation (greater than 500 feet) the site is in a headwater <br />designated area, which triggers the need for stormwater management to address flow control <br />requirements in addition to water quality standards. <br />(1) For the improvements and roadway widening on Capital Drive, all new paving will be <br />pervious pavement (likely asphalt), with an open graded base to match the existing <br />conditions of landscaped and gravel shoulders. This pervious pavement will adequately <br />address treatment (filtration) and flow control requirements for the public road paving <br />improvements. <br />(2) Runoff created from the impervious pavement of the private roadway will be treated by <br />mechanical treatment devices in 2 of the 3 separate outfall locations. A majority of the <br />runoff will be detained in underground storage tanks in order to meet the existing <br />discharge rates offsite, thereby meeting flow control requirements. The runoff <br />discharged to the existing forested area to the east will be released into level spreaders <br />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 <br />submitting for building permits and are not addressed by the improvements proposed <br />above. <br />Due to the sensitivity of development on a hillside, infiltration facilities are not typically <br />recommended and that is why, as noted in (2) above, a combination of catch basins, underground <br />detention tanks, and mechanical treatment devices will be used to treat and filter out sediment <br />and oils in the stormwater collected from the private road. Other reasons infiltration and filtration <br />devices are not recommended for this development are the nature of the steep slopes, the lack of <br />space, and the areas nearby to the east that are prone to landslides. <br />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 <br />point of the roadway. This basin will be collected by catch basins or a trench drain and <br />will go through a mechanical treatment and then enter an underground detention tank. <br />From there the tank will include a flow control structure that will direct runoff to a bubbler <br />on Capital Drive, where it will be released on the roadway where the existing drainage <br />pattern directs runoff. No additional filtration can be applied to this basin area due to the <br />steep slopes and lack of space available. <br />(b) The second discharge location from the private road will be at the bottom of the <br />greenway between lots 12 and 13. There will be various underground storage tanks <br />reducing the peak discharge rate and then releasing the runoff into level spreaders or a <br />Branch Engineering, Inc. <br />2 <br />