Capital Hill PUD, Tentative (14-020) <br />August 22nd, 2017 <br />Though the existing site has many `discharge' locations since it is at the top of a hill and there is <br />no storm infrastructure or system nearby, for our purposes in simplifying the design, we will <br />assume five main points of connection/discharge for designing the post developed condition. <br />1. A portion of the site drains into the eastern gutter line on Capital Drive. <br />2. A portion of the site sheet flows to the east into Hendrick's Park. <br />3. A portion of the site drains into a shallow ditch/channel east of a gravel path north of the <br />Capital Drive existing pavement. <br />4. A portion of the site drains into the western gutter line on Capital Drive. <br />5. A portion of the site sheet drains onto and across Capital Drive and onto the adjacent <br />neighboring properties to the west. <br />The City of Eugene's Stormwater Management Manual defines the flow control storm as the 10- <br />year design storm for a basin area less than 40 acres. For that reason, the post developed condition <br />will need to match pre-development discharge rates up through the 10-year design storm into the <br />destinations described above. <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 be separated into three initial components; <br />(1) Capital Drive Public Improvements, (2) Private Roadway Runoff, and (3) the individual lots. Due <br />to the development's elevation (greater than 500 feet) the site is in a headwater designated area, <br />which triggers the need for stormwater management to address flow control requirements in <br />addition to water quality standards. <br />Due to the sensitivity of development on a hillside, infiltration facilities are not typically <br />recommended and that is why a combination of catch basins, underground detention tanks, and <br />mechanical treatment devices will be used to treat and filter out sediment and oils in the <br />stormwater collected from the private road. Other reasons infiltration and filtration devices are <br />not recommended for this development are the nature of the steep slopes, the lack of space, and <br />the areas nearby to the southeast that are prone to landslides. There are a few basins where the <br />new impervious areas are smaller and do not justify the cost of installing mechanical treatment, <br />though flow control is still applied. Those smaller basins will require higher SDC's as necessary <br />and coordinated with the City of Eugene. <br />3.1.1 Capital Drive Public Improvements Runoff <br />Capital Drive is an existing roadway that sheet flows to the west with intermittent curb <br />along the west side of the roadway. The roadway has a high point toward the middle of <br />the project essentially splitting the roadway into roughly two larger basins; one that <br />drains north and one that drains south. Considering the various aspects of the existing <br />drainage patterns, stormwater management for Capital Drive will be split into several <br />components that discharge to locations 1, 3, 4, & 5 as described in section 2.3 previously. <br />Branch Engineering, Inc. <br />2 <br />