Proposed Stormwater System <br />It is proposed to construct two privately owned infiltration storm planters that collect all impervious runoff from future roofs and <br />proposed paving. To collect stormwater runoff on -site from the proposed paved areas, the applicant is proposing to construct valley <br />gutters in the drive aisle that will direct runoff to catch basins routed to the infiltration storm planters on -site. Roof drainage will be <br />collected via gutters and roof drains which will then be hard piped to the proposed infiltration stormwater planters allowing future <br />stormwater roof runoff to be piped directly into the infiltration storm planters. The physical characteristics of the storm facility is <br />shown in Table 2 of the A & 0 Engineering Stormwater Report, with Table 3 providing the impervious area calculations. <br />Stormwater Calculation Parameters <br />HydroCAD software was utilized to perform hydraulic and hydrology calculations with the parameters taken from the City of Eugene <br />Stormwater Management Manual (2014) unless otherwise noted. Please reference the attached A & 0 Engineering Stormwater <br />Report, page 3 for details. <br />Hydraulic Calculations <br />All Stormwater runoff from the new impervious surfaces is proposed to fully infiltrate during the design 10-year storm even within the <br />proposed stormwater planters. An infiltration rate of 2.50 in/hr was used in the model for the growing medium layer, while 2.67 in/hr <br />was used for all elevations below the growing medium's invert in the rock chamber into native soils. <br />As part of the hydraulic analysis, peak hydraulic grade lines of the infiltration storm planters were analyzed to ensure the safety of <br />the structures and residents. Shown in Table 6, the storm facility contains the 10-year storm event and thus does not endanger <br />surrounding infrastructure, all stormwater is fully treated in the water quality design storm. Also, all stormwater runoff is modeled to <br />be contained within the stormwater planters during the design 10-year storm. Please reference the attached A & 0 Engineering <br />Stormwater Report - page 4, Table 6 - Facility Hydraulic Grade Lines for the analysis results of each storm event. <br />Conclusion <br />With the proposed stormwater system pollution reduction goals are met via infiltrating all stormwater runoff within the stormwater <br />planters. Also Flood Control goals are met by stormwater runoff being modeled to be completely contained within the stormwater <br />system during the design 10-year storm event. The proposed infiltration storm planters will serve the development adequately and <br />are not expected to allow stormwater runoff from the development to damage surrounding structures or present public safety risk. <br />An approved adjustment to a standard pursuant to the provisions beginning at EC 9.8015 of this land use code constitutes <br />compliance with the standard. <br />Response: An adjustment is not requested. All stormwater flood control systems have been designed and will be constructed in <br />accordance with adopted plans and policies, EC Ch 6 and EC Ch 7, as well as the most current Stormwater Management Manual. <br />(i) The proposed subdivision complies with other applicable development standards for features explicitly <br />included in the application. <br />Response: Riverstone Park Subdivision is proposed in compliance with all development standards for features explicitly included <br />the the Subdivision — Tentative plan application. <br />=E: 9.85,16 Subdivision, f eniative Plan Approval Criteria — General/ Viscretionary (continued) <br />(11) The proposal complies with the Traffic Impact Analysis Review provisions of EC 9.8650 through 9.8680 where <br />applicable. <br />Traffic Impact Analysis Review. <br />9.8650 — Purpose of Traffic Impact Analysis Review. <br />15 <br />