DEVELOPMENT STANDARDS SECTION 4 – STORM STANDARDS 3 STORM DRAIN CITY STORM DRAINAGE DESIGN Storm Drainage plans, at a minimum, will be required for all plats, commercial property improvements, and parking lots, meeting the following requirements: 1. All storm drainage plans shall be stamped and signed by a Professional Licensed Engineer (PE). 2. Applicants PE shall use the current Stormwater Management Manual for Eastern Washington, https://fortress.wa.gov/ecy/ezshare/wq/Permits/Flare/2019SWMMEW/2019SWMMEW.htm or approved equivalent for reference in the design of stormwater treatment and flow control for post construction requirements for new development and redevelopment. The design shall at a minimum use the following design storms, or as recommended for the proposed BMP, whichever is greater: A. Volume based treatment BMP’s shall be designed based on the first ½” rainfall (6-month, 24- hour storm event) for all pollution generating impervious surfaces. B. Flow rate based treatment BMP’s shall be designed based on the runoff from the SCS Type II 24-hour storm with a rainfall of 0.59-inches (6-month, 24-hour storm event) for all pollution generating impervious surfaces. C. Flow Control shall be designed based on retaining the full 10-year 24-hour storm event (1.3- inches) on-site and matching the pre-development and post-development 25 year, 24-hour storm events (1.6-inches). D. Where structural BMPs are required, property owners shall operate and maintain the facilities in accordance with an Operation and Maintenance (O&M) plan that is prepared in accordance with the provisions in Chapters 5 and 6 of the Stormwater Management Manual for Eastern Washington (most current version). The O&M plan shall address all proposed stormwater facilities and BMPs, and identify the party (or parties) responsible for maintenance and operation; the O&M plan must also address the long-term funding mechanism that will support proper O&M. At private facilities, a copy of the plan shall be retained onsite or within reasonable access to the site, and shall be transferred with the property to the new owner. Commercial and residential property developers may also develop generic O&M plans, including checklists of actions and procedures for the operators, for BMPs that are commonly used in their projects. 3. Designer shall place catch basins at a maximum spacing of 400’, at all changes in direction, and shall be located upstream of handicap accessible ramps (to prevent gutter line flow through handicap ramps areas). Storm drains shall be constructed to the furthest extent of project, unless otherwise approved by the Public Works Director. 4. Permanent stormwater control facilities shall be maintained and operated in compliance with Chapter 9.100 'Storm Drainage and Surface Water Management Utility' of the Ellensburg Municipal Code and the current Stormwater Management Manual for Eastern Washington. Stormwater control facilities include streams, rivers, ponds, lakes, waterways, groundwater, and functionally related natural and manmade stormwater control facilities that combined constitute the city's stormwater control facility. CONSTRUCTION STORMWATER RUNOFF AND POLLUTION PREVENTION Construction stormwater runoff shall be in compliance with this chapter of the Development Standards, the current Stormwater Management Manual for Eastern Washington, and Chapter 9.100 of the Ellensburg Municipal Code. Applicant and/or applicant's Engineer shall submit to the City, for approval, a Stormwater Pollution Prevention Plan (SWPPP) for all projects with one-acre or more, and from construction projects of less than one acre that are part of a common plan of development or sale, prior to construction. Applicant shall also be required to implement the SWPPP, inspect construction stormwater runoff, erosion control, and pollution prevention methods throughout construction and modify/implement the plan, as necessary, using Best Management Practices in compliance with the current Stormwater Management Manual for Eastern Washington. This chapter also includes the requirement to mitigate migrant dust from leaving the site (dust control).
