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). A. A site-specific geotechnical engineering report is required for all development that disturbs an acre or greater, or any smaller site that the Stormwater Utility Manager deems necessary because of the potential for high groundwater. B. Groundwater is strongly influenced by seasonal irrigation. Depth to groundwater, permeability, and infiltration tests should be conducted during both irrigation and post-irrigation seasons. (Irrigation season April – October, post-irrigation season October – April.) C. The geotechnical report must have been completed within five years of the application date and must specifically address the site and project described in the application. If the report is older than five years or was prepared for a different project on the same site, it may still be accepted if the geotechnical engineer provides an addendum updating the report or a letter confirming its applicability to the current project on the same site. 2. Applicants PE shall use the current Stormwater Management Manual for Eastern Washington, https://fortress.wa.gov/ecy/ezshare/wq/SWMMs/2024SWMMEW/2024_SWMMEW.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 BMPs 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 BMPs 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 Chapter 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. Low Impact Development (LID) is the preferred approach to site development. Designers are required to use Low Impact Development (LID) Principles and BMPs, where feasible. 4. 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 the project, unless otherwise approved by the Public Works Director. 5. 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.
