1710 W MARINE VIEW DR 2024-01-03

BMP C201: Grass -Lined Channels Purpose To provide a channel with a vegetative lining for conveyance of runoff. The purpose of the vegetative lining is to prevent transport of sediment and erosion. Conditions of Use This practice applies to construction sites where concentrated runoff needs to be directed to prevent erosion or flooding. Use this BMP when a vegetative lining can provide sufficient stability for the channel cross sec- tion and at lower velocities of water (normally dependent on grade). This means that the chan- nel slopes are generally less than 5 percent and space is available for a relatively large cross section. . Typical uses include roadside ditches, channels at property boundaries, outlets for diversions, and other channels and drainage ditches in low areas. Channels that will be vegetated should be installed before major earthwork and hydroseeded with a bonded fiber matrix (BFM). The vegetation should be well established (i.e., 75 percent cover) before water is allowed to flow in the ditch unless BMP C 122: Nets and Blankets is used to protect the channel. With channels that will have high flows, erosion control blankets should be installed over the hydroseed. If vegetation cannot be established from seed before water is allowed in the ditch, sod should be installed in the bottom of the ditch in lieu of hydro - mulch and blankets. Design and Installation Specifications See Figure II-3.10: Typical Grass -Lined Channels Locate channels where they can conform to the topography and other features such as roads. Use natural drainage systems to the greatest extent possible . Avoid sharp changes in alignment or bends and changes in grade. . Do not reshape the landscape to fit the drainage channel. The maximum design velocity shall be based on soil conditions, type of vegetation, and method of revegetation, but at no time shall velocity exceed 5 feet/second. The channel shall not be overtopped by the peak volumetric flow rate calculated by one of the following meth- ods: Single Event Hydrograph Method: The peak volumetric flow rate calculated using a 10- minute time step from a Type 1 A, 10-year, 24-hour frequency storm for the worst -case land cover condition. OR o Continuous Simulation Method: The 10-year peak flow rate, as determined by an 2019 Stormwater Management Manual for Western Washington Volume ll - Chapter 3 - Page 332