3800 RIVERFRONT BLVD 2025-07-23 - Laserfiche WebLink

I Design and Installation Specifications Locate the channel where it can conform to the topography and other features such as roads. • Locate them to 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 times shall velocity exceed 5 feet/second. The channel shall not be overtopped by the peak volumetric flow rate calculated using a 10-minute time step from a 10-year, 24-hour storm, assuming a Type 1A rainfall distribution. Alternatively, use 1.6 times the 10-year, 1-hour flow indicated by an approved continuous runoff model to determine a flow rate which the chan- nel must contain. • Where the grass -lined channel will also function as a permanent stormwater con- veyance facility, consult the drainage conveyance requirements of the local gov- ernment with jurisdiction. • An established grass or vegetated lining is required before the channel can be used to convey stormwater, unless stabilized with nets or blankets. • If design velocity of a channel to be vegetated by seeding exceeds 2 ft/sec, a tem- porary channel liner is required. Geotextile or special mulch protection such as fiberglass roving or straw and netting provides stability until the vegetation is fully established. See Figure II-4.2.2 Temporary Channel Liners (p.337). • Check dams shall be removed when the grass has matured sufficiently to protect the ditch or swale unless the slope of the swale is greater than 4 percent. The area beneath the check dams shall be seeded and mulched immediately after dam removal. . If vegetation is established by sodding, the permissible velocity for established vegetation may be used and no temporary liner is needed. • Do not subject grass -lined channel to sedimentation from disturbed areas. Use sed- iment -trapping BMPs upstream of the channel. . V-shaped grass channels generally apply where the quantity of water is small, such as in short reaches along roadsides. The V-shaped cross section is least desirable because it is difficult to stabilize the bottom where velocities may be high. . Trapezoidal grass channels are used where runoff volumes are large and slope is low so that velocities are nonerosive to vegetated linings. (Note: it is difficult to 1 2014 Stormwater Management Manual for Western Washington Volume 11- Chapter 4 - Page 334