1710 W MARINE VIEW DR 2024-01-03

approved continuous runoff model with a 15-minute time step for the worst -case land cover condition.. Worst -case land cover conditions (i.e., producing the most runoff) should be used for analysis (in most cases, this would be the land cover conditions just prior to final landscaping). . Where the grass -lined channel will also function as a permanent stormwater conveyance facil- ity, consult the drainage conveyance requirements of the local jurisdiction. . An established grass or vegetated lining is required before the channel can be used to convey stormwater, unless stabilized with nets or blankets (See BMP C122: Nets and Blankets). . If design velocity of a channel to be vegetated by seeding exceeds 2 ft/sec, a temporary chan- nel 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 11-3.11: Tem- porary Channel Liners. 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 the grass -lined channel to sedimentation from disturbed areas. Use sediment - 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 construct small parabolic shaped channels.) . Subsurface drainage or riprap channel bottoms may be necessary on sites that are subject to prolonged wet conditions due to long duration flows or a high water table. . Provide outlet protection at culvert ends and at channel intersections. . Grass channels, at a minimum, should carry peak runoff for temporary construction drainage facilities from the 10-year, 24-hour storm without eroding. Where flood hazard exists, increase the capacity according to the potential damage. • Grassed channel side slopes generally are constructed 3H :1 V or flatter to aid in the estab- lishment of vegetation and for maintenance. • Construct channels a minimum of 0.2 foot larger around the periphery to allow for soil bulking during seedbed preparations and sod buildup. Maintenance Standards During the establishment period, check grass -lined channels after every rainfall. 2019 Soormwater Management Manual for Western Washington Volume 11 - Chapter 3 - Page 333