Wall heights should conform to the height of the water with one foot of freeboard upstream of the inlet, and be adequate to protect both the fill over the drain and the embankments. Headwall and wingwall fencing and protection barrier or trash rack shall be provided to prevent public entry. The trash rack should be used for inlets 48-inches (diameter or width) and smaller. For inlets larger than 48 inches a special designed trash rack may be required.
If debris is prevalent, barriers consisting of vertical 3-inch or 4-inch diameter steel pipe spaced at 1/3 the main line diameter or width to a maximum of 30 inches on centers should be embedded in concrete immediately upstream of the inlet apron.
Outlet Structures - When a storm drain outlets into a natural channel, an outlet structure shall be provided which prevents erosion and property damage. This outlet structure shall be designed with a cutoff wall having a minimum depth of three (3) feet. Velocity and direction of flow at the outlet should agree as closely as possible with the existing channel velocity. Fencing and a protection barrier shall be provided.
When the discharge velocity is low, or subcritical, the outlet structure shall consist of a headwall wingwalls, and an apron. The apron may consist of a concrete slab, or grouted rip-rap with a minimum 3 foot deep cutoff wall.
When the discharge velocity is high, or supercritical, the designer shall design bank protection in the vicinity of the outlet and an energy dissipater structure.
Protection Barriers and Trash Racks - A protection barrier is a means of preventing access to storm drains. Protection barriers may consist of large, heavy breakaway gates, single horizontal bars across catch basin openings, or chain link fencing around an inlet of an exposed outlet.
Protection barriers shall be provided wherever necessary to prevent unauthorized access to storm drains.
Debris Barriers - A debris barrier or deflector is a means of preventing large debris, such as tree limbs, logs, boulders and refuse, from entering a storm drain and plugging the conduit. The debris barrier should have openings wide enough to allow as much debris as possible to pass through
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