1.25 BMP C200 - Interceptor Dike and Swale
Provide a dike or swale, at the top or base of a disturbed slope or along the perimeter of a disturbed construction area to convey stormwater. Use the dike and/or swale to intercept the runoff from unprotected areas and direct it to areas where erosion can be controlled. This can prevent runoff from entering the work area or sediment-laden runoff from leaving the construction site.
Where the runoff from an exposed site or disturbed slope must be conveyed to an erosion control facility that can safely convey the stormwater.
Locate upslope of a construction site to prevent runoff from entering disturbed area.
When placed horizontally across a disturbed slope, it reduces the amount and velocity of runoff flowing down the slope.
Locate downslope to collect runoff from a disturbed area and direct it to a sediment basin.
1.25.3 Design and Installation Specifications
Stabilize dike and/or swale and channel with temporary or permanent vegetation or other channel protection during construction.
Steep grades require channel protection and check dams.
Channel requires a positive grade to allow stormwater and surface water to drain; steeper grades require channel protection and check dams.
Review construction for areas where overtopping may occur.
Should be used at the top of new fill before vegetation is established.
May be used as a permanent diversion channel to carry the runoff.
Sub-basin tributary area should be one acre or less.
The peak volumetric flowrate calculated using a 10-minute time step from a Type 1A, 10-year, 24-hour frequency storm using a single event model, or
The 10-year return period flowrate, indicated by an Ecology-approved continuous simulation model, using a 15-minute time step.
Design for worst-case land cover conditions.
For permanent facilities, design capacity per Volume 4.
Interceptor dikes shall meet the following criteria:
Horizontal Spacing of Interceptor Dikes:
Stabilization depends on velocity and reach.
Seed and mulch applied within 5 days of dike construction (see BMP C121 - Mulching). Dependent on runoff velocities and dike materials. Stabilization should be done immediately using either sod or riprap or other measures to avoid erosion. See Volume 5, Section 4.3: Open Channel Specifications for additional guidance on channel protection.
The upslope side of the dike shall be graded to ensure stormwater and surface water reach the dike outlet. No erosion shall occur at the outlet. Provide energy dissipation measures as necessary. Sediment-laden runoff must be released through a sediment trapping facility.
Minimize construction traffic over temporary dikes. Use temporary cross culverts for channel crossing.
Interceptor swales shall meet the following criteria:
Maximum 5 percent, and be graded to ensure stormwater and surface water reach a suitable outlet (such as a sediment pond). Seed as per BMP C120 - Temporary and Permanent Seeding, or BMP C202 - Rip Rap Channel Lining, 12 inches thick of riprap pressed into the bank and extending at least 8 inches vertical from the bottom.
Inspect diversion dikes and interceptor swales once a week and after every rainfall. Immediately remove sediment from the flow area.
Repair damage caused by construction traffic or other activity before the end of each working day.
Check outlets and make timely repairs as needed to avoid gully formation. When the area below the temporary diversion dike is permanently stabilized, remove the dike and fill and stabilize the channel to blend with the natural surface.