1.29 BMP C204 - Pipe Slope Drains
To use a pipe to convey stormwater away from or over bare soil to prevent gullies, channel erosion, and saturation of slide-prone soils.
Pipe slope drains can be used when a temporary or permanent stormwater conveyance is needed to move the water down a slope steeper than 10% to avoid erosion (Figure 3 - 12: Pipe Slope Drains).
Connected to new catch basins and used temporarily until all permanent piping is installed;
Used to collect water from aquifers exposed on cut slopes and convey it to the base of the slope;
Used to collect clean runoff from plastic sheeting and direct it away from exposed soil;
Installed in conjunction with silt fence to collect and convey water to a controlled area;
Used to divert small seasonal streams away from construction. They have been used successfully on culvert replacement and extension jobs. Large flex pipe can be used on larger streams during culvert removal, repair, or replacement; and,
Connected to existing roof downspout systems and used to divert water away from work areas during building renovation, demolition, and construction projects.
Used at bridge ends to collect runoff and convey it to the base of the fill slopes along the bridge approaches.
Used on road projects to collect runoff from pavement and pipe it away from side slopes.
There are several commercially available collectors that are attached to the pipe inlet and help prevent erosion at the inlet.
1.29.3 Design and Installation Specifications
Size the pipe to convey the flow. The capacity for temporary pipe slope drains shall be sufficient to handle the peak volumetric flowrate from a 10-year, 24-hour frequency storm assuming a Type 1A rainfall distribution (3.0-inches) using a 10-minute timestep and single event model. Alternatively, use the 10-year return period flowrate, indicated by an Ecology approved continuous simulation model assuming a 15 minute timestep. Size temporary pipe slope drains using the worst-case land cover condition.
Use care in clearing vegetated slopes for installation.
Re-establish cover immediately on areas disturbed by installation.
Use temporary pipe slope drains on new cut or fill slopes.
Use diversion dikes or swales to collect water at the top of the slope.
Ensure that the entrance area is stable and large enough to direct flow into the pipe.
Piping of water through the berm at the entrance area is a common failure mode.
The entrance shall consist of a standard flared end section for culverts 12 inches and larger with a minimum 6-inch metal toe plate to prevent runoff from undercutting the pipe inlet. The slope of the entrance shall be at least 3 percent. Sand bags may also be used at pipe entrances as a temporary measure.
Thoroughly compact the soil around and under the pipe and entrance section to prevent undercutting.
Securely connect the flared inlet section to the pipe slope drain and have watertight connecting bands.
Securely fasten, fuse or have gasketed, watertight fittings for the pipe slope drain sections, and securely anchor them into the soil.
Install thrust blocks anytime 90 degree bends are utilized. Depending on size of pipe and flow, these can be constructed with sand bags, straw bales staked in place, “T” posts and wire, or ecology blocks.
Pipe needs to be secured along its full length to prevent movement. This can be done with steel “T” posts and wire. A post is installed on each side of the pipe and the pipe is wired to them. This should be done every 10-20 feet of pipe length, depending on the size of the pipe and quantity of water to be diverted.
Use interceptor dikes to direct runoff into a pipe slope drain. Ensure the height of the dike is at least 1 foot higher at all points than at the top of the inlet pipe.
Stabilize the area below the outlet with a riprap apron (see BMP C209 - Outlet Protection for the appropriate outlet material). For permanent installations, protect the outfall using guideance in A400 - Outfall Systems.
If the pipe slope drain is conveying sediment-laden water, direct all flows into the sediment trapping facility.
Check inlet and outlet points regularly, especially after storms.
The inlet should be free of undercutting, and no water should be going around the point of entry. If there are problems, the headwall should be reinforced with compacted earth or sand bags.
The outlet point should be free of erosion and installed with appropriate outlet protection.
For permanent installations, inspect pipe periodically for vandalism and physical distress such as slides and wind-throw.
Normally the pipe slope is so steep that clogging is not a problem with smooth wall pipe; however, debris may become lodged in the pipe.
Figure 3 - 12: Pipe Slope Drains