Forest Road Deactivation Practices
in the Pacific Northwest
15 - French Drains	Description, Expectations

Description

A french drain is a subsurface drainage structure installed to diffuse heavy seepage or ground water piping which may be encountered during road construction or deactivation. Use french drains where crossditch installation is impractical because:

• the seepage zone is extensive in length,
• the retrieved fill material may be impermeable, and when placed against the cut, would block natural drainage patterns,
• the stability of the recontoured fillslope may be in question.

Expectations

1. Material in the french drain should consist of cobbles and boulders from the surrounding road fill.
2. The width and height of the french drain depends on the width the seepage zone but should be at least 3 meters (10 feet).
3. The french drain extends down the cutslope and across the road bench to the toe of the pulled back road fill.
4. The drain outfall should have a rock apron to disperse any flows.
5. Never direct water onto unstable slopes/ unprotected erodible soil.
6. Remove all potentially unstable material downslope from the french drain outlet.

Longitudinal french drains can be used in cases where bank sloughing and slide debris will interfere with water flows in an open ditch. They are used primarily in applications to route surface flows along the road in order to avoid saturating unstable slopes below the road.

Blanket drains, french drains, and trench drains are all very useful in steep ground to prevent water accumulations from intercepted flows, and to maintain the natural micro-drainage of the slope.

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Introduction  1   Water Management  2   Waterbars & Ditchblocks  3   Safety and Road Surface  4   Revegetation  5   Sediment Control  6   Erosion Protection  7   Dewatering Streams  8   Cross-Ditches (Road Intact)  9   Cross-Ditches in Pullback  10   Partial Sidecast Pullback  11   Full Sidecast Pullback  12

13  Blanket Drains   14  Trench Drains   15  French Drains   16  Fords   17  Cross-Ditching Practices   18  Cross-Ditching in Pullback   19  Partial Pullback Practices   20  Full Pullback Practices   21  Wood Culvert Removal   22  Metal Culvert Removal   23  Stream Diversion Dams   24  Pre-Work Checklist

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