13 - Blanket Drains / Forest Road Deactivation Practices in the Pacific Northwest

Forest Road Deactivation Practices
in the Pacific Northwest

13 - Blanket Drains	Description, Expectations

Description

A blanket drain is a drainage structure used to accommodate seepage zones on the road cut. The objective is to disperse low-velocity flows over the hillslope rather than concentrating them in cross-ditches.

Figure 7: Blanket Drain Cross-Section
diagram: blanket drain cross-section

Expectations

Remove all potentially unstable material downslope from the blanket drain outlet.
A blanket drain consists of a layer of cobbles or shot rock material placed against seepage zones in cutslopes. Use stones 100 mm (4 inches) or larger in diameter. The blanket thickness should be at least 400 mm (16 inches). The blanket drain, once constructed, can be covered with pullback materials. If this pullback material is fine-textured, you must ensure that the blanket is thick enough that the fines do not plug the pore spaces within the blanket or at the outfall.
The width of the blanket depends on the width of the seepage zone, but should be at least 3 meters (10 feet).
The blanket extends down the cutslope and across the decompacted and outsloped road bench to the toe of the pulled back fill.
Ensure that the blanket does not extend to the top of the cut where surface flows would be intercepted.
The blanket outfall should consist of a horizontal rock apron to further disperse any flows.
Never direct water onto unstable slopes/ unprotected erodible soil.

Figure 8: Blanket Drain
diagram: blanket drain

Trench drains, blanket drains, and french 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.

13 - Blanket Drains