Grade Reversal

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Short reverse gradients in the path surface or up slope on a long downhill gradient, usually a short dip followed by a short rise at regular intervals used to shed surface water off the path.

grade_reversal2

Source: IMBA

Grade reversals are only effective on narrow paths - 1 metre or less, and can be the most unobtrusive of all path drainage features. This technique collects and diverts water off the path before it has a chance to cause erosion. It is most commonly used on mountain bike trails, but can be installed into unbound surface for use by other path users. Grade reversals, however, can not be easily 'retro-fitted' into an existing path surface so its best if they are installed when the path is first constructed.

How to create grade reversals:

  • When marking out the line of the route ready for construction, mark the location of any natural dips or changes in gradient. These locations can be formed into grade reversals as the path is built.

  • When building the path incorporate grade reversals into the sub base layer by reversing the downward gradient to form short dips in the sub base surface with reversed gradients and cross falls.

  • For every grade reversal make sure the outlet is at least 500mm wide, to allow water too freely drain off and away from the path, and to stop the build up of debris, or water flowing back onto the path.

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