Grade reversal

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Grade reversals can be the most unobtrusive of all drainage features and are most effective on narrow paths (1.2m or less). A grade reversal is actually a change in slope to a short incline on a downhill section and this technique diverts water off the path before it has a chance to cause damage. It is most commonly used on mountain bike trails, but could be used on lowland paths with either unbound or semi-bound surface. A grade reversal cannot easily be ‘retro-fitted’ into an existing path surface without excavation works, and even then may not be effective. Grade reversals are best installed when the path is first built.

A grade reversal needs to be combined with a crossfall to ensure that the water does not pond on the path surface. The outlet area should allow water to freely drain away into the ground where it is unlikely to flow back onto the path.

Grade Reversal

Specification
On a sustained incline or across the side slope, mark the location of natural changes in gradient
At each decrease of gradient, reverse the downhill gradient for about 3 to 5m to form a minor incline in the surface
The crossfall will need to be altered to accommodate the dip and incline
Take care not to exceed the maximum long gradient that has been specified for the path (based on the needs of users)
The outlet area should be at least 500mm wide and sloped to prevent debris from building up at the entrance or water ponding on the path surface

 

Maintenance
Annual inspections will be necessary to ensure that the grade reversal is deflecting water off the path without causing surface erosion
Some surface top up may be necessary with unbound surfaces
Remove all debris from the outlet area that is stopping water from draining away from the path

 

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