There are many aspects of trail design that combine to create a smooth-flowing, appealing, and sustainable trail, and where implemented successfully. The importance of and methods for designing trails that promote water drainage are explained in this resource sheet.
The aim of drainage features in the design of the trail is to minimise altering the natural path of runoff to maintain natural hydrology, and prevent pooling of water. Runoff water exacerbates soil erosion, increasing with steepness of slope, and can result in the formation of gullies and sediment deposition downhill.
Side-hill trails avoid steep slopes by following the contours of the landscape and closely aligning to the trail slope, see image below. The lower side of the trail slopes outwards, allowing water to run over and away from the trail, thereby minimising potential degradation. Out-slopes can be implemented consistently along a trail to avoid water building up on the trail. The recommended out-slope for MTB trails is 5%.
One of the most effective and long-lasting drainage features is trail grade reversals which divert water off the trail and downslope. The trail grade changes temporarily, such as a downhill trail inclining slightly then returning down, the low point is where water is shed, see image below. Once established, grade reversals sustainably drain water with little to no maintenance requirements.
Water-bars and grade dips are also drainage features which can be effective when maintained regularly.
Another important aspect of designing well-drained trails is to consider the location of watercourses. Existing crossings or alternate paths should be used where possible as construction of new crossings is where most environmental impact occurs. If low lying boggy terrain is unavoidable, boardwalks or raised reinforced tread are suitable features. Ensuring trails are free-draining and avoid muddy areas helps reduce the spread of soil-borne pathogens (e.g. dieback) which are easily transported in soil on tyres or shoes.