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Trail Construction and Maintenance Notebook
Diverting surface water off the trail should be near the top of your list of priorities. Running water erodes tread and support structures and can even lead to loss of the trail itself. Standing water often results in soft boggy tread or tread and support structure failure. Water is wonderful stuff—just keep it off the trail.
The very best drainage structures are those designed and installed during the original construction. These include outsloping the tread and grade dips. We’ve already discussed outsloping. Let’s move on to the next best drainage choice, grade or drain dips. The classic mark of good drainage is that it is self maintaining, requiring minimal care.
The best grade dips are designed and built during the original construction. These are also called terrain dips, Coweeta dips, and swales. Other versions, often called rolling grade dips, or drain dips, can be built on most sidehill trails or constructed to replace waterbars. The basic idea is to use a reversal in grade to force water off the trail without the need for any other structure.
Terrain dips use grade reversal to take advantage of natural dips in the trail. These need to be planned into the trail when it is first laid out. The grade of the trail is reversed for about 3 to 5 m (10 to 15 ft), then “rolled” back over to resume the descent. A trail that lies lightly on the land will take advantage of each local drainage to remove water from the tread (Figure 18) as the trail winds around trees and rocks. The terrain dip, which uses existing terrain as the control point for the grade reversal, is a natural part of the landscape.
![[diagram] Grade drips](images/fig018.jpg)
Figure 18—Grade dips are much more effective than
waterbars and require less maintenance. Along with
outsloping, they are the drainage structure of choice.
The beauty of terrain dips is that water collected from the hillside is not intercepted and carried by the tread. These grade dips are the most unobtrusive of all drainage structures if constructed with smooth grade transitions, and they require very little maintenance. Be sure to protect the drain outlet by placing guide structures along the lower edge of the tread above or below the outlet.
Another kind of grade dip is the rolling grade dip, which consists of a short reversal of grade in the tread. These can be designed into most sidehill trails. If a trail is descending at 7-percent grade, a short climb of, say, 3 to 5 m (10 to 20 ft) at 3 percent, followed by a return to the descent, constitutes a rolling grade dip (Figure 19). Water running down the trail cannot climb over the short rise and will run off the outsloped tread at the bottom of the dip. The beauty of this structure is that there is nothing to rot or be dislodged. Maintenance is simple.
![[photo] Rolling grade dip on trail](images/fig019.jpg)
Figure 19—Rolling grade dip designed into the
construction of the trail.
If the grade is steep, the tread carries a lot of water, traffic is high, or the soils are erosive, a drain dip may need some additional strengthening. Sometimes a shallow water channel can be constructed in the last several meters of tread leading into the dip. Water follows the channel off the tread without slowing down and depositing soil and debris. A spillway may be needed if there is a potential for headcut erosion in the fillslope. The secret is to keep the water moving at a constant velocity until it is all the way off the tread.
Grade dips should be placed frequently enough to prevent water from building enough volume and velocity to carry off your tread surface. Grade dips are pointless at the very top of grades unless they intercept significant amounts of slope drainage. Usually mid-slope is the best location. Grade dips also should not introduce sediment-laden water into live streams.
Yet another grade dip is the reinforced or armored grade dip. In this dip, a curved water channel is constructed and an angled (like a waterbar) reinforcing bar of rock or wood is placed at the top of the grade reversal. The bar is placed in an excavated trench, with its top edge flush with the existing tread surface so it’s not an obstacle to traffic. Essentially, this is a buried waterbar.
This short reinforced grade dip can be built to replace waterbars on existing trails, especially trails used by wheeled vehicles. Well-located waterbars can be converted by constructing a curved water channel and recontouring the outslope from the top of the bar. For longevity it is best if the bar is reseated so that the top edge is flush with the existing tread surface and the channel is constructed with the correctly angled bar as the reference point.
The outlet is critical. It should be at least 500 mm (1.5 ft) wide, and outsloped. In shallow dips the task is to prevent berms, soil buildup, and puddling. Reinforced spillways may also be needed.
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