Surface Water Control
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 (figure 9) or failure of the tread and support structures. Water is wonderful stuff--just keep it off the trail. Your job is to keep that water off, Off, OFF the tread!
The very best drainage designs are those built into new construction. These include frequent grade reversals and outsloping the entire tread. The classic mark of good drainage is that it's self maintaining, requiring minimal care.
When rain falls on hillsides, after the plants have all gotten a drink, the water continues to flow down the hill in dispersed sheets--called sheet flow (figure 10). All the design elements for a rolling contour trail--building the trail into the sideslope, maintaining sustainable grades, adding frequent grade reversals, and outsloped tread--let water continue to sheet across the trail where it will do little damage.
Figure 10--Design elements for a rolling contour trail let water
sheet across the trail. Sheet flow prevents water from being
channeled down the trail, where it could cause erosion.
(Click here for an enlarged image)
Sometimes, grade reversals are called grade dips, terrain dips, Coweeta dips, or swales. For less confusion, let's call them grade reversals. The basic idea is to use a reversal in grade to keep water moving across the trail. Grade reversals are designed and built into new trails.
A trail with grade reversals and outsloped tread encourages water to continue sheeting across the trail--not down it. The beauty of grade reversals is that they are the most unobtrusive of all drainage features if they are constructed with smooth grade transitions. Grade reversals require very little maintenance.
Grade reversals take advantage of natural dips in the terrain (figure 11). The grade of the trail is reversed for about 3 to 5 meters (10 to 15 feet), then "rolled" back over to resume the descent. Grade reversals should be placed frequently, about every 6 to 15 meters (20 to 50 feet). A trail that lies lightly on the land will take advantage of natural dips and draws for grade reversals. The trail user's experience is enhanced by providing an up-and-down motion as the trail curves up and around large trees (figure 12) or winds around boulders.
Water will always find the path of least resistance--most likely your trail! Gullies form as water eats away the tread material on steep trails. Puddles sit in low-lying areas that leave the water nowhere to go. When water starts destroying your trail, trail users start skirting around the damage. The trail becomes wider or multiple new trails are formed.
Getting water off the trail takes more than digging a drainage ditch. Find out where the water is coming from, then find a way to move it off the trail.
When a crew takes a swipe at the berm with a shovel or kicks a hole through it--that's useless drainage control. These small openings are rapidly plugged by floating debris or the mud-mooshing effect of passing traffic. The erosion lives on.
Puddles that form in flat areas on existing trails may cause several kinds of tread damage. Traffic going around puddles widens the trail (and eventually the puddle). Standing water usually weakens the tread and the backslopes. Water can cause a bog to develop if the soils are right. Traffic on the soft lower edge of a puddle can lead to step-throughs, where users step through the edge of the trail, breaking it down. Step-throughs are one of the causes of tread creep.
The knick is an effective outsloped drain. Knicks are constructed into existing trails (figure 13). For a knick to be effective, the trail tread must have lower ground next to it so the water has a place to drain. A knick is a shaved down semicircle about 3 meters (10 feet) long that is outsloped about 15 percent in the center (figure 14). Knicks are smooth and subtle and should be unnoticeable to users.
Figure 14--A knick is a semicircle cut into the tread, about 3
meters (10 feet) long and outsloped 15 percent in the center.
(Click here for an enlarged image)
If terrain prevents such outsloping, the next best solution is to cut a puddle drain at least 600 millimeters (24 inches) wide, extending across the entire width of the tread. Dig the drain deep enough to ensure that the water will flow off the tread. Feather the edges of the drain into the tread so trail users don't trip. Plant rocks or other large stationary objects (guide structures) along the lower edge of the tread to keep traffic in the center. In a really long puddle, construct several drains at what appear to be the deepest spots.