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.
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.
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.
Another way to force water off existing trails is to use a rolling grade dip. A rolling grade dip is used on steeper sections of trail. It also works well to drain water off the lower edge of contour trails. A rolling grade dip builds on the knick design. A rolling grade dip is a knick with a long ramp about 4 ½ meters (15 feet) built on its downhill side (figure 15). For example, if a trail is descending at a 7-percent grade, a rolling grade dip includes:
- A short climb of 3 to 5 meters (10 to 20 feet) at 3 percent
- A return to the descent (figure 16).
Water running down the trail cannot climb over the short rise and will run off the outsloped tread at the bottom of the knick. The beauty of this structure is that there is nothing to rot or be dislodged. Maintenance is simple.
Rolling grade dips should be placed frequently enough to prevent water from building up enough volume and velocity to carry your tread's surface away. Rolling grade dips are pointless at the top of a grade. Mid-slope usually is the best location. The steeper the trail, the more rolling grade dips will be needed. Rolling grade dips should not be constructed where they might send sediment-laden water into live streams.
Waterbars are commonly used drainage structures. Make sure that waterbars are installed correctly and are in the right location. Water moving down the trail turns when it contacts the waterbar and, in theory, is directed off the lower edge of the trail (figure 17).
Figure 17—Logs used for waterbars need to be peeled
(or treated with preservative), extended at least 300 millimeters
(12 inches) into the bank, staked or anchored, and mostly buried.
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Dips Are In, Bars Are Out
For existing trails with water problems, we encourage the use of rolling grade dips or knicks instead of waterbars. Here's why. By design, water hits the waterbar and is turned. The water slows down and sediment drops in the drain.
Waterbars commonly fail when sediment fills the drain. Water tops the waterbar and continues down the tread. The waterbar becomes useless. You can build a good rolling grade dip quicker than you can install a waterbar, and a rolling grade dip works better.
On grades of less than 5 percent, waterbars are less susceptible to clogging unless they serve a long reach of tread or are constructed in extremely erodible tread material. On steeper grades (15 to 20 percent), waterbars are prone to clogging if they are at less than a 45-degree angle to the trail. Waterbars are mostly useless for grades steeper than 20 percent. At these grades a very fine line exists between clogging the drain and eroding it (and the waterbar) away.
Most waterbars are not installed at the correct angle, are too short, and don't include a grade reversal. Poorly constructed and maintained waterbars become obstacles and disrupt the flow of the trail. The structure becomes a low hurdle for travelers, who walk around it, widening the trail.
A problem with wooden waterbars is that horses can kick them out. Rock, if available, is always more durable than wood (figure 18). Cyclists of all sorts hate waterbars because the exposed surface can be very slippery, leading to crashes when a wheel slides down the face of the waterbar. As the grade increases, the angle of the waterbar (and often the height of its face) is increased to prevent sedimentation, raising the crash-and-burn factor.
Are waterbars ever useful? Sure. Wood or rock waterbars are useful on foot and stock trails where a tripping hazard is acceptable, especially at grades less than 5 percent. Also consider reinforced or armored grade dips where you don't have much soil to work with and in areas that experience occasional torrential downpours.
A variation from the traditional waterbar is the waterbar with riprap tray. The riprap tray is built with rock placed in an excavated trench. The tops of the rocks are flush with the existing tread surface, so they're not an obstacle to traffic. Next, construct a rock waterbar. Use rectangular rocks, chunkers, butted together, not overlapped. Start with your heaviest rock at the downhill side—that's your keystone. Lay rocks in from there until you tie into the bank. Bury two-thirds of each rock at a 45- to 60-degree angle to the trail.
Add a retainer bar of rock angled in the opposite direction from the waterbar. The downhill edge of the retainer bar is at an angle so it nearly touches the downhill edge of the waterbar (figure 19). Fill the space between the waterbar and retainer with compacted tread material.
The number one enemy of simple drains is sediment, especially at waterbars. If the drain clogs, the water you are trying to get rid of either continues eroding its way down the tread, or just sits there in a puddle.
The best drains are self-cleaning; that is, the flow of water washes sediment out of the drain, keeping it clean. In the real world most drains collect debris and sediment that must be removed or the drain will stop working. Because it may be a long time between maintenance visits, the drain needs to handle annual high-volume runoff without failing (figure 20).
The best cure for a waterbar that forces the water to turn too abruptly is to rebuild the structure into a rolling or armored grade dip.
Figure 20—The key to waterbar maintenance is to ensure
that sediment will not clog the drain before the next scheduled
maintenance. Embed the rocks or logs a little deeper, cover
them with soil, and you have a reinforced waterbar.
Click here for a long description.
Walking in the Rain
A lot of learning takes place when you slosh over a wet trail in a downpour and watch what the water is doing and how your drains and structures are holding up. Figure out where the water is coming from and where it's going. Think about soil type, slope, distance of flow, and volume of water before deciding your course of action.
If you've tried various drainage methods and water is still tearing up your trail, it's time to think seriously about rerouting the problem sections. Reroutes are short sections of newly constructed trail. This is your chance to incorporate all the good design features of a rolling contour trail that encourages water to sheet across the trail. Remember the good stuff:
- Locating the new section of trail on a sideslope
- Keeping the trail grade less than half of the grade of the hillside
- Building with a full bench cut to create a solid, durable tread
- Constructing plenty of grade reversals
- Outsloping the tread
- Compacting the entire trail tread
Make sure the new section that connects to the old trail has nice smooth transitions—no abrupt turns.
Some short sections of eroded trails may not be major problems. If the trail surface is rocky—and water, use, and slopes are moderate—this section could eventually stabilize itself. A short section of eroded trail may cause less environmental damage than construction of a longer rerouted section. Weigh your options wisely.