Surface Water Control Cont.
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.
Figure 16--A rolling grade dip builds on the knick design.
It helps direct water off steeper sections of existing trail.
(Click here for an enlarged image)
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).
Drawing of how a waterbar is placed in a trail and different anchoring methods. The drawing includes the text:
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.
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.