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 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.
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.
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.
The waterbar is the second most common drainage structure, after outsloping. Water moving down the trail is turned by contact with the waterbar and, in theory, is directed off the lower edge of the trail. Waterbars are usually the most dysfunctional tread structures in all of the trail world. Yet trail crews annually install or reinstall them by the thousands.
We encourage the use of reinforced grade dips instead of waterbars at most locations where waterbars have been traditionally used. Here's why--
By design, water hits the waterbar and is turned. The water slows down and sediment drops in the drain. The number one cause of waterbar failure is sediment filling the drain until the water tops the bar and continues down the tread. The bar becomes useless. You can build a good grade dip quicker than a waterbar, and it works better.
On grades less than 5 percent, waterbars are less susceptible to clogging (unless they serve a long reach of tread or are in very erodible tread material). On steeper grades (15 to 20 percent), waterbars are very prone to clogging if the bar is at less than a 45° angle to the trail. Waterbars are mostly useless at grades steeper than 20 percent. At these grades a very fine line exists between clogging the drain and eroding it (and the bar) away.
Most waterbars are dysfunctional because they are not installed at the right angle and are too short. The waterbar needs to be anchored 300 mm (12 in) into the cutslope and still extend 300 mm (12 in) into the fillslope. If your tread is 600 mm (24 in) wide, the bar must be 1.7 m (5 ft 6 in) long to be correctly installed at a 45° angle. A bar fitted at an angle of 60° must be 2.4 m (7 ft, 7 in) long. Wider tread requires a longer bar. When the bar is cut too short, the usual response is to install it at a lesser angle. Then it clogs.
Poorly constructed and maintained waterbars also become obstacles. Most waterbars are installed with one-third to one-half of the bar material above the existing tread surface. Some crews even install bars with exposed faces taller than 150 to 200 mm (6 to 8 in). On grades steeper than 7 percent (particularly in erodible soils), the soil placed on the tread below the waterbar is rapidly lost to traffic and water erosion. The structure becomes a "low hurdle" for travelers.
Wimpy little wooden bars less than 150 mm (6 in) in diameter wear or clog quickly into uselessness. Often they rot away in just a few years. Another problem with wooden waterbars is that horses kick them out.
Cyclists of all sorts hate wooden waterbars because of the hazard they present to wheeled traffic. The exposed angled surface can be very slippery, leading to crashes when the wheel slides sideways down the face of the bar. The rider continues down the trail without the cycle. As the grade increases, the angle of the bar (and often the face height) is increased to prevent sedimentation. This raises 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 waterbars where you don't have much soil to work with and in areas that experience occasional torrential downpours (Figure 20).
Figure 20--Reinforced or armored waterbars.
The bar helps keep traffic from wearing a water carrying groove through the drain. Install the bar at an angle of at least 45° and increase the angle as the grade approaches 5 percent or if the soils are very erodible (Figure 21 and Figure 22).
Figure 21--Waterbars need to be constructed at
a 45 to 60° angle to the trail. Water should
run off the trail before hitting the waterbar.
Figure 22--Logs used for waterbars need to be peeled
(or treated with preservative), extended at least
300 mm (12 in) into the bank, staked or anchored,
and mostly buried.
Remember that high-faced bars are barriers to wheeled traffic. On trails that serve wheeled traffic, use either reinforced grade dips or rubber waterbars instead of traditional waterbars. Bikers do not like waterbars because of the "crash factor." It is important to place rubber waterbars such that wheeled vehicles cannot go around them (creating a water channel around the waterbar). Be sure to cut the rubber belting so that it bends easily under the wheel. A stiff rubber bar at a 45° to 60° angle can cause wrecks (Figure 23).
Figure 23--Rubber belt waterbars are good choices
on trails used by wheeled vehicles. They are not
good as reinforced grade dips.
|Think of your waterbar (wood or rock) as a backup to a dip in the trail. Dig the bar first. Make sure it is seated flush, anchored into the cutslope, and at a good angle. Then construct the dip and outlet to match. For rock waterbars, use rectangular rocks," chunkers," butted together, not over- lapped. Start with your heaviest rock at the downhill side--that's your "keystone." Lay rocks in from there until you tie into the bank.|