Surface Water Control (continued)

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.

Waterbars

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.

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.

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