Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Office of Planning, Environment, & Realty (HEP)
PlanningEnvironmentReal Estate

HEP Events Guidance Publications Awards Contacts

Trail Construction and Maintenance Notebook

USDA Forest Service logo   Back | Next Forest Service Technology & Development logo

Surface Water Control Cont.

Rolling Grade Dips

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.

Photo of a trail with a rolling grade dip.
Figure 15--Rolling grade dips direct water
off steeper sections on existing trails.

Drawing of how a rolling grade dip directs water off a trail. Text in the drawing reads, Ramp 3 to 6 m (10 to 12 ft) outslope ramp 5%, original trail alignment, 2 to 3 m (6 to 10 ft), and water.
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

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 log or treated timber waterbar and anchors.
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.

Drawing of how a waterbar is placed in a trail and different anchoring methods. The drawing includes the text:

  • Log or treated timber waterbar and anchors.
  • Embed log 300 mm (12 in) (min.) into bank.
  • Toe of bank
  • Typical outslope 5 to 8%
  • Log flush with tread on downgrade side.
  • Outslope to daylight
  • Skew waterbar 45 to 60 degrees
  • Downgrade
  • Extend log 300 mm (12 in) beyond edge of trail.
  • Top of waterbar is 150 mm (12 in) above surface on upgrade side.
  • Anchoring methods
  • Trail tread
  • Rock anchor
  • Log 300 mm (12 in) diameter
  • Trail tread
  • 40d barbed or ring shank nails
  • 50 by 450 mm (2 by 18 in) square hardwood stakes
  • 100 mm (4 in) (min.)
  • Embed 1/3 (min.) steel pin flush with top, rebar #4 by 450 mm (18 in)
  • Square treated timber 200 mm (8 in)

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.

Drawing of a diagram showing a rock waterbar.  Includes text that reads, Rock Waterbar, rock extends 12 in minimum into bank, skew waterbar 45 to 60 degrees, outslope 5%, rock protrudes 4 in minimum above surface on the upgrade side, and bury two-thirds of each rock.
Figure 18--Waterbars need to be constructed at a 45- to 60-degree
angle to the trail. Rock waterbars are more durable than wood.

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.

Drawing showing a reinforced armored grade grip.  Text in the drawing reads, Reinforced Armored Grade Dip, drainage, top view, direction, riprap tray, rock waterbar, retainer bar, and side view.
Figure 19--A waterbar with a riprap tray.

Maintaining the Drain

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.

Drawing of how a waterbar is placed on a trail.
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.

Drawing of the top view of how a waterbar should be placed in a trail. The drawing includes the text:

  • Downslope-direction of waterflow
  • Trail
  • Thoroughly dig material out of this area-at least two shovel blades wide. Use for backing below waterbar.
  • Reinforce outlet are if eroded.
  • Tread
  • Waterbar constructed at 45° angle. Reset loose or missing rocks and logs.

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.

Relocating Problem Sections of Trail

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.


USDA Forest Service logo Top

Back | Next

Table of Contents
Forest Service Technology & Development logo
Updated: 04/14/2014
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000