Erosion Is Your Enemy

The ideal trail would be designed with a gentle grade, have dips naturally integrated into its original layout, and be hardened where necessary.

However, most trails are far from ideal and require your help to control erosion.

trail erosion

The Appalachian Trail shall be provided with a treadway that is reasonably safe and enjoyable for hiking. The treadway shall be designed, constructed, and maintained so as to minimize its impact on the natural resources of the Trail and its surroundings. (Appalachian Trail Conference, 1979)

Objective

Maintain or restore trail tread to a standard that prevents erosion and is safe for users. Waterbars are constructed to get water off the trail to prevent erosion. Install waterbars or grade dips on all slopes at a frequency that is sufficient to prevent major erosion. Waterbars are installed at a 45-degree angle downhill to the trail.

In areas with adequate drainage, leaf litter should remain on the trail treadway; there should be less than 3 inches of “cupping” of the mineral layer of the trail treadway; no loose rocks or roots resulting from erosion should be apparent. In wet areas, the desired appearance is a stable, non-widening treadway, 12- to 18-inches in width, that uses rock and mineral-soil turnpikes or bog bridges to span saturated soils.

The trail should “wear lightly on the land,” with minimal disturbance to the surroundings. The treadway should reflect a concern for safety, without detracting from the opportunity for hikers to experience wild and scenic lands by their own unaided efforts and without sacrificing aspects of the trail that may challenge their skill and stamina.

Some veteran overseers hold that our first maintenance priority is “water management.” In fact, we are most often involved in “water mitigation”—keeping erosion from destroying the treadway. Our main objective is to separate water from the treadway and get it off of the trail as soon as possible. Where that isn’t possible, we work to slow it down. There are important reasons for this:

  1. Clay is made of very fine particles that, once broken from the ground, are easily washed away. Much of the treadway is built on mineral soil bound together by clay. Clay is held together by a chemical bond that is hard to break, but fast-moving water does it.
  2. “Thick” water flows faster than “thin” water. Water flowing in rivulets or ruts is thick water; water flowing across the trail in a sheet is thin water. Water flowing along the trail generally picks up volume (thickness) from new water flowing onto the trail and, thus, speed.
  3. Gravity works. The ability of water to carry soil particles increases dramatically as it goes faster; water moving at 2 miles per hour has sixty-four times more ability to carry soil particles than water moving at 1 mile per hour.
Thin WaterThin Water
Thick WaterThick Water

Treadway Considerations

Early stages of erosion are easy to spot. Gravel and pebbles cover the center of the treadway after leaf litter, clay, silt, and sand have been washed away. In the worst cases, the center of the trail becomes a jumble of stones and boulders, all smaller particles having been washed downhill.

Culverts Culverts should be clear of obstructions, solid, and cleaned twice per year.
Gravel Graveled sections should be minimized.
Obstructions The treadway should be clear of obstructions unduly impeding foot travel.
Rocks and roots The treadway should be a solid, stable composite of soil, roots, and rocks, free of projecting or hanging roots. Loose rocks should be removed once per year, exposed roots should be covered with mineral soil once per year, and root loops should be removed.
Run-off ditches Waterbar ditches and grade-dip run-off ditches need to be at least a foot wide and continue far enough off the A.T. so the water runs away from the trail. Drainage ditches should be clear of silt and debris and cleaned twice per year. They should be 1-foot wide and 1-foot deep and have sloping sides, so that they do not cave in.
Ruts Mineral soil erosion (rutting) should be less than 3 inches deep.
Safety The treadway should be reasonably safe.
Water Water should run next to or across the treadway, not along the trail.
Water crossings Stepping stones and bog bridges should be stable and sound, with slip-free treads.

Warning Signs

The “early warning” signs of treadway damage include:

Eroding tread Soil washes out into a gully. Compaction alone may cause a 2 – 4-inch gully. If a gully is deeper than that, with silt particles and loose rocks in the gully, it is actively eroding.
Widening trail Trampling of vegetation bordering the treadway, and consequent trail widening, may precede or follow the formation of a gully caused by erosion.
Divided trail Multiple treadways worn by hikers bypassing poor footing or water running in the treadway are undesirable.
Running water Water running down the treadway, especially in good weather, is a problem.
Breaking down Soil breaking down on the outside edge of a graded foot path, causing the treadway to narrow, indicates a problem.
Gullied tread Gullies on a graded sidehill, causing water to run down the trail, rather than off the outsloping edge, indicate an erosion problem.
Short-cut trail Switchback shortcuts—woodland vegetation is trampled and soil eroded—are most undesirable.
Clogged devices Clogged waterbars or grade dips that no longer divert water from treadway need to be repaired.
Deteriorated devices Loose waterbars, steps, or cribs.
Loosened devices Cribs or steps not retaining soil.
Collapsing device Cribs dislodged by saplings.

Maintenance Tasks

Overseers commonly have these tasks to consider, plan, and perform to mitigate erosion and its consequences:

  • Keep all waterbars, channels, culverts, and other water control devices clear of silt and debris.
  • Waterbars must be cleared at least twice a year or they become useless.
  • Checkdams are built to collect sediment on eroded trails in order to restore the tread. Checkdams are installed perpendicular to the treadway where both sides of the trail are higher than the treadway. If done correctly, steps along the treadway will result.
  • Clear debris from the downstream side of streams crossing trails to reduce the potential of flooding.
  • Dig channels for small streams or springs so the water can cross trails, not flow down them.
  • Remove loose stones and other debris from the trail tread that could trip or provide loose footing for hikers.
  • Discourage short-cutting—especially on switchbacks—by blocking with logs, branches, or other such debris.
  • Where needed, build waterbars wide and deep enough to be effective without posing a hazard to hikers. (The most common problems concerning waterbars are that they are too shallow or too few.) Remember that the function of a waterbar is to get large amounts of water off trails during storms. It is a large volume of water running down a trail in a short period that destroys it. Ensure waterbars are big enough to do the job, that there are enough of them, and that they are placed to be most effective.

The standard is an average rise of 7% rise (maximum of 15%) for new trail construction. The rise can increase to 15% for short distances, but the goal is to dig the sidehill on an average grade of 7% using climbing turns and, where necessary, switchbacks and out-sloped tread (by 1% – 3%) to control erosion.

Many existing trail sections exceed a 15% rise, so we must strive to reduce erosion damage as much as possible by building waterbars and checkdams where needed. We also must clean out those waterbars as needed.

Standards

Outsloped tread and coweta dips are the preferred methods for diverting water from a trail. However, where a treadway is depressed or compacted, it will collect water and channel it downhill along the trail, causing increased water speed and consequent erosion. Drainage dips are difficult to add successfully in trail grades above 10%. Most serious erosion problems occur on “old” trail sections that were located along steep ridge lines (usually more than a 10% grade) without provision for drainage. The combination of heavy use, steep grade, and poor or absent drainage creates significant erosion problems in those sections. It is not feasible or desirable to relocate all such sections, so rehabilitation is sometimes necessary.

Appalachian Trail

  • Inspect and clear waterbars in early spring
  • Check erosion control devices periodically throughout the summer and autumn.
  • Restore erosion control devices as necessary.

Blue-blazed trails

Same as the Appalachian Trail.

Wilderness trails

Same as the Appalachian Trail.

Erosion Control Devices

You may have to construct steps, cribs, sidehill construction or reconstruction, grade and coweta dips, or waterbars to remedy the treadway problems discussed above.

Grade Dip

 Existing trail-maintenance devices used to stop erosion and harden the treadway deserve close attention from Trail maintainers to ensure they are functioning properly. Deteriorating trail construction may cause more damage than it prevents.

Coweta Dip

 This technique is possible only on sidehill trails, where one side of the trail can discharge water downhill.

These are usually installed during trail construction, when a continuous uphill grade may be interrupted by short 10- to 15-foot-long and 2- to 3-foot-deep downhill grades. Water flowing down the trail is forced to leave the treadway at the dips.

Coweta Dip - Profile

The coweta dip is a permanent, “maintenance-free” drainage technique. On gentle trail sections, coweta dips may be added successfully after construction by using wire flags to lay out a dip above an anchor point, such as a tree or boulder, then digging it out as in new construction.

Coweta Dip - Top view
  1. This requires a lot of digging! Start 15 feet above the point where you want the grade to dip back up slightly.
  2. Cut down—lower—the outside edge throughout the length of the dip.
  3. Gently grade the cut mineral soil back to the up-slope side, spreading it along the 15 feet of the coweta dip.
  4. Ensure that the cutbank (uphill) is back-sloped by about 45%.

 Completely remove all duff from the construction area.

  1. Grade the tread so that it is out-sloped downhill from the cutbank by 1-3%!
  2. Using the tried-and-true, “Mark 1 Mod A”-eyeball method, grade the tread up by 10% from the lowest point of your coweta dip.
  3. From the lowest point of the dip, grade gently back up on a 10% grade to the apex of the lower berm.
  4. “Sift” the mineral soil from side-to-side and lengthwise to find and remove bumps and humps that will catch rain water and hold it on the tread.
  5. Pack dirt tightly with a McLeod.

 This should form an area, more or less shaped like a pork chop with the bone as the trench and the small end forming the drain.

Why Leaf Litter?

Because it controls splash erosion.

  1. Ensure the drain flows down-slope widely and openly so that rain water disperses, or sheets, off of the tread quickly without backing up and flooding the trail.
  2. Scatter leaf litter over the new tread in the coweta dip.
    • If the soil is very dry, walk over the leaf-littered tread in the dip several times.

 It is important to have a wide basin because it gives wind a chance to blow away leaves and light debris and help keep the grade dip somewhat clear.

Waterbars

If the length of the grade is great, or particularly steep, waterbars are essential. The number and spacing of waterbars depends on slope steepness, the amount of water on the trail, the construction of the treadway (whether it was sidehill or steps, etc.) and the availability of places to divert water off the trail. Generally, the steeper the slope, the more waterbars you will need. Roughly, you need one waterbar for every 8 – 10 feet of elevation change. If the trail rises 100 feet, you'll need at least ten waterbars.

Waterbar Elevation

Materials

You can use either logs or rocks to create a waterbar; the construction technique is the same. Considerations include:

Log waterbar

  • Use black locust log whenever possible; otherwise, select an oak log.
  • Try not to use green wood.
  • Remove the bark with a draw knife—very important.

Rock waterbar

  • Collect relatively large rocks, 2 – 6 inches thick with large, flat sides.
  • Place them from largest to smallest, starting at the beginning of the trench (extending a foot or so past the trail on the up-slope end and down the length of the trench until the rock line reaches two or three feet off the trail on the drainage side.
  • Overlap the rocks to create a solid line—overlap the larger stone over the next smaller—descending toward the drain side.
  • Shim or chock the rocks with smaller stones so they do not shift when the top of the line is walked on.
  • The larger the rocks, the better stability they will provide.

Construction

  1. Place water-diversion structures immediately below any obvious sources of water, to remove water before it causes damage.
  2. Grade the treadway uphill from the bar gradually into the bar in a “banked turn” so that water does not contact the bar.
  3. Pack leftover mineral soil over the bar as a ramp to begin its transformation into a grade dip.
  4. On very steep slopes, it may be necessary to install a step immediately below the waterbar to prevent undercutting and retain the soil below the bar.

 The waterbar itself should not be used as a step.

  1. Water will drain easily from the trail at the outside of sharp bends and corners, which are good locations for water-diversion structures.

Lock the Angle

  1. Lay out a line in the dirt (use the handle of a McLeod or a rock bar) angled at exactly 45° to the direction of travel.
  2. Lock in the bar's position by positioning rocks or by driving stakes into the ground a couple of feet from either end of the bar and exactly in line with it.

 Placing the log nearly perpendicular to the direction of travel is the most common mistake in waterbar construction.

Dig the Trench

  1. Dig a trench on the uphill side of the bar a bit over 7 feet in total length. Clip or saw roots out of the way and dispose of them well out of sight off the trail.

 Take care not to disturb the tread downhill from the bar.

    • The finished trench should be only an inch or two longer, deeper, and wider than the log itself.
    • Save any extra-large rocks for anchors.

Place the Log

  1. Position the log in the trench with its flattest side up and with its top surface flush with the downhill side of the trail and at exactly the target slope.

 A rock bar laid across the trench in the center of the trail and along the direction of travel should just touch the top of the log and guarantee that it isn't too high—the second most common mistake in waterbar construction.

  1. Extend the waterbar off the trail—a long, wide ditch above the bar should clearly channel water away from the trail.

Anchor the Ends

  1. Fill any gaps under or behind the log with rocks from your pile.
  2. Firmly tamp rocks behind the log until they're flush with the downhill trail and hammer them in front of the log until they come up about a third of its diameter.
  3. Firmly tamp lots of dirt from your pile on top of the rocks behind the log to make a solid, flush surface.

The Outflow Channel

  1. Form the uphill face of the waterbar by digging, filling, raking, and tamping as necessary in order to make a smooth, even surface extending about a foot down from the top of the log and tilted so that the handle of the tamper leans about 30° – 45° from plumb.

 The steeper the trail, the steeper the tilt needs to be.

  1. At least 10 feet of trail leading down into the face of the waterbar should be graded with a 5% outslope and at a uniform trail slope, which will be slightly steeper than its original slope to allow for the excavation at the face of the waterbar.

 Take particular care to rake the area smooth and tamp it thoroughly, leaving no rocks, roots, or dirt in the water’s path.

  1. Dig an outflow channel off the side of the trail parallel to the buried log, at least 24 inches wide, and long enough that water won’t run back onto the trail.

 Narrow channels can be overwhelmed in downpours and will clog more easily after even light rains.

  1. Dig through whatever berm you have to in order to create a channel that’s completely clear of obstructions.

 An inadequate outflow is perhaps the third most common mistake in waterbar construction.

  1. Finally, tamp everything thoroughly and mulch any disturbed surfaces with leaves for a natural look and for protection from splash erosion.

Construction Gallery

Figure 1 - Surface: rock, gravel
Figure 1 - Surface: rock, gravel
Figure 2 - Below seepage
Figure 2 - Below seepage
Figure 3 - Natural anchors
Figure 3 - Natural anchors
Figure 4 - Needed Tools
Figure 4 - Needed Tools
Figure 5 - Clinometer
Figure 5 - Clinometer
Figure 6 - 45° angle
Figure 6 - 45° angle
Figure 7 - Gauge width and depth
Figure 7 - Gauge width and depth
Figure 8 - Collect material
Figure 8 - Collect material
Figure 9 - Anchor rocks
Figure 9 - Anchor rocks
Figure 10 - Target slope
Figure 10 - Target slope
Figure 11 - Direction of travel
Figure 11 - Direction of travel
Figure 12 - Target depth
Figure 12 - Target depth
Figure 13 - Flush surface
Figure 13 - Flush surface
Figure 14 - Set anchor rock
Figure 14 - Set anchor rock
Figure 15 - 30°–45° from plumb
Figure 15 - 30°–45° from plumb
Figure 16 - 5% outslope
Figure 16 - 5% outslope
Figure 17 - 24-inch channel
Figure 17 - 24-inch channel
Figure 18 - Leaf litter
Figure 18 - Leaf litter
Figure 19 - Re-do this?
Figure 19 - Re-do this?

Maintenance

  • Remove leaves, twigs, loose dirt and rocks, and other material from the trench.
  • Reset and re-secure rocks or logs as required. Replace logs or stakes if they show signs of deterioration.
  • Ensure that the trench extends sufficiently off the trail such that the full length of the trench will effectively drain water off the treadway.
  • Re-rake dirt to grade the downhill and uphill sides of the trail.

 Do not waste dirt by raking off the trail.

  • Compact dirt into damaged tread areas.
  • Rake loose rocks from the tread above and/or between waterbars.
  • Survey the need for additional waterbars or checkdams and construct them as required.

Check dams

Check dams are used to arrest further erosion and to hold material placed during site restoration. Check dams are intended to slow and hold surface water long enough to deposit transported sediment. They should be used with waterbars or drainage (coweta) dips to reduce overall erosion from the abandoned tread.

Check dams

Check dams are best used as holding structures for fill used to re-contour the old tread.

  • Seat the material in the dam in an excavated footing that extends into the sides of the trench.
  • Add successive levels (more dams up and below) to the dam with enough batter to offer stability against the pressure of the fill as material behind the dam naturally builds up.
  • Make sure that the top of the dam's level or slightly higher than the trench walls.
  • For watertightness, use chinking and tamped fill to complete the uphill face of the dam.
  • Next, add fill behind the dam to finish the process.

It generally takes a long time for these trenches to fill up. Most never do. If they do, add fill below the dam to finish the process.

Check dam - Top View

Spacing between dams depends on the steepness of the old grade and the degree of restoration desired.

  • If the check dam is intended only to slow down erosion, then relatively wide spacing is sufficient (every 65 feet on a 25% grade).
  • If the intent is for half of the old trench to be filled back in, the bottom of each dam should be level with the top of the next lower dam.
  • On steeper grades the dams need to be closer together. If the intent is to approach complete re-contouring of the trench, the dams should be closer still, especially on grades above 25%.

Construction

Building a check dam employs most of the same techniques as the rock or log waterbar.

  1. Dig the trench for the dam (log or stone) perpendicular across the treadway.
  2. Do not create a drain!
  3. Slope the downhill and uphill portions of the trail the same as a waterbar, forming a collection pocket on the uphill side of the trail.
  4. Be certain to pin logs, score log tops, and stabilize rocks.

Maintenance

There is no maintenance to perform with a check dam; it is supposed to fill up with sediment.

  • When a check dam is full, build another 4 – 6 feet down-slope and another 4 – 6 up-slope.
  • When these are full, repeat the process.

 Over time, this helps rebuild a water-damaged trail section.

  • Install waterbars above checkdams to minimize further damage.

Equipment:

  • McLeods
  • Pick or cutter mattocks
  • Fire rakes
  • Engineer hammer or small sledge hammer (for driving log pins)
  • Pulaski
  • Saws (chain or hand)
  • Draw knife (to remove bark)
  • Surveying stakes (for pinning logs)
  • Shovels (optional)