Principles of Trail Layout and Design

When hikers or trail runners accelerate, they lower their center of gravity, lean forward, rise up on the balls of their feet, and push downward with their leg muscles. These actions transfer the weight of the user to the balls of their feet, which increases the pounds per square inch applied to the trail tread. The angle of the foot also becomes more acute increasing the impingement angle. (See Photo 5.4.)

When hikers and trail runners brake they lower their center of gravity, lean backward, transfer their weight to the heels of their feet, and push backward with their leg muscles. These actions transfer the weight of the user to the heel of the foot, which increases the pounds per square inch applied to the trail tread. Similarly, the angle of the foot also becomes more acute increasing the angle of impingement.

When trail runners come to a curve in the trail they lower their center of gravity, lean to the inside of the curve, and transfer their weight to the inside of one shoe and the outside of the other. These actions transfer the weight of the user to the sides of the feet, which increases the pounds per square inch applied to the trail tread. The angle of the foot also becomes more acute, increasing the angle of impingement.

When horses accelerate, they lower their center of gravity, transfer most of their weight to the rear hooves, and push forward with their powerful hind legs. These actions increase the pounds per square inch applied to the trail tread. In addition, the impingement angle of the hooves becomes more acute increasing the potential to cut, fragment, and plow the trail tread. (See Photo 5.5.)

When horses brake they lower their center of gravity, transfer most of their weight to their rear hooves, and push backward with their hind legs, causing the same effect on the tread surface as acceleration.

When horses are trotting or loping and come to a turn they shift their center of gravity, lean to the inside of the turn, and transfer their weight to the inside of their hooves on the outside legs and to the outside of their hooves on the inside legs. These actions transfer the weight of the horse to a smaller surface area, which increases the pounds per square inch applied to the trail tread. The angle of the hooves also becomes more acute, increasing the angle of impingement.

When mountain bikers accelerate, they apply more downward force to the bike's pedals, which transfers to the rear wheel in the form of torque. Torque results in greater cutting and fragmenting of the trail tread. If the mountain biker stands up on the bike pedals to apply more force, they transfer their weight to the center of the bike, which decreases the weight on the rear tire. These actions combined with the increased torque to the rear tire cause the rear tire to slip, further increasing the cutting and fragmenting of the trail tread.

When mountain bikers brake, they decrease their speed by applying force to (squeezing) the brake levers, activating the brake pads on the front and rear wheels, and reducing the speed of the tires' rotation. The harder the brakes are applied, the less wheel rotation occurs. These actions can also cause the wheels to lock up and the tires to slide across the trail, resulting in cutting, plowing, and fragmenting of the trail tread.

When mountain bikers come to a curve in the trail they lower their center of gravity, lean to the inside of the turn, and transfer their weight to the inside of the bike's tires. These actions decrease the surface area of the tire coming into contact with the trail tread, which increases the pounds per square inch applied to the trail tread. The angle of the tire also becomes more acute increasing the angle of impingement. At fast speeds, the rear tire can lose traction causing the tire to skid across the trail tread. If the user brakes coming into the turn and accelerates coming out of it, the mechanical wear rate is compounded, increasing the cutting, fragmenting, and plowing on the trail tread. (See Photo 5.6.)

ATVs and similar OHVs are light weight vehicles that can generate high amounts of torque to their tires. The amount of torque and its corresponding effects on the trail tread are directly related to the degree of acceleration applied by the rider. When a rider accelerates an OHV, more torque is applied to the vehicle's tires. As the torque increases, the cutting and fragmenting of the trail tread also increases. Rapid acceleration can cause tires to slip, further increasing the cutting and fragmenting of the trail tread.

When OHV riders brake their vehicles, they decrease their speed by first applying their rear brakes and then squeezing the grips on the handle bars to compress the front brake pads against the wheel drums, reducing the speed of tire rotation and slowing the vehicle. The harder the brakes are applied, the less wheel rotation occurs until the wheels lock up. When the wheels are locked-up, the tire slides across the trail tread, resulting in cutting, plowing, and fragmenting of the trail tread.

When an OHV rider comes to a curve in the trail, they turn their tires into the turn, lean to the inside of the turn, and transfer their weight to the inside of the vehicle. This weight transfer increases the pounds per square inch applied to the outside of the tires on the inside of the turn and to the inside of the tires on the outside of the turn. The angle of the tires also becomes more acute increasing the angle of impingement. At fast speeds, the rear tires can lose traction causing the tires to skid across the trail tread, further increasing the cutting, fragmenting, and plowing effects on the trail tread. When braking and accelerating in a turn, cutting, fragmenting, and plowing are further increased. (See Photo 5.7.)

CALIFORNIA STATE PARKS

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Watercourse crossings include wet crossings (such as armored stream crossings) and fords or dry crossings (such as bridges). Some wet trail crossings in low volume watercourses can be crossed even during peak flows. These streams are usually ephemeral. Appropriate crossing locations have a mild stream gradient that is controlled by a feature such as bedrock, boulders, or large trees, often referred to as “nick points”, in or near the channel. These features stabilize the channel gradient and make it easier to construct and maintain the crossing. The channel must be straight and not subject to lateral scour, undercutting, or deposition. Streambanks must be stable with moderate slopes for successful construction of the trail in and out of the channel. Photo 5.22 depicts a wet crossing with a low gradient, straight channel with bedrock at the base of the crossing to control the stream gradient and stabilize moderately sloped approaching banks.

Fords are primarily intended for use by horses. They are located in streams that have low to moderate flows, or are closed to use when high flows occur, usually in winter and early spring. Good ford locations exist where the stream gradient levels off after a steep run and the channel is comparatively wide. This leveling off causes small rocks and coarse aggregate to drop out of suspension and deposit in the streambed creating a streambed that is easy for horses to cross without slipping on large, smooth, slick rocks. The wide channel reduces the depth of the water and the level stream gradient reduces the velocity of the water. Streambanks must be stable and have moderate slopes for construction of a trail in and out of the active stream channel. This combination of geomorphic and hydrologic characteristics provides the safest and most sustainable ford crossing. Photo 5.23 depicts an equestrian ford across a moderate flow, low gradient, and wide channel with good approaching banks.

Dry watercourse crossing sites are usually located on large and deep bodies of water or on Class I trails, where user expectations do not include getting wet. These sites have bridges with specific design requirements. A bridge site is located where the stream channel narrows and the banks are high above the stream. A narrow channel limits the length and size of the bridge, and the high banks keep the bridge above future flood events. Careful investigation of the stream channel is required to ensure that the bridge is above future flood events (100-year flood event minimum). The channel should be inspected up- and downstream of the crossing site to find indications of past flood events such as scour marks on streambank walls, vegetation loss or changes, driftwood debris deposited on ledges or flat areas, pieces of grass and flotsam left hanging on vegetation, silt lines left on trees or rocks, and tree bark scarred by flood debris. (See Photos 5.24 and 5.25.) To determine the high water mark, design for the largest woody debris that might be carried downstream in a flood event. Floating logs and trees can project several feet above the high water mark, so additional freeboard is required for these objects to pass under a bridge.

The general condition of the stream channel must also be evaluated. Banks need to be stable and, if possible, composed of scour-resistant material such as bedrock. In the absence of bedrock, the bank material should not be subject to sloughing or crumbling. The banks adjacent to the bridge abutments must not be subject to erosion or undercutting from lateral scour. Any bridge site should be located away from the outside bend of a channel where banks will be eroded by the current. (See Figure 5.7.)

The general health of the watershed should also be evaluated. Channels lacking exposed bedrock, boulders, or large woody debris may be in a “depositional” mode. This condition is exemplified by small aggregate and silt deposited in terraces adjacent to the channel and reflects an unstable watershed. The true scour line, “thalweg”, of the channel may be substantially lower than what is observed in the field. Knowing the true thalweg is critical in determining the depth of the bridge abutments that are to be constructed adjacent to the channel. In addition, a stream channel filled with aggregate may cause lateral bank scour that could undermine abutments adjacent to the channel. (See Photo 5.26.)

Prior to the final selection of a bridge site, a hydrologist or a qualified engineer should be consulted to evaluate the crossing and calculate the water discharge levels during a 100-year flood event. See Chapter 14, Drainage Structures, for further information on these methods.

When designing a watercourse crossing, it is important to make certain that the trail is fully separated from the inner gorge before beginning a descent. If the downhill descent occurs within the inner gorge of the watercourse, the trail will likely traverse through wet and unstable ground. Depending on the gradient of the watercourse, the traverse can be a considerable distance. It is preferable to layout the trail so that it levels off or gradually climbs out of the influence of the inner gorge before the trail begins to descend. Photo 5.27 illustrates a trail alignment that descends too quickly (red line) and a trail alignment that climbs out of the influence of the inner gorge (yellow line).

Turns are trail features used to gain additional linear run to reduce linear grades. If landbase, resources, aesthetics, or construction feasibility prohibits lengthening the trail in a curvilinear fashion, then trail features such as topographic turns, climbing turns, and switchbacks are used to overcome elevation gains between control points. If a turn is required, the designer must find the appropriate location for it. The appropriate location for a turn will depend on a number of criteria related to the design, function, and sustainable performance of the feature. See Chapter 12, Topographic Turn, Climbing Turn, and Switchback Construction, for further details on designing and constructing these trail structures.

The first option is a topographical turn, facilitated by a small hill or knoll, which allows the trail to contour around a small hill while maintaining an outsloped trail bed. This feature facilitates a change in direction without the additional construction and drainage design associated with climbing turns and switchbacks. It is often so small that it does not appear on a 40-foot contour map and can only be located through field reconnaissance. The hill conceals one trail leg from the other, so cutting across the turn does not occur. An outsloped trail bed cannot be accommodated where the trail crosses over the saddle after it completes its circumnavigation of the hill. At this location, the trail needs to be crowned, insloped, or have a drain dip installed for drainage. Usually these drainage features are very short in length because saddle crossings are narrow. (See Photo 5.28.)

Another option is a climbing turn or a switchback. Generally, a climbing turn is preferable to a switchback because it requires less excavation and retaining wall construction. Typically, a climbing turn is located on a hillslope of 30% or less, and a switchback is located on a hillslope steeper than 30%. Switchbacks located on steep slopes require more excavation into the hillside to construct the upper leg and the inside (upper) corner of the turn. The lower portion of the switchback usually requires a retaining structure to support the fill material used to build the lower or downhill portion of the turn. Located on steep hillslopes, the change of direction associated with the upper and lower leg of a switchback is more acute than those of a climbing turn. However, the design and location for both structures are very similar. Both are built into the hillslope and located on the landform where they can be properly drained. Since the upper legs of both the climbing turn and switchback need to be insloped to prevent water from draining onto the lower leg, they must be placed where the upper leg can drain freely off the corner of the turn. The best place for these types of turns is on the nose of a ridge or near the upper flanks of a watercourse. Both locations allow the water collected on the upper leg to drain freely off the corner of the turn. When a turn is located near a watercourse the apex (corner) of the turn should be located close to the top of the slope leading into the watercourse. This placement allows water from the upper leg to drain into the watercourse. No portion of the turn should be constructed within the watercourse. Figure 5.8 illustrates topographically appropriate locations (marked by an X) for climbing turns and switchbacks.

Another important location criterion for these structures is a break in slope. By locating the corner of the turn where there is a distinct change or break in the slope of the hillside, one leg can be located above the break and one leg can be located below the break. This break in slope can be used to obscure the lower leg from the field of vision of a trail user coming down the upper leg. The trail user coming down the upper leg will not see the lower leg until they reach the turn, which is a very effective method of preventing users from cutting between the two legs of a turn. This technique can be further enhanced by taking advantage of natural barriers such as trees, large rocks, or dense brush. The turn is placed where these barriers are located between the two legs. Natural barriers also obscure the lower leg from the trail user's line of sight and can prevent cutting between the two legs of the turn.

Another design technique that will reduce the cutting of turns is to place the corner of the turn where the turn provides the user with a scenic view of the surrounding countryside. Incorporating a view at the corner of the turn draws the trail user to the turn and rewards them with a scenic vista.

It may not be possible to locate a turn where all of these features are present, but the more of these features that can be included the more functional and sustainable it will be. Photos 5.29 demonstrate the appropriate location for a climbing turn on a ridge nose with a slope less than 30% (left). Note no retaining wall was used at the bottom of the landing. On the right of Photo 5.29, the appropriate location of a switchback on the flank of a watercourse with a slope greater than 30% is shown. Note the retaining wall at the bottom of the landing. Photo 5.30 provides examples of using a break in the slope and natural barriers to prevent cutting of the turn by trail users.

Sometimes there are topographic features on the landform that can serve as control points. The most common of these features is a low point or saddle on an extended ridge. These locations control the elevation of a potential trail alignment simply by being the lowest point of land that a trail can pass through. Recognizing these control points early in the layout and reconnaissance process can expedite the determination of the trail corridor, the location of control points, and the grades between those control points. Photo 5.31 demonstrates how for a trail alignment with a starting location (A) and an ending location (B), the saddle on the ridge between those two locations (arrow) can serve as an elevation control.

Certain locations on the landform should be avoided by trail designers whenever possible. One of these locations is a ridge top. Trail designers often locate trails along the tops of ridges for the view and to minimize the amount of required brushing, clearing, and trail construction. If the ridge is comprised of very durable bedrock, this layout practice is acceptable. However, if the ridge top is comprised of soil, the construction of the trail tread (even light brushing and clearing) will result in the trail bed being lower than the surrounding soil horizon. With the trail bed lower than the terrain adjacent to it, surface runoff cannot flow off the trail, and it will begin to flow down the trail. The trail effectively becomes a ditch and the combination of water erosion and mechanical wear quickly incises the trail bed. This condition cannot be remedied with drainage structures such as water bars or grade reversals. (See Photo 5.32.)

If a trail is to be located along a ridge, it is much better design practice to locate it just below the ridge top where hillside construction will facilitate sheet flow across the trail bed. The trail is close enough to the top of the ridge to offer good views yet facilitate natural surface runoff. This layout practice may also provide opportunities to have the trail cross the ridge top where a low point or saddle occurs. This design offers the trail user different views in a dramatic fashion rather than just seeing the same view for an extended period of time. (See Figure 5.9.)

A similar problem occurs when trail designers layout trail on flat ground. As soon as the trail is constructed, the trail bed is lower than the surrounding soil horizon. Even if the sod is not removed, the initial user traffic will compact the soil in the trail bed below the adjacent terrain. Sheet flow accumulates in the trail bed and ponds or flows down the trail. (See Photo 5.33.) Again, the combination of soil saturation, mechanical wear, and water erosion creates an entrenched trail that cannot be corrected with water bars or grades reversals.

The best solution to this problem is to avoid this type of topography during the design and layout process. If flat, poorly drained areas cannot be avoided then the trail must be hardened or elevated using construction techniques or structures such as aggregate surfacing, turnpikes and causeways, stone pitching, timber planking, puncheons, or boardwalks. See Chapter 14, Drainage Structures, and Chapter 15, Timber Planking, Puncheon, and Boardwalk Structures, for further information on these prescriptions and techniques.

Another common trail design and layout mistake is to route a trail through a meadow. Again, most meadows are on flat and poorly drained ground. Once the trail is constructed, the trail bed is lower than the surrounding soil horizons. The trail collects water, causing the trail bed to become saturated, deformed, and eroded. Once the trail reaches this condition, trail users will no longer stay within the trail bed and will develop a trail parallel to the degraded one. If left uncorrected, this situation often leads to multiple parallel trails developing across the meadow, each one replacing the entrenched and degraded trail that preceded it. (See Photo 5.34.)

The solution to this problem is to locate trail alignments outside the meadow on stable well-drained soil. The alignment is close enough to provide good views of the meadow but does not encroach upon the fragile meadow environment. The meadow becomes a control point that the trail must avoid. If routing the trail outside the meadow is not feasible due to overriding social or political issues then the trail segment through the meadow must be elevated or hardened through the use of trail structures previously mentioned.

Another important factor to consider when designing and laying out a trail is its orientation to the sun ("aspect"). In California, a southern aspect will often provide more direct exposure to sunlight while a northern aspect will provide less. If the trail is at a high altitude with significant snowfall and cool temperatures, a southern aspect will facilitate a quicker snow melt and provide warmer and dryer conditions to trail users. Conversely, a northern exposure will provide cooler and moister conditions. At low elevations that receive warm to hot summer temperatures, a northern aspect will often provide trail users with cooler and more shaded conditions.

Due to the different temperatures and moisture conditions between southern and northern aspects, they frequently have different vegetative communities. The southern aspect may have more shrub and brush species while the northern aspect may have more trees and ground cover.

Taking advantage of the landform's aspect can greatly improve the availability and performance of the trail and the comfort of the trail user.

Please note that the attributes associated with the landform's aspect can vary throughout California. Temperature and vegetation conditions can be highly variable and must be verified during the research and reconnaissance process. (See Photo 5.35.)

CALIFORNIA STATE PARKS

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Once the flagger is on grade, the shooter and flagger use their clinometers or Abney hand levels to sight at the horizontal reference points on each other's bodies. The instruments should read the linear grade for that segment, and both team members must be within 1% of each other. If the two grades are off more than one percentage point, one person is reading their instrument incorrectly, shooting at the wrong reference point, or one instrument is defective. The team re-sights the grade to determine the cause of the error and takes the appropriate corrective action. On an ascending trail, if the instrument readings are within one percentage point of each other and the linear grade is too low, the flagger in front moves up the slope to increase the grade. If the linear grade is too high, the flagger in front moves down the slope to lessen the grade.

Once the flagger is at the correct linear grade, they make a scuffmark where they are standing (downhill foot). This mark represents the known elevation for that shot, and the shooter will occupy that location during the next grade shot. The flagger either ties a flag on a piece of vegetation directly above the place they are standing, or places a wire flag in the ground at that location. When tying a flag on vegetation, select a stem or branch that is substantial enough to be around for at least one year, and tie the flag high enough that it can be easily seen the next time the alignment is walked. The initial flagging should be spaced so that a person standing at a flag station can always see the flags in front and behind them. If the flagging is expected to survive more than one year, it is recommended that a durable grade of flagging be used. Once the flagger has scuffed the ground and tied or placed a flag representing trail grade, they move forward for the next shot, and the shooter occupies the mark where the flagger previously stood. This process is repeated until the next control point is reached. Once at the next control point, the process repeats itself except the linear grade prescription may change for the new segment, based on prescriptions established during reconnaissance. (See Photo 5.37.)

In some locations the ground (topsoil or A horizon) may be covered by layers of organic material or vegetation (tundra mat) that is too thick to establish an accurate elevation for linear grade identification. In these situations, the flaggers can use two lengths of rebar to perform the flagging. Once the rebar is marked to reflect the horizontal reference point of the flagger, it is shoved through the organics or tundra mat until it strikes soil. The flagger then sights across the reference point on one piece of rebar to the reference point on the other piece. Since both pieces of rebar are resting on topsoil, an accurate linear grade for the underlying soil can be obtained. (See Photo 5.38.)

Curvilinear alignment should be carefully followed during flagging. So that the trail is kept nearly perpendicular to overland sheet flow, linear grade shots should be taken between all topographic breaks in the landform including subtle breaks. To ensure the trail will not accumulate or divert water, natural drainage patterns should be maintained, including dipping the trail in and out of topographic watercourse features, such as small swales and undulations. Additionally, linear grades should be adjusted relative to changes in the percent of hillslope to prevent the trail from becoming fall line and able to capture and convey the hillside sheet flow. A properly laid out trail will be nearly hydrologically invisible on the landform and will prevent water from entering and running down the trail.

In Figure 5.39, two flaggers shoot across a small rounded ridge (yellow lines) that does not follow curvilinear alignment, resulting in an alignment close to the fall line. An intermediate station (pink flag) at the center of the ridge would produce an alignment slightly longer (green line) but closer to the contour of the landform.

It is important to note that when placing a flag at a scuff marking the correct linear grade, its location represents the outboard hinge of the trail bed and not the centerline of the trail bed. In manuals following traditional trail construction practices, the centerline is influenced by the percent of grade of the hillslope where the trail is constructed. If the hillslope grade is 50% or steeper, the trail bed will be nearly 100% native bench, which means the flag represents the outside edge of the trail bed. For a hillslope grade of 30%, the trail bed is approximately 50% native bench and the flag represents centerline of the trail bed. For a hillslope grade of 10%, the trail bed is approximately 25% native bench and the flag represents the inside quarter of the trail bed. These estimates are based on using fill material for constructing the trail bed. (See Figure 5.13.) Partial bench construction is not a recommended practice as a trail bed comprised of fill material will be subject to differential settling, more susceptible to mechanical wear, and less sustainable. For these reasons trail designers should always strive for full bench construction.

When full bench construction is prescribed, the flag represents the outside edge of the trail. To achieve full bench construction (regardless of the percentage of the hillslope), the trail bed is simply constructed further into the hillside. (See Photo 5.40.) It is important for the designer to understand this concept, since identifying the outside edge of the trail bed will enable the construction crew to locate the trail travelway for brushing and clearing (2 feet beyond the top of the cut bank to 2 feet beyond the outboard hinge) and determine where to start the top of the cut bank on the hillslope.

Photo 5.40 illustrates how, following traditional construction practices, a trail constructed on a 30% hillslope would be half native bench and half fill bench (yellow line). However, by constructing further into the hillslope the entire trail bed is comprised of native material (red line).

Upon completion of the initial flag line, the flagging team re-traces the alignment and re-evaluates the route. It is good practice to re-evaluate work and make adjustments to improve the alignment. Once re-evaluation has been completed, the route is “tight flagged” by spacing the flags 20 to 30 feet apart.

If the trail supervisor, equipment operators, and hand crews are skilled enough to adjust the clearing limits of the travelway to the hillslope, this flag line is sufficient to initiate the clearing and brushing of the trail. If not, additional flag lines representing travelway are installed. These additional lines show the clearing limits for trail construction crews. These outer flag lines are established by using a combination of trail construction standards, percent of hillslope, and construction methods to be used for the project.

After completing the initial flag line, a Trail Work Log is developed for construction. See “Developing Trail Work Logs and Cost Estimates” below for details on how to develop a Trail Work Log.

50%
SIDE SLOPE

40%
SIDE SLOPE

30%
SIDE SLOPE

20% - 5%
SIDE SLOPE

NOTE: AMOUNT OF TRAIL BENCH VARIES LINEARLY W/ % OF SIDE SLOPE. ALL SOIL SHOULD BE MINERAL AND CONTAIN NO ORGANIC MATERIAL.

CALIFORNIA STATE PARKS

NOT TO SCALE