Measuring slopes
The following methods can be used for measuring the gradient or angle of a slope.
- Ordnance Survey maps are only useful for large schemes, because even the most detailed information on the 1:25,000 map only shows the contour interval every five metres. It does help with understanding the general topography and drainage patterns of an area, but is not often of use for the detailed route finding that may be necessary.
- The most accurate form of measurement is made by using surveyors’ levelling equipment. It may be possible to arrange for the local authority or college group to do such a survey, but this degree of accuracy is only occasionally needed for designing bridges and steps. See the relevant chapters for details.
- The quickest and easiest method of measuring degrees of slope is to use a clinometer. This is a small, hand-held instrument, from which the angle is read off in degrees. Clinometers are available from surveying equipment suppliers. They are expensive, but very useful for anyone involved in path planning work, as it is difficult, even with experience, to make an accurate estimate by eye.
A cheap but effective clinometer can be made with a spirit level, straight pole and plastic protractor. Sight along the pole, and tilt it until it is parallel with the slope. Then measure the angle between the pole and the spirit level, checking the latter is horizontal.
Frequently tasks involve re-routing an eroded path up a slope where most, if not all of the slope is visible from the bottom. A new route can be chosen by finding the appropriate gradient (see below) with the clinometer, and then marking it on the ground with pegs. Even if the choice of line appears obvious, it is always worth walking the slope and looking at it thoroughly, not only for the gradient, but for soil depth, wetness, vegetation and lines of sight. It is particularly important to look at the slope from the top downwards, as it is the downhill use that causes most of the problems.
The optimum gradient for a stable path is one that drains quickly without causing erosion. The diagram below indicates the range of different gradients that affect path construction and repair. These gradients apply to the gradient of the slope itself, not the path. A path taking an oblique line up a slope will have a gradient less than that of the slope.
Problems on slopes
Erosion becomes more damaging as a slope steepens, because the erosive power of water increases with increasing speed of flow. As it is a natural process which occurs even on vegetated slopes, soils tend to get thinner as the slope becomes steeper.
The likelihood of damage to slopes increases when vegetation cover decreases. Most hill and mountainsides in Britain have grassland or moorland vegetation, often suffering from overgrazing. Trees are not only the best natural barrier against erosion, but also make the management of people on paths much easier as lines of sight and movement are restricted. The re-establishment of native woodland species in fenced plots, particularly on heavily eroded lower mountain slopes, is a way of enhancing the landscape, restoring areas of erosion and easing the provision of a durable path system.
The following diagram shows different gradients with corresponding characteristics and uses. Note the difference between gradients as they apply to slopes, and as they apply to paths. Conversions to % and 1 in x values are given as these may be encountered in other literature. Note also that the actual slope in the field always appears much steeper than a diagrammatic representation. The 30 degree slope shown in the diagram would be considered a very steep slope to climb, and many people would use their hands for assistance in climbing it.
Several studies have been made of paths in upland areas in Great Britain, and estimates made of the threshold angle above which path formation problems become acute.
The theoretical optimum for a path up a slope is seven degrees. A path of this gradient a comfortable climb, and can be constructed with drains to prevent water running down the path and eroding it. Such a path is easy to route if the seven degree line coincides with the ‘desire line’, but often the obvious destination will be directly up a slope much steeper than seven degrees. Between about seven and 10 degrees, durable direct ascent paths can be made, provided drainage is effective. Above about 10 degrees, pitching may become necessary, depending on the situation and amount of use. Above the threshold of about 17 degrees of slope, a choice has to be made between a steep, direct ascent which will require steps or pitching, or ‘swinging’ the path to lessen its gradient while still taking a fairly direct line (with or without pitching). The construction of formal zigzags with limbs of a lower gradient, typically 12-20 degrees, may be needed on slopes over 25 degrees.
On many sites, the topography will limit the range of choices, for example where streams or rock faces constrain the path to a steep, narrow zone, and no alternative lower gradient route is possible. Many paths in Britain have gradients of over 25 degrees, for which pitching is the only durable option.
Swinging the path line
On slopes steeper than about 17 degrees, the path can be ‘swung’ to lessen the gradient and take a natural line up the hillside, without the formation of either a straight staircase, or formal zigzags. Rather than the ‘Z’ pattern of zigzags, informal ‘S’ lines are followed. Most slopes are broken by outcrops of bedrock, streams and other natural features, which prevent the path line becoming too regular, and make sections either directly above or below difficult to see, so discouraging short-cutting.
Water must be diverted from flowing onto or down the path. This can be done by digging small ditches to protect the top side of path traverses, and directing the water away from the path line. Cut-offs and cross drains can also be constructed as necessary. Excess material dug from ditches can be used for slope repair and landscaping, or mounded into humps or ‘blisters’, strategically placed to discourage short-cutting.
Whether or not pitching is required will depend on the type of subsoil, and the amount of use the path receives. Where bedrock is exposed, either naturally or by trampling and erosion, this will form a durable path surface.
Zigzags
Zigzags can be constructed in order to keep the gradient down to one which can be made stable with its natural surface, or by the laying of aggregate surfacing material. A path gradient of about 15 degrees is usually the maximum for aggregate paths, although steeper path gradients have been achieved using artificial geogrids to reduce slippage of the aggregate.
There are many old stalking paths, as well as more recently constructed vehicle tracks in the Scottish mountains which follow zigzag routes. Many tracks were constructed in the 19th Century to give access for shooting parties, and have long gentle gradients alternating with spectacular sets of zigzags to gain height. The zigzags often follow the nose of a ridge, where run-off from the mountain is less than in a gully.
A study made of some of the zigzags on the stalkers’ tracks in the Mamores, Highland Region (John MacKay; unpublished paper) showed the following limb gradient and length of limb for 48 different sections of zigzag.
These zigzags are amongst the steepest in Britain, climbing slopes of over 25-30 degrees. The histogram therefore shows a higher mean gradient than would normally be found, with limb gradients of 15 to 16 degrees being more typical.
Zigzags tend to appear spontaneously on slopes of 30 degrees and over, where the direct ascent becomes uncomfortable to the walker.
The gradient of a zigzag has to be such that the walker views it as being less tiring and so quicker than the direct ascent or descent. The length of the limb of the zigzag is also significant. Repetitive limbs of the same length, such as on the zigzag path up Selborne Hanger, Hampshire, are tedious to walk, and two obvious short cuts develop down either side. Drainage is also a problem, as each limb directs water down the short cut.
On the other hand, too long a limb may appear to lead away from the direction of the destination. Varying the length of limb is the best pattern, taking advantage of any spurs, outcrops, boulders or slight lessening of gradient to make the turn.
Ideally, the corner of the next zigzag above should not be visible, but this is not often possible. Where necessary, short cutting can be discouraged by steep revetments or barriers.
The corner of a zigzag is the point most vulnerable to erosion, and should be positioned on a stable part of the slope. This may be an area of bedrock exposed on a scree slope, or above an outcrop or bluff on a vegetated slope. It should also be positioned at a point where the slope is less steep, to allow more room for the turn to be made. The turning area should only have a very slight gradient. Alternatively, the turn can be made with steps.
