Information and Advisory Note Number 43 Back to menu
1.1 The occurrence of erosion depends on the balance between the soil's shear
strength (its ability to resist moving downslope) and the shear stresses (the
forces effecting change) applied to it. The shear strength is a function of the
physical and chemical properties which give the soil cohesion e.g. soils with
little clay mineral content tend to have low shear strengths. Shear stress can
be a product of a variety of factors. It is greater on steeper slopes, where
pore water pressure is high (e.g. after infiltration of storm water), and where
physical loading is high (e.g. after rock falls) Saturation of the soil on a
slope after an unusually heavy rainfall event is a common cause of slope
failure. Removal of the base of a slope e.g. by stream erosion, can reduce
support for higher parts of the slope and can sometimes lead to slumping.
Erosion of soil surfaces is the result of factors which expose, loosen and
transport soil particles: slope failure, peat cutting, overgrazing or burning
can expose bare soil; frost action is important in loosening particles; and
wind, rain and snow melt-water can remove loosened particles.
1.2 Erosion can be caused by prolonged wet weather, by brief intense rainstorms,
or by fairly insignificant rain following very dry conditions. Plant cover
reduces erosion principally by reducing the physical impact of rainfall or
drainage water and by reinforcing the soil structure with its roots. Much
erosion by water and frost, and all that caused by wind, occurs only if plant
cover is reduced or removed from the soil.
1.3 High annual precipitation, or a high frequency of severe rain events, may lead
to
an increased potential for erosion. Intense or prolonged freeze-thaw conditions
will have a similar effect. There was a long period of very variable but
generally colder and wetter summers and more severe winters from the 14th
century until the 19th century (the "Little Ice Age") when there was probably
more geomorphological activity than now in the uplands. Shorter term changes may
also be significant. In central Scotland there was a steady increase of up to
40% in annual rainfall totals from 1971 to 1990, mostly in the winter half of
the year. Between 1959 and 1994 on Rum the frequency of heavy rainfall (> 30 mm
24 hrs'1) was 45 % higher during 1981 to 1994 than during 1959 to 1980.
1.4 While there is general agreement on the factors involved in erosion of
mineral soils, opinions differ on the causes of upland peat erosion. One view is
that it is the consequence of unnatural damage, the other is that it is a
natural process. Both may be true. Peat depth at any location is ultimately
limited by a balance between accumulation and decay. Stability is determined by
the shear strength of the peat and the shear stresses generated by its mass, its
topographic position, and its pore water pressure. Bog slides and gullying
("nagging") may represent a natural culmination of bog growth in certain
situations. Alternate periods of erosion and recovery may occur, it has also
been suggested that desiccation of peat during the Early Mediaeval Warm Period
(from 1150 to 1300 AD) may have changed the drainage patterns in bogs such as to
prime them for subsequent erosion during the generally cold, wet conditions of
the succeeding six centuries. Nearly all areas of blanket bog above 700 m in
Scotland appear to display some erosion features
1.5 Whatever the significance of natural
events, it is clear that human activity which
modifies or removes plant cover is a very
important influence on the occurrence,
severity and duration of erosion. The principal
anthropogenic factors which contribute to
accelerated erosion are discussed below.
1.6 It is difficult to separate the effects of
grazing from other factors but it has been
shown in several areas that increased sheep
numbers may lead to the creation of bare
ground subject to erosion. Peat soils,
particularly on cut-over sites, are more
vulnerable than mineral soils
1.7 Even where overall stocking rates are causing no effect, localised erosion can
be initiated if stock are concentrated. This can occur around supplementary
feeding sites, gates, and well-used pathways between favoured grazing areas.
1.8 By preventing or hindering revegetation sheep grazing may maintain or
exacerbate erosion initiated by other causes. For example, this can be a danger
on recently burnt areas particularly if the area burnt is small relative to the
stock numbers which may be attracted by the fresh regrowth.
Scars on slopes, created by sheep rubbing
and used by them for shelter and resting,
may extend annually and may result in
considerable amounts of soil erosion. On
mineral soils in the Lake District, grazing
pressures of 2 5 sheep ha"1 (year round) or
5 sheep per ha (summer grazing only) are
the critical densities for initiating scars.
Peat is several times more vulnerable, and
the critical threshold may be about 0.5
sheep ha" (summer grazing only).
1 9 Both scars and other forms of grazing induced erosion, if not severe, may
become largely inactive and ultimately revegetate if stocking rates are reduced
on the areas affected.
1.10 Cases of severe erosion are clearly associated with gross overstocking
{e.g. 2.5 sheep ha'1 year on blanket bog in Argyll). However, data which relates
stocking levels to all the relevant factors (including soil, topography,
altitude, precipitation, burning regime, drainage) is lacking. At present,
site-specific stocking limits can only be set by experience and observation.
1.11 The evidence for red deer causing increased peat hagging and other types of erosion is anecdotal and no objective data exist. However, being heavier than sheep, they may have the potential to cause similar effects at somewhat lower stocking densities, especially on steep slopes
1.12 The burning of heather or grass is a potential cause of erosion but, while
catastrophic fires under very dry conditions can result in severe erosion which
may persist for decades, the effect of well planned and well controlled muirburn
is small and has not been shown to produce significant long-term impacts
1.13 Following burning, loss of plant cover reduces both interception of
precipitation and evapotranspiration, increasing the possibility of soil
saturation. Movement of soil particles by raindrop impact is enhanced However,
the absence of plant cover means that wind speeds at the ground surface are
higher and this may aid evaporative loss. Water infiltration is reduced in some
soils by clogging of the soil pores with fine ash, the development of a crust of
charred organic matter and ash, or the distillation and deposition of organic
compounds within the soil during the fire. This can lead to greater surface
flows and concentration of drainage water, increasing the potential for erosion.
114 The impact of burning is dramatically increased if it damages or destroys
the root
mat which binds and protects the soil This is particularly so if the fire is
severe enough to burn into the humus or peat horizons of the soil. The resulting
bare mineral soil or peat surfaces are readily eroded by wind and water Also,
the loss of the seed bank, continuing instability of the soil, and excessively
variable moisture conditions unfavourable for seedling growth mean that
revegetation may be long delayed.
1.15 Periodic cool burns, which merely
remove above ground vegetation and loose
plant litter and are followed by rapid
revegetation, are preferable to a situation in
which there is a large accumulation of
biomass which is potentially combustible in
dry periods. In the latter situation any fire will
be dangerously hot and may easily ignite the
underlying peat, especially during a period of
drought when the peat itself may be partially
desiccated.
1.16 Drainage ditches or "grips", about 30 - 40 cm deep and about 70 cm wide, are a common feature in many hill areas of high rainfall or where soil drainage is impeded. Where the rainfall is high and peats are well- humified, these grips have little effect on the water table at the spacings commonly used (15-35 m apart). The water table is reduced only within a metre or two of the dram. However, drains can initiate erosion.
Concentration of drainage water from moor grips has probably contributed to the
formation of this gully in the Moorfoots (A. MacDonald)
A study by the Game Conservancy in Swaledale found a 98% increase over three
years in average cross-sectional area, and a doubling of depth, of new drains
cut in peat. Where the drains were arranged in herringbone fashion, the spinal
drain carried largest flows and suffered greater erosion. The cutting of new
moor grips is now much reduced since the ending of grant aid (except in the
crofting counties) in 1985.
1.17 Pre-afforestation ploughing and ditching lead to increased erosion and
suspended sediment yields up to 4 times those in unforested catchments. The
level of increase and time taken for loads to return to normal depend on soil
and climatic factors and forestry practice In some cases, sediment loads may
remain elevated due to continuing erosion of the drains.
1 18 Mechanised harvesting, timber dragging, road improvement and heavy vehicle
traffic all impose pressures on the soil, often leading to a second period of
increased erosion
1.19 Though often a cause for comment,
recreational impacts are highly localised and
small scale. Path width increases with
wetness, roughness and steepness but
decreases with roughness of adjacent ground.
Unsurfaced access tracks and associated
unvegetated spoil may erode readily. Skiing
(and associated operations) may cause
damage to vegetation, leading to a reduction
in plant cover and increased susceptibility to
erosion. There is growing experience of
treatment by appropriate reseeding and
alteration of practice, restricting the extent of
the most damaging activities.
1.20 Though it is accepted that historic
pollution by sulphur dioxide deposition led to
loss of Sphagnum moss in the Pennines, there
is no information to indicate whether or not
atmospheric pollution is implicated in erosion
in the Scottish uplands There may have
been, and may still be, effects in parts of
Scotland south of the Great Glen.
2.1 Relevant published studies are limited in number and usefulness. This is
because of differences in methodology, difficulties in determining the amount of
erosion attributable to different impacts or land types, and by uncertainties
about the natural heritage significance of given amounts of soil loss. Slight
surface erosion at high altitude may be more significant than greater erosion
losses at altitudes where soil formation occurs more rapidly. Deposition of
eroded material may be locally harmful (e.g. destroying salmonid spawning beds)
but beneficial elsewhere (e.g. creating intertidal flats).
2.2 Average rates of soil erosion of up to 2 tonnes ha year'1 have been reported
for entire upland catchments in central and southern Scotland. The following
provide guideline figures on more specific rates of loss.
3.1 Peat hagging. Erosion of blanket peat into blocks of varying size and shape
separated by gullies In the early stages, the gullies may be narrow and
well-defined, actively cutting down into the peat With increasing slope, a
characteristic branching pattern develops as each gully cuts back at its upper
end. When the gullies reach the underlying mineral layer, by which time they are
often more than 3 m wide, downward erosion reduces or stops and this layer may
become vegetated. The bare sides of the adjacent peat hags continue to erode and
slump until physical equilibrium is restored. Revegetation and new peat
formation can then occur. In extreme cases only isolated blocks of peat may
ultimately remain as visible evidence of former peat cover.
3.2 Gullying Mostly occurs in glacial drift deposits on lower hill slopes and is
common in areas with steep slopes and high rainfall.
3.3 Sheet erosion. Extensive surface erosion of bared soil under the influence
of gravity, aided by frost, wind, rain and the movements of animals
3 4 Slope failure or landslide. The sudden movement of a large mass of material,
usually due to waterlogging, leaving a characteristic break at the upper edge
and a ridge of debris at the lower. In small slides, the exposed face is often
concave due to rotational movement of the material.
3.5 Wind erosion and solifluction. Particularly at higher elevations on
relatively low-angle slopes where high wind speeds are common and cycles of
freeze and thaw loosen material The effects of alternate freezing and thawing,
and post thaw soil saturation, lead to soil flow (solifluction) on steeper
slopes.
3.6 Debris flows and debris cones. Movement of very wet, relatively coarse
material. Associated with steep slopes of drift and scree at higher altitudes.
3.7 Bog bursts. The causes of bog bursts are not fully understood but they are
associated with apparent hydrological overloading during extremely heavy
rainfall. They may be initiated at a point where other erosive forces are also
operating e.g. where stream erosion cuts into the foot of a peat covered slope.
Bursts may occur even on very gentle slopes.
3.8 River processes. Normal fluvial processes mean that watercourses attempt to
cut back their beds. The rate of this depends on the nature of the underlying
substrate, the bed slope and the volumes of water involved. Where rocks or soil
are in transport, they abrade and undercut the banks downstream and, in steep
terrain, this may induce valley slope slides. Often such slides are initiated by
storm events when soil saturation may be a contributory cause.
4.1 A recent survey based on interpretation of 1988/89 aerial photographs
provides estimates of the extent of different types of erosion in the Scottish
uplands. This was based on a 20% random sample of 5 x 5 km grid squares from
sixteen regions within the Scottish uplands.
Areas surveyed Most of the main blocks of hill land were covered except for the
northern Mamores and the Nevis range, the Forest of Atholl, the Grampian Ms norm
of the Cairngorms, Applecross and the Coulin forest, Ben Armine forest, the Outer
Hebrides, and me Northern Isles The islands of Arran, Islay, Jura, Mull and Skye
were treated as one region
4.2 Of the total area surveyed
4.3 There were significant regional variations
in the percentage of the land area surveyed
affected by erosion Three regions stood out
as being more eroded: Monadhliaths (24%),
Trossachs (24%) and Easter Ross (18%).
There was also variation in the occurrence of
different types of erosion.
4 4 Peat erosion was most marked in the Monadhliaths where it occurred on 20% of
the land surveyed but, with the exception of Lorn and Lochaber, it was also
widespread throughout the western Highlands (5-10% of the area).
4.5 Gullying was most marked in the Trossachs (15% of the area) but was also
notable on the hills around the central lowlands, Lorn and Lochaber, and the
islands of Arran, Mull and Skye (5-10% of the area).
4.6 Debris erosion was most marked in the Cairngorms (7% of the area) and to a
lesser extent in Easter Ross (3% of the area).
4.7 The extent of landslides was insignificant (<1% of the area) in all regions.
4.8 Sheet erosion was not substantial in any region. It was most marked in the
Cairngorms (3%) and west Sutherland (2%).
Grieve, I C , Hipkin, J.A. & Davidson, D.A. (1994) Soil erosion sensitivity in upland Scotland Research, Survey and Monitoring Report No 24, SNH, Perth.
Comments were kindly provided by Dr Ian Grieve (University of Stirling), Dr Robert Evans (consultant), Dr John Gordon (SNH), Andrew Taylor (SNH), Dr Des Thompson (SNH) and Dr Helen Armstrong (SNH).
John Andrews
Andrews Ward Associates
17 West Perry
HUNTINGDON
Cambs PE18 0BX
Angus MacDonald
Uplands Group, Advisory Services
Scottish Natural Heritage
2 Anderson Place
EDINBURGH
EH6 5NP
Tel. 0131-447 4784