Report 4 (of 6): Holding
up the Himalayas - Nepal
Introduction
The Himalayas experience some of the highest rates
of erosion in the world and the adverse geology,
topography and land use are made more acute by the
humid sub-tropical temperature zones. Rapid rock
weathering and heavy rainfall are predominant features
of this area and are responsible for inducing landslides
and erosion. Landslides block and cause damage to
roads; farming land is swept away and houses are
destroyed.
In Nepal, the road network has expanded rapidly
since the 1950s and the design, construction and
maintenance of roads needs to focus on the long and
steep slopes which are subject to erosion and very
shallow slope failure. Bioengineering can be used
to protect almost all slopes against erosion through
slope protection and stabilisation which dramatically
reduces landslides and the devastation they cause.
Bioengineering
Bioengineering is the use of living vegetation for
engineering purposes. It aims to protect and stabilise
slopes by preventing erosion and shallow mass movement.
It operates in the same way as civil engineering
structures and carries out the same functions. Bioengineering
offers an additional set of tools for the engineer,
adding to the options available and increasing the
scope of works. It is usually used in conjunction
with civil engineering structures and non-living
plant materials, rather than replacing them. There
are limitations in using vegetation by itself, for
example, deep-seated landslides, where the rupture
plane is deeper than about 50 centimetres below the
surface, cannot be stabilised.
The skill in using vegetation in engineering is
to combine it carefully with civil works to give
the best results in terms of cost and effect. Bioengineering
is relatively low in cost and the materials and skills
are all available in rural areas. It takes some time
for the materials to reach their maximum strength,
but unlike civil engineering systems, they tend to
become stronger over time.
Effects of Vegetation on Slopes
A cover of vegetation protects the soil against
rain splash and erosion, and prevents the movement
of soil particles down the slope under the action
of gravity. Vegetation increases the soil infiltration
capacity, helping to reduce the volume of runoff.
Plants transpire considerable quantities of water,
reducing soil moisture and increasing soil suction.
Stems and leaves cover the ground surface and absorb
the impact of material moving down the slope, therefore
protecting it. This network of surface fibres produces
a tensile mat effect and restrains the underlying
strata. During high velocity flows, foliage is flattened
and covers the soil surface, providing protection
against erosive flows. The foliage also acts as a
barrier to rainfall and creates a significant reduction
in the kinetic energy of raindrops and thus, diminishes
their power of erosion. Plant roots bind the soil
which increases the shear strength through a matrix
of tensile fibres, resulting in increased resistance
to deformation. The roots of plants penetrate deeply
giving anchorage into firm strata, bonding the soil
mantle to stable sub-soil or bedrock; and support
to upslope material through buttressing and arching.
These factors make a significant contribution to
slope stability, but only once the roots are mature
enough to reach deeply into the ground which takes
about five years.
Vegetation encourages other plants and animals to
live on the slope and therefore, bioengineering helps
to improve the environment as well as providing useful
products, such as, firewood and fruit.
Characteristics of Bioengineering Plants
Vegetation can be selected and arranged on the slope
to perform specific engineering functions. A mixture
of plant types should be introduced so as to give
a range of rooting depths which create an irregular
structure. This tends to prevent continuous shear
planes from developing in the upper soil layers,
discouraging shearing from taking place. The plants
on the site should be a mixture of ages so that they
do not all need to be replaced at the same time and
to ensure that there will always be strong healthy
plants protecting the slopes. Local species should
be used rather than imported materials because native
plants are more likely to be adapted to the growing
conditions of the general environment and be resistant
to local diseases.
An ideal bioengineering vegetation community (figure1)
has large trees which root deeply, giving the maximum
anchorage effect. Shrubs with strong, woody roots
which are shallower than the tree roots form an intermediate
level and large clumping grasses with a dense network
of fibrous roots close to the soil surface, provide
a thick surface cover to prevent erosion.
To increase the light penetrating through the canopy, the trees can either be
pollarded' (as shown in figure 1) or coppiced'. Pollarding is where the main
trunk is cut off, about two or three metres above the ground and new, smaller
shoots can grow. Coppicing is where the trunk is cut off about 30 centimetres
above the ground to allow new shoots to grow from the stump. Both treatments
act as a form of thinning and the trees become lighter in weight and more flexible
but retain their strong root systems. The plants growing on the ground improve
because they are able to develop an extensive root network.
Function of Vegetation in Slope Stabilisation
Plants are used for specific purposes in bioengineering and there are seven
main functions for vegetation found on the steep, fragile and eroding slopes
in Nepal.
Catching material that is moving down the slope in the process of erosion,
as a result of gravity or with the aid of water, is done using the stems of
vegetation. It can be used on slopes where there is a risk of shallow failure
or where there are other civil engineering structures or in rehabilitation
areas, for example, quarry or tipping sites, to catch materials or debris.
Armouring the slope against surface erosion, such as runoff and rain splash,
requires a continuous surface vegetation cover. Armouring can be used in areas
where there is bare soil or where there is a risk of gullying on the slopes
due to the surface material being weak or poorly protected against erosion.
It can also be used as a slope component where other civil engineering structures
are employed to armour the surface between inert structures.
Supporting the slope from below, by propping from its base, using large mature
plants with deep dense root systems, such as fully grown bamboo or trees. This
is usually used in conjunction with other engineering works to improve support
to the slope.
Reinforcing the soil by increasing its shear strength. This depends on the
strength and density of the plant roots. On slopes where the failure depth
is less than about 50 centimetres deep, the most likely causes are debris flow
or transitional slips which can be limited by reinforcing the soil. Where general
rehabilitation is required, reinforcement of the soil is often carried out.
Drainage of the slope to avoid the slumping of saturated surface material
is determined by the distribution of plants or the planted configuration on
the slope, for example, using vertical or diagonal planting to direct water
down the slope. The root systems of the plants can also be used as a drainage
tool, by carrying water down into the soil as well as drawing it out by transpiration.
The risk of shallow failure on slopes can be decreased by draining the soil
and vegetation can be used to drain excess runoff safely.
Limiting the extent of slope failure by using plant roots to hold the surface
together.
Improvement of the local environment, particularly the soil and micro-climate,
encourages better growth of other vegetation, either naturally or through management.
Bioengineering Techniques for Roadside Slopes
Planted grass lines: grass sprigs are planted in lines on the slope. The lines
can be either on the contour (horizontal) which will trap materials moving
down the slope; or downslope (vertical) which maximises surface drainage while
protecting against erosion; or diagonal which combines the benefits of horizontal
and vertical planting.
Grass seed: grass seed is spread over the ground surface to give complete
but random surface armouring. It is often covered in mulch - stems and leaves
of unwanted plants are cut up and placed around seedlings to keep the soil
cool and moist - to aid its development but it still takes considerable time
to grow.
Turfing: a surface is covered with turf which gives complete and instant surface
armouring although, this is an expensive method to use.
Shrub and tree planting: seedlings of shrub and trees are planted at intervals
throughout a site. This is usually only feasible on less steep slopes. They
grow to reinforce and anchor the slope but it will be about five years before
they contribute significantly to slope strengthening. Care and protection are
required in the first three years of growth.
Shrub and tree seeding: the seeds of shrubs and trees are inserted into cracks
on steep, rocky slopes and grow to reinforce and anchor the slope. These plants
take about five years before they make a significant contribution to slope
strengthening and during this period need protection.
Large bamboo planting: large clumping bamboos are planted, usually close to
the base of the slope. This establishes a very strong line of plants which
provide excellent reinforcement, trapping and support. Bamboos take about five
years to contribute significantly to slope strengthening and require protection
in the early years.
Brush layering: the lower end of live woody cuttings (usually hardwood) is
laid in shallow trenches across a slope, usually following the contour, and
the aerial part is left sticking out above the ground. These form a strong
barrier to prevent the development of rills and to trap material moving down
the slope. Excess debris can roll over brush layering with minimal damage which
helps it to survive long enough to take root and grow into strong shrubs. If
angled at the right position, brush layering can serve as a drainage function.
Palisades: live woody cuttings are planted in horizontal lines across a slope,
usually following the contour, and make fences consisting of closely-spaced
upright cuttings. They provide a strong and low cost barrier to slow the development
of rills, trap material moving downwards and reinforce the soil with minimum
disturbance to the slope. Palisades can be used for drainage if positioned
at the right angle.
Live check dams: a variety of woody cuttings are used to build a live check
dam which is an effective low cost structure to reduce erosion in smaller gullies
through armouring and reinforcement. They also trap materials moving down the
slope and will act as drainage if positioned at the right angle.
Fascine constructions: bundles of live branches are laid in trenches just
below the surface, usually following the contour. This is a very strong and
low cost barrier to trap material and reinforce the slope once it has grown
through the interlocked root system. It can also be used for draining if it
is angled at the right position.
The bioengineering methods chosen and the plants used determine whether or
not the slope will be sufficiently stabilised. The techniques are complicated
and each slope needs individual assessment. Consideration needs to be given
to the angle, length, drainage and moisture levels of the slope. Usually a
combination of methods is used for stabilising landslides because this will
produce the most resilience and the best effects.
Managing Bioengineering Communities The vegetation needs to be managed
otherwise it may not provide the functions required for engineering and might
even have a negative impact on the protection and stabilisation of the slopes.
Unmanaged vegetation tends to produce a dense canopy of trees which reduces
the amount of sunlight reaching the grasses and shrubs. The trees may have
deep roots, but with relatively little vegetation able to survive beneath them,
they are not able to stop erosion on the surface. They may also grow too big
and start to destabilise the slopes on which they are growing by toppling in
strong winds.
The aim is to establish a vegetation community which does not need too much
intervention from outside to maintain it. For example, plant species which
regenerate naturally; species which do not grow too fast or too tall (thereby
reducing the need for frequent cutting and removal); and species which live
longer.
During the initial implementation, the vegetation on the slope should be trimmed
back and the loose soil chipped off and removed. The plants are prepared ready
for use and a line is plotted for them to take. After planting, the slopes
must be kept in good order with regular maintenance to remove undergrowth and
weeds. The plant nursery must be kept well stocked with the right species for
the next planting season.
The information provided is a summary explanation of bioengineering which
is extremely site specific for each slope. The decision to implement one of
the alternatives documented requires careful consideration of a wide range
of situation specific parameters, many of which are not addressed here.
For more information, please contact:
John Howell FRR Ltd Brockley Combe Backwell Bristol BS48 3DF United Kingdom
Tel: +44 (0) 1934 862861 Fax: +44 (0) 1934 863666
Geo-Environmental Unit Department of Roads Babar Mahal Kathmandu Nepal
Tel/Fax: +977 (0) 1 231981
ITDG would like to thank the Transport Research Laboratory, Old Wokingham
Road, Crowthorne, Berkshire, RG45 6AU, United Kingdom and His Majesty's Government
of Nepal who prepared the original material on bioengineering in Nepal.
|
Related
Links 
