K.M. Sthapit and L.C. Tennyson
In 16 years of activity, the Nepalese Department of Soil Conservation and Watershed Management has developed and implemented erosion control techniques suitable to mountainous topography characterized by intensive monsoon rains. This article presents examples of bio-engineering techniques used in Nepal to reduce erosion caused by landslides and the inappropriate construction of forest access roads.
Keshar Man Sthapit is Chief of the Watershed Management Project.
Lawrence C. Tennyson is Chief Technical Adviser of the Watershed Management Project and of the Shivapuri Watershed Management and Fuelwood Plantation Project.
Nepal is a mountainous country; approximately 86 percent of the area is characterized by steep hills and mountains. Elevation ranges from 200 m to 8848 m (Mount Everest) within a radius of less than 200 km. The country comprises five distinct physiographic regions: the High Himalayas (23.7 percent of the total area), the High Mountains (19.7 percent), the Middle Mountains (29.5 percent), the Siwalik (12.7 percent) and the Terai (14.4 percent), (LRMP, 1986).
The rate of natural erosion in the geologically young and seismically active mountains is high, as is that of the subsequent down-slope transport and deposition of sediments. Laban (1979) estimated that approximately 74 percent of landslides occur under natural conditions. However, intensive use of the land resource for agriculture, grazing and fuelwood and development of infrastructure such as roads, without adequate conservation measures, has accelerated surface, gully and mass wasting erosion in Nepal. Therefore, although statistics are not available, it is reasonable to assume that the percentage of landslides caused by human activities is significantly higher in the more densely populated Middle Hills and Siwaliks. One of the major causes of slope instability produced by human activities is deforestation in the hills and mountains.
In 1974, His Majesty's Government of Nepal established the Department of Soil Conservation and Watershed Management (DSCWM) under the Ministry of Forests and Soil Conservation. From an initial three projects, the Department has grown steadily and at present provides services in 26 of the 75 districts. Currently the DSCWM employs 130 professional, 335 technical and 281 administrative staff, making it one of the largest watershed management operations in the entire Asia-Pacific region.
In 1988, the government completed development of the Master Plan for the Forestry Sector, setting out programme directions through to the season 2010/11. Soil conservation and watershed management (SCWM) is one of the major elements in the Plan; the establishment of 18 district soil conservation offices and eight integrated watershed management offices is foreseen by the year 1994. However, SCWM services to protect land resources from soil erosion, flooding, sedimentation and river shifting will be provided in all of the districts of the country.
Landslides, i.e. down-slope movement of land mass as a result of slope failure, occur when the shear stresses of the land mass exceed the shear resistance forces. Many of the landslides in Nepal are triggered by toe cutting of the slope and when the land mass becomes saturated with water.
Landslide treatment at Labok, Biring watershed, Ilam district
The Labok landslide, which comprises approximately three hectares, is located within the Biring River catchment in the eastern district of Ilam (Figure 1). The average elevation of the slide area is about 800 m. The climate is subtropical with an average annual rainfall of 3000 mm. A significant portion of this rainfall occurs from June through August. The maximum recorded 24-hour rainfall is 352 mm. The soils are sandy loams. The vegetation surrounding the slide area is characteristic of mixed hardwood forests.
Several landslides have occurred in the Biring watershed. When the slide at Labok was first recorded by aerial photography in 1954 it had apparently already been active for several years. From the up-slope agricultural land, water that flowed on to the slide area produced severe rill and gully erosion. In addition, the water from up-slope sources that infiltrated into the slide residuum, coupled with toe cutting of the slide area, created conditions for continued mass movement during the wet season. Up-slope agricultural land was frequently destroyed, and domestic dwellings located below the scarp area of the slide were in constant danger.
Nepal's physiographic regions
The Terai region is a gently sloping plain of alluvial deposits. Forty-four percent of the country's population resides in this region. The subtropical climate and gentle topography are conducive to agricultural production. However, severe flooding, river shifting and river bank cutting occur frequently, thus threatening the stability of agriculture in many areas of this region.
The Siwalik is composed of north dipping interbedded sandstone (hard and soft), silt stone, shale and conglomerate. The area has a subtropical climate. The soils are highly vulnerable to water erosion, and flash floods occur frequently in the low-lying areas. The rivers flowing in the Siwalik region transport tremendous volumes of debris during the summer monsoon season.
The Middle Mountains are a complex of phyllites, schist, granites, quartzites and limestone. Climate in this region ranges from subtropical in the valleys to temperate on the slopes and ridge tops. The High
Mountains consist of highly metamorphosed phyllite, schist, gneiss and quartzite. Climate in the High Mountains ranges from warm temperate in the valleys to alpine at the ridge tops. More than half of the population resides in these regions. High population density and intensive land use (steep land cultivation, deforestation and heavy grazing) have created conditions for accelerated erosion here. The predominant erosion processes are mass wasting and gully erosion. The majority of the sediment load contribution to the rivers which originate or flow through these areas is derived from these types of erosion. Surface erosion (rill and inter-rill) on sloping agricultural land is prevalent in the Middle Hills and, to some extent, the High Mountains. However, compared with mass wasting and gully erosion, the contribution to in-stream sediment from erosion of agricultural land is considered to be minor.
The geology of the High Himalayas consists of gneiss, schists, limestone and shales. Alpine climate occurs in the low areas with an arctic environment at the higher elevations. Owing to the cold climate, physical weathering predominates: thus soil development is slow and most soils are very stony in texture. Population is very sparse in this region. Rock slides, which dam the major rivers, and glacial outbursts are quite common phenomena, resulting in disastrous floods in the High and Middle Mountain regions.
FIGURE 1 - Map of the physiographic regions of Nepal
The DSCWM began the task of stabilizing the slide area in 1975. During the first year, a detailed survey of the area was conducted to determine the causes of the initial slide and reasons for the continued mass movement. Also during the first year, some bamboo-cum-loose-stone check dams (see Figure 2) were constructed to begin control of rill and small gully erosion on the slide surface.
The Labok slide in 1975. Note the structural erosion control works in the left foreground
During the 1976/77 season, a series of stone-lined diversion channels were constructed to prevent excess up-slope surface water from flowing on to the slide surface. Water from these channels was dispersed on to bedrock portions of the natural drainage to dissipate energy of the concentrated flow and minimize erosion at the disposal sites. To mitigate gully erosion and superficial mass movement, a combination of bamboo, bamboo-cum-loose-stone and gabion check dams and retaining walls were constructed along the toe of the slide.
Labok slide area in 1986, with dense vegetation cover
Complementing these essential physical structures, thousands of utis (Alnus nepalensis) seedlings were planted (2 x 2 m spacing). Utis is a suitable tree for landslides because of its aggressive growth on exposed soils. Rhizomes of bamboo and amliso (broom grass, Thysanolaene maxima), were also planted intensively throughout the slide area. More than 600 bamboo rhizomes were planted in relatively stable pockets where adequate soil and moisture were available. Approximately 40000 plugs of amliso were planted in rows (1 x 1 m spacing) along the contour. Broadcast seeding of amliso (20 kg/ha) and utis (20 kg/ha) was carried out on the inaccessible portions of the slide.
FIGURE 2.a. Examples of erosion control structures used at the Labok slide - Diversion channel
FIGURE 2.b. Examples of erosion control structures used at the Labok slide - Gabion retaining wall
FIGURE 2.c. Examples of erosion control structures used at the Labok slide - Gabion check dam
FIGURE 2.d. Examples of erosion control structures used at the Labok slide - Bamboo check dam
FIGURE 2.e. Examples of erosion control structures used at the Labok slide - Bamboo-cum-loose-stone check dam
Nevertheless, some sporadic mass movement continued after treatment. Therefore, in 1977/78 maintenance of structures and construction of additional check dams and retaining walls were undertaken. Replanting of bamboo, amliso and utis plus additional broadcasting of seed were also required during this period.
After the 1977/78 season, only minor maintenance of the site continued. Observation of the site in 1986 by the authors revealed that the area was fully covered by a dense cover of grass and trees, effectively controlling surface erosion. The combination of vegetation and structures had stabilized the gullies, and mass movement had ceased.
Over the past several years, the local people have utilized the grass fodder produced on the site. The leaves are mixed with other fodder to feed livestock, and the stalks are used for making brooms - hence their common name.
Development of forest access roads in the mountainous topography of Nepal is a challenging undertaking. Construction of a stable road with minimal impact on the surrounding environment requires the consideration of many factors, including balanced cut and fill, proper drainage and disposal of water from the road prism, and conservation measures on exposed slopes. Several big-engineering methods have been developed to mitigate erosion of cut-and-fill slopes on forest roads (FAO, 1985; 1989).
A landslide is a downward movement of a part of a slope of rock detritus or soil, along a sliding surface where shear failure occurs. A moving area can be distinguished from an unmoving area: the soil mass above the sliding surface moves and the soil mass below the sliding surface does not.
The soil mass in any slope is subjected to the shear stress of gravity. Usually this is balanced by the shear strength of the soil mass, making the slope stable. However, if shear stress from any source (excessive groundwater, earthquake, inappropriate construction, etc.) exceeds the shear strength of the soil along a potential sliding surface in the slope, shear failure or movement occurs.
The most important natural cause of landslides is an increase in the groundwater. Water saturation decreases the shear resistance of the soil mass and increases the shear stress by increasing the weight of the soil mass. These also occur along rivers or torrents where the stability of the slope is damaged by loss of support at its lower end. Landslides can also be triggered by earthquakes.
In recent years, large-scale construction in mountain areas, particularly road and reservoir development, has become a major cause of landslides.
Although landslides may be sudden and spectacular, they also may be slow and discontinuous. In many cases the existence of a landslide may be determined only by sophisticated tests or by aerial photographs taken at intervals of months or even years. The justification for landslide prevention and control work, irrespective of the originating factors, is closely linked to the on-site and downstream values to be protected. The scale ranges from wilderness areas where the occurrence of landslides may have no adverse effects, to catchment areas where landslides may threaten relatively few lives but menace important water supply systems, to urban areas (e.g. parts of Hong Kong and Japan) where the lives and homes of hundreds of people may depend on the stability of a single slope.
Shivapuri watershed-road erosion control
The Shivapuri watershed located north of Kathmandu is an important water supply catchment for the city. The watershed was placed under protected status in 1975 and the government began construction of a circular access road (105 km) to facilitate management of the area in 1977. The road was constructed through moderate-to-steep topography in sandy, stony soils at elevations ranging from 1740 m to 1860 m.
The road was constructed with balanced cut-and-fill design, excess material being deposited, in most cases, on the valley side. The exposed, unconsolidated fill material was highly vulnerable to rill and inter-rill erosion and, in some places, mass movement. To keep gabion wall construction to a minimum and to reduce cost and time of construction, embankment filling on the valley side was conducted. The valley-side embankments were constructed with the criterion of keeping side slopes between 67 and 80 percent, gradients considered to provide stable slopes for most fill materials (Hansen, 1986).
In areas where embankment filling was not practical and mass movement of road material might occur, gabion retaining walls were constructed according to standard engineering design. A series of loose stone-filled gabion boxes (usually of 1 x 1 m cross-section) were used to construct retaining walls to prevent down-slope movement of material.
To control surface erosion on fill slopes by vegetation measures, Napier grass (Pennisetum purpureum), also known as elephant grass or Uganda grass, was planted. At some sites, utis (Alnus nepalensis) was planted with Napier. The planting began in 1986 and continued through 1989. During this period approximately 20 km of road-fill slope was planted with Napier grass.
A road drainage system was constructed consisting of stone-lined drains on the cut-slope side of the travelway, pipe culverts, cross-drain box culverts, causeways, and surface cross-drains with water disposal into natural drainages and on to land surfaces. Construction of energy dissipators and establishment of vegetation cover were undertaken at disposal points to minimize the erosion potential of concentrated flow. Loose stone in conjunction with planting of utis and Napier was used at these disposal points (Hansen 1986, 1989).
Napier grass, indigenous to tropical Africa, has been used for surface erosion control in many countries. The grass has good soil-binding qualities and is easily propagated by planting of cuttings, which is advantageous on steep-sloping, loosely consolidated soil. The grass is an aggressive reproducer and spreads with short stout underground stems. It is multipurpose in that it is effective for surface erosion control and the leaves and young stems are good-quality fodder. Because the grass is good for fodder, exclusion of grazing animals is essential for initial growth and survival.
Planting Napier grass on a road-fill slope In the Shivapuri watershed
Napier cuttings were planted at 45 x 45 cm spacing in a series of parallel single rows with two cuttings per spot. The cuttings were planted at an angle to the surface with two to three nodes buried in the soil and one node above ground, and with sound tamping of the soil to prevent air pockets (Ness, 1989).
Planting was done at the end of the rainy season, from June through August depending on the year. High survival of cuttings occurred on most sites. Failures occurred on very stony soils and on waterlogged sites, and when cuttings were not planted properly.
The grass has grown well in Shivapuri over a wide range of fertility and moisture conditions. Although not observed in Shivapuri, apparently rodents like to feed on the roots. Owing to low temperatures at higher elevations the grass was dormant during the cool-season months in Shivapuri. The dormancy period had no effect on erosion control, because the dormant plants continued to provide good soil cover and growth occurred before the pre-monsoon and monsoon rains. However, fodder production during the winter period was adversely affected
Following one year of growth, sampling of biomass production in 1987 et ton sites with differing elevations revealed an average annual yield of fresh matter of 127 tonnes/hectare with a range of values from 80 t/ha on steep fill slopes at 1850 m elevation to 190 t/ha on relatively flat area at 1790 metres (Nunkoo and Ness, 1989).
In Shivapuri, the grass can be harvested at intervals of six to eight weeks from May to October. The people who live within and adjacent to the protected area are encouraged to harvest the grass for fodder at no cost under the supervision of the project. Through the project extension services, many farmers have also begun planting Napier grass on small plots near their homes for stall feeding of animals. These plantings were usually on marginal land or in a portion of the home vegetable garden.
Investigation of rooting depth after two years of growth revealed depth of superficial roots to 0.3 m with maximum depth to 1.0 m (Hansen, 1989).
The success of Napier grass as an effective control of surface erosion on road-fill slopes and as a high-quality fodder crop has led to the use of this form of vegetation control in many parts of the country.
A Nepalese woman collecting Napier grass for fodder from planted roadside areas
Control of severe landslide and gully erosion usually requires engineering structures to avoid slope failure and gully cutting. However, structural measures involve high investment and a high degree of technology. In some cases in Nepal, the lack of timely and adequate maintenance has resulted in spectacular failures of this type of control measure. The resultant erosion is often more serious than before the treatment.
The exclusive use of vegetative measures for control of major landslides and severe gully erosion often results in failure. However, when vegetative measures are combined with engineering measures, i.e. big-engineering, the end result can be effective stabilization of the area from mass movement and gully cutting, and dramatic reduction of surface erosion, at relatively low cost and high sustainability, and with the added benefit of fodder and fuelwood production for local populations.
The successful results of bio-engineering techniques have been observed in gully control, waste land reclamation and slope stabilization in many places in the country. Bio-engineering control measures have been observed in Nepal to be economically desirable and most effective for erosion control in degraded areas.
FAO. 1985. Vegetative and soil treatment measures. Conservation Guide 13/1. Rome.
FAO. 1989. Road design and construction in sensitive watersheds. Conservation Guide 13/5. Rome.
Hansen, Joakim. 1986. Consultancy in road design, planning and construction. FO: GCP/NEP/041/NOR, Consultant Report No. 2. Kathmandu, FAO.
Hansen, Joakim. 1989. Consultancy in road design, planning and construction. FO: GCP/NEP/041/NOR, Consultant Report No. 4. Kathmandu, FAO.
Laban, P. 1979. Landslide occurrences in Nepal. FO: NEP/74/020, IWM/WP/13. Kathmandu, FAO.
LRMP (Land Resource Mapping Project). 1986. Land systems report. Kathmandu, FAO.
Nunkoo, P. & Ness, B. 1989. A study of Napier grass (Pennisetum purpureum) production in Shivapuri watershed area. FO: DP/NEP/85/008, Working Paper No. 28. Kathmandu, FAO.
Ness, Bjorn. 1989. Production potential of Pennisetum purpureum. FO: DP/NEP/85/008, Working Paper No. 29. Kathmandu. FAO.