Bio-engineering for slope stabilization in Nepal
Soil erosion and slope instability have long been major challenges in Nepal. The country’s steep topography, intense rainfall, and fragile geological conditions contribute to frequent landslides, erosion, debris flows, rockfalls, riverbank cutting, and slope failures. Human activities within the natural environment have further aggravated these problems, often triggering mass movements that lead to slope instability. As a result, Nepal has suffered significant losses of life, property, and infrastructure, particularly roads and bridges.
The natural ground gradually loses its initial strength and becomes unstable due to both natural and human-induced activities. Natural processes include landslides, mass movement, soil erosion, and the slow weathering of rocks. Human-induced activities include blasting in surrounding areas; construction of roads, dams, and high-rise structures that generate ground vibrations; haphazard cut-and-fill operations on otherwise stable ground; and the addition of excessive loads on soil masses. From a soil protection perspective, civil engineering structures are often constructed to protect unstable soil from weathering. However, building such structures is not always feasible or cost-effective.
As an alternative or complementary approach, living plants are systematically planted using standard methods to gradually improve soil strength over time—either alongside or independent of civil engineering structures. This approach is known as bio-engineering. Large-scale civil engineering solutions are often expensive and sometimes socially unacceptable. Bio-engineering, as a low-cost slope stabilization technique, offers an efficient alternative for controlling shallow-seated slope failures. In Nepal, which has active geomorphology, steep mountain slopes, intense rainfall, and limited economic resources, bio-engineering plays a particularly important role and should be more widely adopted.
Bio-engineering refers to the partial or complete use of living vegetation, with or without civil engineering structures, to stabilize soil in its natural setting. The proportion of vegetation to civil structures depends on site-specific conditions and engineering requirements. The core principle of bio-engineering is to provide initial support through civil structures, where necessary, while allowing vegetation to progressively strengthen the soil mass. Over time, as vegetation matures, the contribution of civil engineering structures becomes minimal, with the overall stability largely maintained by plant root systems.
It must be emphasized that bio-engineering cannot entirely replace civil engineering structures in terms of strength, economy, or durability. Instead, bio-engineering requires appropriate support from civil structures depending on site conditions. However, bio-engineering is more flexible, environmentally adaptable, and resilient to variable loads than rigid civil structures. It is therefore particularly effective for small-scale sediment control on steep slopes, habitat restoration projects, and landslide mitigation in seismic regions. Studies have shown that vetiver grass is one of the most effective bio-engineering plants when compared to other vegetation types. This article focuses on the valuable bio-engineering plant known as vetiver grass.
Nepal’s first application of bio-engineering dates back to 1980 during the Dharan–Dhankuta road project, where it was used to protect roadside slopes.
Typical bio-engineering applications in Nepal include soil bio-engineering at Krishna Bhir, bio-terracing along roadsides, bio-engineering measures along the Dipayal–Mellekh road, roadside bio-engineering on the Muglin–Narayanghat road, the Dhangadi–Dadeldhura section, and slopes in Dhankuta, among others.
Vetiver grass is an inexpensive, fast-growing, and highly versatile plant capable of withstanding a wide range of environmental conditions. Since the 1980s, the World Bank has promoted vetiver grass for preventing landslides and soil erosion through slope stabilization. As a vegetation-based system, vetiver is environmentally beneficial from the outset. Its exceptionally long and dense root system reinforces soil and makes it highly resistant to erosion caused by high-velocity water flow. The deep and rapidly developing roots also provide excellent drought tolerance, making vetiver suitable for stabilizing steep slopes.
Vetiver can withstand extreme weather conditions, including prolonged droughts, flooding, submergence, and temperature variations ranging from –14°C to 55°C. It also shows high tolerance to soil acidity, salinity, and acid sulfate conditions, outperforming many conventional vegetation types. Moreover, vetiver can regenerate once adverse conditions are removed.
Vetiver is widely used to consolidate cut slopes, primarily by reducing erosion caused by surface runoff that would otherwise damage downslope areas. It effectively prevents shallow surface failures, thereby reducing the likelihood of deeper slope failures. Even in rare cases where deep failures occur, vetiver helps reduce the volume and velocity of sliding material. Additionally, it preserves the natural and aesthetic character of road corridors.
In Nepal, bio-engineering has long been practiced for erosion control along riverbanks, unstable retaining structures, sloping terrain, road batters, and agricultural land protection. Native species such as bamboo, kans (wild sugarcane), kush (halfa grass), amriso (broom grass), and khayar (black catechu), often combined with non-living materials like wood and stone, have traditionally been used. However, due to limited scientific understanding of bio-engineering principles and applications, its use in infrastructure development remained limited until conventional civil engineering structures proved inadequate.
Vetiver systems can also be effectively combined with conventional structural measures such as retaining walls, rock protection, and concrete riprap. For example, vetiver hedgerows may protect the upper portion of a slope while civil engineering structures reinforce the lower section. Key characteristics—such as tolerance to temperature extremes, rapid root growth, deep anchorage, low cost, longevity, high survival rate, and minimal maintenance—make vetiver one of the most successful bio-engineering plants in comparative assessments.
Comparative studies consistently show vetiver to be superior to many other bio-engineered vegetation types. Its suitability for steep slopes highlights its strong potential for application along hill roads in Nepal. In conclusion, multiple studies demonstrate that the use of vetiver grass on slopes has significantly enhanced bio-engineering practices in Nepal and holds great promise for future slope stabilization efforts.
