Trees/plants prevent the direct flow of rainwater and fallen stone during the landslide and collect them near the root by multi steamed shrubs or bamboos.
Roots of trees penetrate deeply through the soil and help the trees to stain rigidly in slopy land.
3. Dense roots (Reinforcement)
The root which makes a dense below the surface of soil helps to hold soil and stone providing rigidity to plants.
The combined action of main and branch root of trees provides support to loose soil against sliding and erosion.
Vegetables and shrubs provide protective cover against the erosion of soil due to the surface flow of water.
Trees help to dispose of the excess water to the higher depth below the surface through their roots system and provide drainage facility.
B. The hydrological function of Bio-engineering
Vegetation plays a major role in balancing the hydrological cycle by changing quantities of water circulating from land and water bodies to the atmosphere . Vegetation influence the hydrological parameters like evaporation, transpiration, runoff, infiltration thus maintaining the hydrological balance.
C. Ecological function
Unlike engineering structures, bio-engineering performance as ecological function by transforming degrade slope environment to better ecological condition.
The vegetative system is widely used in civil engineering as a way of reducing the visual impact of civil engineering works and improve the landscape.
Past experience and knowledge from erosion and landslide control program have led us to believe that engineering practice alone is not always the solution to problems of slope stabilization. In many cases, engineering practice is expensive and need a high level of skill and technology that are not always affordable by the users and implementer. Living vegetation can be used for their engineering, hydrological, ecological properties.
Seeding with grasses and legumes reduce surface erosion, which can under certain condition lead to a landslide. Planting with shrubs provides vegetative cover and stronger root systems, which in turns will enhance slope stability. If not control, surface erosion and small shallow slope failure can lead to large problems that cannot be controlled.
The use of vegetation helps in slope protection by arresting and preventing slope failure and surface erosion. Planning is required for the successful implementation of a vegetation program. Before undertaking to seed, a person with local experience should be mobilized for advice. Local knowledge best on success and failure of the project is important. Seed application should begin approximately six weeks before the period of drought or damaging frost.
A slope is made as stable as possible before seeding. Controlling surface water drainage removing cut bank overhangs reducing slope angle and benching all should be done before seeding.
There are two basic types of seeding:
i. Dry seeding
It is done with rotary disk and air blown seeders. This method is less costly but limited to rough sol surface and gentle slope. Rotary disk seeders are used to spread seed and fertilizer by centrifugal force. Airblown seeders use air to blow or shoot seed and fertilizer at a distance of 5-8 mm. Equipment can be adopted for motorized vehicle.
ii. Hydraulic seeding
This types of seeding are the application of seed in a water slurry that contains fertilizer, soil binder, etc. These system requirements a mixing tank with mechanical hydraulic agitation and volume pumping. Hydraulic seeding is effective for seeding on a stiff slope where tracking of seeds to the slope is necessary.
Suitability of seeds depends upon soil type, climatic condition, nature of species, local condition, etc.
Selection of appropriate plant species for bio-engineering
The correct selection of suitable plant species is essential for the identification of engineering functions and structural characteristics of the plant for the success of bio-engineering practice. While selecting the plant for bio-engineering following criteria should be considered:
1. Purpose, site condition, and local situation.
2. Physical and climatic factors.
3. Engineering functions required.
4. Bio-engineering techniques required.
5. Structural characteristics of the plant.
6. Availability of planting material.
7. Techniques and species of bio-engineering.
Selection of plants should be based on careful studies of engineering functions such as armor, anchor, dense root, support, drain, etc. and structural characteristics of the plant such as size, shape, forms and structure leaf, stem, roots, etc.
The bio-engineering species should have the following properties:
1. Establish rapidly
2. Grow vigorously
3. Should have the deep and dense root system
4. Should resist drought
5. Propagate easily
6. Resist insects and disease
7. Withstand harsh environment