CIVIL WORKS GUIDELINES FOR MICRO-HYDROPOWER IN NEPAL
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9. Slope stabilisation
9.1 Overview
Nepal’s mountain slopes, particularly the slopes of the
Middle Mountains, are undergoing rapid changes due to
river cutting, weathering, and soil erosion. The rate of soil
erosion is very intense in the Middle Mountains because of
the subtropical climate and intense rainfall (2000 to 2500
mm per year falling within 3 to 4 months). This area is also
widely cultivated using irrigated terraces and heavily
deforested due to population pressures. Poor water
management and forest mismanagement in this area have
led to further decline of the hill slopes. The natural
processes coupled with man’s influence have led to
landslides, and degradation of hill slopes affecting the
sustainability and durability of irrigation channels, water
supply systems, micro-hydro schemes and other
development work.
It is not possible to completely check these natural
processes, however it is possible to control them by
appropriate choice of management, design and construction
practices. The underlying principle behind slope
stabilisation measures is to stabilise hill slopes and river
banks so as to protect the micro-hydro schemes.
Most slope stabilisation problems can be effectively tackled
by making sure that the hill slopes are dry (by diverting the
surface water from the slopes), constructing retaining walls
as well as undertaking bio-engineering measures such as
planting appropriate vegetation. Note that dry slopes are
more stable than saturated ones and landslides generally
occur on wet slopes.
Photo 9.1 Unstable slopes are a threat to schemes. This powerhouse
was destroyed by a landslide
Retaining structures such as dry stone masonry walls,
gabions and terracing are the most common method used to
stabilise slopes in micro-hydro schemes. In most micro-hydro
schemes constructing reinforced concrete retaining walls is not
feasible due to their cost. In the long term, preventive bio-
engineering measures would be more effective, sustainable and
cheaper than remedial works. These measures will often need
the continued maintenance commitment of the community.
9.2 Implications of natural geological
processes
Soil erosion, river cutting, weathering, and slope failures
have implications for design, construction, operation and
maintenance of micro-hydro schemes. River cutting can
affect intakes in several ways, besides triggering slope
failures, that may damage a portion of headrace canal,
foundations of settling basin, crossings and powerhouse.
For example, meandering rivers can leave intakes high and
dry. Similarly, degrading rivers can render intakes useless.
When headrace canals are built on hill slopes where surface
erosion has advanced to a stage where gullies have already
formed, there are greater risks of canal failure due to
deepening and enlargement of the gullies.
The main cause of gully formation is excessive run-off due
to deforestation, overgrazing and burning of the
vegetation.
Excavation work can also trigger soil erosion. The following
is recommended to reduce the risk of slope failure due to
excavation work:
Catch drains can be constructed above the top of an
excavation, diverting surface water to a safe area.
When excavating for canal construction, to prevent
surface erosion fresh hill cuts and exposed slopes of
channel banks must be quickly covered with topsoil so
that vegetation can be re-established.
Spoil from excavations should be carefully disposed of so
that soil erosion is not initiated.
Wherever possible canals should have balanced cut and
fill sections to avoid too much excavation and exposure
of fragile layers.
Provide adequate berm width on the hillside of headrace
canals, to stop shallow landslips blocking the flow and
causing overtopping, which leads to erosion of downhill
slopes.