rainfall and their effect in hillslope
water can move over the surface of ground, into the surface to a depth of a few centimetres, further down into the soil profile, and deep into rocks. all these pathways can lead to instability in various forms.
surface water movement
conditions that lead to overland flow are:
- when the soil has a capping (compacted surface). a soil cap will prevent
infiltration even if the soil itself is highly permeable;
- when the rate of precipitation exceeds that of infiltration (when the soil
is not saturated);
- when the soil is saturated;
- when impermeable rock or impermeable soil is at the surface;
- slope, to a limited extent, can determine whether or not overland flow takes
place. If the slope is very steep, water will flow over it however permeable
the surface is. however, for most practical situations slope does not cause
overland flow, although it certainly influences the rate of overland flow.
the result of water flowing over the surface is:
- flow without any damage to surface or
- sheet erosion, or
- rill erosion.
sheet erosion is the removal of soil, mineral, or rock particles evenly over the whole surface. sheet erosion persists in firm ground that resists rilling, e.g. very weak rocks. mudstone and soft sandstone, the Siwalik rocks, are typical examples of this material. a thin weathered skin develops on these rocks, which is then removed by rain-wash, to expose firmer rock beneath. thus, softening and removal of the rock continues only on the surface. Rill erosion is the removal of soil, mineral or rock particles along water channels. this is by far the commonest form of erosion. as rills become larger and deeper, they develop into gullies. fine soil of low cohesion, e.g. silt or silty soil, is most susceptible to rill erosion. this is because fine, cohesionless particles are very easily detached by water and carried in suspension.
sub-surface water movement
when water percolates into the soil, it enters the voids and starts to fill them up. as a result, pore water pressure starts to rise. pore water pressure is the pressure acting on soil grains by water held in the pores. pore water pressure can be positive or negative. it is negative when the voids are only partially filled with water. (this state is also known as soil suction). pore water pressure becomes neutral just before the point at which the voids become completely filled with water. pore water pressure becomes positive at the point when all the air has been expelled from the voids and the water phase in the soil-water mix becomes continuous. at that point, the water phase becomes a column and hydrostatic pressure, equivalent to the height of the column, is exerted within the pores. the pressure is transferred to the soil grains.
if the hydrostatic pressure is sufficiently high it will force the grains apart and the mixture will start to behave as a liquid. hydrostatic pressure developed near the soil surface, as when the upper layer becomes saturated during heavy rain, causes the soil to flow.
when pore water pressure becomes positive along the walls of a fissure underground, a 'pipe' develops. a pipe is an enlarged fissure that forms underground in fine-grained, non-cohesive soil, especially silty or fine sandy soils. enlargement of the fissure takes place when water, flowing along the fissure or into the fissure from the side walls, detaches particles of soil and carries them away in suspension. pipes that have not broken through to the surface can still sometimes be detected by the presence of an elongated hollow of subsided ground pointing down the slope. the trench may be above the head of a gully and in the same alignment as the gully, indicating that water is moving into the gully head as ground water through a pipe.
if water travels downwards to the bottom of the soil profile, it commonly becomes halted in its path by the impermeable surface of the rock beneath. It then migrates downhill along the interface until it emerges as a spring at a point where the soil becomes shallower or the rock outcrops at the surface.
pore water pressure may become positive at the base of the soil profile, resulting in a deep translational landslide (the commonest deep type) or circular failure.
water in rocks
if water goes deeper than the soil profile, it goes into the bedrock. In horizontal rocks, it will move sideways, slowly, along the surface of an impermeable layer. in tilted beds, it will move more rapidly down the slope. if the rock is fractured, the water will continue to go deeper. hydrostatic pressure is exerted within the open joint systems of rocks in exactly the same way as in soils. if the water cannot escape as spring water, high pressure can develop and force the joints apart. this is the cause of many rockslides.
the permeability of rocks is controlled by:
- the permeability of the rock;
- the angle of bedding; and
- the number, orientation, openness and continuity of fractures.
conclusion
bioengineering can be applied to:
- rill erosion;
- shallow liquefaction failures;
- shallow mass movements (slides); and
- some kinds of shallow rock failure.
sheet erosion, although shallow, is difficult to arrest because plants will not grow in the rock beneath the weathering skin, and the skin itself is not thick enough to support plants. even if plants can be established, they are in constant danger of being washed bodily off the slope along the interface with the firmer rock beneath.
nepalese soils and rocks are generally very permeable, containing voids and many fractures. these allow water in to various depths, all of which can cause instability of various kinds.
rainfall and its effect in nepal
water is the main agent in triggering hazards in Nepal. to understand these water related hazards the knowledge on occurrence of precipitation, its spatial and temporal distribution, distribution of water over the ground and below the surface as rainwater come in contact to ground is necessary.
distribution of rainfall in nepal
the temporal and spatial variation of rainfall in Nepal is very high. 80-90% of rainfall occurs during June to September. Nevertheless, rainfall in Nepal occurs with different mechanisms.
(a) Orographic Rainfall: This type of rainfall is due to the condensation of moisture-laden vapour as it meet orographic barrier (mountains). the monsoon rainfall in Nepal (June- Sept) is mainly by this mechanism. during these months the moisture-laden vapour from Arabian Sea and Bay of Bengal get intercepted by high mountains of Nepal causing condensation as it rises up. This results in high rainfall across the southern flanks of Annapurna range (windward face) and low rainfall along trans Himalayan region (Leeward face).
(b) Frontal Rainfall: when dry cold wind and light moisture laden hot wind meet at front, heavy cold wind compels to rise the latter resulting in condensation. this mechanism prevails in Nepal during winter as hot moisture laden wind from Mediterranean sea meets cold wind from central Asia. as a consequence winter rainfall is much significant in the western part of Nepal.
(c) Convective Rainfall: this type of rainfall mechanism is very local in nature. during April month the temperature in Terai is high causing significant evaporation from water bodies and soil and evapotranspiration from vegetation. however the environment at high altitude is still cold enough to condense convectively the evaporated moisture locally resulting in rainfall.
(d) Cyclonic Rainfall: cyclones are the large concentric low-pressure zones. such zones appear in Bay of Bengal and its vicinity during autumn resulting in rapid condensation of moisture. This mechanism is mainly responsible for rainfall in Nepal during autumn.
estimation of maximum runoff
the estimation of maximum runoff is important in order to know the extent of hazard as well as to design the mitigation measures. the following classical Rational method could be used for its estimation.
rational formula:
Qpeak = CItcA
where,
Qpeak = Peak discharge from the catchment
C = Runoff Coefficient
Itc = Rainfall intensity for duration equalling time of concentration of the catchment, tc
in this method, determination of maximum rainfall intensity for duration equal to time of runoff concentration is sought. In most of hilly catchment of Nepal this time is of the order of 15 to 30 minutes. therefore, a long-term continuous precipitation records is required to estimate the peak intensity for such duration with given return period. In the absence of such data, 24-hr maximum rainfall can be used with extrapolation for estimating peak intensity of smaller duration. 24 hour maximum rainfall amounts are available for 277 stations distributed across the country. the Weighted average method could be used to estimate the same for particular location from the stations in the vicinity. the main philosophy behind the extrapolation of 24-hr max rainfall to achieve intensities for different durations is that the log-log plot of intensity versus duration for different stations of Nepal shows sets of parallel straight lines with two distinct slopes broken at the same duration as shown in the figure below. therefore, the point representing maximum intensity for 24 – hr duration, plotted in the same graph could be extended parallel to those lines to get intensity duration curve for the location of our concern. with the help of this curve the intensity for any duration matching the travel time (concentration time) for watershed of our concern is determined to be used in rational formula.
fig: log-log plot Intensity duration curve
movement of water
water can move over the surface of ground, into the surface to a depth of a few centimetres, further down into the soil profile, and deep into rocks. all these pathways can lead to instability in various forms.
surface water movement
conditions that lead to overland flow are:
- when the soil has a capping (compacted surface). a soil cap will prevent
infiltration even if the soil itself is highly permeable;
- when the rate of precipitation exceeds that of infiltration
(when the soil is not saturated);
- when the soil is saturated;
- when impermeable rock or impermeable soil is at the surface; and
- slope, to a limited extent, can determine whether or not overland flow
takes place. if the slope is very steep, water will flow over it however
permeable the surface is. however, for most practical situations slope
does not cause overland flow, although it certainly influences the rate
of overland flow.
the result of water flowing over the surface is:
- flow without any damage to surface or
- sheet erosion, or
- rill erosion.
sheet erosion is the removal of soil, mineral or rock particles evenly over the whole surface. sheet erosion persists in firm ground that resists rilling, e.g. very weak rocks. mudstone and soft sandstone, the Siwalik rocks, are typical examples of this material. a thin weathered skin develops on these rocks that is then removed by rain-wash, to expose firmer rock beneath. thus, softening and removal of the rock continues only on the surface. Rill erosion is the removal of soil, mineral or rock particles along water channels. this is by far the commonest form of erosion. as rills become larger and deeper, they develop into gullies. fine soil of low cohesion, e.g. silt or silty soil, is most susceptible to rill erosion. this is because fine, cohesionless particles are very easily detached by water and carried in suspension.
subsurface water movement
when water percolates into the soil, it enters the voids and starts to fill them up. as a result, pore water pressure starts to rise. pore water pressure is the pressure acting on soil grains by water held in the pores. pore water pressure can be positive or negative. it is negative when the voids are only partially filled with water. (this state is also known as soil suction). pore water pressure becomes neutral just before the point at which the voids become completely filled with water. pore water pressure becomes positive at the point when all the air has been expelled from the voids and the water phase in the soil-water mix becomes continuous. at that point, the water phase becomes a column and hydrostatic pressure, equivalent to the height of the column, is exerted within the pores. the pressure is transferred to the soil grains.
if the hydrostatic pressure is sufficiently high, it will force the grains apart and the mixture will start to behave as a liquid. hydrostatic pressure developed near the soil surface, as when the upper layer becomes saturated during heavy rain, causes the soil to flow.
when pore water pressure becomes positive along the walls of a fissure underground, a 'pipe' develops. a pipe is an enlarged fissure that forms underground in fine-grained, non-cohesive soil, especially silty or fine sandy soils. enlargement of the fissure takes place when water, flowing along the fissure or into the fissure from the side walls, detaches particles of soil and carries them away in suspension. pipes that have not broken through to the surface can still sometimes be detected by the presence of an elongated hollow of subsided ground pointing down the slope. the trench may be above the head of a gully and in the same alignment as the gully, indicating that water is moving into the gully head as ground water through a pipe.
if water travels downwards to the bottom of the soil profile, it commonly becomes halted in its path by the impermeable surface of the rock beneath. it then migrates downhill along the interface until it emerges as a spring at a point where the soil becomes shallower or the rock outcrops at the surface.
pore water pressure may become positive at the base of the soil profile, resulting in a deep translational landslide (the commonest deep type) or circular failure.
water in rocks
if water goes deeper than the soil profile, it goes into the bedrock. In horizontal rocks, it will move sideways, slowly, along the surface of an impermeable layer. in tilted beds, it will move more rapidly down the slope. if the rock is fractured, the water will continue to go deeper. hydrostatic pressure is exerted within the open joint systems of rocks in exactly the same way as in soils. if the water cannot escape as spring water, high pressure can develop and force the joints apart. this is the cause of many rockslides.
the permeability of rocks is controlled by:
- the permeability of the rock;
- the angle of bedding; and
- the number, orientation, openness and continuity of fractures.
conclusions
bioengineering can be applied to:
- rill erosion;
- shallow liquefaction failures;
- shallow mass movements (slides); and
- some kinds of shallow rock failure.
sheet erosion, although shallow, is difficult to arrest because plants will not grow in the rock beneath the weathering skin, and the skin itself is not thick enough to support plants. even if plants can be established, they are in constant danger of being washed bodily off the slope along the interface with the firmer rock beneath.
Nepalese soils and rocks are generally very permeable, containing voids and many fractures. these allow water in to various depths, all of which can cause instability of various kinds.
hydrology and drainage structures
problem on drainage structures
there were some problems in a small gully near Kalimati, in the Lamosangu-Jiri Road in the Charnawati area, after the rainfall of 1984. later on, the gully was protected by gabion check dams, and the water coming from the gully above the road was diverted to the adjacent Kalimati gully about 50 m away for safety reasons. however, in 1987, after the high intensity rain, the Kalimati Gully was heavily damaged. the gully became almost 3 times wider than before below the road and there were several bank failure problems along the gully above the road. however, the small gully to the left to the Kalimati gully did not show such problems. what may be the regions behind it? also, give your solution.
problem on drainage structures
in the Naubise sector of the Prithvi Highway, the water started to flow through the road surface after the heavy rain of 1993. the water damaged the road surface. however, the side drains and a hume pipe culvert at the problematic area were in good condition. what may be the causes and solutions to the problems?
problem on drainage structures
a series of stone masonry check dams were constructed in a gully in the Daunne Hills. between the two check dams, the vegetation is growing. the lower portion of most of the check dams is damaged and is about to collapse. what may be the causes and solutions to the problems?
problem on drainage structures
the water flows in the drain regularly throughout the year. the downhill side slope is generally wet all the time. now, the soil slope on the downhill side is going to collapse. what could be the causes and your solution to this problem?
problem on drainage structures
a cross-drain was constructed at Lumle, just above the Bamboo Lodge, near a switchback. at present, the outlet of the culvert is directed down to the courtyard of the Lodge and it is creating problems to the hotel. what is wrong with the drain? how could it be constructed properly? give your solutions.
problem on drainage structures
a bridge was constructed over an alluvial fan. after a heavy flood of 1988, the bridge was choked by big boulders and debris. the flood also deposited about 50 cm thick debris on the bridge deck. what was wrong with the bridge and how could you correct it?
problem on drainage structures
there is a small soil slide (length: 20 m, width: 10 m) in the Hetaunda-Thingan Road. the road authority constructed a catch drain around it. but after the rainfall of 1995, the slide became bigger and about half of the catch drain near the crown of the slide collapsed. to solve this problem, the authority made a cement masonry drain at the collapsed part of the catch drain. however, after the rainfall of 1996, again cracks are seen in the cement masonry catch drain. what could be the reason behind it? and, give your solution to the problem.
hydrological and hydraulic factors to be considered when designing the drainage are the following:
· frequency, intensity, and duration of rainfall;
· runoff peaks and their frequencies;
· distribution of precipitation;
· manning’s coefficient;
· velocity of runoff; and
· slope angle and gradient.
drainage structures fail even though the above factors are considered in the design. so, the following points should be considered when designing and constructing the drainage structures.
| problems |
solutions |
| ad hoc size selection from standard drawings |
follow hydraulic and hydrological calculations. |
| long side drains |
break the drains at an interval of 150 to 200 m. |
| lack of water management below the drains |
provide appropriate aprons and water management systems. |
| clogging of drains |
provide proper gradient and drain size. |
| drainage structures in the wrong place |
correct it by following the natural slopes. |
| concentration of runoff from various gullies in a single drain |
construct appropriate drains in each gully |
| seepage of water through drains |
line the drains properly |
| accumulation of debris in the engineering structures |
manage debris risers, debris deflectors, debris racks. |
| incision of the gully below the drain |
there is more water than the gully can manage. protect the gully, and/or cut short the water diverted from other gullies |
| incision of the gully above the drain |
the gully is unstable. protect it by constructing check dams and chutes. |
| others |
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