geomorphology and landforms
geomorphology
geomorphology studies the outer surface of the earth (Geo means the earth and Morphos means the shape). it studies the shapes of mountains, plateaux, rivers, glaciers, and also such smaller features as landslides, fans, terraces, gullies, and streams.
landform classification of the Himalayas
the major landforms of the Himalayas are described below (see the block diagram, figure 1):
figure 1: block diagram of Himalaya showing main landforms
and rock units
Gangetic Plain (Terai)
the Terai Plain lies to the south of the Himalayan mountain Range. it is made up of sand, silt, and clay. the northern border of the Terai Plain consists of boulders and pebbles, and it is called the Bhabar Zone.
the Terai Plain is very gentle (slope is less than 2 degrees). the altitude of the Terai Plain is less than 150 m. rivers meander on the Terai Plain and deposit sediments.
Siwaliks (Chure or Churia Hills)
the Siwaliks are the first foothills lying immediately north of the Terai Plane. the Siwalik rocks are very soft. they are represented by mudstone, soft sandstone, and conglomerate. Shallow slides and alluvial fans are found in the Siwaliks. there are many dun valleys (also called as Inner Terai) within the Siwaliks. the Siwalik Hills range in altitude from 500 m to 1000 m.
Lesser Himalaya (Lower Himalaya)
the Lesser Himalayan rocks lie to the north of the Siwaliks. this zone is further subdivided into the Mahabharat Range and the Midlands (figure 1).
Mahabharat Range
the Mahabharat Range lies immediately north of the Siwaliks. It is made
up of stronger rocks such as limestone, quartzite, dolomite, and schist.
the Mahabharat Range is very active and there are many landslides.
as the Mahabharat Range is actively rising, the rivers make gorges
and have steep to vertical banks. the altitude of the Mahabharat Range
varies from 1000 m to 2000 m.
Midlands
Midlands are situated between the Mahabharat Range in the south
and the Higher Himalayas in the North. The topography of the Midlands
is strongly dissected by many streams and rivers. There are old river
terraces and thick residual soils. The altitude of the Midlands ranges
from 700 m to 1500 m.
Higher Himalaya
the Higher Himalaya includes the snow peaks and the mountains surrounding them. there are many glaciers, large rockslides, and talus deposits. the Higher Himalaya is made up of such rocks as schist, quartzite, a gneisses, and granites. they have rugged topography with altitudes ranging from 1500 m to 8000 m and more.
Tibetan-Tethys Zone (Trans Himalaya)
the area north of the Higher Himalaya is called the Trans Himalaya. Many soil slides, debris flows, and talus deposits characterise the area. it is made up of soft sedimentary rocks and its altitude ranges from 2500 m to 500 m. figure 1 shows the various subdivisions of the Himalaya.
exercise on Geomorphic Features of Nepal
on the block diagram given below identify the following places:
Daman, Tansen, Sagarmatha, Kodari, Tatopani,
Annapurna, Dhaulagiri, Kathmandu,
Panchkhal, Daunne, Dhan Khola
Biratnagar, Dang, Ilam,
Nepalgunj, Dharan, Kurintar
Muktinath, Butwal,
Jhapa, Birgunj, Jomsom,
Hetaunda
answer to exercise on Geomorphic Features of Nepal
Terai Zone: Biratnagar, Birgunj, Nepalgunj, Dharan, Butwal, Jhapa
Siwaliks: Dhan Khola, Daunne, Hetauda, Dang
Mahabharat Zone: Tansen, Daman
Midlands: Kathmandu, Kurintar, Panchkhal, Ilam
Higher Himalaya: Sagarmatha, Kodari, Tatopani, Annapurna, Dhaulagiri
Trans Himalaya: Jomsom, Muktinath
micro-morphology in relation to instability
study of small-scale landforms
the following small-scale landforms are observed in a watershed.
Geological plane: term utilised for any more or less plane discontinuity or set of discontinuities cutting a body of rocks: bedding, foliation, fractures, joints, cracks and faults.
dip slopes and counter dip slopes: the gentle slope of the mountain parallel to the dip of the geological plane is called the dip slope whereas the slope on the opposite direction to the dip is called the counter dip slope. the counter dip slope is always steeper than the dip slope and is formed by joints (see figure below).
landslide: it is the term used for the downward and outward movement of rock or soil mass.
Talus deposits or talus cones: Talus deposits are loose rock fragments deposited at the bottom of a steep slope. the main agent is gravity.
Colluvial soils: Colluvial soils are angular in shape and area the disintegration product of the rock due to gravity (landslides) and running water.
alluvial soils and river terraces: the river deposits are called the alluvial soils. generally the soil contains rounded pebbles and cobbles together with sand and silt. the terraces are relatively flat and horizontal or gently inclined, and sometimes long and narrow. steeper ascending and descending slopes on each side bound them. In the Himalayas, the deposits form various levels called the river terraces. In this case, the older terraces are at a higher altitude and the younger terraces are in a lower portion.
alluvial fans: when a river, stream, or gully emerges from steep areas and flows through gentler slopes or plains, the sediment load carried by it is no longer possible to move. consequently, the coarse sediments are deposited in the form of an alluvial fan.
mountain slope classification
when considering the alignment, design, construction, rehabilitation, and maintenance of irrigation canals in the Hills of Nepal, it is important to recognise where landsliding or soil erosion is occurring and to distinguish between different types of slope failure and erosion processes.
the terrain through which the canal passes can be classified into mountain zones and these in turn can be separated into several land units based on criteria of slope stability/instability. all slopes within a land unit display similar characteristics and require similar geotechnical inputs. figure below illustrates a mountain zone classification and it is also described in the table 1 below.
table 1: mountain slope classification
| zone |
land unit |
description |
| 1 snow peaks |
|
snow peaks. |
| 2 zone of glaciers |
|
slopes with glaciers and moraines. |
| 3 degraded middle slopes (less than 35 degrees) |
3A |
ancient erosional terrace. covered with in-situ weathered profile of soil up to 3 m thick. slope angle generally less than 35 degrees. relatively stable. often farmer terraced. surface water erosion high. |
| 3B (4D) |
degraded colluvium. transported slope debris or landslide debris comprising gravel, cobbles, and boulders bound in silt/clay matrix. slope angle less than 35 degrees. relatively stable. often farmer terraced. Variable permeability. |
| 4 active lower slopes (more than 35 degrees) |
4A |
bare rock slopes. steep slopes angle often more than 60 degrees. stability dependent on orientation of discontinuities (bedding/joints) |
| 4B |
rock slopes with shallow (less than 2 m) loose debris cover. slope angle 45 degrees - 60 degrees. shallow instability and debris slides. mass instability as 4A. |
| 4C |
active colluvium. thick landslide debris often with toe being eroded seasonally by river. slope angle more than 35 degrees. actively degrading, highly unstable. |
| 4D(3B) |
degraded colluvium. see 3B but less stable. |
| 5 recent side terraces and valley floor (less than 20 degrees) |
5A |
higher terraces. oldest terraces often heavily dissected by drainage gullies. thick weathering profile up to 3 m. slope angle generally 0 degrees to 20 degrees. generally stable but can be highly erosive if surface residual soil cover is penetrated. |
| 5B |
terrace scarp faces. steep, often sub-vertical. actively degrading usually as sudden slumps. highly erosive forming cones of loose debris. |
| 5C |
low terraces. usually above normal flood levels, but susceptible to periodic high floods. soil cover is generally less than 30 cm. highly permeable and erosive. slope angle 0 degrees - 10 degrees. |
| 5D |
flood plain. gravel/cobble banks, highly mobile and covered by annual flood levels. no established soil cover. highly permeable and erosive. |
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