Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 50

mass movement of landslide, avalanche, slumping and surface creep, as material is moved from higher
elevations to lower elevations under the action of gravity. In small scale erosion, gravity may cause a rock
to drop from a height, such that it falls to the ground and breaks into pieces. Large scale gravitational
erosion includes slope creep, avalanche, and landslide. Avalanche, landslide, and debris flow are discussed
in Chapter 3. Slump is a gravitational erosion of solid material slipping down along a curved slope
surface, which occurs when the slope becomes too steep, and the base material cannot support the rock and
soil above. A curved scar is left where the slumped materials originally rested. Creep is the movement of
rock and sediment slowly shifting downhill, which is caused by gravity alone. Creep is extremely slow
and would be difficult to see without a lot of measurements over time.
Glacier erosion —A glacier is a moving mass of ice of large volume on the land surface. It moves by
gravity, as a consequence of its extraordinary weight. A glacier steadily moves forward, carrying pieces
of rock, soil, and vegetation with it. Under certain conditions, a glacier may have a layer of melted water
surrounding it, which greatly enhances its mobility. In a wet and warm maritime climate a glacier moves
at a speed of about 300 m/yr. By contrast, in the dry, exceptionally cold, inland climate of Antarctica, the
Meserve Glacier moves at a speed of just 3 m/yr (www.answer.com). Figure 2.1 shows the Hailuo Gully
Glacier on Gongga Mountain on the east margin of the Qinghai-Tibet Plateau. The glacier has eroded the
gully bed and banks and carries a lot of solid material on its surface flowing down the gully at a speed of
170 m/yr to 350 m/yr.
Fig. 2.1 The Hailuo Gully glacier on the Gongga Mountain on the east margin of the Qinghai-Tibet Plateau has eroded
the gully bed and carries a lot of solid material on its surface (See color figure at the end of this topic)
Freezing and thawing also causes erosion. Changes in temperature and moisture cause expansion and
contraction of materials, as when water seeps into a crack in a rock and then freezes, expanding and splitting
the rock. The solid particles detached from the rock deposit on the hillside at an angle approximately
equal to the angle of repose of the grains. Figure 2.2(a) shows the rock erosion resulting from freezing
and thawing on a 4700 m high mountain at Luhuo on the Qinghai-Tibet Plateau. Figure 2.2(b) shows the
fans of deposits or eroded sediment at the toe of the Rocky Mountains in Canada resulting from freezing
and thawing. The sediment from the freezing and thawing erosion is relatively uniform with a median
diameter around one centimeter.
Shattering erosion —As a consequence of freezing and thawing shattering erosion of rocks and
underlying stratified talus deposits has been reported from temperate upland environments as well as
cold environments worldwide (Saas and Krautblatter, 2007). Stratified scree deposits with rich fine