DEFORESTATION AROUND THE WORLD - India Environment Portal

Landslides Caused Deforestation
rainfall threshold is a power-law curve that fit the points in the scatter plot (actually a lower
envelope in that is a point lies to the right upper of the curve, landslides may occur), usually
b
take the form I aD
, where a and b are positive constants that vary with soil, vegetation
and land use.
Godt et al. (2006) suggested that landslide-triggering rainfall must be considered in terms of
its relationship with antecedent rainfall. For example, a heavy rainfall event within a dry
period is not likely to trigger shallow landslides, while the opposite is true for lighter
rainfall within a wet period. As it directly affects soil moisture conditions, Godt et al. (2006)
correctly claim that antecedent rainfall must be included in an empirical model’s assessing
of a rainfall’s potential in causing landslide. Godt et al. (2006) therefore is a significant
improvement over Caine’s (1980) seminal rainfall intensity-duration threshold line
approach. The antecedent rainfall index is usually defined as a red noise of the accumulative
rainfall amount 3 days (for tropics) and 7 days (temperate climate zone) prior the landslide
event. Recent empirical methods also compile many soil hydrological parameters by using
water-balance models with little physical basis but are convenient for estimating soil
moisture conditions. For example, Godt et al. (2006) uses a detailed assessment of rainfall
triggering conditions, hill slope hydrologic properties, soil mechanical properties, and slope
stability analyses. The accumulation of sliding material is a slow process (either rockfalls or
aeolian processes or damaging erosion processes from weathering) compared with the
sliding. Previous sliding will reshape the sliding material profile and may even completely
remove the sliding layer. These will increase the stability of the slope and a similar rainfall
amount may cause sliding on a reduced scale, or not at all. Thus, the empirical parameters (a
and b) in the ID approach vary not only spatially but also temporally. In this sense, all
previous ID approaches still lack the important time varying features.
A synthetic consideration of preparatory and triggering factors, however, demands a more
comprehensive modeling of the physical processes involved in landslides (Costa, 1984;
Iverson, 1997). The overview by Iverson (1997) suggested several criteria for dynamic
landslide models, including that a model should be capable of simulating the full startmovement-spread-cessation
cycle of the detached material, and should cover a wide
spectrum of debris flows. With continued growth and expansion of human population, raintriggered
shallow landslides increasingly result in loss of life and significant economic cost.
From an ecological viewpoint, landslides are an important factor in desertification over
mountainous regions because they are very effective in transferring biomass from live to
dead respiring pools (Ren et al., 2009).
Along these lines of walking, there are physically-based slope stability models to simulate
the transient dynamical response of pore pressure to spatiotemporal variability of rainfall
(e.g., Transient Rainfall Infiltration and Grid-based Regional Slope-Stability Analysis—
TRIGRS, Baum et al. 2008); commercially available numerical modeling codes for
geotechnical analysis of soil, rock and structural support in three dimensions (e.g., FLAC-
3D, www.itascacg.com/flac3d), and fully three dimensional, full Navier-Stokes and multirheological
modeling systems such as the scalable, extensible geo-fluid model, known as
SEGMENT (Ren et al., 2008; Ren et al., 2009, Ren et al. 2010; Ren et al. 2011a,b).
Slope stability models are based on the following reasoning: On a sloping surface, the
gravitational force can be partitioned into a component normal to the slope (Fn),
contributing to friction that resists sliding erosion, and a component parallel to the slope (Fp)
that promotes sliding. A stability parameter, S, is defined as SF / F , where is the
n p
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