152
10 A Sensitivity Analysis on the Gravity …
little influence (Au 1998). The erosional history and the consequent morphology are
also much more important except for the trimming induced by occasional very large
run-off events (Thornes and Alcántara-Ayala 1998).
Because gravity erosion is affected and constrained by so many factors, its quantification is complicated and difficult to achieve. Furthermore, gravity erosion is a
stochastic, non-continuous process, and usually occurs as a combination of soil transportation with sheet flow and mass failure on the steep slope (Keefer and Larsen 2007;
Benda and Dunne 1997). Although, the process is readily observed on natural hill
slopes, quantifying it in a natural environment is significantly challenging given the
extended timeframe between the occurrence of the process, and variability in rainfall, soils, and other factors (Acharya et al. 2011). Site-specific and real-time measurement is almost impossible due to the uncertainty and non-continuity of gravity
erosion. Hence the volume of individual failure was normally calculated by multiplying the slide area by the thickness of the slide mass after the rainfall events
(Guzzetti et al. 2009; Haflidason et al. 2005). Nevertheless, the calculated volume
involves an amount of tinkering, for shallow debris-flow scars rapidly heal and are
difficult to detect after as few as years (Montgomery and Dietrich 1994). Moreover,
erosion volumes caused by water and gravity could not be distinguished from the
above calculation approaches. Landslide activity maps represent a short-cut in the
assessment of mass movement hazards (Parise and Wasowski 1999). While valuable,
these inventory maps usually do not provide information on the timing of the events,
making it difficult to correlate landslide occurrences with specific triggering events
(Kirschbaum et al. 2010).
The specific processes of rain-induced mass failures are most easily studied and
quantified in a flume using a rainfall simulator under controlled laboratory conditions
(Acharya et al. 2011). Here, we employed a topography meter designed by us to
quantitatively measure the process of gravity erosion, and we utilized the increaserate-analysis method to analyze the sensitivity of gravity erosion. The experimental
activity was focused on processes related to gravity and to the interaction between
rainfall and topography.
10.2 Gravity Erosion on the Loess Plateau
Areas of the Loess Plateau, especially the Loess Hill Ravine Region and the Loess
Mesa Ravine Region, are severely affected by gravity erosion (Figs. 6.1 and 10.1). All
types of mass failure are abundant in the area, and locally cover 30–50% of the land
(Wang et al. 1993). In the area, rainstorm-induced gravity erosion frequently occurs,
because the undulating terrain on the Loess Plateau is characterized by crisscrossing
gullies, the vegetation is so sparse, and especially the loess is collapsible and in
vertical joints. On the Loess Plateau, a steep bank with the slope of more than 70°
in the upper reaches of the small watershed is the main source of gravity erosion.
Forms of gravity erosion on the Loess Plateau include avalanche, landslide, earth
flow, and creep (Tang 2004).
10 A Sensitivity Analysis on the Gravity …
little influence (Au 1998). The erosional history and the consequent morphology are
also much more important except for the trimming induced by occasional very large
run-off events (Thornes and Alcántara-Ayala 1998).
Because gravity erosion is affected and constrained by so many factors, its quantification is complicated and difficult to achieve. Furthermore, gravity erosion is a
stochastic, non-continuous process, and usually occurs as a combination of soil transportation with sheet flow and mass failure on the steep slope (Keefer and Larsen 2007;
Benda and Dunne 1997). Although, the process is readily observed on natural hill
slopes, quantifying it in a natural environment is significantly challenging given the
extended timeframe between the occurrence of the process, and variability in rainfall, soils, and other factors (Acharya et al. 2011). Site-specific and real-time measurement is almost impossible due to the uncertainty and non-continuity of gravity
erosion. Hence the volume of individual failure was normally calculated by multiplying the slide area by the thickness of the slide mass after the rainfall events
(Guzzetti et al. 2009; Haflidason et al. 2005). Nevertheless, the calculated volume
involves an amount of tinkering, for shallow debris-flow scars rapidly heal and are
difficult to detect after as few as years (Montgomery and Dietrich 1994). Moreover,
erosion volumes caused by water and gravity could not be distinguished from the
above calculation approaches. Landslide activity maps represent a short-cut in the
assessment of mass movement hazards (Parise and Wasowski 1999). While valuable,
these inventory maps usually do not provide information on the timing of the events,
making it difficult to correlate landslide occurrences with specific triggering events
(Kirschbaum et al. 2010).
The specific processes of rain-induced mass failures are most easily studied and
quantified in a flume using a rainfall simulator under controlled laboratory conditions
(Acharya et al. 2011). Here, we employed a topography meter designed by us to
quantitatively measure the process of gravity erosion, and we utilized the increaserate-analysis method to analyze the sensitivity of gravity erosion. The experimental
activity was focused on processes related to gravity and to the interaction between
rainfall and topography.
10.2 Gravity Erosion on the Loess Plateau
Areas of the Loess Plateau, especially the Loess Hill Ravine Region and the Loess
Mesa Ravine Region, are severely affected by gravity erosion (Figs. 6.1 and 10.1). All
types of mass failure are abundant in the area, and locally cover 30–50% of the land
(Wang et al. 1993). In the area, rainstorm-induced gravity erosion frequently occurs,
because the undulating terrain on the Loess Plateau is characterized by crisscrossing
gullies, the vegetation is so sparse, and especially the loess is collapsible and in
vertical joints. On the Loess Plateau, a steep bank with the slope of more than 70°
in the upper reaches of the small watershed is the main source of gravity erosion.
Forms of gravity erosion on the Loess Plateau include avalanche, landslide, earth
flow, and creep (Tang 2004).