182
11 Detecting Fingerprints of Gravity Erosion Drivers …
other scar morphologies occurred, including Tf, Ps, and Ia. There is significant evidence that the mass failure depends critically on the shear strength, tensile strength,
and effective stress of the slope material (Goulding 2006). In addition, although the
initial slope gradients were 70° or 80° in the experiments, the instantaneous slope
gradients of the individual mass failures were different, as the failures frequently
occurred. Hence, the dynamic slope gradients might affect the shear strength of the
slope surface (Meng 1996). However, rainwater infiltration triggers the variations in
the soil water content, leading to increased pore water pressure, and in turn causing
re-distribution of the macroscopic stress and strength of the potential scar, together
with the dynamic changes of instantaneous slope and the cohesion, and ultimately
resulting in different scar morphologies.
11.3.5 Effects of Parameters on Scar Forms
Failure scars could reflect the method of failure triggering, and the frequency and size
of the gravity erosion (Zhang et al. 1997; Millar and Quick 1997). In the study, Tf,
Uc, and Dc were the three major types of failure surfaces occurring in the processes
of gravity erosion on the steep loess slope, among which the Tf was the most crucial.
The result is close to other similar studies on the Loess Plateau. Shi et al. (2016)
also found that the arc-shaped scar and the translational scar were two major types
of scars in the northern bank of the Weihe River on the Loess Plateau. In fact, most
of the well-preserved arc-shaped scars on the natural slopes are Uc scars, because
the stability of the Dc scar is weak, which tends to induce subsequent mass failures.
Meanwhile, profound and obvious discrepancies were seen among the mass failures
for different scars in the experiments. Indeed, changing the scar geometry while
keeping the upper surface of the released mass constant induces a change in the
released volume (Lucas et al. 2011). Our study reveals that the total volume of the
failure masses with the scars Tf, Uc, and Dc accounted for a large percentage of the
total volume for the mass failures, of which the Tf was the most decisive.
Different triggering methods can explain why the failures volume of the Tf was
larger than those of the Uc and Dc. In this study, there were three ways to trigger
mass failures, including the method of crack propagation (Fig. 11.6), the deformation
of partially saturated soil (Fig. 11.7), and the combined effects of the crack propagation and deformation of partially saturated soil (Fig. 11.8). In comparison, the
third triggering approach (i.e., the method driven by the combined effects of crack
propagation and saturated soil), which was prone to form a Tf scar, might result in the
maximum volume in the experiments. A failure scar is not only a fingerprint of the
slide trace or a vertical profile formed by the soil block separating from a sloped face
(Waldmann et al. 2011). It is also a measure of geomorphic effectiveness of rainfall
as a function of climate change (Convertino et al. 2013). In turn, a mass failure is
affected and constrained by rainfall parameters, particularly the rainfall intensity and
duration (Guzzetti et al. 2008).
11 Detecting Fingerprints of Gravity Erosion Drivers …
other scar morphologies occurred, including Tf, Ps, and Ia. There is significant evidence that the mass failure depends critically on the shear strength, tensile strength,
and effective stress of the slope material (Goulding 2006). In addition, although the
initial slope gradients were 70° or 80° in the experiments, the instantaneous slope
gradients of the individual mass failures were different, as the failures frequently
occurred. Hence, the dynamic slope gradients might affect the shear strength of the
slope surface (Meng 1996). However, rainwater infiltration triggers the variations in
the soil water content, leading to increased pore water pressure, and in turn causing
re-distribution of the macroscopic stress and strength of the potential scar, together
with the dynamic changes of instantaneous slope and the cohesion, and ultimately
resulting in different scar morphologies.
11.3.5 Effects of Parameters on Scar Forms
Failure scars could reflect the method of failure triggering, and the frequency and size
of the gravity erosion (Zhang et al. 1997; Millar and Quick 1997). In the study, Tf,
Uc, and Dc were the three major types of failure surfaces occurring in the processes
of gravity erosion on the steep loess slope, among which the Tf was the most crucial.
The result is close to other similar studies on the Loess Plateau. Shi et al. (2016)
also found that the arc-shaped scar and the translational scar were two major types
of scars in the northern bank of the Weihe River on the Loess Plateau. In fact, most
of the well-preserved arc-shaped scars on the natural slopes are Uc scars, because
the stability of the Dc scar is weak, which tends to induce subsequent mass failures.
Meanwhile, profound and obvious discrepancies were seen among the mass failures
for different scars in the experiments. Indeed, changing the scar geometry while
keeping the upper surface of the released mass constant induces a change in the
released volume (Lucas et al. 2011). Our study reveals that the total volume of the
failure masses with the scars Tf, Uc, and Dc accounted for a large percentage of the
total volume for the mass failures, of which the Tf was the most decisive.
Different triggering methods can explain why the failures volume of the Tf was
larger than those of the Uc and Dc. In this study, there were three ways to trigger
mass failures, including the method of crack propagation (Fig. 11.6), the deformation
of partially saturated soil (Fig. 11.7), and the combined effects of the crack propagation and deformation of partially saturated soil (Fig. 11.8). In comparison, the
third triggering approach (i.e., the method driven by the combined effects of crack
propagation and saturated soil), which was prone to form a Tf scar, might result in the
maximum volume in the experiments. A failure scar is not only a fingerprint of the
slide trace or a vertical profile formed by the soil block separating from a sloped face
(Waldmann et al. 2011). It is also a measure of geomorphic effectiveness of rainfall
as a function of climate change (Convertino et al. 2013). In turn, a mass failure is
affected and constrained by rainfall parameters, particularly the rainfall intensity and
duration (Guzzetti et al. 2008).