6.3 Methods and Materials
79
where V is the landslide volume (m
3 ) and A is the landslide area (m
2 ) (Qiu et al.
2015). This relation, of which the correlation coefficient was 0.91 and the significance level was less than 0.05, was developed from a catalogue of 52 landslides
surveyed in Yan’an area, Shaanxi Province. Both the Lvliang area investigated in
this study and the Yan’an area are located in the Loess Hill Ravine Region, and the
average thickness value of the individual landslides in Lvliang section is close to
that in Yan’an. Hence, the volume-area scaling relation can be employed to calculate
the individual landslide volumes of disturbed failure blocks. The sediment yield is
known as a net soil loss, which is calculated by subtracting in-field deposition from
gross soil loss (Tsai et al. 2012).
According to the failure volumes, death tolls and economic losses of the landslides,
the Ministry of Land and Resources of the People’s Republic of China (2004) divided
the landslide disasters into four grades: small-scale loss event, moderate loss event,
severe catastrophe and devastating catastrophe. The landslides analyzed here were
the same as or more serious than a moderate event with a collapsed volume larger
than 10,000 m
3 and/or death toll less than 1. In this study, the effects of rainfall,
human activities, freeze-thaw and earthquake on the occurrence of the landslide are
calculated by the equal-weight method when one landslide event was preceded by
two or more kinds of triggers. For instance, if a landslide was triggered only by
rainfall, the weighting coefficient of the factors is 1; if a landslide was induced by
rainfall events and human activities together, both weighting coefficients of the two
factors are assumed to be 0.5.
6.4 Results and Discussion
6.4.1 Analysis of the Catastrophic Loess Landslides
The catastrophic landslides that happened on the Loess Plateau from 1980 to 2015
were chosen for a detailed case study (Table 6.1). All the accidents were beyond
moderate events and had definite records of date and location in official documents.
In total, there were 53 fatal landslides, causing 717 casualties. The proportion of
landslide induced by each factor could be found via a statistical analysis of the
data. It turned out that rainfall, inducing 40% of the catastrophic events, was the
most important trigger factor for initiating catastrophic landslides. The result was
particularly linked to the intense and concentrated summer rainfalls on the Loess
Plateau. For example, the landslides in the Baiyuli Village and the Wangjiagou Village
of Shanxi Province and the debris flow in the Min County of Gansu Province were all
directly triggered by storms. On the other hand, landslides induced by the earthquake
and freeze-thaw process accounted for 1 and 23% of the total, respectively. Hence,
the process of freeze-thaw has a greater impact on landslides than earthquakes.
79
where V is the landslide volume (m
3 ) and A is the landslide area (m
2 ) (Qiu et al.
2015). This relation, of which the correlation coefficient was 0.91 and the significance level was less than 0.05, was developed from a catalogue of 52 landslides
surveyed in Yan’an area, Shaanxi Province. Both the Lvliang area investigated in
this study and the Yan’an area are located in the Loess Hill Ravine Region, and the
average thickness value of the individual landslides in Lvliang section is close to
that in Yan’an. Hence, the volume-area scaling relation can be employed to calculate
the individual landslide volumes of disturbed failure blocks. The sediment yield is
known as a net soil loss, which is calculated by subtracting in-field deposition from
gross soil loss (Tsai et al. 2012).
According to the failure volumes, death tolls and economic losses of the landslides,
the Ministry of Land and Resources of the People’s Republic of China (2004) divided
the landslide disasters into four grades: small-scale loss event, moderate loss event,
severe catastrophe and devastating catastrophe. The landslides analyzed here were
the same as or more serious than a moderate event with a collapsed volume larger
than 10,000 m
3 and/or death toll less than 1. In this study, the effects of rainfall,
human activities, freeze-thaw and earthquake on the occurrence of the landslide are
calculated by the equal-weight method when one landslide event was preceded by
two or more kinds of triggers. For instance, if a landslide was triggered only by
rainfall, the weighting coefficient of the factors is 1; if a landslide was induced by
rainfall events and human activities together, both weighting coefficients of the two
factors are assumed to be 0.5.
6.4 Results and Discussion
6.4.1 Analysis of the Catastrophic Loess Landslides
The catastrophic landslides that happened on the Loess Plateau from 1980 to 2015
were chosen for a detailed case study (Table 6.1). All the accidents were beyond
moderate events and had definite records of date and location in official documents.
In total, there were 53 fatal landslides, causing 717 casualties. The proportion of
landslide induced by each factor could be found via a statistical analysis of the
data. It turned out that rainfall, inducing 40% of the catastrophic events, was the
most important trigger factor for initiating catastrophic landslides. The result was
particularly linked to the intense and concentrated summer rainfalls on the Loess
Plateau. For example, the landslides in the Baiyuli Village and the Wangjiagou Village
of Shanxi Province and the debris flow in the Min County of Gansu Province were all
directly triggered by storms. On the other hand, landslides induced by the earthquake
and freeze-thaw process accounted for 1 and 23% of the total, respectively. Hence,
the process of freeze-thaw has a greater impact on landslides than earthquakes.