The M_w 8.0 Wenchuan earthquake on 12^th May 2008 caused severe loss of human lives and properties, i.e. 69,227 fatalities and an estimated direct economic loss of USD150 billion in the seismic area. Beside the short-term impact to residents and properties, the earthquake also influenced the rehabilitation of victims and the evolution of landscape in the stricken area for a long period. For instance, the earthquake triggered more than 56,000 landslides, leaving about 5–15 km³ of erodible material in the mountain area. The loose deposits of landslides are retained on mountain slopes and in gullies, prone to reactivating and transforming into debris flows and landslide dams in rainy seasons.

After the 2008 earthquake, devastating debris flows erupted frequently in the seismic area. O...[ Read more ]

The M_w 8.0 Wenchuan earthquake on 12^th May 2008 caused severe loss of human lives and properties, i.e. 69,227 fatalities and an estimated direct economic loss of USD150 billion in the seismic area. Beside the short-term impact to residents and properties, the earthquake also influenced the rehabilitation of victims and the evolution of landscape in the stricken area for a long period. For instance, the earthquake triggered more than 56,000 landslides, leaving about 5–15 km³ of erodible material in the mountain area. The loose deposits of landslides are retained on mountain slopes and in gullies, prone to reactivating and transforming into debris flows and landslide dams in rainy seasons.

After the 2008 earthquake, devastating debris flows erupted frequently in the seismic area. On 14 August 2010, 3 July 2011, 13 July 2013, 6 July 2016, and 20 August 2019, five heavy storms hit the seismic area, numerous landslides and debris flows were triggered, completely blocking the Minjiang River, burying nearby villages, flooding Yingxiu Town, and ruining the roadbed of National Road G213 and Province Road S303. Till 2019, 11 years after the 2008 earthquake, post-earthquake disasters including landslides, debris flows, landslide dams and flooding were still disturbing the seismic region. To advance the rehabilitation of residents and recovery of landscape, great efforts have to be made.

Building complete, updated and quantitative landslide inventories is an essential work to conduct post-earthquake landslide evaluation. In this PhD work, by means of interpreting satellite images, field investigation, monitoring rainfall and debris flows, a series of event-based landslide and debris flow inventories covering the period from 2008 to 2019 have been compiled. By analysing these inventories, it is found that in the selected study region in the epicentral area east (E), southeast (SE) and south (S) are the dominant aspects that landslides are more likely to occur. Granite and diorite with intensive joints are the most common lithological conditions that landslides occur. The number and volume of landslides and debris flows present a declining trend, the locations of landslides gradually move to lower altitudes, the mobility and runout distance of debris flows also decrease with time.

To explicate how much loose material still exists in mountains and how such material moves in catchments is essential for quantitative risk assessment and mitigation. In this PhD work, a flow-direction based partitioning method is proposed to calculate the volume of co-seismic landslide clusters. Combining with field measurements, the hillslope, channelized, and runout volumes of landslide deposits after the earthquake and each rainstorm are quantified. A mass balance framework is also proposed to illustrate the whole process of mass movement in the study area since the earthquake. It is found that substantial amount of hillslope deposits moved to channels and subsequently ran out of catchments in the form of debris flows in the first five years. Simultaneously, the major part of co-seismic landslide deposits was retained on hillslopes and got inactive. The volume of active landslide deposits kept declining in the past 11 years. The channel deposit accumulated soon after the earthquake but gradually ran out of valleys as it played a role of carrier that transports hillslope deposits to the river system. Overall, in our study area, more than 80% of the denuded mass by the 2008 Wenchuan earthquake still remains on the hillslopes, which poses a long-term threat to the residents and properties.

Many factors have influenced the evolution process of post-earthquake landslides and debris flows, such as seismic factors, topographic factors, biophysical factors and geomechanics factors. The large magnitude of the 2008 Wenchuan earthquake and the rugged topography in the study area determined the long period of post-earthquake evolution. This study focuses on the biophysical factors and geomechanics factors, i.e. grain coarsening, revegetation, soil densification and rainfall intensity. Grain coarsening was observed to be closely related to the shrinking debris flow scale. Revegetation remarkably increased the soil strength with the help of the root system, but the hydraulic conductivity also increased by plant roots. Soil densification in gentle slopes increased the shear strength of soils, and cementation of mineral particles was detected under Scanning Electron Microscope (SEM). The influence of rainfall intensity faded since the rainfall threshold of debris flows gradually recovered soon after the earthquake. However, extreme rainstorms sometimes can deteriorate the landscape evolution in view of the 2013 debris flows.

The prediction of future hazard scenarios is still scarce for the seismic area, but nonetheless has paramount importance for the risk assessment and management. At the last part of this thesis, numerical simulations and stress testing of future hazards are conducted. The adopted program EDDA 2.0 well reproduces the debris flows in 2010, 2013 and 2019. It is shown that under 1,000-year return period and 10,000 return period rainfall, the same area will experience much larger landslides and debris flows compared with the 2019 scenario. Considering the influence of grain coarsening, revegetation and soil densification, the extreme hazards will be alleviated by about 14% in the amounts of landslides and debris flows. To conclude, the long-term evolution of post-earthquake landslides and debris flows in the epicentral area of the 2008 Wenchuan earthquake still continues, although the geohazard activity has declined significantly.