PhD Defence Chenxiao Tang

monitoring landslide risk dynamics after a major earthquake in a mountainous environment

Chenxiao Tang is a PhD student in the department of Earth Systems Analysis. His supervisor is dr. C.J. van Westen from the faculty of Geo-Information Science and Earth Observation.

Major disasters, such as earthquakes, have a large impact on society, causing massive direct and indirect losses. Large earthquakes may also seriously affect the natural environment, in the form of secondary hazards. In mountainous regions one of the most severe secondary effects is the triggering of co-seismic landslides. These may result in the loss of vegetation and the production of large volumes of landslide deposits, which drastically change the susceptibility to rainfall-induced landslides and flooding after the earthquake. In such circumstances human settlements are not only threatened by co-seismic hazards such ground shaking or landslides, but also face a continuous threat of landslides, debris flows and flash floods during and after reconstruction phase. A monitoring of changes in post-seismic hazards, exposure and vulnerability is important for addressing changing risk as a basis for recovery planning.

We analysed changes in landslide activity in a period of seven years after the 2008 Wenchuan earthquake, with five multi-temporal landslide inventories, which we interpreted stereoscopically from high resolution images, followed by field investigation. The results show that most of the post-seismic landslide activities were concentrated within the first three years following the earthquake. The landslide activity decreased considerably from May 2008 until April 2015, from 6727 co-seismic landslides to 66 active landslides in 2015. After the 2010 monsoon season, the activity of the post-earthquake landslides dropped considerably and most of the active landslides became dormant. Of the total of 6727 co-seismic landslides, 2221 had one or more phases of reactivation. Apart from the reactivation of co-seismic landslides also 660 new landslides occurred after 2008. In April 2015, the number of active landslides had gone down to 66, less than 1% of the co-seismic landslides, still much higher than the pre-earthquake situation. We expect that the landslide activity will continue to decay, but may be halted if extreme rainfall events occur.

Due to the lack of adequate pre- and post—Wenchuan-earthquake Digital Elevation Models, the landslide volume estimation was done either using empirical area-volume relationships over large areas or by field surveys in a few catchments with debris flow threats. The trend of the change of volume of loose materials in the earthquake affected area over the decade since the earthquake remains largely unknown. In this study we were able to address this issue using nine DEMs taken at different years and from different sensors to study the change in loose material volume caused by co-seismic and post-seismic landslides over a period of 9 years. The area around the towns of Yingxiu and Longchi, for which also multi-temporal landslide inventories were available, was selected for this study. Methods to register the DEMs and minimize their vertically bias were applied. The quality of the DEMs was assessed through GCPs and terrain representation. As could be expected, high resolution DEMs showed more realistic volume estimates than the low-resolution ones. The analysis showed that the frequency and magnitude of the landslide volume dynamics decreases significantly after the early post-seismic period (< 6 years), and in the last years (2014 – 2017) human activities became a more dominate factor than mass movements. The post-seismic material loss from 2008 to 2014 was close to the gained volume of the co-seismic landslides, and the depletion of the materials was mostly at the toes of the co-seismic landslides. The analysis was done based on gain and loss calculated from the DEMs, and actual volumes could not be calculated due to unknown failure surface depths of the landslides.

In order to address the issue caused by unknown failure surface depth in landslide volume estimation, we developed a model that integrates MATLAB and ARCMAP functions to simulate the geometry of failure surfaces with exposed scarps. To test our model, a series of analogue experiments were designed to acquire data for model testing in a controlled environment, before applying the model to real landslide cases. The results from the analogue experiments showed that it is essential to use some additional elevation points other that scarps, that could be obtained by overlaying the pre-and post-earthquake DEMs and trace the outcropping failure surface along the bottom of the slope. The model was then applied on real landslides. The total landslide volume obtained using the model showed a poor agreement with the results using empirical equations, and for both individual landslides and total volume there were significant differences. Due to lack of measurement from borehole or geophysical investigation it is not possible to know the which method is more accurate.

We monitored the changes in the Longxi valley during ten years related to the impact of the Wenchuan earthquake and the subsequent recovery process, with seven inventories from different years containing buildings, roads, land use and mitigation measures. It was found that the loss caused by post-seismic debris flows was slightly more than the loss caused by the Wenchuan earthquake, due to fast reconstruction of buildings that were more in numbers and value than pre-earthquake situation. Most of the stronger building construction types were only implemented after the areas was impacted by the earthquake, and mitigation structures were only installed after being impacted by debris flows and floods. A greater awareness to avoid living in hazard prone areas was observed after the 2010 debris flows. Despite the extensive and repeated damage, the earthquake, and subsequent landslides, debris flows and floods gave Longchi town a chance to increase its resistance to these hazards in future, and to improve economically.

Finally, a catchment-based multi-temporal (semi)quantitative debris flow risk analysis was carried out. The results show that the risk increased rapidly in the first three years after the 2008 earthquake due to the ongoing reconstruction, then decreased suddenly due to the impact of the 2010 debris flows that affected 59 of the 66 investigated watersheds. After that the society adapted by relocating buildings and installing mitigation works. In the later years the risk decreased gradually due to the recovery of the environment, and further protection works. The region showed a low disaster awareness before the catastrophic debris flows of 2010, then quickly learned and adapted. the study showed how risk components can vary over time in a dynamic environment, and the importance to update hazard and risk maps frequently under such circumstances. It is necessary to use up-to-date data and hazard and risk assessments in urban planning, before carrying out reconstruction work after large earthquakes.