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The Mw 8.0 Wenchuan earthquake on 12th 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 li

Mass flows such as landslides and floods pose great risks to the built environment. In the literature the assessment of the building vulnerability to these different mass flows is evaluated separately and little effort has been made to investigate the interaction mechanisms between mass flows and buildings, leading to a lack of explicit failure mechanisms in many vulnerability models. In this study, the failure modes of a typical reinforced concrete building upon impact by debris flows, debris floods, floods and earth flows are discovered through an explicit finite element platform LS-DYNA. Five impact cases with increasing solid concentration; namely, low-intensity water flow, high-intensity water flow, debris flood, debris flow and earth flow, are considered. The intrinsic physics of building damage by these flow-like mass movements is compared. The frontal walls and side walls are vulnerable to any type of flow. A forward pushover failure mode, which is a global failure of the building frame, is observed in the high-intensity water flow and debris flood cases. Upon impact by an earth flow, progressive local failure of the columns at the ground floor can occur and lead to backward collapse of the building. Upon impact by a debris flow, both the local column failure and the global pushover collapse of building can occur, as debris flows with high solid contents are an intermediate flow sharing characteristics of both floods and earth flows. The in-depth study of the physics of building damage provides a solid basis to develop generic vulnerability models for such flow-like mass movements of varying solid contents. © 2020 Elsevier B.V.

Understanding the evolution of post-seismic geohazards with vegetation recovery is of great significance for long-term geohazard risk assessment. The Wenchuan earthquake in 2008 triggered numerous landslides and severely damaged the vegetation in the earthquake-stricken region. Between 2008 and 2017, we evaluated the vegetation recovery process, the evolution of rain-induced landslides and debris flows, and the relationship between them by analyzing satellite images and conducting detailed field investigations. Shortly after the earthquake, numerous landslides and debris flows were triggered by rainstorms and the vegetation was severely damaged near the epicenter. In the first 5 years (2008–2013) the vegetation recovered quickly but was not well rehabilitated. Rain-induced landslides and debris flows occurred actively but exhibited a clear declining trend in both frequency and magnitude. After 2013, the vegetation gradually recovered to the pre-seismic normal level in terms of quantity, but with higher variability and isolated spots to be further rehabilitated. Over this period, rain-induced geohazards mainly occurred in the catchments where the level of vegetation recovery was uneven, with occasional bare deposits. Therefore, a close relation is observed between declining rain-induced geohazards and the recovery of vegetation, in both time and space. The findings provide important information for post-seismic infrastructure re-construction and disaster risk reduction in future mountain earthquake events. © 2019 Elsevier B.V.

Intense earthquakes can trigger tens of thousands of landslides in the stricken area (e.g. 1999 Chi-Chi earthquake; 2008 Wenchuan earthquake; 2018 Hokkaido earthquake). The loose deposits of these landslides provide abundant source materials for debris flows in rainy season. Therefore, determining the deposit volume of co-seismic landslides is essential for debris flow simulation, risk assessment and risk management as well as evaluation of long-term post-earthquake hazards evolution. However, during an earthquake, co-seismic landslides occur concurrently and shelter the boundaries of each other. The overlapping in clustered co-seismic landslide boundaries makes a single landslide display much large area from satellite images interpretation, either manually or automatically. Consequently, the calculated landslide volumes are overestimated when applying landslide area-volume power laws. To solve this problem, we propose a partitioning approach of clustered landslides to find the boundaries of individual landslides, by means of generating the Flow Directions of clustered landslides from a local digital elevation model (DEM). Three severely damaged catchments in the Wenchuan earthquake region are chosen as our study area. Each clustered landslide in these catchments is partitioned into several individual landslides and then calculated by power law scaling relationship. The adopted power law scaling relationship is selected out of six widely used power laws and verified by 11 local landslide cases by random distribution matching in MATLAB. For comparison validation, the post-earthquake images displaying boundaries of individual landslides two years later are interpreted and detailed field survey results of these gullies are also deployed. The results show that the proposed approach works well in volume calculation. The calculated landslides volume is only 3% larger than that derived by the post image method and is 1.3 times of the field investigation results. Before the

A strong earthquake can trigger thousands of co-seismic landslides. The evaluation of the volume of such co-seismic landslides is essential for landslide risk assessment and long-term post-earthquake hazard evaluation. Empirical volume-area power laws are often applied to estimate the landslide volume. When several small landslide deposits form a large cluster on the hillslope, blurry boundaries of the clustered co-seismic landslides may lead to seemingly one large landslide and grossly overestimated landslide volume. In this paper, we propose a flow direction-based method to identify the boundaries of a cluster of landslides and to partition co-seismic landslides. Three watersheds that were severely impacted by co-seismic landslides in the Wenchuan earthquake region are chosen for case studies. Each co-seismic landslide cluster in these watersheds was interpreted from images taken shortly after the earthquake, and partitioned into individual landslides in accordance with a flow direction map that is processed based on a local digital terrain model. Finally, the volume of each individual landslide is calculated using an empirical volume-area power law. The adopted power law scaling relationship is selected from six widely used empirical relations and verified by 11 local landslide cases. The proposed flow-direction based partitioning method is further verified by clear boundaries of landslides that were exposed after storms, as well as supplementary field survey results. The proposed method results in similar landslide boundaries and volumes as the field survey results. The uncertainty of this approach is below 19%. If no partitioning is conducted, the total volume of the landslide cluster can approach twice that from the field investigations. © 2019

The 2008 Wenchuan earthquake triggered numerous landslides in the stricken area. The loose landslide materials on mountain slopes and in gullies are prone to reactivation and may transform into debris flows in the rainy season. Nine years after the 2008 Wenchuan earthquake, landslide activities are still active in the study area near the epicenter of the earthquake in Sichuan, China. The debris flows blocked the Minjiang and Yuzixi river and caused severe damage to nearby villages and reconstruction sites. How did the mechanisms evolve in these repeated debris flows? We evaluated the loose deposit volumes in catchments of the triangle study area before and after each heavy storm since the earthquake, and analyzed the triggering rainfall intensities, initiation mechanisms and runout characteristics of these debris flows. The preliminary results of a case study in Gaojiagou Ravine show that the rainfall threshold for debris flows increased with time, the initiation mechanisms evolved from landslides to channel-bed failure and subsequently to channel-bank erosion. The mobility of the debris flows decreased from 2010 to 2016 as the initiation positions moved lower and the particle size of the runout materials became coarser.

The 2008 Wenchuan earthquake in Sichuan, China triggered numerous landslides in the stricken area. The loose landslide materials retained on mountain slopes and in gullies are prone to reactivation and may transform into debris flows in the rainy season. Nine years after the Wenchuan earthquake, debris flows are still active in Gaojiagou Ravine 16.5 km north of the epicentre. On 14 August 2010, 3 July 2011, 13 July 2013, and 6 July 2016, four large-scale debris flows were triggered by heavy storms in Gaojiagou Ravine. The four debris flows blocked the Minjiang River twice and caused severe damage to nearby villages and reconstruction sites. Several questions arise from these repeated debris flows. Did the level of triggering rainfall change in the four debris flow events? How did the initiation mechanism evolve over time? What are the differences in the runout characteristics in the four events? How did the check dams constructed in 2012 function in the subsequent debris flows? In this paper, we evaluated the loose deposit volumes in Gaojiagou Ravine before and after each of the four debris flows and the runout volumes, and analysed the triggering rainfall intensities, initiation mechanisms and runout characteristics of the four debris flows. The rainfall threshold for the debris flows in Gaojiagou Ravine increased over time, and the initiation mechanisms evolved from landslides to channel-bed failure, and subsequently to channel-bank erosion. The mobility of the debris flows decreased from 2010 to 2016 as the initiation positions moved lower and the particle size of the runout materials became coarser.