Xuanmei Fan

Characteristics and classification of landslide dams associated with the 2008 Wenchuan earthquake

BackgroundStrong earthquakes are among the prime triggering factors of landslides, which may block rivers, forming landslide dams. Some of these dams may pose serious threats to people and property due to upstream inundation and downstream dam-breach flooding. Evaluating the stability and potential hazard of landslide dams is significant for the mitigation measures, but remains challenging. The 2008 Wenchuan earthquake (Mw 7.9) in China triggered numerous landslides over a broad area, some of which dammed rivers, posing severe threats to downstream settlements. Our previous study created one of the most complete landslide dam inventories including detailed geomorphic parameters of 828 landslide dams induced by the Wenchuan earthquake. This paper presents the study of a number of representative landslide dams associated with the 2008 Wenchuan earthquake.ResultsThe coseismic landslides were classified into rock/debris avalanches, debris flows, rock/debris slides and rock falls. According to dam composition material and sedimentological features, landslide dams were categorized into three types: dams mainly composed of large boulders and blocks; dams composed of unconsolidated fine debris; and dams with partly intact rock strata at the base topped by large boulders and blocks or soil with rock fragments, showing two-layered or three-layered depositional structure. This classification is linked to the typology of damming landslides and considered to be a preliminary indicator of dam stability. In addition, dam stability also largely depends on valley morphometry as well as landslide runout distance and mechanism. The post-earthquake debris flow damming events induced by subsequent rainfalls are also introduced. It was found that there is still a large amount of loose sediment remaining on the slope, which may continue promoting heavy debris flows and dams in the coming years or decades.ConclusionsThe classification of landslide dams proposed in this study can be used as a preliminary indicator of dam stability. More reliable assessment requires a geotechnical approach taking into account a variety of dynamic loading scenarios, and also relies on knowledge about the accurate dam and barrier-lake geometry. There is still a large amount of loose sediment remaining on the slopes, which may be reactivated and remobilized during the heavy post-earthquake rainstorms. Therefore, predicting the post-earthquake debris flows and evaluating their potential for damming rivers are still of great concern and remain as a main challenge.

Did the 2008 Wenchuan Earthquake Lead to a Net Volume Loss

The topographic evolution of mountain landscapes is a coupling process of tectonic rock uplift, landslide erosion, and valley incision etc. A widely accepted notion is that an earthquake will build up the mountainous topography, whereas some researchers suggest that the 2008 Wenchuan earthquake tumbled down the mountain because the wasting mass volume due to landsliding is two to six times larger than the gain volume caused by rock uplift. The purpose of this paper is to compare the wasting mass volume due to seismic landsliding with the gain volume caused by rock uplift related to the 2008 Wenchuan event based on a new detailed landslide inventory prepared by visual interpretation of aerial photos and satellite images of high resolutions. The results show that about 5.9 km3 materials, generated by nearly 200,000 landslides triggered by the Wenchuan earthquake, are distributed in the landslide intensity area. Although the landslides volume is larger than the published volume of tectonic rock uplift (2.6 ± 1.2 km3), it is rather smaller than that from the previous study. We think it is not enough only to account for the co-seismic landslide volume and uplifted volume in the study of landscape evolution of the Longmenshan mountain area where the Wenchuan event took place. Orogenic evolution is affected by a variety of factors, such as co-seismic and interseismic crustal uplift, and isostatic compensation of mass removed from the surface of the earth which leads to orogenic growth, whereas co-seismic landslides and river erosion can destroy mountainous topography.

Hazard and Risk Related to Earthquake-Triggered Landslide Events

First, we analyse how important earthquake-induced landslide hazards are compared to other geohazards at world-wide scale. Then, we try to estimate where these hazards may have the strongest impacts—at regional and local scale. In this regard, we also consider the short- and long-term effects of geological, tectonic, climatic and morphological conditions. Hazard and risk related to these processes was also analysed based on series of case histories: e.g., the 1920 Haiyuan earthquake-landslide disaster in China, the 1970 Nevado-Huascaran rock avalanche, as well as the 1999 events in Taiwan, 2001 in El Salvador, 2005 in Pakistan, and 2008 in China. Detailed report was provided for events in Central Asia: the 1911 earthquakes in Kemin, Sarez in 1911, Khait in 1949, Gissar in 1989 and Suusamyr in 1992. Particular focus is on mega-events such as the Usoy rockslide in Tajikistan as well as giant prehistoric rockslides in other parts of Central Asia and in the world (including Europe). We will try to answer several questions such as: how likely is a seismic versus climatic origin for giant landslides; how is the general geohazards level affected by these low-frequency earthquake-triggered mega-events. One conclusion is that in semi-arid mountain regions marked by a strong seismic activity, such as those in Central Asia, seismogenic landslides and related long-term effects may represent the most important geohazards. Further, the susceptibility to seismic slope instability is highest along active fault zones and on convex slopes made of soft or fractured materials.

Spatial Distribution of Landslide Dams Triggered by the 2008 Wenchuan Earthquake

Landslide dams are a common type of river disturbance in tectonically active mountain belts with narrow and steep valleys. Here we present an unprecedented inventory of 828 landslide dams triggered by the 2008 Wenchuan earthquake, China. Of the 828 landslide dams, 501 completely dammed the rivers, while the others only caused partial damming. The spatial distribution of landslide dams was similar to that of the total landslide distribution, with landslide dams being most abundant in the steep watersheds of the hanging wall of the Yingxiu-Beichuan Thrust Fault, and in the northeastern part of the strike-slip fault near Qingchuan. We analyzed the relation between landslide dam distribution and a series of seismic, topographic, geological, and hydrological factors.

The iRALL Doctoral School 2018: advanced studies on large landslides on the 10th anniversary of the Wenchuan earthquake

The research association on large landslides The iRALL, the International Research Association on Large Landslides, is a non-governmental, non-political, non-profit organisation that promotes research and knowledge on large landslides and gives particular attention to the chain of geohazards in earthquake-affected areas. The association was founded on 11 November 2015 and benefits of a prestigious scientific committee of world-renowned academicians. The iRALL secretariat is hosted at the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (SKLGP), within the campus of the Chengdu University of Technology in Chengdu, China (http:// irall.sklgp.com/en/introduction/index.html).

The Classification of Damming Landslides and Landslide Dams Induced by the Wenchuan Earthquake

The devastating 2008 Wenchuan earthquake with a magnitude of Mw 7.9 was the largest seismic event in China in more than 50 years. It triggered numerous landslides over a broad area, some of which dammed rivers, posing severe threats to downstream settlements. The coseismic landslides are classified into rock/debris avalanches, debris flows, rock/debris slides and rock falls. A number of representative examples of each type and the corresponding landslide dam features were studied. According to dam composition material and sedimentological features, landslide dams were categorized into three types: dams mainly composed of large boulders and blocks; dams composed of unconsolidated fine debris; and dams with partly intact rock strata at the base topped by large boulders and blocks or soil with rock fragments, showing two-layered or three-layered depositional structure. This classification is linked to the typology of damming landslides and considered to be a preliminary indicator of dam stability. In addition, dam stability also largely depends on valley morphometry as well as landslide runout distance and mechanism.

Distinctive controls on the distribution of river-damming and non-damming landslides induced by the 2008 Wenchuan earthquake

The 2008 Wenchuan earthquake (China, Mw 7.9) highlighted the importance of assessing and mitigating the hazards from co-seismic landslides and landslide dams. The seismic shaking triggered hundreds of thousands of landslides, about 800 of which dammed the course of rivers. To understand whether distinctive factors concurred with the river-damming events, we analyzed the spatial patterns of the river-damming landslides and the non-damming landslides separately, with reference to a number of possible controlling factors. Then, we quantified the significance of these factors using the weight of evidence method, and we used the results to perform a susceptibility assessment in a portion of the earthquake-affected region to verify the effectiveness of the method. We find that the distance to the fault surface rupture, peak ground acceleration (PGA) and lithology play a controlling role for co-seismic landslides of any type. The occurrence of river-damming landslides, rather than by a specific lithology or topography, is more related to hydrological factors, while topographic controls (slope, internal relief and terrain roughness) are more significant for the non-damming landslides.

A Conceptual Event-Tree Model for Coseismic Landslide Dam Hazard Assessment

Earthquakes may trigger a series of multiple cascading geohazard phenomena. For example, coseismic landslides may block rivers and form landslide dams, which occur frequently in tectonically active mountains with narrow and steep valleys. The catastrophic release of water masses from landslide-impounded lakes is capable to produce outburst floods and debris flows, causing loss of lives, housing and infrastructure. Quantifying the probability of these cascading phenomena following a triggering event has been a main research challenge. This study creates a conceptual event tree model for hazard assessment of earthquake-induced landslide dams, with the involvement of many discussions amongst specialists in different fields. Event tree (ET) is a graphical, hierarchical and tree-like representation of possible events, which has been successfully applied in the volcano hazard assessment, but the application in landslide research is rather limited. We attempt to elaborate the event tree model by applying it in estimating the hazard of landslide dams induced by the Wenchuan earthquake. The model starts from a scenario-earthquake on a known possible active faults; the model then progressively assesses the susceptibility to coseismic landslides and landslide dams, and, finally, provides an estimate of dam-break flood hazard. According to the literature and our best understanding of the seismic hazard, we suggested or proposed possible methods to estimate the probabilities at successive nodes, the cascading events.