Abstract
Abstract. Floods from failures of landslide dams can pose a hazard to people and property downstream, which have to be rapidly assessed and mitigated in order to reduce the potential risk. The Tangjiashan landslide dam induced by the Mw = 7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3 × 108 m3, and the potential catastrophic dam breach posed a serious threat to more than 2.5 million people in downstream towns and Mianyang city, located 85 km downstream. Chinese authorities had to evacuate parts of the city until the Tangjiashan landslide dam was artificially breached by a spillway, and the lake was drained. We propose an integrated approach to simulate the dam-breach floods for a number of possible scenarios, to evaluate the severity of the threat to Mianyang city. Firstly, the physically-based BREACH model was applied to predict the flood hydrographs at the dam location, which were calibrated with observational data of the flood resulting from the artificial breaching. The output hydrographs from this model were inputted into the 1-D–2-D SOBEK hydrodynamic model to simulate the spatial variations in flood parameters. The simulated flood hydrograph, peak discharge and peak arrival time at the downstream towns fit the observations. Thus this approach is capable of providing reliable predictions for the decision makers to determine the mitigation plans. The sensitivity analysis of the BREACH model input parameters reveals that the average grain size, the unit weight and porosity of the dam materials are the most sensitive parameters. The variability of the dam material properties causes a large uncertainty in the estimation of the peak flood discharge and peak arrival time, but has little influence on the flood inundation area and flood depth downstream. The effect of cascading breaches of smaller dams downstream of the Tangjiashan dam was insignificant, due to their rather small volumes, which were only 2% of the volume of the Tangjiashan lake. The construction of the spillway was proven to have played a crucial role in reducing the dam-breach flood, because all the other natural breach scenarios would have caused the flooding of the downstream towns and parts of Mianyang city. However, in retrospect improvements on the spillway design and the evacuation planning would have been possible. The dam-break flood risk will be better controlled by reducing the spillway channel gradient and the porosity of the coating of the channel bottom. The experience and lessons we learned from the Tangjiashan case will contribute to improving the hazard mitigation and risk management planning of similar events in future.
Highlights
Natural damming of rivers by mass movements are very common and potentially dangerous phenomena (Costa and Schuster, 1988; Evans et al, 2011), which have been documented all over the world, e.g. in Japan (Swanson et al, 1986), Canada (Clague and Evans, 1994), China (Chai et al, 1995), the northern Apennines in Italy (Casagli and Ermini, 1999) and New Zealand (Korup, 2005)
The predictions of peak discharge of the Tangjiashan dam from empirical equations proposed by previous studies in Table 6 are compared with those from the Scenario 4 of the BREACH model, as both of them consider the whole breach of the dam
The Tangjiashan landslide dam was the most dangerous one of the approximately 800 coseismic landslide dams induced by the Wenchuan earthquake, and impounded the largest lake whose catastrophic outburst flooding threatened ∼ 2.5 million people downstream
Summary
Natural damming of rivers by mass movements are very common and potentially dangerous phenomena (Costa and Schuster, 1988; Evans et al, 2011), which have been documented all over the world, e.g. in Japan (Swanson et al, 1986), Canada (Clague and Evans, 1994), China (Chai et al, 1995), the northern Apennines in Italy (Casagli and Ermini, 1999) and New Zealand (Korup, 2005). The largest flood in recorded history was caused by the failure of the earthquake-induced Raikhot landslide dam in 1841 on the Indus River in Pakistan, which has an estimated peak discharge of ∼ 540 000 m3 s−1 (Mason, 1929; Shroder Jr., 1998). Schneider (2009) described the largest landslide triggered by the Kashmir earthquake (Mw = 7.6, 2005) in Pakistan, which formed a natural dam impounding two lakes in the Karli river. On overview of research work on landslide dams was made by Korup (2002) and Evans et al (2011)
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