Simulating dam-breach flood scenarios of the Tangjiashan landslide dam induced by the Wenchuan Earthquake

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 <i>M</i><sub>w</sub> = 7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3 × 10<sup>8</sup> m<sup>3</sup>, 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

Earthquake‐Induced Chains of Geologic Hazards: Patterns, Mechanisms, and Impacts

“…The longevity of landslide dams ranges from minutes to thousands of years, depending on many factors, including dam volume, size, shape, and sorting of blockage material, rates of seepage through the blockage, and rates of sediment and water flow into the newly formed lake (Costa & Schuster, 1988). Studies of the longevity of landslide dams (Costa & Schuster, 1988, 1991Fan, Tang, et al, 2012;Peng & Zhang, 2012a, 2012b can be summarized as follows: 85- Figure 16. A proposed hazard assessment model and mitigation workflow for landslide dams modified from .…”

Analysis of changes in post-seismic landslide distribution and its effect on building reconstruction

“…Geohazards after major mountain earthquakes not only pose an imminent threat to lives by coseismic landslides and quake-dam breach floods (Cui et al, 2011;Fan et al, 2012a) but may also persist for more than 10 years before recovery to pre-earthquake conditions (Cui et al, 2011;Huang, 2011). Post-seismic reconstruction often occurs quickly during that time, while geodynamic activity still continues.…”

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

“…Wang et al (2008) and Liu et al (2009) numerically modelled the Tangjiashan landslide, and Liou et al (2010) as well as Xu et al (2010) detected the changes of its barrier lake based on satellite image classification. Fan et al (2012b) created an integrated approach that combines the physically-based BREACH model and the 1D-2D SOBEK hydrodynamic model to simulate the potential dam-break floods of the Tangjiashan landslide dam in different scenarios. Given the relatively short longevity of landslide dams, evaluating their stability and potential hazard is significant for the mitigation measures.…”