Rapid episodic erosion of a cohesionless landslide dam: Insights from loss to scour of Yangjia Gully check dams and from flume experiments

Highlights

• Erosion-induced collapse of check dams was studied in field investigations and flume experiments.

• The erosion rate of loose landslide deposits is easily underestimated.

• Erosion rate increases once a hydraulic drop forms on the sediments across the check dam.

• Countermeasures to mitigate the rapid erosion are suggested.

Abstract

An underestimation of local scouring in loose cohesionless landslide deposits may cause rapid and episodic failure of a hazard mitigation structure. A rock avalanche triggered by the 2008 Wenchuan earthquake generated an abundance of erosion-sensitive sediment in Yangjia Gully, resulting in several debris flows. Twelve concrete pile check dams were constructed along the gully to mitigate debris flow hazard in 2017. However, three of the downstream check dams progressive collapsed during the next two years. Field investigations revealed at the downstream of most check dams an accelerating scour, which quickly led to their instability and failure. Energy dissipation structures built at the toe of the dams moderated the local scour but did not prevent it. Repeated field investigations and flume experiments were carried out to better understand the process of bed erosion. The erosion rate is substantially high, and appears to be irreversible without artificial intervention once a hydraulic drop is formed. The presented case in Yangjia Gully shows the consequence of underestimating rapid erosion. Furthermore, several suggestions are proposed for hazard mitigation engineering in erosion-sensitive deposit.

Introduction

The 2008 M_w 7.9 Wenchuan earthquake triggered many landslides in the Longmenshan area of Sichuan Province, China (Tang et al., 2011; Huang and Fan, 2013). Landslide dams, resulted from Co-seismic landslides and rock avalanches that blocked rivers, led to disaster-chain effects such as disastrous dam-break floods (Xu et al., 2009). A total of 828 landslide dams identified in the populated mountainous area (Fan et al., 2018a). Over 30 dams were artificially breached to mitigate catastrophic floods that may be resulted from natural dam failures (Xu et al., 2009).

The Yangjia Gully rock avalanche occurred in Beichuan Qiang Autonomous County during the 2008 Wenchuan event. Dam-break floods, debris flows, and raised riverbeds related to the enhanced geomorphic activity in the Yangjia Gully area threatened 3000 lives and infrastructure in the downstream town of Chenjiaba. Therefore, after the natural breaching of the Yangjia Gully rock avalanche dam, a series of check dams have been built along the gully to mitigate the debris flow hazard. The latest 12 dams were completed in 2017. However, during a flood one year later, several check dams in the steepest section of the gully became unstable, as the rapid local scour caused the dam piles to topple.

Check dams, diversion channels, flexible barriers, and deposition basins are conventional engineering countermeasures to mitigate debris flow hazards (Wang et al., 2012; Chen et al., 2019; Hu et al., 2020). Check dams, which can be classified as closed-type and open-type dams (including slit dams, beam dams, grid dams, and so on), are more effective for managing torrential floods and debris flows (Remaître and Malet, 2010). A wide variety of check dams have been studied via theoretical analyses, field investigations, numerical simulations, and physical models in flumes. What's often under discussion include barrier efficiency for a variety of dam structures, the weakening and retardation of peak discharge, boulders and sediments trapped by dams, and upstream soil erosion, etc. (Armanini et al., 1991; Li et al., 2019; Chen et al., 2019; Hu et al., 2020). Some check dams are able to reduce debris flow hazard at some localities for a long time, such as the dams in the town of Port Alice on Vancouver Island North (Nasmith and Mercer, 1979). However, in many cases, check dams were damaged. These include dams which were filled beyond capacity, or which were suffered from overtopping, burial, water and debris impacts, or dam-body erosion (Remaître and Malet, 2010; Fan et al., 2018). For example, in the Faucon torrent in the French Alps, nearly half of the 100 check dams were buried, damaged, or totally destroyed (Remaître et al., 2008).

Check dams may rapidly fill up and get damaged by large debris flows in post-earthquake areas because of the massive source materials. Unsuitable designs may even intensify the catastrophic consequences of disasters; an example includes the Wenjia Gully, near the town of Qingping, where 5 × 10⁷ m³ of loose deposit accumulated during the Wenchuan earthquake. Nineteen closed-type check dams were built in the gully at the beginning of 2010 (Chen et al., 2015). On August 13th, 2010, an extremely large debris flow destroyed all of them, caused seven deaths, 39 injuries and buried 497 houses (Liu et al., 2017). The materials previously trapped behind the dams were mobilized so that they instantly increased the flow discharge and run-out distance (Tang et al., 2012; Chen et al., 2015; Zhou et al., 2015). Another tragic example is the August 7th, 2010 debris flow event in Zhouqu County, Gansu Province, China, which caused 1765 fatalities and destroyed seven large masonry closed-type check dams. The loose material trapped upslope of the dams was incorporated into the debris flow and the flow volume was amplified (Tang et al., 2011). Dam collapse may produce increasingly serious consequences, especially for regions where the soil is easily eroded (Liu et al., 2020).

The interaction between debris flow and check dams is related to the geometry of a countermeasure structure and its corresponding features (Hu et al., 2020). Open-type check dams have an advantage in maintaining filling space (Armanini et al., 1991). Closed-type concrete check dams built in the lower reaches of Yangjia Gully were totally buried before 2010. Thus, a series of open-type concrete pile dams were built to maintain reservoir capacity in 2017. The new open-type check dams soon encountered a major erosion problem below their downstream face owing to overtopping and consequently turbulent scour. As the height differential between the two faces increased, the scour rate accelerated. The scouring exposed the bases of the dam piles and caused three check dams to topple during massive debris flows or floods. There are many examples in the literature that document the importance of the scouring and consequent retrogressive erosion in streams controlled by check dams. After a debris flow in 2003, 0.5–4.0 m deep scour erosion occurred in check dams in the Faucon torrent (Remaître et al., 2008). A similar phenomenon also occurred in Xiaogangjian Gully check dams, where erosion scoured a hole beneath the dams and was eventually stopped by steel-pipe pile foundations (Chen et al., 2015). Scouring occurs in nearly all check dams. Many theories and formulas have been proposed to calculate the scour depths and temporal evolution, but there is still no universal formula capable of covering all conditions (Conesa-García and García-Lorenzo, 2009). The scouring effect is more intense in erosion-sensitive materials than in general stream beds; for example, part of the check dams (constructed in the 1970s) in the Cárcavo catchment still has not reached the predicted scour depth (1.48 m) after nearly half a century. By comparison, two of the check dams in Yangjia Gully were destroyed in just one year, and rapid scour below the dam outfalls was an obvious preparatory factor, as some piles were left hanging above the riverbed. Another example that illustrats the extremely erosion rate on loose material is the Barlin check dam in Taiwan. An energy dissipator dam failed in 2007; consequently, the Barlin check dam collapsed during a typhoon in September 2007. Retrogressive erosion was deemed a probable cause (Wang and Kondolf, 2014).