Can a human mechanism be developed to accurately predict floods in the rivers of the Tibetan Plateau and the Himalayan Mountains?

Landslide-dammed lake outburst floods (LLOFs) may pose serious safety threats to nearby residents and their livelihoods, as well as cause major damages to the downstream areas in mountainous regions. This study presents the Diexi ancient landslide-dammed lake (DALL) in the Upper Minjiang River at the eastern margins of the Tibetan Plateau, which was known to an estimated previous maximal lake area of 1.1 × 107 m2 and an impounded volume of 2.9 × 109 m3 . Then, at approximately 27 ka BP, the ancient landslide dam failed and catastrophic LLOFs occurred. It was determined that the peak discharge of the Diexi ancient LLOFs could be reconstructed using regression, parametric, and boulder competence approaches. The reconstructed maximum peak discharge might be 72,232.66 m3 /s, with an average velocity of 17.23 m/s, indicating that the Diexi ancient LLOFs were the most gigantic outburst floods to occur in the Upper Minjiang River Valley since the Late Pleistocene Period. The differences in the widths and slopes within the former and the later reaches of the dam indicated that the geomorphic influences on the river channel resulting from the DALL and its LLOFs have existed for tens of thousands of years. These findings were of major significance in deepening the understanding of the existence and disappearances of important river-knickpoints on a time scale of tens of thousands of years. Keywords: Landslide dam; Ancient dammed lake; Landslide-dammed lake outburst floods (LLOFs); Peak discharge.Introduction Natural processes, such as tectonic movements, climate changes, volcanic activities, biological forces, and chemical processes, can form different types of natural dams and dammed lakes. These include landslide-dammed lakes, glacier lakes, moraine-dammed lakes, volcanic lakes, and organic lakes (Ermini and Casagli, 1988; Evans, 1986; Clague and Evans, 2000; Dai et al., 2005; Korup and Montgomery, 2008; Pierce et al., 2010; Chen et al., 2013; van Gorp et al., 2013; Delaney and Evans, 2015; Emmer, 2017; Kataoka et al., 2018). In the category of natural dammed lakes, landslide-dammed lakes are widely found to exist in various parts of the world (Ermini, 2003; Korup, 2004; Dai et al., 2005; Dong et al., 2009; Butt et al., 2013; Zhou et al., 2013; Stefanelli et al., 2018; Chen et al., 2018; Fan et al., 2020). Landslide dams and dammed lakes are common geomorphic phenomena in the eastern and southern margins of the Tibetan Plateau in southwestern China (Dai et al., 2005; Korup and Montgomery, 2008;Cui et al., 2009; Chen et al., 2013; Wang et al., 2014a). Among the aforementioned landslides, many large-scaled ancient landslides had blocked rivers, forming ancient landslide-dammed lakes, as illustrated in Table 1. The longevity of landslide-dammed lakes may vary from several minutes to millennia (Costa and Schuster, 1988; Korup, 2002; Ermini, 2003; Butt et al., 2013; Delaney and Evans, 2015; Chen et al., 2018). A stable dammed lake may exist for thousands of years and be utilized for hydroelectric generation or tourist attractions. However, unstable dams may fail and generate landslide-dammed lake outburst floods (LLOFs) causing destructive hazards. It is essential to assess the mechanism and processes related to the triggers and breaches of landslides and dammed lakes, and are of major significance in the reconstructions of previous events, as well as effective hazard mitigation. China’s Diexi ancient landslide-dammed lake (DALL) is a well-known case, and is located the Upper Minjiang River at the eastern margin of the Tibetan Plateau. This lake initially received widespread attention in the research studies which were conducted regarding the Diexi earthquake of 1933. A large number of in-depth examinations of the lacustrine sediment in the upstream area of the landslide dam were completed in the decades following the aforementioned earthquake event (Wang et al., 2011; Wang et al., 2014b; Wei et al., 2015). However, few research reports have presented data regarding the LLOFs and the outburst deposits in the downstream areas. In our previous field research studies, several outburst deposit bars were discovered to be distributed in the downstream areas of the relict ancient landslide dam, and were subsequently examined and analyzed (Ma et al., 2018; Chen et al., 2019). In this study, a summarization of the characteristics of the outburst floods of the DALL was presented, and the maximum peak discharge of the LLOFs was reconstructed based on geomorphic and sedimentary characteristics. Furthermore, this study discussed the failure mode of the DALL, along with the geomorphological effects of the DALL and its outburst flooding actions.Regression Equations The models relied on a single or series of regression relationships derived from test case studies or observed historical dam failures, which were determined to be related to the observed peak discharge levels in order to accurately measure the impounded water volumes. These included the water heights or depths (Kirkpatrick, 1977; Cenderelli, 2000; Pierce et al., 2010); water storage levels or volumes (Evans, 1986; Walder and Costa, 1996; Walder and O’Connor, 1997); or combinations of the water height (depths) and storage (volumes) ( Macdonald and Langridgemonopolis, 1984; Costa, 1985; Costa and Schuster, 1988). It was found that such regression relationships were convenient for application purposes, but limited in their suitability. This was due to the fact that they generally neglected the inclusion of the basic hydraulic principles related to the breach initiation and enlargement processes (Westoby et al., 2014), and thereby only provided order-of-magnitude predictions of the probable peak discharge levels (Dai et al., 2005). In the cases where reliable estimates of the time-to-peak flow were unavailable, providing that the geometric characteristics of the dam structure and its lake drainage basin were known, it was considered that the analysis of the empirical relationships could accurately provide an expeditious and simple approach to estimating the peak flows, and would be suitable for relatively basic hazard assessments (Morris et al., 2007). Therefore, in spite of the limitations of the regression equations, they may provide constructive guidance for the general evolution law of the LLOFs’ peak discharge levels. In this study, the equations developed by Evans (1986), Costa and Schuster (1988), and Cenderelli (2000) were adopted for the reconstruction of the peak flow of Diexi ancient LLOFs, which were described as follows: (i) Evans, 1986 Qp = 0.72(Vw) 0.53 (1) (ii) Costa and Schuster, 1988 Qp = 0.0158PE0.41 (2) (iii) Cenderelli, 2000 Qp = 24(d) 1.73 (3) Qp = 3.4(V) 0.46 (4) Qp = 1.9(V∙d) 0.4 (5) In this study, the results of the field research revealed that a number of lacustrine deposits directly covered the slope of the relict ancient landslide dam (D-I) located in Jiaochang Village. The thicknesses of these deposits were determined to be between 1 and 3 m, and 2,302 m a.m.s.l. respectively (Figs. 2b and 3). It was supposed that the elevation of the lake surface after failure was 2,302 m. The water surface elevations of Xiahaizi Lake and the previous DALL were 2,122 m a.s.l. and 2,355 m a.s.l., respectively (Fig. 2). Therefore, the previous dam height (Hd) and the drop in the lake level depth (d) after breaching were determined to be 233 and 53 m, respectively. Finally, the peak discharge (Qp) of Diexi ancient LLOFs were successfully calculated using the aforementioned Eqs. (1) to (5).Parametric Equations It is noteworthy that the models had considered the semi-physical processes, although they were simplified. Generally, several parameters were taken into consideration, including the lake surface area and volume, lake geometric characteristics, water depth, breach geometric characteristics, flood routing time, and the erosion rate of breach development. The obtained data was applied in the simulations of the dam-break formation, lake drainage characteristics, and maximum peak discharge level (Fread, 1996; Morris et al., 2009; Capart, 2013). The models generally required detailed geometric data of the dam structure, as well as its material properties. The hydraulic conditions of the breach flood were also important in order to achieve accurate simulations. However, it has been found to be difficult to obtain all of the aforementioned parameters for ancient dammed lakes, since their failures were rarely recorded in historic literature. Among these previously used models, the physically based predictive equations developed by Walder and O’Connor (1997) were frequently adopted in the assessments of the peak discharge levels LLOFs. For example, Dai et al. (2005) estimated the peak flow (Qp) of the LLOFs in southwestern China’s Dadu River using Walder and O’Connor’s predictive equations. This study also adopted the approach developed by Walder and O’Connor (1997) to estimate the peak flow of the Diexi LLOFs as follows: Qp =1.51(g 0.5d 2.5) 0.06 (kV/d) 0.94 if kV(gd) -0.5d -3