Barrier lake bursting and flood routing in the Yarlung Tsangpo Grand Canyon in October 2018

Journal Pre-proofs Research papers Barrier lake bursting and flood routing in the Yarlung Tsangpo Grand Canyon in October 2018 Chen Chen, Limin Zhang, Te Xiao, Jian He PII: DOI: Reference:

S0022-1694(20)30063-9 https://doi.org/10.1016/j.jhydrol.2020.124603 HYDROL 124603

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Journal of Hydrology

Received Date: Revised Date: Accepted Date:

7 August 2019 23 November 2019 19 January 2020

Please cite this article as: Chen, C., Zhang, L., Xiao, T., He, J., Barrier lake bursting and flood routing in the Yarlung Tsangpo Grand Canyon in October 2018, Journal of Hydrology (2020), doi: https://doi.org/10.1016/j.jhydrol. 2020.124603

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Barrier lake bursting and flood routing in the Yarlung

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Tsangpo Grand Canyon in October 2018

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Chen Chena, Limin Zhanga,b,c*, Te Xiaob, Jian Heb

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* Corresponding author

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Email addresses: [email protected] (C. Chen), [email protected] (L. M. Zhang),

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[email protected] (T. Xiao), [email protected] (J. He).

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a

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Resource and Hydropower, Sichuan University, Chengdu, China

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b

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and Technology, Hong Kong

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c

State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water

Department of Civil and Environmental Engineering, The Hong Kong University of Science

HKUST Shenzhen Research Institute, Shenzhen, China

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Abstract

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Ice-soil mixture landslide dams formed frequently in the Tibetan Plateau in response to global

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warming, which pose great threats to both upstream and downstream areas due to inundation

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and lake bursting. On 17 October 2018, a large landslide, induced by an ice-avalanche at the

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Sedongpu Basin of the Yarlung Tsangpo, blocked the main course of the river near Gyalha.

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The barrier lake level rose quickly and the dam was overtopped naturally at 13:30 on 19

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October 2018, generating a dam-breaching flood with a peak flow rate of 32,000 m3/s. This

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paper presents a comprehensive study of the disaster chain of landslide-barrier lake-dam

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breaching-river flooding in the Yarlung Tsangpo Grand Canyon, detailed geological and

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hydrological characteristics of the study region, rapid prediction of the dam breaching

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hydrograph using an erosion-based numerical model, and analysis of the flood routing in a 460

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km canyon reach along the Yarlung Tsangpo. The simulated peak discharge at the dam site is

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over 30,000 m3/s and the corresponding dam-breaching and flood routing hydrographs agree

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well with the observations. Two additional scenarios with larger inflow rates are also

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considered. Results show that with a larger inflow into the barrier lake, the erosion of the dam

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body becomes more rapid. When the inflow rate is increased by six times, the peak dam-

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breaching discharge can be doubled. The study serves as basis to manage the flood risks due to

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landslide dam bursting on the Yarlung Tsangpo or similar rivers.

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Keywords: Ice avalanche; Landslide; Landslide dam; Barrier lake; Dam breaching; Flood

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routing.

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1. Formation and bursting of the Gyalha barrier lake

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The Yarlung Tsangpo is the highest large river in the world with an average elevation

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of 4000 m. The river originates from Chemayungdung Glacier in southwestern Tibet and flows

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eastward on the Tibetan Plateau. It bends sharply in southeastern Tibet, where it passes between

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two peaks: the 7,782 m Namcha Barwa and the 7,294 m Gyala Peri (Fig. 1). The river then cuts

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its way through the Yarlung Tsangpo Grand Canyon and into Arunachal, where it is called

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Siang. Down the river from Arunachal, the river becomes broader, and after reaching Assam it

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is also known as the Brahmaputra. The river further flows into Bangladesh from Assam, where

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it is called Padma. It crosses China, India and Bangladesh with a length of about 3,350 km, a

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catchment area of 66.6×104 km2, and an annual mean runoff of 66.29×1010 m3 (Liu, 1999).

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At around 5 am, 17 October 2018, a very large landslide struck the Yarlung Tsangpo in

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southeastern Tibet, China, 7 km downstream the Gyalha Village and about 175 km upstream

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Medog, in the vicinity of 29°47’7.20’’ N, 94°55’24’’ E (Fig. 1). The landslide originated from

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the left bank of the Yarlung Tsangpo inside the Sedongpu Basin, formed a landslide dam and

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blocked the main course of the Yarlung Tsangpo (Fig. 2a). The landslide dam was composed

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of massive ice and debris 40-60 million m3 in volume, 310-620 m in width, and 79 m in height

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(Jin, 2019; Tong et al., 2018; CCTV News, 2018a). The Gyalha landslide was featured with

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mixtures of ice and soil materials, which marched into the Yarlung Tsangpo and spread

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laterally along the river by 2.3 km. The barrier lake level increased very quickly; at the noon

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of 17 October, the lake behind the landslide dam was 15 km long, the water level rose 40 m

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above the original water level, and the reservoir volume reached about 150 million m3. On the

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morning of 18 October, the water level of the barrier lake was already 50 m higher than the

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ground surface of the Gyalha Village and the lake was at risk of breaking. Figure 3 shows the

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inundated Gyalha Village after the formation of the landslide dam, where the access road to

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the village was completely submerged. More than 20,000 people in the Mainling County and

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Medog County could be affected by the barrier lake due to either inundation or lake bursting.

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Authorities in Tibet activated the highest level of emergency response and evacuated at least

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6,000 residents. The breach of the landslide dam could also bring massive floods along the

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Siang River in Arunachal, posing a great threat to the people in the region. Thus, the Chinese

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and India governments exchanged information on 18 October to cope with this disaster. The

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upper Siang district administration has warned residents in the Siang Valley to take

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precautionary measures (NDTV, 2018). By 7 am, 19 October, the lake water level had risen

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about 75 m above the original level and was still rising at an average rate of 0.61 m per hour

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(CCTV News, 2018a). Eventually, at 13:30 pm on 19 October, the Gyalha landslide dam was

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overtopped (Fig. 2b), with a lake capacity larger than 500 million m3 (Xinhua Net, 2018a). The

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peak breaching flow was as large as 32,000 m3/s at the dam site and resumed its base flow on

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20 October (Jin, 2019).

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Landslides frequently occurred in the Sedongpu Basin due to glacial activities. The

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Yarlung Tsangpo near Sedongpu had been partially blocked after the Nyingchi earthquake in

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2017, thus can be blocked more easily. The mixtures of ice and debris from ice avalanches are

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very unstable, hence the barrier dam may break quickly, releasing a large quantity of water.

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Once a landslide dam forms and blocks a major river, it may pose a great threat to residents

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both upstream and downstream the dam. For instance, an extremely large landslide on 9 April

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2000 at Zhamu Creek, 48 km northeast of the Gyalha landslide (30°12’03’’ N, 94°58’03’’ E,

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Fig. 1) blocked the Yigong River that is a tributary of the Yarlung Tsangpo (Lu et al., 2003).

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The landslide dam breached on 10 June 2000, releasing a massive impounded water volume of

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about 3 km3, resulting in a dam-breaching flood with a peak discharge of 120,000 m3/s (Shang

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et al., 2003). The burst of the barrier lake affected 4,000 people in the downstream. Since most

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landslide dams are short-lived with 51% of landslide dams breaching within one week, timely

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evacuation of downstream people is extremely important (Peng and Zhang, 2013a,b). The

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prediction of the dam breaching time, flow rate, and the downstream flood routing process is

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important for evaluating the dam-breaching flood risks and making a successful emergency

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management plan.

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The focus of this study is to conduct a comprehensive analysis of the Gyalha landslide-

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barrier lake-dam breaching-flooding disaster chain in the Yarlung Tsangpo Grand Canyon.

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Taking the Gyalha landslide as a benchmark, the processes of dam breaching and flood routing

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with plausible larger inflows into the barrier lake are also simulated. Results of dam breaching

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process, flow rate, final breach size, and subsequent flood routing are analyzed and compared.

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Detailed information of the geometric and hydraulic conditions of the dam, the dam breaching

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simulation, the flooding routing process, and the resulting outflow hydrographs are presented

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in this paper.

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2. Geological and geomorphological characteristics 2.1. Topography of the studying river reach and the Sedongpu Basin

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The studying river reach for flood routing analysis starts from the Gyalha landslide dam

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site to the downstream Pasighat Village (Fig. 1), which is the main portion of the Yarlung

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Tsangpo Grand Canyon. The canyon is the deepest, and longest canyon in the world, which

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stretches 496 km with an average depth of 5,000 m, passing between the peaks of the Namcha

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Barwa (7,782 m) and Gyala Peri (7,294 m). The elevation of the canyon entrance at Pei Village

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in Mainling County is about 2880 m, whereas the elevation at Pasighat Village is around 150

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m, corresponding to a water level drop of 2,730 m. The deep cut of riverbed is caused by the

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rapid uplift of the eastern Tibet. The canyon crosses the eastern Himalayan syntaxis, where the

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continental collision between the Eurasian plate and the India plate is at its strongest.

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The topography for the flood routing analysis can be extracted from a digital elevation

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model, such as ALOS World 3D-30m (AW3D30) in this study (Tadono et al., 2014; Takaku et

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al., 2016, downloadable from https://www.eorc.jaxa.jp/ALOS/en/aw3d30/data/index.htm). As

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shown in Fig. 4, the elevation at the Gyalha landslide dam site is about 2,742 m, while the

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elevation decreases sharply to about 150 m at Pasighat. Figure 4b shows three typical cross

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sections A-C along the river reach, representing cross sections at the upper, middle and lower

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river reach. Slopes of riverbanks become gentler from the upstream to the downstream.

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On 17 October 2018, the Gyalha landslide occurred on the left bank of the Yarlung

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Tsangpo in the Sedongpu Basin (29°47’7.20’’ N, 94°55’24’’ E). The Sedongpu Basin has a

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catchment area of 66.89 km2 and an average elevation of 4,540 m. The highest point in the

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basin is the Gyala Peri Peak of 7,294 m, while the lowest point is 2,746 m, with a large elevation

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difference of 4,548 m. The basin strata are mainly composed of Proterozoic Namjagbarwa

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Group, and the primary bedrocks are Proterozoic marble and gneiss (Pt), which are interbedded

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and can lose stability under glacier erosion and freeze-thaw weathering (Huang et al., 2007).

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The Sedongpu Basin has a special steep-gentle-relatively steep ladder terrain from the top to

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the bottom (Fig. 5); 50% of the basin is steeper than 30° with an average slope of 34.89° (Tong

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et al., 2018). The middle area of the basin is gentle with slopes smaller than 15°, which is also

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the area covered with glaciers and moraines. Due to the ladder terrain, the collapse of rocks

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and glaciers from the upper steep part accumulated in the gentle area, providing massive loose

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materials to move along the lower relative steep area once triggered by snow melt or intense

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rainfall. Thus, the basin with massive loose materials is susceptible to large-scale debris flows.

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There are 12 branches of well-developed glaciers in the upper Sedongpu Basin, and the Gyalha

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landslide was believed to be induced by large avalanches. The runout materials were a mixture

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of glacier and rocks, which impacted into the accumulations and moraines in the lower part,

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formed a large debris flow and dammed the Yarlung Tsangpo (Tong et al., 2018; Liu et al.,

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2019). The avalanches were triggered under the conditions of steep terrain, broken rock masses,

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global warming, regional rainfall, glacier ablation, and antecedent earthquake activities.

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2.2. Structure of the Gyalha landslide dam

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The Gyalha landslide was triggered by avalanches from the upper glaciers of the

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Sedongpu Basin (Fig. 6). The mixtures of ice and disintegrated soils marched approximately

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4-7 km into Yalung Tsangpo (Hu et al., 2018). The nearby Yigong landslide in April 2000

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showed the similar feature, which was triggered by snow melt and rainfall and the debris

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marched about 8 km into the Yigong River (Zhou et al., 2016), entraining the colluvium with

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snow and ice. The Yigong landslide dam consisted mainly of angular coarse clasts in a matrix

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of gravel and finer fractions. The lithology was composed of disintegrated granite, marble and

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gneiss (Shang et al., 2003). Figure 7 shows some cut and scour features left by mass movement

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through Zhamu Creek Valley and the materials left in the deposition zone of the Gyalha

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landslide. The landslide deposits at both Yigong and Gyalha are loose fine debris with high

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erodibility. Therefore, in this study, the soil materials of the Gyalha landslide dam was

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benchmarked against the Yigong landslide dam, which will be introduced later.

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The size of the Gyalha landslide dam is interpreted based on satellite images and remote

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sensing (IMHE, 2018; Hu et al., 2018). The profiles of the longitudinal section and the cross-

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section of the dam are shown in Fig. 8. Along the river, the crest length of the landslide dam is

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about 300 m, the upstream and downstream lengths of the dam are estimated to be 720 and 880

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m, respectively. The elevations at the base and crest of the dam are 2,758 m and 2,837 m,

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respectively, giving a dam height of 79 m. Other information about the dam can be referred to

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Table 1.

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3. Hydrological characteristics

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The Yarlung Tsangpo is an important international river with several major tributaries,

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including Lhasa River, Nyang River, Nimu Maqu River and Parlung Tsangpo. It is a major

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freshwater resource for China and downstream South Asian countries (Ren et al., 2018; Sun et

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al., 2019), having an important influence on Asian hydrology and affecting millions of people.

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3.1. Glacier retreat in Tibetan Plateau

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The Tibetan Plateau is the largest glacier area in the world with a total glacier area of

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46,640 km2, of which 9,014 km2 is in the Yarlung Tsangpo Basin (Jia et al., 2008). Glaciers

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are very sensitive to climate change. Figure 9 shows changes of annual mean air temperature

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of the Yarlung Tsangpo Basin (from the source to the Pasighat) from 1957 to 2004 (Liu et al.,

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2007), showing a gradually increasing trend. Glaciers in the Tibet-Himalaya region show

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accelerated retreat with a substantial decrease of snow cover due to global warming

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(Oerlemans, 2005; Yao et al., 2007; Prasad et al., 2009). Figure 10 shows significant glacier

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reduction in the region of the Gyalha landslide from 19 Jan. 2017 to 19 Jan. 2018. Melting

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water from glacier/snow accounts for a considerable proportion of the runoff of the Yarlung

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Tsangpo. The annual glacier meltwater is 148.8×108 m3, accounting for about 9% of its annual

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runoff (Jia et al., 2008). Glacier related debris flows are consequences of general glacier retreat

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due to global warming and exposure of large quantities of unconsolidated, unvegetated, and

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sometimes ice-cored glacial sediments (Chiarle et al., 2007). These sediments are easily

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mobilized by floods from heavy precipitation, snowmelt, or glacial lake outbursts, which help

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to initiate rock avalanches and landslides. Glaciological phenomena as ice/snow avalanches

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and glacial floods can have significant impacts upon society over a very short time scale

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(minutes-days) (Richardson and Reynolds, 2000).

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3.2. Precipitation and runoff in the studying region

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Precipitation in the Yarlung Tsangpo Basin is dominated by the Indian summer

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monsoon circulation. The monsoonal moisture penetrates into the southern Tibetan Plateau and

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moves upward along the main course of the Yarlung Tsangpo due to the block of the

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Himalayas.

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The long-term annual mean runoff in the downstream part of the Yarlung Tsangpo is

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about 1,500-3,000 mm, which becomes as large as 5,000 mm at Pasighat and decreases sharply

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at the “horse-show bend” area near the Namcha Barwa (Jia et al., 2008). Figure 11 shows the

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annual runoff from 1970 to 2013 and the long term intra-annual distribution of the runoff from

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1960 to 2009 at the Nuxia Hydrological Station located at 76 km upstream of the Gyalha

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landslide dam site (Liu et al., 2011; Wang et al., 2016b). Although the annual runoff only shows

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a slightly increasing trend, the monthly runoff within a year varies significantly, showing a

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notable climate-dependent feature. The runoff is large in the summer and autumn. In the rainy

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August, the monthly runoff can be as large as 23.4% of the annual runoff.

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3.3. Past barrier lakes near Sedongpu Basin

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The Sedongpu Basin is a region where rock-avalanches, landslides and debris-flows

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occur frequently. Based on the literature (Tong et al., 2018; Liu et al., 2019) and satellite

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images, the blockages of the Yarlung Tsangpo in the Sedongpu Basin in the history are

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summarized in Table 2. Most of the blockages were caused by ice-avalanches. Some typical

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satellite images of Sedongpu Basin with different remaining landslide dams are shown in Fig.

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12. The Gyalha landslide on 17 October 2018 was believed to be affected by the Ms 6.9

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Nyingchi earthquake at Mainling County on 18 Nov. 2017 (Hu et al., 2018; Tong et al., 2018;

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Liu et al., 2019). The earthquake triggered at least 529 landslides, including rockslides,

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avalanches and rock falls (Hu et al., 2018). It induced three large landslide dams, one of which

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was located at the identical location of the Gyalha landslide dam. These landslide dams

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breached afterward. The residuals of these dams make the river narrower (Fig. 12). It should

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be noticed that about two weeks after the Gyalha landslide on 17 October 2018, a new landslide

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occurred at the same location on 29 October, which also blocked the Yarlung Tsangpo and the

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dam was overtopped shortly afterwards (Fig. 12d).

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3.4. Characteristics of the Gyalha barrier lake in 2018

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After the formation of the Gyalha landslide dam on 17 October 2018, a barrier lake

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formed momentarily. The water level and flow rate at the Dexing Hydrological Station (Fig.

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1) in Medog County, 173 km downstream the dam, dropped from 74.28 m at 8 pm on 16

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October, to 73 m at 8 am on 17 October, and further to 71.27 m at 2 pm on 17 October. The

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corresponding flow rate reduced from 3,430 m3/s to 2,620 m3/s from 8 pm 16 October to 8 am

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17 October, and to 1,580 m3/s by 2 pm on 17 October (Xinhua Net, 2018b). The inundation

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areas and the lake volumes at different lake water levels are determined using a digital elevation

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model, AW3D30. Figure 13 shows the relation between the lake surface elevation and the lake

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volume derived from the digital elevation model. The final lowest elevation of the crest of the

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dam is 2,837 m, determining a maximum water depth before overtopping of 79 m and a lake

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capacity of about 4.9×108 m3. After the formation of the barrier lake, the water volume stored

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in the first hour was about 8×106 m3 (Xinhua Net, 2018c), indicating an initial incoming flow

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rate of 2,222 m3/s. Thus, the time required to fill the barrier lake was about 2.6 days, implying

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a short lifespan of the landslide dam. Figure 14 further shows the inundation area of the barrier

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lake just before breaching. The barrier lake stretches 27 km upstream with an inundation area

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of around 11 km2. The Gyahla Village was partially inundated, and the access road to the

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Gyahla Village was completely submerged (Fig. 3b).

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4. Modelling of barrier lake bursting and flood routing process

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4.1. Principle of dam breaching simulation

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The dam breaching process can be simulated as a process of erosion of the dam

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materials by the overtopping flow (e.g. Jiang et al., 2017; Zhong et al., 2018; Begam et al.,

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2018; Zhang et al., 2019). A simple linear equation is widely used to estimate the erosion rate

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(E) of both cohesive and gravelly landslide dam materials in water flow (e.g. Hanson and

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Simon, 2001; Briaud, 2008; Shi et al., 2015; Okeke et al., 2016; Zhong et al., 2017 etc.):

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𝐸 = 𝐾𝑑(𝜏 ― 𝜏𝑐)

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where Kd is the coefficient of erodibility; τ is the shear stress at the soil/water interface; and τc

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is the critical shear stress at initiation of soil erosion. Kd and τc can be measured in-situ or

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estimated using several empirical equations (e.g. Mitchener and Torfts, 1996; Julian and

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Torres, 2006; Annandale, 2006; Thoman and Niezgoda, 2008; Chang et al., 2011). The shear

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stress at the soil-water interface can be calculated as:

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𝜏 = 𝛾𝑤𝑅ℎ𝑆

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where γw is the unit weight of water (N/m3); Rh is the hydraulic radius (m); S is the energy

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slope, which is equal to the slope gradient. For a trapezoid breach cross-section, Rh can be

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obtained as:

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𝑅ℎ =

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where H is the elevation of the water surface (m); Z is the elevation of the breach bottom (m);

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Wb is the breach bottom width (m); and α is the angle of the side slope.

(1)

(2)

(𝐻 ― 𝑍)cos 𝛼 + 𝑊𝑏sin 𝛼 2(𝐻 ― 𝑍) + 𝑊𝑏sin 𝛼

(3)

(𝐻 ― 𝑍)

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The breach discharge through a trapezoid breach can be calculated based on the broad-

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crested weir flow assumption which satisfies the condition of landslide dams with a large

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channel length but a small slope (Singh and Scarlatos, 1988):

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𝑄𝑏 = 1.7[𝑊𝑏 + (𝐻 ― 𝑍)tan 𝛼](𝐻 ― 𝑍)

258

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(4)

The hydrodynamics is modelled by solving the continuity equation of the lake together

259

with the breach outflow through a broad-crested weir:

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𝐴𝑠 𝑑𝑡 = 𝑄𝑖𝑛 ― 𝑄𝑜𝑢𝑡

𝑑𝐻

(5)

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where As is the lake water surface area (m2); Qin and Qout are the inflow and outflow rates of

262

the reservoir.

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In this study, an erosion-based numerical model DABA (Chang and Zhang, 2010;

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Zhang et al., 2016) is used to simulate the breaching process. The model has three advantages

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compared with other models (i.e. DAMBRK, BREACH, BEED): (1) the variations in soil

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erodibility along depth can be considered; (2) the steepening process of the landslide dam can

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be taken into account; and (3) the final size of the breach does not need to be assumed.

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The breach evolution in the DABA model is divided into three phases in the cross

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section and the longitudinal section. On the cross section (Fig. 15a), at the beginning, the soil

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at the dam crest can erode easily when overtopped naturally. During the first phase, the width

271

of the breach will not change but the breach depth and breach bottom width increase slightly

272

until the bank slope reaches a critical angle, αc. Then, in phase II, the breach size develops

273

significantly both horizontally and vertically, while keeping the bank slope angle at αc. This

274

process continues until the shear stress by overtopped water flow cannot overcome the soil

275

erosive resistance at the channel bed or the bank side, indicating that the breach process comes

276

to the final phase. Then in phase III, when the soil at the channel bed cannot be eroded, the

277

breach only develops in the horizontal direction, the breach width increases gradually at a

278

constant slope angle αc while the breach erosion depth does not change significantly. When the

279

soil at the two side walls do not erode, the erosion process can only cut in the vertical direction;

280

the breach depth increases but the breach bottom width decreases.

281

On the longitudinal section (Fig. 15b), in the beginning, water flows over the dam crest

282

and cuts the toe of the downstream slope until it reaches a limit angle, βf. Then, the downstream

283

slope keeps eroding while maintaining the slope angle at βf in the phase II. This process

284

continues until the downstream slope eventually meets the upstream slope, which implies the

285

longitudinal breach process goes into phase III. In phase III, the breach of the downstream

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slope develops dramatically with a sharp decrease of the dam crest elevation.

287 288

4.2. Erodibility of the dam materials

289

Referring to Eq. (1), soil erodibility can be described by the coefficient of erodibility

290

(Kd) and the critical shear stress at initiation of soil erosion (τc). τc indicates the potential of

291

erosion in soils, while Kd represents how fast the soil erodes. Kd and τc are functions of basic

292

soil properties and soil state, which can be obtained by laboratory tests, field tests or empirical

293

equations. Kd and τc in this study were obtained based on the empirical equations proposed by

294

Chang et al. (2011), which are particularly for broadly graded landslide deposits:

295

𝐾𝑑 = 20075𝑒4.77𝐶𝑢―0.76

(6)

296

𝜏𝑐 = 6.8(𝑃𝐼)1.68𝑃 ―1.73𝑒 ―0.97

(7)

297

where e is the void ratio; Cu is the coefficient of uniformity; PI is the plasticity index; and P is

298

the fines content with particle size smaller than 0.063 mm. When the fines content is smaller

299

than 10%, τc is recommended to be estimated as (Annandale, 2006):

300

𝜏𝑐 = 3𝑔𝑑50(𝜌𝑠 ― 𝜌𝑤)𝑡𝑎𝑛𝜙

301

where g is the gravitational acceleration; ρs is the soil mass density; ρw is the water mass

302

density; ϕ is the friction angle; and d50 is the mean particle size.

2

(8)

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The soil materials of the Gyalha landslide are benchmarked against the Yigong

304

landslide materials, both being ice-soil mixtures in the Tibet Plateau. The materials of the

305

Yigong and Gyalha landslides travelled long distances, and were highly disintegrated. Thus,

306

the distributions of Kd and τc with depth for the upper part of the Gyalha landslide dam can be

307

referred to those for the materials of Yigong landslide dam, while the lower part that was not

308

fully disintegrated is referred to the Tangjiashan landslide dam (Shi et al., 2015). The soil grain

309

size distributions at the deposition zone of the Yigong landslide (Hu et al., 2015; Kang et al.,

310

2017; Wang et al., 2017; Wang et al., 2016a) are summarized in Fig. 16a. For the Gyalha 13

311

landslide dam, Liu et al. (2019) reported a soil-rock ratio of about 8:2, and Cai (2019) found

312

soil-rock ratio values in the range of 6:4 - 7:3, both of which are in the range of 10:0 - 3.5:6.5

313

for the benchmark Yigong materials (Fig. 16a). Here the boundary between soil and rock is

314

defined as 10 mm (Cai et al., 2019). The bulk density of the Yigong landslide deposits is 1.845

315

g/cm3, the specific gravity is 2.718, and the friction angle is about 37º (Hu et al., 2009, 2015;

316

Wang et al., 2016a). Using Eqs. (6)-(8), the variations of Kd and τc for the Gyalha landslide

317

dam with depth are shown in Figs. 16b and c.

318

Furthermore, a sensitivity analysis is performed to evaluate the influence of soil

319

erodibility on the overtopping process. The analysis is conducted by (1) changing Kd by ±10%

320

and ±20% while maintaining τc as a constant; and (2) changing τc by ±10% and ±20% while

321

keeping Kd unchanged. The hydrographs of the overtopping process are quite similar when Kd

322

is changed (Fig. 17a), with variations of the peak flow rate up to 16% and a maximum time

323

difference of 48 min. The influence of τc is very small as shown in Fig. 17b. With the

324

development of overtopping, the shear stress τ at the soil-water interface is much larger than

325

the initiation critical shear stress τc; thus the effect of τc is not significant.

326 327

4.3.

Principle of flood routing simulation

328

The unsteady flooding routing process can be simulated based on the one-dimensional

329

Saint-Venant equations for the conservation of mass and momentum (e.g. Horritt and Bates,

330

2002; Oubanas et al., 2018; Lei et al., 2019):

331

∂𝑄 ∂𝑥

+

∂𝐴 ∂𝑡

332

∂𝑄 ∂𝑡

+

∂(𝑄2/𝐴) ∂𝑥

333

where Q is the flow rate; x is the distance along the channel bed; A is the cross-sectional area;

334

t is time; g is the gravitational acceleration; h is the flow depth; S0 is the bed slope; and Sf is the

335

friction slope and can be calculated using Chezy and Manning’s formulas as:

(9)

=0 +𝑔𝐴

(∂ℎ∂𝑥－𝑆0 + 𝑆𝑓) = 0

(10)

14

𝑄2𝑛2

336

𝑆𝑓 =

337

in which n is Manning’s coefficient of the roughness of the cross-section; and R is the hydraulic

338

radius of the cross-section. For mountain streams with steep banks, if the riverbed is primarily

339

covered with cobbles with large boulders but no vegetation, Manning’s n value ranges from

340

0.04 to 0.07 (Chow, 1959). For the riverbanks with medium intensity of vegetation and

341

constructed materials, the recommended n values are between 0.06 and 0.14 (USACE, 2016).

342

In this study, the n values are assumed to be 0.06 and 0.12 for the channel bed and riverbank,

343

respectively.

𝑅

43 2

𝐴

(11)

344

The study river reach is from the Gyalha landslide dam site to the downstream Pasighat

345

Village (Fig. 1), with a river length of 460 km. Based on the aforementioned digital elevation

346

model, the river is divided into two parts according to the rive gradient, as shown in Fig. 4a. In

347

the first river reach, the water elevation changes from 2742 m to 1800 m with a river length of

348

110 km and a large water level drop of 8.6 m per kilometer. Thus, the cross sections in this

349

reach are set densely at a small interval of 0.05 km. In the second river reach, the elevation

350

changes from 1800 m to 150 m in a river length of 370 km, corresponding to a smaller water

351

level drop of 4.5 m per kilometer. The cross sections are then set at a larger interval of 1 km.

352

A total of 2764 cross sections along the river are finally determined.

353

Based on the breaching hydrograph at the dam site given by DABA and the input

354

topography information of the studying river reach, the flooding routing process in this study

355

is simulated using a one-dimensional hydrodynamic model HEC-RAS (USACE, 2016).

356 357

5. Results of barrier lake breaching and flood routing analyses

358

5.1. Barrier lake bursting on 19 October 2018

359

The Gyalha landslide dam was overtopped naturally at 13:30 on 19 October 2018. The

360

inflow rate into the barrier lake was 2222 m3/s by then. A small initial channel with a gentle 15

361

slope angle is assumed in the DABA model as shown in Table 1. The hydrograph at the dam

362

site, simulated by using DABA, is presented in Fig. 18. The simulated peak discharge at the

363

dam site is 31,685 m3/s, which is almost the same as the reported value of 32,000 m3/s (Jin,

364

2019). Based on the flood routing results, the time at the peak outflow at the Gyalha landslide

365

dam site is determined at 19:11 on 19 October. Figure 18 also illustrates the attenuation of the

366

peak discharge along the river. At Dexing Hydrological Station, the predicted peak discharge

367

is 25,741 m3/s, occurring at 23:37 on 19 October, whereas the reported maximum discharge is

368

23,400 m3/s at 23:40 on 19 October (China Daily, 2018). The simulated peak flow rate

369

decreases to about 21,216 m3/s at Tuting by 3:04 on 20 October, and to 11,235 m3/s at Pasighat

370

by 18:32 on 20 October. Moreover, at 7:00 on 20 October (i.e. about 17.5 hours after the dam

371

breach), the discharge at Dexing Hydrological Station dropped significantly to 6230 m3/s

372

(CCTV News, 2018b), while the predicted discharge at that time is 6635 m3/s, with a small

373

difference of 6.5%. By 7:00 on 20 October, the total water volume passing Dexing was 550

374

million m3. After deducting 40 million m3 from other tributaries, approximately 510 million

375

m3 of water has been drained (CCTV News, 2018b). The reservoir level was lowered by about

376

56 m and the threat of the flooding was dismissed.

377

Figure 19 further shows changes of the relative water level at Tuting. The water level

378

started to decrease at about 11:30 on 17 October, and the river flow diminished at 2:30 on 18

379

October with limited flow from other tributaries (i.e. about 21.5 hours after the formation of

380

the Gyalha landslide dam) (SANDRP, 2018). It dramatically rose up to about 15 m at 2:30 on

381

20 October and returned to the base flow on the early morning of 21 October. The predicted

382

maximum water level rise at Tuting is 16.4 m, occurring at 3:04 am on 20 October, which is

383

close to the observations.

384 385

16

386

5.2. Analysis of dam breaching scenarios with larger river inflow rates

387

The runoff of Yarlung Tsangpo shows a strong climate-dependent feature and mainly

388

concentrates in the summer (Jia et al., 2008; Liu et al., 2011). Summer is also the time for

389

glacier melting (Jansson et al., 2003). There are a large number of glaciers in the lower Yarlung

390

Tsangpo Basin where the studying area is located. Hence the meltwater from the glaciers in

391

summer is of particular importance (Yao et al., 2010; Ren et al., 2018). If ice avalanches

392

develop in the glaciers on the high mountains of Sedongpu, and the landslide debris blocks the

393

Yarlung Tsangpo at a time of large runoff, the damming may cause more catastrophic

394

consequences. Therefore, it is critical to explore impacts of landslide lake bursts under possibly

395

large runoff conditions, which directly affect the duration and development of the dam

396

breaching process, the final breach size, and the subsequent flood routing process.

397

Figure 20 presents the runoff of the latest large flood in the Yarlung Tsangpo in 1998

398

and the intra-annual distribution of runoff from 1960 to 2009 at Nuxia Hydrological Station

399

(Liu, 1999; Liu et al., 2011). The maximum flow rate in 1998 is 13,487 m3/s, which is 2.6 times

400

the maximum monthly mean flow rate of 5119 m3/s in August. In this study, the maximum

401

flow rate of 13,487 m3/s and the maximum monthly mean flow rate of 5119 m3/s are used to

402

represent the possibly large runoffs of the downstream Yarlung Tsangpo. Table 3 shows results

403

of the dam breaching simulations. Taking the Gyalha landslide dam as a benchmark, with a

404

larger inflow rate, the time of overtopping decreases dramatically. The time for evacuating

405

downstream residents also shortens from 62.5 h to 27.1 h, and even to only 10.3 h when the

406

inflow rate increases from 2222 m3/s to 5119 m3/s, and further to 13,487 m3/s. Hence the dam

407

breaching risk will intensify when the river blockage occurs in the rainy season.

408

When the dam is overtopped naturally, the subsequent peak flow increases obviously

409

with the increasing inflow rate. Comparing the conditions of the Gyalha landslide and the

410

maximum flood flow in 1998, the dam-breaching peak flow rate is nearly doubled when the

17

411

inflow is increased by five times. The total breach duration and the erosion degree of the dam

412

body also differ greatly under the three different inflow conditions. With a larger inflow rate,

413

the breach initiation time becomes shorter, the materials erode more rapidly, and the dam

414

breaching process develops very quickly. On the cross section, when the initial runoff is larger,

415

the breach process goes into phase II and phase III (Fig. 15a) more quickly, causing a

416

significant enlargement of breach size in both horizontal and vertical directions. On the

417

longitudinal section, the breaching process will go into phase III (Fig. 15b) sooner, the erosion

418

of the downstream slope will develop more dramatically, and the dam crest elevation will lower

419

sharply. It can be found that in the case of the maximum runoff of flood in 1998, the landslide

420

dam can be completely eroded in 13.8 h.

421

Figure 21 shows results of the flood routing process along the river. The trends of the

422

attenuation of peak discharge are quite similar for the three inflow conditions. However, the

423

magnitude and arrival time of the peak discharge along the river are quite different. Taking the

424

Gyalha landslide dam as a benchmark, the peak discharge along the river is 23% larger at the

425

maximum monthly runoff condition and increases dramatically by 83% at the 1998 flood flow

426

condition. The corresponding average arrival times of the peak flow at Dexing, Tuting and

427

Pasighat are about 2.8 h and 1.4 h earlier, respectively, which shortens the time for emergency

428

actions. Thus, it is suggested to install a monitoring and warning system for the Sedongpu

429

Basin to assist emergency management.

430 431

6. Summary and conclusions

432

This paper reports the formation and breach of the Gyalha landslide dam on Yarlung

433

Tsangpo in 2018, and a comprehensive analysis of a typical mountain landslide-barrier lake-

434

dam breach-flooding disaster chain. The dam breaching hydrograph is simulated using an

435

overtopping-erosion based dam breaching model, DABA, and the flood routing in the 460 km-

18

436

long Grand Canyon is analyzed using one-dimensional Saint-Venant equations. The simulated

437

peak discharges at the dam site and Dexing are 31,685 and 25,741 m3/s, respectively, which

438

agree well with observed values of 32,000 and 23,400 m3/s, respectively.

439

Investigations of the dam breach and flood routing process under two larger plausible

440

inflow conditions are further conducted: the maximum flow of the flood in 1998 and the annual

441

maximum mean monthly runoff. Under a larger inflow, the duration of dam breaching

442

decreases significantly and the scour and erosion of the dam body become more rapid. When

443

the inflow rate is increased by five times, the peak discharge can be doubled.

444

The Sedongpu Basin has 12 well developed glaciers, where landslides induced by ice

445

melting occurred frequently and blocked the Yarlung Tsangpo in recent years. Once an ice-soil

446

mixture dam breaks, the flood routing process can be very fast in the Yarlung Tsangpo Grand

447

Canyon. Thus, a long-time monitoring and warning system is suggested to assist risk

448

management in the region.

449 450

Acknowledgements

451

This research is supported by the National Key Technologies Research and

452

Development Program of the Ministry of Science and Technology of China (Project No.

453

2018YFC1508600).

454 455

CRediT author statement

456

Chen Chen: Data curation, Formal analysis, Writing- Original draft preparation Limin

457

Zhang: Conceptualization, Methodology, Funding acquisition, Supervision, Writing - review

458

& editing Te Xiao: Software, Validation, Investigation Jian He: Software, Validation,

459

Investigation.

460

19

461

Declaration of competing interest

462

None.

463 464

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617 618

List of table captions

619

Table 1 Input parameters for the analysis of dam breaching and flood routing

620

Table 2 Blockage of the Yarlung Tsangpo in the Sedongpu Basin in the recent history

621

Table 3 Results of dam breaching and flood routing analysis

622 623

List of figure captions

624

Fig. 1. Location of the Gyalha landslide dam and the river reach for flood routing simulation.

625

Fig. 2. The Gyalha landslide dam: (a) formation; and (b) after breaching (Photo credit: Xinhua

626 627 628 629 630

Net). Fig. 3. Inundation of the Gyalha village: (a) before and (b) after formation of the Gyalha landslide dam (Photo credit: Xinhua Net). Fig. 4. Profile of the studying river reach: (a) drop of water surface elevation with distance from the dam site; and (b) three typical cross-sections.

631

Fig. 5. Profile of the Sedongpu basin (modified from Tong et al., 2018).

632

Fig. 6. A planar map of the Gyalha landslide dam.

633

Fig. 7. Materials at landslide dams: (a) Yigong landslide dam (Photo credit: Xu et al., 2012);

634 635

and (b) Gyalha landslide dam (Photo credit: Xinhua Net). Fig. 8. Cross sections of the Gyalha landslide dam: (a) along the river; (b) across the river.

26

636 637 638 639 640 641 642

Fig. 9. Changes of annual mean air temperature of the Yarlung Tsangpo basin from 1957 to 2004. Fig. 10. Glacier reduction around the Gyalha landslide dam: (a) on 19 Jan. 2017; and (b) on 19 Jan. 2018 (Source: Sentinel-2). Fig. 11. Annual average flow rate from 1970 to 2013 and the long term intra-annual distribution of runoff from 1960 to 2009 at Nuxia Hydrological Station. Fig. 12. Examples of landslides at the Sedongpu Basin: (a) on 18 Feb. 2017; (b) on 05 Nov.

643

2017; (c) on 08 June 2018; and (d) on 31 Oct. 2018 (Source: Sentinel-2).

644

Fig. 13. Lake storage capacity-water surface elevation curve for the barrier lake.

645

Fig. 14. The barrier lake behind the landslide dam.

646

Fig. 15. Breach enlargement process: (a) cross section; and (b) longitudinal section.

647

Fig. 16. Soil erodibility parameters: (a) particle size distributions of materials from Gyalha

648

landslide dam and benchmark materials at Yigong landslide dam; (b) assumed Kd at

649

Gyalha landslide dam; and (c) assumed τc at Gyalha landslide dam.

650

Fig. 17. Sensitivity of soil erodibility to the overtopping process: (a) Kd is changed by ±10%

651

and ±20% while τc remains the same; (b) τc is changed by ±10% and ±20% while Kd is

652

kept the same.

653

Fig. 18. Observed and simulated hydrographs along the Yarlung Tsangpo.

654

Fig. 19. Observed and simulated rises of the relative water level at Tuting.

655

Fig. 20. The long period mean monthly runoff from 1960 to 2009 and the 1998 flood at the

656 657 658

Nuxia Hydrological Station. Fig. 21. Simulated flooding process along the Yarlung Tsangpo with different runoffs: (a) hydrographs; and (b) peak discharge with distance.

659

27

660 661 662

Fig. 1. Location of the Gyalha landslide dam and the river reach for flood routing simulation.

663

28

(a)

664 (b)

665 666 667 668

Fig. 2. The Gyalha landslide dam: (a) formation; and (b) after breaching (Photo credit: Xinhua Net).

29

(a)

669 (b)

670 671 672 673

Fig. 3. Inundation of the Gyalha Village: (a) before and (b) after formation of the Gyalha landslide dam (Photo credit: Xinhua Net).

674

30

(a)

675 676

Relative height (m)

200 0 400

B

200 0 400 200 0 -900

677

(b)

A

C -600

-300

0

300

600

900

1200

Distance from the river center (m)

678 679 680 681

Fig. 4. Profile of the studying river reach: (a) drop of water surface elevation with distance from the dam site; and (b) three typical cross-sections.

682

31

7000 6600

Rocks andofglaciers Collapse rocks and are easy to collaspe glaciers in the upper along the upper steep part steep part

Elevation (m) (m) Elevation

6200 5800 5400 5000

Steep

4600

Accumulation massive Massive looseof materials accumulat in theinmiddle loose materials the middlegentle gentlearea area

Steep

4200

Glacier-debris Glacier-debris flow flowison triggered and slide the relatively steepalong the relative steep terrain terrain

3800 3400 Proterozoic marble and gneiss (Pt)

3000

Gentle Gentle Relative-steep Relatively-steep

2600 0

683

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 Distance (m)

684 685 686

Fig. 5. Profile of the Sedongpu Basin (modified from Tong et al., 2018).

687

32

Source zone

Passage and entrainment zone

Gyalha landslide dam

688 689 690 691

Fig. 6. A planar map of the Gyalha landslide dam.

692

33

(a)

693 (b)

694 695 696 697 698

Fig. 7. Materials at landslide dams: (a) Yigong landslide dam (Photo credit: Xu et al., 2012); and (b) Gyalha landslide dam (Photo credit: Xinhua Net).

699

34

2900

(a) Elevation (m)

2850

300 m

Yarlung Tsangpo

2800

2837 m

79 m River bed

2750

Gyalha landslide dam

2758 m

1.9 km

2700 0

500

700

1000 Distance (m)

1500

2000

3200

(b) Elevation (m)

3100

Namcha Barwa

3000

Sedongpu Basin

2900

2837 m

2800

79 m

Proterozoic marble and gneiss

2700 0

701

Gyalha landslide dam

500

1000

1500

Zhibai HPGbearing gneiss 2000

2500

Distance (m)

702 703

Fig. 8. Cross sections of the Gyalha landslide dam: (a) along the river; (b) across the river.

704

35

Annual mean temperature (°C)

9

8

7

6

5 1957

705

1967

Year

Average mean temperature (°C)

1961-1970 1971-1980 1981-1990 1991-2000 2001-2004

7.2 7.3 7.44 7.78 8.02

1977

1987

1997

2007

Year

706 707 708

Fig. 9. Changes of annual mean air temperature of the Yarlung Tsangpo Basin from 1957 to 2004.

709

36

2017/01/19

(a)

Gyalha landside dam

710 711

2018/01/19

(b)

Gyalha landside dam

712 713

37

714 715

Fig. 10. Glacier reduction around the Gyalha landslide dam: (a) on 19 Jan. 2017; and (b) on 19 Jan. 2018 (Source: Sentinel-2).

38

Month 1

2

3

4

5

6

7

8

9

10

11

12 50

Annual average flow rate (m3/s)

Intra-annual distribution of runoff

2500

Annual average flow rate

40

2000 30 1500 20 1000 10

500

0

0 1970

716 717 718

Intra-annual distribution of runoff (%)

3000

1975

1980

1985

1990

1995

2000

2005

2010

Year

Fig. 11. Annual average flow rate from 1970 to 2013 and the long term intra-annual distribution of runoff from 1960 to 2009 at Nuxia Hydrological Station.

719

39

Sedongpu Valley Sedongpu Valley

(a)

(b)

720

Sedongpu Valley Sedongpu Valley

(c)

(d)

721 722 723 724 725

Fig. 12. Examples of landslides at the Sedongpu Basin: (a) on 18 Feb. 2017; (b) on 05 Nov. 2017; (c) on 08 June 2018; and (d) on 31 Oct. 2018 (Source: Sentinel-2).

726

40

Lake surface elevation (m)

2910 2880 Crest of the Gyalha landslide dam

2850 2820 2790

River bed 2760 0 727

2

4 6 Storage capacity (108 m3)

8

10

728 729

Fig. 13. Lake storage capacity-water surface elevation curve for the barrier lake.

730

41

N

94°58'0"E

29°42'0"N

29°44'0"N

Gyalha landslide dam

29°38'0"N

Service Layer Credits: Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID,

0 731

2.5 94°54'0"E

5

29°38'0"N

29°40'0"N

29°40'0"N

Gyalha Village

29°44'0"N

94°56'0"E

29°42'0"N

94°54'0"E

km 94°56'0"E

94°58'0"E

732 733

Fig. 14. The barrier lake behind the landslide dam.

734

42

Cross-section Phase Ⅰ

αc αc

735

αc

Cross-section Phase Ⅲ

αc

(a)

Cross-section Phase Ⅲ

Cross-section Phase Ⅱ

736

βf

737

(b)

βf

βf

Longitudinal-section Phase Ⅲ

βf

βf

βf

βf

Longitudinal-section Phase Ⅱ

Longitudinal-section Phase Ⅰ

738 739 740

Fig. 15. Breach enlargement process: (a) cross section; and (b) longitudinal section.

741

43

Percentage passing by weight (%)

100

Materials of Gyalha from Liu et al., 2019 Materials of Gylha from Cai, 2019 Materials of Yigong from Hu et al. 2015 Materials of Yigong from Wang et al. 2017 Materials of Yigong from Wang et al. 2016 Materials of Yigong from Kang et al. 2016

80

60

40

20

(a) 0 0.01

742

0.1

1 Particle diameter (mm)

10

100

743 Coefficient of erodibility, Kd (mm3/N-s) 1 10 100 1000 10000

Critical erosive shear stress, Tc (Pa) 1 10 100 1000 10000

0

0

(b)

(c) 20 Depth (m)

Depth (m)

20

40

60

80

744

100

40

60 Gyalha (field) from Cai, 2019 Gyalha (field) from Liu et al., 2019 Gyalha used in the study

80

100

Gyalha (field) from Cai, 2019 Gyalha (field) from Liu et al., 2019 Gyalha used in the study

745

44

746 747 748

Fig. 16. Soil erodibility parameters: (a) particle size distributions of materials from Gyalha landslide dam and benchmark materials at Yigong landslide dam; (b) assumed Kd at Gyalha landslide dam; and (c) assumed τc at Gyalha landslide dam. 4

The Gyalha landslide dam Kkd d increased by 10% Kkd d increased by 20% Kkd d decreased by 10% Kkd decreased by 20% d

Discharge (104 m3/s)

(a) 3

2

1

0 13:30 19 Oct

13:30 20 Oct

01:30 20 Oct

01:30 21 Oct

Time

749 4

The Gyalha landslide dam ττc c increased by 10% ττc c increased by 20% ττc c decreased by 10%

Discharge (104 m3/s)

(b) 3

ττc c decreased by 20% 2

1

0 13:30 19 Oct

750

13:30 20 Oct

01:30 20 Oct

01:30 21 Oct

Time

751 752 753 754

Fig. 17. Sensitivity of soil erodibility to the overtopping process: (a) Kd is changed by ±10% and ±20% while τc remains the same; (b) τc is changed by ±10% and ±20% while Kd is kept the same.

755

45

4

Reported peak discharges at the dam site and Dexing Another reported discharge at Dexing

Discharge (104 m3/s)

Dam site 3

Dexing Tuting

2

Pasighat 1

0 13:30 19 Oct

756

01:30 20 Oct

13:30 20 Oct

01:30 21 Oct

13:30 21 Oct

01:30 22 Oct

Time

757 758

Fig. 18. Observed and simulated hydrographs along the Yarlung Tsangpo.

759

46

20

Relative water level (m)

Observed water level Simulated water level 15

10

Gyalha landslide occurred Overtopping started

5

0 02:30 17 Oct

760

02:30 18 Oct

02:30 19 Oct

02:30 20 Oct

02:30 21 Oct

02:30 22 Oct

Time

761 762

Fig. 19. Observed and simulated rises of the relative water level at Tuting.

763

47

1

2

3

4

5

Month of a year 6 7

8

9

10

11

12

15000 Intra-annual runoff of flood in 1998 Average monthly intra-annual distribution of runoff from 1960 to 2009

Runoff (m3/s)

12000

9000

6000

3000

0 0 15th Jun.

764

15 30th

30 15th Jul.

30th 45

15th 60 Aug.

30th 75

15th 90 Sept.

30th 105

Month of 1998

765 766 767

Fig. 20. The long period mean monthly runoff from 1960 to 2009 and the 1998 flood at the Nuxia Hydrological Station.

768

48

7

Discharge (104 m3/s)

6

Maximum runoff of the 1998 flood

(a)

Maximum monthly runoff of a year

Dam site

Runoff of the Gyalha landslide

5

Dexing

4

Tuting 3

Pasighat

2 1 0

13:30 19 Oct

13:30 21 Oct

01:30 21 Oct

13:30 20 Oct

01:30 20 Oct

Time

769 7

Maximum runoff of the 1998 flood Maximum monthly runoff of a year Runoff of the Gyalha landslide

Peak discharge (104 m3/s)

(b) 6 Dexing

5

Tuting

4 3

Pasighat

2 1 0 0

770

100

200

300

400

500

Distance from the Gyalha landslide dam site (km)

771 772 773 774

Fig. 21. Simulated flooding process along the Yarlung Tsangpo with different runoffs: (a) hydrographs; and (b) peak discharge with distance.

775 776

49

777 778

Table 1 Input parameters for the analysis of dam breaching and flood routing. Analysis Dam breaching

Flood routing

Parameter Overtopping time, T0 Dam bottom elevation, Db (m) Dam crest elevation, Dt (m) Water level, Hw (m) Inflow rate, Qin (m3/s) Initial breach top width, Wt (m) Initial breach bottom width, Wb (m) Initial breach depth, Hb (m) Critical breach side slope, αc (º) Initial dam crest length, Dc (m) Initial dam upstream length, Lu (m) Initial dam downstream length, Ld (m) Initial breach bottom gradient, Bg Critical dam downstream slope, βf (º) Manning’s coefficient for channel bed Manning’s coefficient for riverbank

Value 13:30, 19 Oct. 2018 2758 2837 2836.1 2222 5 2 1 45 300 720 880 0.006 30 0.06 0.12

779

50

780 781

Table 2 Blockage of the Yarlung Tsangpo in the Sedongpu Basin in the recent history. Time (year/month/day)

Possible cause

Blocking Yarlung Tsangpo?

2018/10/29 2018/10/18 2018/10/17 2018/07/26 2018/01 2017/12/21 2017/11/18 2017/11/03 2017/10/22 2014 1968 1950

Ice-avalanche Ice-avalanche Ice-avalanche Debris-flow Debris-flow Ice-avalanche The Ms 6.9 Nyingchi earthquake Debris-flow − Ice-avalanche Ice-avalanche The Ms 8.6 Modog earthquake

Blocked Blocked Blocked Partially blocked Blocked Blocked Blocked Not blocked Blocked Blocked Blocked Blocked

782

51

783 784

Table 3 Results of dam breaching and flood routing analysis. Simulation results

Inflow rate, Qin (m3/s) Time to fill barrier lake, Tf (h) Breach initiation duration, Ti (h) Breach development duration, Td (h) Total breaching duration, Tt (h) Final breach depth, Hb (m) Final breach top width, Wt (m) Final breach bottom width, Wb (m) Peak flow rate at dam site (m3/s) Peak flow rate at Dexing (m3/s) Peak flow rate at Tuting (m3/s) Peak flow rate at Pasighat (m3/s)

Runoff conditions Actual runoff at the Gyalha landslide dam in 2018 2222 62.5 3.8 21.2 25.0 65.7 280.5 149.2 31685 25268 20848 11093

Average monthly runoff in August

Maximum runoff of the flood in 1998

5119 27.1 3.1 15.9 19.0 71.2 305.0 162.7 39005 31401 25525 13529

13487 10.3 2.2 11.6 13.8 79.0 349.8 191.8 57257 46775 37619 20763

785 786

52

787 788

Declaration of interest statement

789 790

None

791 792

53

793 794

Highlights: 

which breached within 2 days with a peak flood over 30,000 m3/s.

795 796

A large landslide dam formed in October 2018 in the Yarlung Tsangpo Grand Canyon,



797

This paper presents a disaster chain of landslide-barrier lake-dam breaching-river flooding in the 460 km canyon.

798



An erosion-based numerical model was used to simulate the dam-breaching process.

799



Plausible dam breaching scenarios at larger river flows are analysed to help risk

800

management.

801 802

54