Glacial burst triggered by triangular wedge collapse: a study from Trisul Mountain near Ronti glacier valley

Figures & data

Figure 1. Standard False Color Composite (FCC) map of the study area highlighting impact location.

Figure 2. Image representing slope and impact location (Google Earth Pro image of 11-Feb-2021).

Figure 3. Wedge structure captured by Sentinel-2 MSI.

Table 1. Details of offset-tracking pairs from Sentinel-1A.

Sentinel-1A Level-1 Datasets
Timeline	Master Data	Slave Data	Microwave Band/Frequency	Data Type	Beam Mode / Polarization	Spatial Resolution/ Width	Frame/Path/ Flight Direction	Temporal Baseline (Days)
Pre-Monsoon	16 Mar 2020	2 July 2020	C-Band/ 5.405 GHz	GRD	IW/VV + VH	5m × 20m/250 Km	92/129/Ascending	108
Monsoon	2 July 2020	18 Oct 2020
Post-Monsoon	18 Oct 2020	3 Feb 2021
Annual (2020-2021)	16 Mar 2020	3 Feb 2021						324
ALOS PALSAR Dataset
Data	Date	Spatial Resolution	Microwave Band/Frequency	Polarization		Beam Mode/ Path	Frame/Flight Direction	Off-Nadir Angle
Hi-Res Terrain Corrected DEM	3 Sept 2007	12.5 m	L-Band/1.27 GHz	HH + HV		FBD/520	590/Ascending	34.3°

Figure 4. Flowchart indicating the adopted methods.

Figure 5. Sub-pixel offset tracking process flow for surficial displacement using Sentinel-1 GRD wide swath imagery.

Figure 6. Generalized physical model of the wedge structure found in the study area.

Table 2. Inputs to analysis of slope stability.

Rock properties (Intact)
Specific weight:	0.027 MN/m^3
Modulus of Elasticity:	27.65 GPa
mi:	15.934
s:	1
a:	0.5
Poisson’s Ratio:	0.30
Rock Mass Properties
Geological Strength Index (GSI): 52
Modulus of Elasticity:	0.009 MN/m^3
Cohesion:	1320 KPa
Tension:	124.61 KPa
Angle of Friction:	50.47°
Poisson’s Ratio:	0.30

Table 3. Inputs for numerical simulation of rock/ice fall.

Seeder properties	Values (Rock)	Values (Ice)
Units	100	100
Mass (kg)	550.6 million	45.85 million
Density (kg/m^3)	2753	917
Total Potential Energy (tera joules)	1019

Table 4. Stochastic parameters of rockfall.

Parameter	Horizontal velocity (rock/ice)	Coefficient of restitution
Standard deviation	0.01 m/s	0.05
Mean value	0.10 m/s	0.5
Relative max value	0.11 m/s	0.55
Relative min value	0.09 m/s	0.45

Figure 7. Map showing annual mobility of the glacial bodies in the Rishiganga and the Dhauliganga valley between 2020 and 2021.

Figure 8. Map showing annual (a) and seasonal (pre-monsoon: (b); monsoon: (c); post-monsoon: (d)) mobility of the Ronti glacier and wedge between 2020 and 2021.

Figure 9. Indicating the mobility trend of (a) trunk, (b-d) flange-I/II/III of Ronti glacier and (e) wedge respectively.

Figure 10. Kinematic analysis showing results for planar sliding (left) and wedge sliding (right) failure.

Table 5. Orientation of joint sets and rock slope.

Joint Set	Joint-1	Joint-2	Slope
Dip Direction/Dip	N205° (SW) / 34°	N278° (SW) / 42°	N190° / 55°

Figure 11. Result from limit equilibrium analysis of rock slope (at 20% CoV).

Figure 12. Result from Finite element SSR (left; at 20% CoV) and Rockfall analysis (right) of rock slope.

Table 6. Probabilistic back analysis results (LEM and FEM-SSR).

Method		LEM (Slope)			FEM-SSR (RS^2)
Coefficient of Variation (CoV)		10%	15%	20%	10%	15%	20%
Mean FOS	Bishop Simplified	1.255			1.05	1.05	1.07
	Janbu Simplified	1.192
	GLE/Morgenstern-Price	1.247
Standard Deviation FOS		0.168	0.258	0.377	0.140	0.223	0.321
Probability of Failure (PF)		3.6%	11%	17.6%	35.39%	41.56%	41.06%
	Normal	1.67	1.14	0.87
Reliability Index (RI)	Lognormal	1.82	1.21	0.89
	Cohesion (kPa)	1294.19	1280.09	1305.86	1307.11	1308.09	1307.11
At FOS = 1
	Angle of Internal friction (º)	42.87	42.80	42.87	46.60	47.23	46.99

Figure 13. Illustration for (a) BH, (b) TV and (c-d) TKE of rock and ice fall at mean, maximum, 75 percentiles and 95 percentiles respectively.

Data availability statement

The data used for the study has been collected from different agencies that can be assessed using the following links.

SENTINEL-2 MSI: https://scihub.copernicus.eu/dhus/#/home

ALOS PALSAR: https://search.asf.alaska.edu/#/?zoom=5.915&center=96.406,16.491&dataset=ALOS

RANDOLPH GLACIER INVENTORY: https://www.glims.org/RGI/rgi60_dl.html