Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya

Medwedeff, William; Clark, Marin K.; Zekkos, Dimitrios; West, A. Joshua; Chamlagain, Deepak

doi:10.1029/2021jf006240

Cited by 5 publications

(3 citation statements)

References 87 publications

(165 reference statements)

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“…The combined effects of fault-driven near-surface deformation and extreme ground shaking make this zone near the fault particularly susceptible to RSS and consequent slope failure. However, these aspects also require further investigations and seismic surveys, which have been used to assess hillslope strength 39,75 .…”

Section: Discussionmentioning

confidence: 99%

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Estimating weakening on hillslopes caused by strong earthquakes

Xi,

Tanyas,

Lombardo

et al. 2024

Commun Earth Environ

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The weakening of hillslopes during strong earthquakes increases landsliding rates in post-seismic periods. However, very few studies have addressed the amount of coseismic reduction in shear strength of hillslope materials. This makes estimation of post-seismic landslide susceptibility challenging. Here we propose a method to quantify the maximum shear-strength reduction expected on seismically disturbed hillslopes. We focus on a subset of the area affected by the 2008 Mw 7.9 Wenchuan, China earthquake. We combine physical and data-driven modeling approaches. First, we back-analyze shear-strength reduction at locations where post-seismic landslides occurred. Second, we regress the estimated shear-strength reduction against peak ground acceleration, local relief, and topographic position index to extrapolate the shear-strength reduction over the entire study area. Our results show a maximum of 60%–75% reduction in near-surface shear strength over a peak ground acceleration range of 0.5–0.9 g. Reduction percentages can be generalized using a data-driven model.

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Section: Discussionmentioning

confidence: 99%

“…To address these limitations, some methods coupling active geotechnical seismic surveys with outcrop-scale engineering desciptions/tests, which are used to classify rock masses based the geological strength index 34 , have been proposed 38,39 . However, even those methods fail to capture slope shear strength at a regional scale and thus do not address the RSS.…”

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confidence: 99%

Estimating weakening on hillslopes caused by strong earthquakes

Xi,

Tanyas,

Lombardo

et al. 2024

Commun Earth Environ

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“…The 2021 Melamchi Floods occurred in the Melamchi Khola Valley, where topography changes from more subdued in the south to high mountains in the north associated with a pronounced gradient in rock uplift and exhumation driven by Himalayan tectonics (Medwedeff et al, 2022). Approximately 6 years before the ood, the 2015 Mw7.8 Gorkha earthquake produced more than 1500 landslides in the catchment (Roback et al, 2018).…”

Section: Main Textmentioning

confidence: 99%

The 2021 Melamchi Flood: A massive erosional cascade in the Himalayan Mountains of central Nepal

Chen

Hollingsworth

Clark

et al. 2023

Preprint

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Large, sediment-laden floods in mountainous terrain can have disastrous consequences and play important roles in landscape evolution. These events often unfold as a series of interconnected processes, but understanding of such “hazard cascades” has been hampered by lack of quantitative data on sediment movement. Here, we use a time series of high-resolution satellite imagery to quantify erosion and aggradation during the 2021 Melamchi Khola Floods in the Himalaya of central Nepal, providing a unique sediment budget for such an event. Our analysis reveals massive headwater erosion via remobilization of Gorkha landslides, gullying, debris-flows, and incision of glacial deposits. Unlike many other high mountain floods, the widely distributed erosion suggests this event was not primarily driven by a single source, e.g., glacial lake or landslide dam failure. High sediment supply caused aggradation in a high-elevation, low-relief glacial valley and triggered catastrophic incision into associated ancient fills. As this material was transported downstream, it caused further riverbed incision that in turn resulted in failures of surrounding hillslopes. Further downstream, as river steepness diminished, the main channel in the lower basin was widened by 3-5-fold and aggraded by ~ 5–20 m. However, deposition in the Melamchi Khola was not enough to accommodate the vast amount of flood material, and over 70% was delivered from the Melamchi Khola to the downstream Indrawati basin. Our sediment budget provides rare insight into the chain of events involved in a massive flood and helps shed light on how such floods can magnify hazard and reshape the fluvial landscape.

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A Computationally Efficient Method to Determine the Probability of Rainfall-Triggered Cut Slope Failure Accounting for Upslope Hydrological Conditions

Robson,

Milledge,

Utili

et al. 2023

Rock Mech Rock Eng

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We present a new computationally efficient methodology to estimate the probability of rainfall-induced slope failure based on mechanical probabilistic slope stability analyses coupled with a hydrogeological model of the upslope area. The model accounts for: (1) uncertainty of geotechnical and hydrogeological parameters; (2) rainfall precipitation recorded over a period of time; and (3) the effect of upslope topography. The methodology provides two key outputs: (1) time-varying conditional probability of slope failure; and (2) an estimate of the absolute frequency of slope failure over any time period of interest. The methodology consists of the following steps: first, characterising the uncertainty of the slope geomaterial strength parameters; second, performing limit equilibrium method stability analyses for the realisations of the geomaterial strength parameters required to calculate the slope probability of failure by a Monte Carlo Simulation. The stability analyses are performed for various phreatic surface heights. These phreatic surfaces are then matched to a phreatic surface time series obtained from the 1D Hillslope-Storage Boussinesq model run for the upslope area to generate Factor of Safety (FoS) time series. A time-varying conditional probability of failure and an absolute frequency of slope failure can then be estimated from these FoS time series. We demonstrate this methodology on a road slope cutting in Nepal where geotechnical tests are not readily conducted. We believe this methodology improves the reliability of slope safety estimates where site investigation is not possible. Also, the methodology enables practitioners to avoid making unrealistic assumptions on the hydrological input. Finally, we find that the time-varying failure probability shows marked variations over time as a result of the monsoon wet–dry weather.

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Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya

Cited by 5 publications

References 87 publications

Estimating weakening on hillslopes caused by strong earthquakes

Estimating weakening on hillslopes caused by strong earthquakes

The 2021 Melamchi Flood: A massive erosional cascade in the Himalayan Mountains of central Nepal

A Computationally Efficient Method to Determine the Probability of Rainfall-Triggered Cut Slope Failure Accounting for Upslope Hydrological Conditions

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