Experimental and Numerical insight into Scale effects in granUlaR slides in the mountainous Environment

Project Information

ENSURE

Grant agreement ID: 101029080

DOI

10.3030/101029080

Project terminated on 25 October 2023

EC signature date 13 March 2021

Start date 1 August 2021

End date 8 July 2024

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 224 933,76

EU contribution

€ 224 933,76

Coordinated by

THE UNIVERSITY OF NOTTINGHAM
United Kingdom

Periodic Reporting for period 1 - ENSURE (Experimental and Numerical insight into Scale effects in granUlaR slides in the mountainous Environment)

Reporting period: 2021-08-01 to 2023-07-31

Summary of the context and overall objectives of the project

The European Environment Agency listed 212 fatalities from catastrophic landslides in 1998-2009, which directly resulted in total costs of €551 million. European mountainous regions will face an increased rate of landslide activity due to climate change. Landslides incorporate a wide spectrum of sediment-fluid mixtures between hyper-concentrated flows and dry rockfalls. Landslides are often modelled as granular slides, representing rock avalanches, rockslides and cliff collapses. A granular slide can be defined as a gravity-driven rapid movement of discrete particles assembly consisting of sediment and air, and sometimes also water, relative to its surroundings. This ENSURE project focuses on these granular slides which may cause devastation and human losses in their run-out path.

Well defined and controlled laboratory experiments are often used by researchers to investigate granular slides. However, such experiments have limitations: small-scale laboratory experimental results cannot currently be extrapolated to reliably predict larger-scale natural events. This is due to so-called scale effects resulting in different granular slide characteristics at different geometric scales (sizes). For example, the slide runout distance changes if the slide size is reduced, resulting in a dangerous underestimation. The physical reasons behind scale effects are currently not well understood. A better understanding and quantification of scale effects would enable (i) laboratory experiments to be conducted under conditions where scale effects are negligible or (ii) the removal of scale effects from laboratory experiments in the upscaling process to reliable predict real granular slides in nature. The detailed analysis in a cost-effective manner of the relevance of scale effects in granular slides with a better understanding of the physical processes is therefore a central requirement on the route towards safer mountainous urbanisation and more efficient landslide disaster mitigation strategies in the future.

This project aimed to address these scale effects challenges with the following research objectives (RO):
RO1: To develop a large experimental data base by conducting laboratory granular slide scale series experiments under relevant conditions to understand and quantify scale effects.
RO2: Complementing the work carried out under RO1, develop a fully calibrated and validated numerical simulation model by using CFDEM Coupling to ensure that scale effects are captured and complement the data base with additional slide scenarios.
RO3: Utilising the database compiled in RO1 and RO2, perform an application-based feasibility evaluation to establish a correct upscaling method accounting for scale effects by linking and validating laboratory-scale properties with real field-scale events to ensure the predictive capability of laboratory experiments.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Dr Sazeda Begam (research fellow) modified an existing slide ramp previously used for 2D experiments by Kesseler et al. (2020) (Grain Reynolds number scale effects in dry granular slides. J. Geophys. Res. 125(1), 1-19) to be used for 3D experiments by expanding the ramp sidewards, modifying the release mechanism and by updating the measurement system e.g. with two new laser distance sensors (LDSs) to measure the slide profiles and two cameras (Figure 1a). Dr Begam conducted then novel laboratory experiments under various experimental conditions (3 different scales, dry and moist slides, different slide volumes, smooth and rough ramp surface) resulting in a new large experimental data base (Figure 1). All parameters were carefully scaled between the different scales and all experiments were conducted 3 times to investigate repeatability and to work with averaged values. The granular material parameters (internal and ramp bed friction angles) were also quantified. The slide profiles were measured with two LDSs at two different locations on the ramp and recorded with a camera and the slide parameters after deposition were analysed with the photogrammetry software Agisoft based on still pictures and manual measurements of some key deposition features. Whilst the dry granular slides resulted in consistent results, the moist slide experiments were less consistent such that only the dry experimental results were further processed (RO1). Dr Begam also obtained access and got familiar with a supercomputer and conducted initial tests with the software LIGGHTS-DEM 3.8 coupled via CFDEM coupling with OpenFOAM to work towards RO2. However, this work was interrupted by the long-term absence of Dr Begam such that RO3 could not be achieved during the fellowship period.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The experimental data set at different sizes is unique and pushes the state-of-the-art in scale effects investigation in granular slides. They are valuable for the quantification of scale effects and allowed to analyse the following parameters at 3 different scales, different slide volumes and for smooth and rough ramp surfaces:
• Slide profiles including its thicknesses at two locations on the ramp
• Slide front and back velocities
• Slide lateral spread angles on the ramp
• Slide back and maximum slide front runout distances
• Slide deposit lateral expansion
• Maximum slide deposit thickness
• Repeatability of all these parameters
Many of these parameters showed clear “scale effects” for the smooth ramp surfaces, i.e. results such as the maximum runout distance, changed in dimensionless form with the size of the experiment. However, this behaviour significantly changes for the rough ramp surfaces where many parameters show smaller scale effects. This confirms and expands our previous findings based on 2D experiments (e.g. Kesseler et al., 2020) and some of these new findings were presented at the EGU conference in spring 2022 and in several internal research seminars. A peer-review article is also in preparation. This article will improve our physical understanding of scale effects in granular slides, help to provide an upscaling method accounting for scale effects to ensure the predictive capability of laboratory experiments and provide a valuable data base for future studies.

The work carried out addressed issues related to the environment and some studies suggest an increased rate of landslide activities due to climate change. The quantification of scale effects is a key aspect of landslide modelling and enables laboratory experiments to be conducted under conditions where scale effects are negligible or enables the removal of scale effects from laboratory experiments in the upscaling process to reliable predict real granular slides in nature. The work also results in a better understanding of the physical processes and is therefore a central requirement for safer mountainous urbanisation and more efficient landslide disaster mitigation strategies in the future. Potential users of the project results are other researchers including numerical modellers requiring high-quality laboratory data, engineers, earth scientist and urban planners (urbanisation plans). In the longer term, the work may also have an impact on policy making (e.g. related to natural disaster mitigation strategies such as settlement planning in EU countries and beyond).

(a) Medium scale experiment and (b,c,d) slide deposit small, medium and large scale on smooth ramp