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Predicting debris flow pathways using volume-based thresholds for effective risk assessment

2025-01-21  |   Editor : houxue2018  
Category : News

Abstract

Investigating the preferential flow path of a debris flow is crucial for quantifying the risk and developing mitigation strategies. Here, we examined 66 debris flows from the Western Ghats in India employing Rapid Mass Movement Simulation (RAMMS)::Debris Flow software to understand the kinematics of run-out. Our analysis revealed that the debris flow run-out in the study area follow two main routes: 60 along the existing stream channels (SC) and six following the steepest hill slope (SH). We further simulated these debris flows to identify their drivers, and derived a threshold that distinguishes between SC and SH-type debris flows. Our results indicate that the debris flow volumes greater than 7072 cu. m is SH-type, whereas those with smaller volumes are more likely to follow SC paths. The model’s accuracy was validated against field observations, achieving a success rate of 93% for SH-type flows and 85% for SC.

Content

The run-out characteristics of catastrophic debris flows of the study area have been assessed using a computer program called Rapid Mass Movements Simulation (RAMMS):Debris Flow. Previously, RAMMS has been successfully used for modeling the run-out scenarios of a few independent slides in the Western Ghats. This computer program predicts the flow path, distance, and velocities based on slope, soil and depleting mass conditions. The simulated output from the model predicts the slope-parallel velocities and flow heights using depth-averaged equations26. The model requires basic input parameters such as initial release volume and a digital elevation model (DEM). Two types of release information are available for use in the model: block release and hydrograph. This study adopted the block release method in this study, as the data availability limits the use of the hydrograph model. The input DEM used is ALOS PALSAR with 12.5 m spatial resolution, which can be freely downloaded (www.asf.alaska.edu/). RAMMS uses the Voellmy-fluid friction model, which is controlled by two friction parameters: dry-Coulomb type friction (μ) that scales with normal stress, and a velocity-squared drag or viscous-turbulent friction (ξ). Dry-Coulomb type friction ranges between 0.01 and 0.2, and viscous-turbulent friction between 200 and 1100 m/s2.

These rheological parameters are estimated through a trial and error approach such that the simulation best fits the debris flow footprint (observed best-fit was utilized) whereas the volume estimation is based on the soil thickness in the scarp area, given as input release depth (m) in RAMMS. This helps in identifying the drivers of each long run-out debris flow.

The results show that the release volume is crucial in determining the run-out path. The debris flow with a higher release volume than the threshold value (7072 cu. m) tends to follow SH-type. The high energy associated with the material volume might have caused high velocity for SH-type in contrast to SC-type that follows a flow direction path with low release volume. The high release volume makes SH debris flow more hazardous than SC. The threshold derived in this study to determine the path of debris flow can be used to classify them and assess their potential impacts on communities residing in areas with different susceptibility levels. Hence, this threshold can be employed to create landslide risk maps. The results of this study establish a threshold volume of 7072 cu. m to distinguish between SC and SH paths in debris flows. This threshold, based on RAMMS modeling of 66 debris flows in the Western Ghats, allows for predicting flow paths using only terrain data, such as digital elevation models (DEMs), rather than running computationally intensive simulations for each event.

Although RAMMS simulations can predict flow paths accurately, our threshold provides a practical alternative, particularly valuable for local decision-makers who may lack expertise in specialized modeling software. By using this volume threshold, practitioners can quickly determine the likely flow path based only on debris volume, supporting cost-effective and timely risk assessment and planning. Thus, this threshold-based approach serves as a useful tool in hazard mitigation, enabling regions with limited resources to assess debris-flow dynamics and plan accordingly without extensive computational demands. However, the creation of such maps is beyond the scope of this study, and therefore warrants further research. Attempts to create susceptibility maps with run-out area were attempted earlier, but its reliability in comparison with the existing susceptibility maps need to be validated in areas like the Western Ghats. Based on the outcomes, it should be noted that the RAMMS model exclusively takes into account the volume and material properties of the landslides and neglects other significant influencing factors. The simulation of the narrow run-out path for SC-type debris flow using 12.5 m elevation data is also challenging and caused over-prediction of debris flow area.

Sources:

npj Natural Hazards

https://www.nature.com/articles/s44304-024-00055-2 .

Provided by the IKCEST Disaster Risk Reduction Knowledge Service System

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