Grain-size segregation and levee formation in geophysical mass flows

Particle size segregation can have a significant feedback on the motion of many hazardous geophysical mass flows such as debris flows, dense pyroclastic flows and snow avalanches. This paper develops a new depth-averaged theory for segregation that can easily be incorporated into the existing depth-averaged structure of typical models of geophysical mass flows. The theory is derived by depth-averaging the segregation-remixing equation for a bi-disperse mixture of large and small particles and assuming that (i) the avalanche is always inversely graded and (ii) there is a linear downslope velocity profile through the avalanche depth. Remarkably, the resulting ‘large particle transport equation’ is very closely related to the segregation equation from which it is derived. Large particles are preferentially transported towards the avalanche front and then accumulate there. This is important, because when this is combined with mobility feedback effects, the larger less mobile particles at the front can be continuously shouldered aside to spontaneously form lateral levees that channelize the flow and enhance run-out. The theory provides a general framework that will enable segregation-mobility feedback effects to be studied in detail for the first time. While the large particle transport equation has a very simple representation of the particle size distribution, it does a surprisingly good job of capturing solutions to the full theory once the grains have segregated into inversely graded layers. In particular, we show that provided the inversely graded interface does not break it has precisely the same solution as the full theory. When the interface does break, a concentration shock forms instead of a breaking size segregation wave, but the net transport of large particles towards the flow front is exactly the same. The theory can also model more complex effects in small-scale stratification experiments, where particles may either be brought to rest by basal deposition or by the upslope propagation of a granular bore. In the former case the resulting deposit is normally graded, while in the latter case it is inversely graded. These completely opposite gradings in the deposit arise from a parent flow that is inversely graded, which raises many questions about how to interpret geological deposits.

Download Full-text

Multi-Mesh-Scale Approximation of Thin Geophysical Mass Flows on Complex Topographies

Communications in Computational Physics ◽

10.4208/cicp.oa-2019-0184 ◽

2021 ◽

Vol 29 (1) ◽

pp. 148-185

Author(s):

Yih-Chin Tai

Keyword(s):

Mass Flows ◽

Geophysical Mass Flows

Download Full-text

A two-fluid model for avalanche and debris flows

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2005.1596 ◽

2005 ◽

Vol 363 (1832) ◽

pp. 1573-1601 ◽

Cited By ~ 245

Author(s):

E. Bruce Pitman ◽

Long Le

Keyword(s):

Debris Flows ◽

Fluid Model ◽

Solid Material ◽

Fluid Inertia ◽

Two Phase ◽

Engineering Research ◽

Mass Flows ◽

Two Fluid Model ◽

Two Fluid ◽

Geophysical Mass Flows

Geophysical mass flows—debris flows, avalanches, landslides—can contain O (10 6 –10 10 ) m 3 or more of material, often a mixture of soil and rocks with a significant quantity of interstitial fluid. These flows can be tens of meters in depth and hundreds of meters in length. The range of scales and the rheology of this mixture presents significant modelling and computational challenges. This paper describes a depth-averaged ‘thin layer’ model of geophysical mass flows containing a mixture of solid material and fluid. The model is derived from a ‘two-phase’ or ‘two-fluid’ system of equations commonly used in engineering research. Phenomenological modelling and depth averaging combine to yield a tractable set of equations, a hyperbolic system that describes the motion of the two constituent phases. If the fluid inertia is small, a reduced model system that is easier to solve may be derived.

Download Full-text

Discrete and continuum modelling of grain size segregation during bedload transport

Journal of Fluid Mechanics ◽

10.1017/jfm.2020.274 ◽

2020 ◽

Vol 895 ◽

Cited By ~ 1

Author(s):

Rémi Chassagne ◽

Raphaël Maurin ◽

Julien Chauchat ◽

J. M. N. T. Gray ◽

Philippe Frey

Keyword(s):

Grain Size ◽

Bedload Transport ◽

Size Segregation ◽

Continuum Modelling ◽

Image Position

Download Full-text

Controls on the grain size distribution of landslides in Taiwan: the inﬂuence of drop height, scar depth and bedrock strength

10.5194/esurf-2021-19 ◽

2021 ◽

Author(s):

Odin Marc ◽

Jens M. Turowski ◽

Patrick Meunier

Keyword(s):

Grain Size ◽

Size Distribution ◽

Grain Size Distribution ◽

Block Size ◽

Fragmentation Model ◽

Drop Height ◽

Size Segregation ◽

Key Factor ◽

Grain Size Distributions ◽

Strong Segregation

Abstract. The size of grains delivered to river by hillslopes processes is thought to be a key factor to better understand sediment transport, long-term erosion as well as sedimentary archives. Recently, models have been developed for the grain size distribution produced in soil, but they may not apply to active orogens where high erosion rates on hillslopes are driven by landsliding. Until now relatively few studies have focused on landslide grain size distributions. Here we present grain size distribution 5 (GSD) obtained by the grid-by-number sampling on 17 recent landslide deposits in Taiwan, and we compare it to the geometrical and physical properties of the landslides, such as their width, area, rock-type, drop height and estimated depth. All slides occurred in slightly metamorphosed sedimentary units, except two, which occurred in younger unmetamorphosed shales, with rock strength expected to be 3 to 10 times weaker from their metamorphosed counterparts. We found that 4 deposits displayed a strong grain-size segregation on their deposit with downslope toe deposits 3 to 10 times coarser than apex 10 deposits. In 3 cases, we could also measure the GSD inside the landslides that presented percentiles 3 to 10 times finer than the surface of the deposit. Both observations could be due to either kinetic sieving or deposit reworking after the landslide failure but we cannot explain why only some deposits had a strong segregation. Averaging this spatial variability we found the median grainsize of the deposits to be strongly negatively correlated to drop height, scar width and depth. However, previous work suggest that regolith particles and bedrock blocks should coarsen with increasing depth, opposite to our observation. 15 Accounting for a model of regolith coarsening with depth, we found that the ratio of the original bedrock block size and the D50 was proportional the potential energy of the landslide normalized to its bedrock strength. Thus the studied landslides agree well with a published, simple fragmentation model, even if that model was calibrated on much larger and much stronger rock avalanches than those featured in our dataset. This scaling may thus serve for future model of grain size transfer from hillslopes to river, trying to better understand landslide sediment evacuation and coupling to river erosional dynamics.

Download Full-text

r.avaflow & r.randomwalk: two complementary and comprehensive open source GIS simulation tools for the propagation of rapid geophysical mass flows

10.7287/peerj.preprints.2224v2 ◽

2016 ◽

Author(s):

Martin Mergili ◽

Matthias Benedikt ◽

Julia Krenn ◽

Jan-Thomas Fischer ◽

Shiva P Pudasaini

Keyword(s):

Open Source ◽

Open Source Software ◽

Flow Model ◽

Rock Avalanche ◽

Two Phase ◽

Simulation Tools ◽

Physically Based ◽

Open Source Gis ◽

Mass Flows ◽

Geophysical Mass Flows

We present two GIS model applications for simulating the propagation of rapid geophysical mass flows: r.avaflow employs an advanced physically-based two phase flow model intended for in-detail case studies, r.randomwalk a conceptual model suitable for studies at various scales. Both tools are implemented in open source software environments serving for the needs of both research and practice. They offer a range of visualization, validation, parameter sensitivity analysis and parameter optimization functions. Some of the key functionalities of both tools are demonstrated for the Acheron rock avalanche in New Zealand.

Download Full-text

Controls on the grain size distribution of landslides in Taiwan: the influence of drop height, scar depth and bedrock strength

Earth Surface Dynamics ◽

10.5194/esurf-9-995-2021 ◽

2021 ◽

Vol 9 (4) ◽

pp. 995-1011

Author(s):

Odin Marc ◽

Jens M. Turowski ◽

Patrick Meunier

Keyword(s):

Grain Size ◽

Size Distribution ◽

Grain Size Distribution ◽

Block Size ◽

Fragmentation Model ◽

Size Distributions ◽

Drop Height ◽

Size Segregation ◽

Grain Size Distributions ◽

Strong Segregation

Abstract. The size of grains delivered to rivers by hillslope processes is thought to be a key factor controlling sediment transport, long-term erosion and the information recorded in sedimentary archives. Recently, models have been developed to estimate the grain size distribution produced in soil, but these models may not apply to active orogens where high erosion rates on hillslopes are driven by landsliding. To date, relatively few studies have focused on landslide grain size distributions. Here, we present grain size distributions (GSDs) obtained by grid-by-number sampling on 17 recent landslide deposits in Taiwan, and we compare these GSDs to the geometrical and physical properties of the landslides, such as their width, area, rock type, drop height and estimated scar depth. All slides occurred in slightly metamorphosed sedimentary units, except two, which occurred in younger unmetamorphosed shales, with a rock strength that is expected to be 3–10 times weaker than their metamorphosed counterparts. For 11 landslides, we did not observe substantial spatial variations in the GSD over the deposit. However, four landslides displayed a strong grain size segregation on their deposit, with the overall GSD of the downslope toe sectors being 3–10 times coarser than apex sectors. In three cases, we could also measure the GSD inside incised sectors of the landslides deposits, which presented percentiles that were 3–10 times finer than the surface of the deposit. Both observations could be due to either kinetic sieving or deposit reworking after the landslide failure, but we cannot explain why only some deposits had strong segregation. Averaging this spatial variability, we found the median grain size of the deposits to be strongly negatively correlated with drop height, scar width and depth. However, previous work suggests that regolith particles and bedrock blocks should coarsen with increasing depth, which is the inverse of our observations. Accounting for a model of regolith coarsening with depth, we found that the ratio of the estimated original bedrock block size to the deposit median grain size (D50) of the deposit was proportional to the potential energy of the landslide normalized to its bedrock strength. Thus, the studied landslides agree well with a published, simple fragmentation model, even if that model was calibrated on rock avalanches with larger volume and stronger bedrock than those featured in our dataset. Therefore, this scaling may serve for future modeling of grain size transfer from hillslopes to rivers, with the aim to better understanding landslide sediment evacuation and coupling to river erosional dynamics.

Download Full-text