Debris flows: Experiments and modelling

Turnbull, Barbara; Bowman, Elisabeth T.; McElwaine, J. N.

doi:10.1016/j.crhy.2014.11.006

Cited by 61 publications

(41 citation statements)

References 112 publications

(210 reference statements)

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“…Following common practice, we use the term gully or gully landform for the alcove-channel-apron assemblages on Mars reported by Malin and Edgett (2000), although under terrestrial definitions (e.g., Neuendorf et al, 2005) "gulch" or "ravine" would be more accurate for many of these kilometer-scale features. Additionally, although terrestrial "debris flows" are commonly defined as wet flows with a wide range of grain sizes (e.g., Iverson, 1997;Turnbull et al, 2015), this usage is not always followed in planetary science, so we refer to "wet" or "aqueous" debris flows to emphasize this aspect. We use the Mars Year calendar defined by Clancy et al (2000).…”

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confidence: 99%

The formation of gullies on Mars today

Dundas

McEwen

Diniega

et al. 2017

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A decade of high-resolution monitoring has revealed extensive activity in fresh Martian gullies. Flows within the gullies are diverse: they can be relatively light, neutral, or dark, colorful or bland, and range from superficial deposits to 10-meter-scale topographic changes. We observed erosion and transport of material within gullies, new terraces, freshly eroded channel segments, migrating sinuous curves, channel abandonment, and lobate deposits. We also observed early stages of gully initiation, demonstrating that these processes are not merely modifying pre-existing landforms. The timing of activity closely correlates with the presence of seasonal CO2 frost, so the current changes must be part of ongoing gully formation that is driven largely by its presence. We suggest that the cumulative effect of many flows erodes alcoves and channels and builds lobate aprons, with no involvement of liquid water. Instead, flows may be fluidized by sublimation of entrained CO2 ice or other mechanisms. The frequent activity has likely erased any features dating from high-obliquity periods, so fresh gully geomorphology at middle and high latitudes is not evidence for past liquid water. CO2 ice-driven processes may have been important throughout Martian geologic history, and their deposits could exist in the rock record, perhaps resembling debris-flow sediments. Gully landforms on Mars resemble water-formed features on Earth, with channels transporting material from an alcove to a depositional apron. From their discovery (Malin and Edgett, 2000), they have generally been interpreted as evidence for wet debris flows or flowing liquid water (e.g., Carr, 2006). Such liquid would have major implications for Martian climate, geology, the possibility of life, and the definition of Special Regions for planetary protection (Rummel et al., 2014). Understanding the formation of gullies has thus been a major focus of recent Mars science, as shown by the work in this volume. Numerous hypotheses for gully formation have been considered. Martian surface conditions are not favorable for the existence of liquid water, so initial models focused on release of groundwater from shallow or deep aquifers (Malin and Edgett, 2000; Mellon and Phillips, 2001; Gaidos, 2001), possibly aided by geothermal heating melting permafrost (Hartmann, 2001; Hartmann et al., 2003) or the occurrence of brines (Knauth and Burt, 2002). However, the occurrence of gullies on sand dunes and isolated peaks argued against significant input from aquifers, and led to the development of models based on insolation-driven melting of snow or shallow permafrost at times when the

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confidence: 99%

The formation of gullies on Mars today

Dundas

McEwen

Diniega

et al. 2017

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121

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“…Sharp & Nobles 1953;Johnson 1970Johnson , 1984Takahashi 1980;Costa & Williams 1984;Pierson 1986;Iverson 2014;Turnbull, Bowman & McElwaine 2015). Figure 1 shows large boulders deposited at the front of a debris flow in Arizona, USA.…”

Section: Introductionmentioning

confidence: 99%

Asymmetric breaking size-segregation waves in dense granular free-surface flows

et al. 2016

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Debris and pyroclastic flows often have bouldery flow fronts, which act as a natural dam resisting further advance. Counter intuitively, these resistive fronts can lead to enhanced run-out, because they can be shouldered aside to form static levees that self-channelise the flow. At the heart of this behaviour is the inherent process of size segregation, with different sized particles readily separating into distinct vertical layers through a combination of kinetic sieving and squeeze expulsion. The result is an upward coarsening of the size distribution with the largest grains collecting at the top of the flow, where the flow velocity is greatest, allowing them to be preferentially transported to the front. Here, the large grains may be overrun, resegregated towards the surface and recirculated before being shouldered aside into lateral levees. A key element of this recirculation mechanism is the formation of a breaking size-segregation wave, which allows large particles that have been overrun to rise up into the faster moving parts of the flow as small particles are sheared over the top. Observations from experiments and discrete particle simulations in a moving-bed flume indicate that, whilst most large particles recirculate quickly at the front, a few recirculate very slowly through regions of many small particles at the rear. This behaviour is modelled in this paper using asymmetric segregation flux functions. Exact non-diffuse solutions are derived for the steady wave structure using the method of characteristics with a cubic segregation flux. Three different structures emerge, dependent on the degree of asymmetry and the non-convexity of the segregation flux function. In particular, a novel 'lens-tail' solution is found for segregation fluxes that have a large amount of non-convexity, with an additional expansion fan and compression wave forming a 'tail' upstream of the 'lens' region. Analysis of exact solutions for the particle motion shows that the large particle motion through the 'lens-tail' is fundamentally different to the classical 'lens' solutions. A few large particles starting near the bottom of the breaking wave pass through the 'tail', where they travel in a region of many small particles with a very small vertical velocity, and take significantly longer to recirculate. †

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“…Low grain Reynolds numbers indicate viscous (i.e. velocity dependent) interaction between individual grains and the air, where large grain Reynolds numbers indicate turbulent form drag (which is velocity-squared dependent) between the particles and air (Jackson et al 2013;Turnbull and McElwaine 2008;Turnbull et al 2015). The type of drag has implications on other mechanisms within the flow such as fluidisation (which in turn can affect the basal friction on the slide) and internal energy dissipation (due to changes in porosity and collision networks).…”

Section: Discussionmentioning

confidence: 99%

A laboratory-numerical approach for modelling scale effects in dry granular slides

2018

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Granular slides are omnipresent in both natural and industrial contexts. Scale effects are changes in physical behaviour of a phenomenon at different geometric scales, such as between a laboratory experiment and a corresponding larger event observed in nature. These scale effects can be significant and can render models of small size inaccurate by underpredicting key characteristics such as flow velocity or runout distance. Although scale effects are highly relevant to granular slides due to the multiplicity of length and time scales in the flow, they are currently not well understood. A laboratory setup under Froude similarity has been developed, allowing dry granular slides to be investigated at a variety of scales, with a channel width configurable between 0.25 and 1.00 m. Maximum estimated grain Reynolds numbers, which quantify whether the drag force between a particle and the surrounding air act in a turbulent or viscous manner, are found in the range 10 2 − 10 3 . A discrete element method (DEM) simulation has also been developed, validated against an axisymmetric column collapse and a granular slide experiment of Hutter et al. (Acta Mech 109:127-165, 1995), before being used to model the present laboratory experiments and to examine a granular slide of significantly larger scale. This article discusses the details of this laboratory-numerical approach, with the main aim of examining scale effects related to the grain Reynolds number. Increasing dust formation with increasing scale may also exert influence on laboratory experiments. Overall, significant scale effects have been identified for characteristics such as flow velocity and runout distance in the physical experiments. While the numerical modelling shows good general agreement at the medium scale, it does not capture differences in behaviour seen at the smaller scale, highlighting the importance of physical models in capturing these scale effects.

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Debris flows: Experiments and modelling

Cited by 61 publications

References 112 publications

The formation of gullies on Mars today

The formation of gullies on Mars today

Asymmetric breaking size-segregation waves in dense granular free-surface flows

A laboratory-numerical approach for modelling scale effects in dry granular slides

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