Gert Lube

Axisymmetric collapses of granular columns

Experimental observations of the collapse of initially vertical columns of small grains are presented. The experiments were performed mainly with dry grains of salt or sand, with some additional experiments using couscous, sugar or rice. Some of the experimental flows were analysed using high-speed video. There are three different flow regimes, dependent on the value of the aspect ratio a = hi/ri ,w herehi and ri are the initial height and radius of the granular column respectively. The differing forms of flow behaviour are described for each regime. In all cases a central, conically sided region of angle approximately 59 ◦ , corresponding to an aspect ratio of 1.7, remains undisturbed throughout the motion. The main experimental results for the final extent of the deposit and the time for emplacement are systematically collapsed in a quantitative way independent of any friction coefficients. Along with the kinematic data for the rate of spread of the front of the collapsing column, this is interpreted as indicating that frictional effects between individual grains in the bulk of the moving flow only play a role in the last instant of the flow, as it comes to an abrupt halt. For a< 1.7, the measured final runout radius, r∞, is related to the initial radius by r∞ = ri(1 + 1.24a); while for 1.7 <a the corresponding relationship is r∞ = ri(1 + 1.6a 1/2 ). The time, t∞, taken for the grains to reach r∞ is given by t∞ =3 (hi/g) 1/2 =3 (ri/g) 1/2 a 1/2 ,w hereg is the gravitational acceleration. The insights and conclusions gained from these experiments can be applied to a wide range of industrial and natural flows of concentrated particles. For example, the observation of the rapid deposition of the grains can help explain details of the emplacement of pyroclastic flows resulting from the explosive eruption of volcanoes.

Kinematic characteristics of pyroclastic density currents at Merapi and controls on their avulsion from natural and engineered channels

We herein report an example of pyroclastic density current avulsion on 14 June 2006 at Merapi, Indonesia. Four discrete series of multiple dome collapses led to the generation of four individual block-and-ash flows into Kali Gendol valley. All four pyroclastic density currents locally overflowed the channel margins to devastate cultivated terraces along each side of the box-shaped canyon while propagating as much as 2.2 km within adjacent tributaries. The largest destruction was caused by the second and third pyroclastic density currents. Both of these flows partially spilled out of the Gendol valley at a travel distance of 4.9 km, bypassing a sabo dam just upstream of the village of Kaliadem and leaving the village almost completely destroyed and buried by several meters of overbank deposits. The main mechanism of flow avulsion on June 14 was the overflow of up to 20 vol% of the dense, basal part of the pyroclastic density currents onto interfluves. The relative proportions of valley-escaped material increased dramatically in the succession of each of the four pyroclastic density currents. The main geometric parameters controlling flow avulsion and their critical values were quantified through high-resolution real-time kinetic–global positioning system (RTK-GPS) data of the pre-event topography for each of the 14 June flows. This case highlights the way in which a sabo dam can significantly increase the potential of flow avulsion. Key lessons are derived for the future hazard mitigation in valleys subject to volcanic mass flows. Flow-observational and geometric data are combined into a model to derive the kinematic characteristics of the basal, valley-ponding avalanche and the valley-escaping veneer flows, which are otherwise hidden by overriding clouds of elutriated ash.

Static and flowing regions in granular collapses down channels

Through laboratory experiments we investigate inertial granular flows created by the instantaneous release of particulate columns into wide, rectangular channels. These flows are characterized by their unsteady motion, large changes of the free surface with time, and the propagation towards the free surface of an internal interface separating static and flowing regions. We present data for the time-dependent geometry of the internal interface and the upper, free surface for aspect ratios, a, in the range from 3 to 9.5 (where a=hi∕di is the ratio of the initial height to basal width of the column). The data were analyzed by two different approaches. First, by integrating under the entire internal interface we obtained data for the static area, AD, as a function of time for different a. Second, in order to characterize vertical deposition rates, we measured the thicknesses of the flowing region, hF(x,t), and the static region, hD(x,t), at fixed horizontal positions, x, and time, t, since the initiation of the experiment. We also determined detailed velocity profiles with depth at distances scaled to the final maximum runout distance to analyze the kinematic behavior of the flowing layer. In the initial free-fall phase, the temporal variation of the static area is independent of hi and scales as gdit. During the subsequent lateral spreading phase, AD(t) varies linearly with time and the nondimensional deposition rate (dAD/dt)/(gd3i)1∕2 is a linear function of a. The thickness of the interface hD(x,t) at constant x depends on a and varies linearly with time. The local deposition rate ∂hD∕∂t is not constant along the flow length. Data show that for the major part of the flow length ∂2hD/∂t∂x is constant. In the lateral spreading phase, the velocity profiles are characteristically linear with a basal exponential region, a few grains in thickness, which separates static from moving regions. The shear rate is a constant dependent on a modified initial height h i as (g/h i)1∕2, where h i is a characteristic length scale in the system describing the fraction of the granular column actually involved in the flowing region.

Quantifying the geomorphic impacts of a lake-breakout lahar, Mount Ruapehu, New Zealand

At 11:18 h (New Zealand time, GMT +12) on 18 March 2007 an impoundment of 0.01 × 10 6 m 3 of tephra collapsed, releasing 1.3 × 10 6 m 3 of water from Crater Lake at 2536 m elevation on Mount Ruapehu. The lahar traveled 200 km along the Whangaehu River. Aerial LiDAR surveys of the upper 62 km of flow path were made before and after the lahar. We present here the first large-scale quantification of the geomorphic impact of the dam-break flood along with the rates and controls on its sediment entrainment and deposition. The flood mobilized a net value of 2.5–3.1 × 10 6 m 3 of boulders, gravel, and sand over the first 5 km of travel to form a lahar of at least 4.4 × 10 6 m 3 passing instruments at 6.9 km. LiDAR volume-transfer calculations match dynamic measurements made. After a logarithmic increase in cumulative net sediment entrainment, the lahar appeared to reach its maximum sediment-carrying capacity at 22 km. Patterns of alternating sediment erosion and deposition occurred that dominantly reflect a combination of channel morphology and confinement on the local sediment-carrying capacity of the flow.

Static and flowing regions in granular collapses down channels : Insights from a sedimenting shallow water model

A two layer model for the collapse and spreading of a granular column is presented. This model builds upon that of Larrieu et al. [J. Fluid Mech. 554, 669 (2006)] where the free fall collapse of the column and subsequent flow of material onto a plane is represented by a “raining” mass source term into a thin flowing layer of constant density. These modified shallow water equations with Coulomb friction capture the free surface of the flows and key scaling laws for initial sand columns of aspect ratios up to a<10. However, unrealistically high coefficients of friction of μ=0.9 are required to reproduce run-outs observed. Key scaling laws for high aspect ratio columns are also not captured. We thus extend the model of Larrieu (2006) to include an estimation for the interface between the static and flowing regions observed within granular collapses in the laboratory by Lube et al. [Phys. Fluids 19, 043301 (2007)]. An empirical sedimentation term Ls and the instantaneous removal of a static deposit wedge, see...

Energy growth in laharic mass flows

Lahars, debris flows, and sediment-rich floods are frequent and deadly hazards at all mountain-forming volcanoes. Their hazard potential is traditionally assessed through mass-conserving closed-system models, where peak conversion rates of potential energy to mechanical energy and hence maximum destruction potential are predicted to occur on the steepest volcano flanks. This belies evidence of extremely high-energy and deadly catastrophes caused by such flows at large distances from volcanoes. Here we use the first high-resolution record of a moving lahar to develop a new model of the temporally and spatially variable mass-flow structure. We show that bulk flow energy can grow dramatically in such systems over tens to hundreds of kilometers via momentum transfers from the lahar into water and particles along its path. We also demonstrate that dynamic transformations of such flows and their ultimate runout are primarily controlled by the mass flow front.

Explaining the extreme mobility of volcanic ice-slurry flows, Ruapehu volcano, New Zealand

The near-invisibility of ice-slurry flows in the geological record belies their signifi-cant hazard at snow-capped volcanoes. These four-phase flows exhibit extreme rates of volumetric bulking and unusually high mobility. Mechanisms of their motion are clarified through two examples generated on 25 September 2007 at Mount Ruapehu, New Zealand. Brief explosions through Crater Lake ejected 5700 m 3 of acidic water that entrained 60 times this volume of snow as it traveled over a snow-covered glacier. The resulting ice-slurry traveled up to 7.7 km (height/length [H/L] ratio of 0.16). A cogenerated second flow took a more tortuous initial path before riding over the already frozen deposits of the first unit and beyond (H/L 0.13). For the first time, downstream evolution of the kinematic properties of propagating ice-slurry fronts could be characterized as well as the longitudinal variation of the physical properties of their resulting deposits. The chemistry and composition of the deposits show that during flow, vertical percolation of water through the porous ice–particle–water–air mixture generated a basal zone of high internal pore pressure. This effect was particularly strong when a thick, high-density flow front formed, which raced ahead of the tail to control runout and consequent hazard.

Synthesizing large-scale pyroclastic flows: Experimental design, scaling, and first results from PELE

Pyroclastic flow eruption large-scale experiment (PELE) is a large-scale facility for experimental studies of pyroclastic density currents (PDCs). It is used to generate high-energy currents involving 500–6500 m3 natural volcanic material and air that achieve velocities of 7–30 m s−1, flow thicknesses of 2–4.5 m, and runouts of >35 m. The experimental PDCs are synthesized by a controlled “eruption column collapse” of ash-lapilli suspensions onto an instrumented channel. The first set of experiments are documented here and used to elucidate the main flow regimes that influence PDC dynamic structure. Four phases are identified: (1) mixture acceleration during eruption column collapse, (2) column-slope impact, (3) PDC generation, and (4) ash cloud diffusion. The currents produced are fully turbulent flows and scale well to natural PDCs including small to large scales of turbulent transport. PELE is capable of generating short, pulsed, and sustained currents over periods of several tens of seconds, and dilute surge-like PDCs through to highly concentrated pyroclastic flow-like currents. The surge-like variants develop a basal <0.05 m thick regime of saltating/rolling particles and shifting sand waves, capped by a 2.5–4.5 m thick, turbulent suspension that grades upward to lower particle concentrations. Resulting deposits include stratified dunes, wavy and planar laminated beds, and thin ash cloud fall layers. Concentrated currents segregate into a dense basal underflow of <0.6 m thickness that remains aerated. This is capped by an upper ash cloud surge (1.5–3 m thick) with 100 to 10−4 vol % particles. Their deposits include stratified, massive, normally and reversely graded beds, lobate fronts, and laterally extensive veneer facies beyond channel margins.

Effects of volatile behaviour on dome collapse and resultant pyroclastic surge dynamics: Gunung Merapi 2010 eruption

Abstract In 2010, Gunung Merapi (Central Java, Indonesia) generated two violent eruption sequences on 26 October and 5 November culminating in widespread pyroclastic density currents (PDCs) associated with the destruction of lava domes. Tephra from PDC deposits were analysed to examine pre-dome collapse volatile behaviour and evidence of carbonate assimilation. Secondary-ion mass spectroscopy (SIMS) depth profiles of plagioclase phenocrysts reveal that the 7Li/30Si ratios in 26 October products are higher in the glass compared to the crystal, indicating a build-up of Li in the groundmass not observed in the 5 November samples. Higher Li in the groundmass suggests gas accumulation and rapid development of conduit overpressure in the shallow plumbing system prior to the initial 26 October explosion, which was only captured through the behaviour of quickly diffusing Li and not H2O. Following the explosion-induced decompression, juvenile magma rapidly ascended in great volume to generate extremely destructive PDCs following subsequent dome collapses, particularly on 5 November. Additionally, 26 October tephras contain carbonate grains in the ash component and abundant CO2 within the lava lapilli groundmass glass, which supports previous studies indicating assimilation of calc-silicate lithologies by the Merapi magma at depth in the plumbing system prior to the onset of 2010 activity. supplementary material: Feldspar microlite compositions and SIMS volatile data for the glass measurements are available at http://www.geolsoc.org.uk/SUP18762.

A major ice-calving event at Tasman Glacier terminus, Southern Alps, 22 February 2011

ABSTRACT Terminus calving of icebergs is a common mass-loss mechanism from water-terminating glaciers globally, including the lake-calving glaciers in New Zealand’s central Southern Alps. Calving rates can increase dramatically in response to increases in ice velocity and/or retreat of the glacier margin. Here, we describe a large calving event (c. 4.5 × 106 m3) observed at Tasman Glacier, which initiated around 30 min after the MW 6.2 Christchurch earthquake of 22 February 2011. The volume of this calving event was equalled or exceeded only once in a subsequent 13-month-long study. While the temporal association with the earthquake remains intriguing, the effects of any preconditioning factors remain unclear.