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 = h i /r i , where h i and r i 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 ∞ = r i (1 + 1.24a); while for 1.7 < a the corresponding relationship is r ∞ = r i (1 + 1.6a 1/2 ). The time, t ∞ , taken for the grains to reach r ∞ is given by t ∞ = 3(h i /g) 1/2 = 3(r i /g) 1/2 a 1/2 , where g 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.

[1] Geological carbon dioxide (CO 2 ) storage is a means of reducing anthropogenic emissions. Dissolution of CO 2 into the brine, resulting in stable stratification, increases storage security. The dissolution rate is determined by convection in the brine driven by the increase of brine density with CO 2 saturation. We present a new analogue fluid system that reproduces the convective behaviour of CO 2 -enriched brine. Laboratory experiments and high-resolution numerical simulations show that the convective flux scales with the Rayleigh number to the 4/5 power, in contrast with a classical linear relationship. A scaling argument for the convective flux incorporating lateral diffusion from downwelling plumes explains this nonlinear relationship for the convective flux, provides a physical picture of high Rayleigh number convection in a porous medium, and predicts the CO 2 dissolution rates in CO 2 accumulations. These estimates of the dissolution rate show that convective dissolution can play an important role in enhancing storage security. [2] The storage of carbon dioxide (CO 2 ) in geological formations has been proposed as a technological means to reduce anthropogenic emissions of this greenhouse gas [Orr, 2009;Benson and Cook, 2006]. The positive buoyancy of supercritical CO 2 relative to the ambient brine filling the pore spaces may lead to leakage along imperfections in the geological seal, which is of considerable concern for the security of long-term storage [Gasda et al., 2004;Pruess, 2005;Neufeld et al., 2009]. One of the primary mechanisms for stable long-term geological storage of CO 2 is the dissolution of injected CO 2 within ambient brine. Under typical conditions injected CO 2 dissolves into the ambient brine thereby increasing the density of the brine [Teng et al., 1997]. This layer of dense, saturated brine forms by the processes of diffusion, dispersion and mechanical mixing during injection and, once it has reached sufficient thickness, becomes rapidly unstable to convective overturning [Ennis-King et al., 2005;Riaz et al., 2006]. The process of convective dissolution of CO 2 has recently been imaged at ambient conditions in a Hele-Shaw cell [Kneafsey and Pruess, 2009], and enhanced mass transfer has been measured at reservoir conditions [Yang and Gu, 2006;Farajzadeh et al., 2007]. Convection is therefore expected in most sequestration sites, and controls the dissolution rate and hence the long-term risk of leakage. Geochemical observations in natural CO 2 reservoirs require large amounts of CO 2 dissolution into the ambient brine and provide field evidence for sustained convective transport of dissolved CO 2 [Gilfillan et al., 2008[Gilfillan et al., , 2009. Convective dissolution of CO 2 is therefore expected in most natural and anthropogenic CO 2 reservoirs, and controls the mobility of carbon in the subsurface. It is therefore an important mechanism in the deep carbon cycle [Sherwood and Ballentine, 2009], and controls the long-term risk of leakage of CO 2 from geological storage.[3] Despi...

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The release from a point source of relatively heavy fluid into a saturated porous medium above an impermeable boundary is considered. A theoretical relationship is compared with experimental data for the rate of propagation of the front of the resulting gravity current and its shape. A motivation of the study, the problem of carbon dioxide sequestration, is briefly discussed.

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