Assessing debris-flow hazard focusing on statistical morpho-fluvial susceptibility models and magnitude-frequency relationships. Application to the central-eastern Pyrenees

Occurrence of debris flows has received little attention in the Pyrenees, probably due to the small risk faced by most of the debris-flow prone sites in this mountain range. Nevertheless, the event of Biescas, which occurred in august 1996 and causing 87 casualties, demonstrates the existence of high-risk spots in the region and justifies the elaboration of the debris-flow hazard assessment presented in this thesis. Five debris flows, which occurred in 2008, are selected; and site-specific descriptions and analysis, regarding geology and morphology, were performed. The results are compared with worldwide data and some conclusions on hazard assessment are presented. The preliminary analysis of some major Eastern Pyrenean debris flows represents the background for this thesis. The necessity of possessing an inventory of past occurrences is of crucial importance when assessing debris-flow hazard. Criteria of reconnaissance were thought to be visible from aerial viewing. 691 tracks through which debris flows are thought to have travelled have been revealed. Based on debris-flow inventories and using a geographical information system, the debris-flow hazard assessment presented in this thesis takes into account fluvio-morphologic parameters, gathered for every 1st-order catchment as well as every 2nd-order catchment. Mountainous headwaters are a common subject in geomorphological studies. Often investigated at local scale, the geomorphological context in which headwaters evolve has been poorly reported in the Central-Eastern Pyrenees or worldwide. A series of parameters obtained for Central-Eastern Pyrenean headwaters catchments consisting of 3005 1st- and 655 2nd-order catchments are presented. Acquired from a digital elevation model, these catchments have been digitalised, identified and attributed a value for each parameter. Previously reported parameters¿ ranges agree with those presented in this study. For the first time, the ranges of values give details about the Central-Eastern Pyrenees headwater catchments. Data mining techniques are used on the morphometric parameters, to calculate and test three different models. The first model is a logistic regression. The other two are classification trees, which are rather novel susceptibility models associated with debris flows. Results related to the training dataset show that the optimized model¿s performance lies within existing reported range although closer to the lowest end (near 70%). When the models are applied to the test set, the logistic regression seems to offer the best prediction, as training and test set results are very similar in terms of performance. Trees are better at extracting laws from a training set, but validation through a test set gives poorer results for a prediction at regional scale. The determination of magnitude of a historic event can be done by distinguishing its deposits. However this is not a trivial task in debris fans that accumulate deposits, corresponding to consecutive debris flows, especially if only a conventional geomorphological analysis is carried out. The event deposits can be mapped and, subsequently, trees damaged by the flows sampled for dating events. A magnitude-frequency relationship was prepared for El Rebaixader site, at local scale, and is compared to that of the Tordó creek. Moreover, a debris-flow inventory was created in the "Aigüestortes i Estany de Sant Maurici" National Park in the Central Pyrenees, Spain, and this regional magnitude-frequency relationship is compared to that of Rebaixader. Both curves include a strong rollover effect at about 2000 m2, and events larger than this magnitude can be represented by a power law, with an exponent between -1.5 and -1.9. This thesis is a first step toward the assessment of debris-flow hazard in the Central-Eastern Pyrenees. Although a lot of information is provided, more work is still to be done, in order to fully capture debris-flow importance in landscape evolution

Diffusive foam wetting process in microgravity

We report the experimental study of aqueous foam wetting in microgravity. The liquid fraction $\ell$ along the bubble edges is measured and is found to be a relevant dynamical parameter during the capillary process. The penetration of the liquid in the foam, the foam inflation, and the rigidity loss are shown all to obey strict diffusion processes.Comment: 4 pages, 6 figures, submitted to Phys. Rev. Let

On the mass of the neutron star in V395 Car/2S 0921-630

We report high-resolution optical spectroscopy of the low-mass X-ray binary V395 Car/2S 0921-630 obtained with the MIKE echelle spectrograph on the Magellan-Clay telescope. Our spectra are obtained near superior conjunction of the mass donor star and we exploit the absorption lines originating from the back-side of the K-type object to accurately derive its rotational velocity. Using K0-K1 III templates, we find vsini=32.9 +/- 0.8 km/s. We show that the choice of template star and the assumed limb darkening coefficient has little impact on the derived rotational velocity. This value is a significant revision downwards compared to previously published values. We derive new system parameter constraints in the light of our much lower rotational velocity. We find M_1=1.44 +/- 0.10 Msun, M_2=0.35 +/- 0.03 Msun, and q=0.24 +/- 0.02 where the errors have been estimated through a Monte-Carlo simulation. A possible remaining systematic effect is the fact that we may be over-estimating the orbital velocity of the mass donor due to irradiation effects. However, any correction for this effect will only reduce the compact object mass further, down to a minimum mass of M_1=1.05 +/- 0.08 Msun. There is thus strong evidence that the compact object in this binary is a neutron star of rather typical mass and that the previously reported mass values of 2-4Msun were too high due to an over-estimate of the rotational broadening.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

Penetration and cratering experiments of graphite by 0.5-mm diameter steel spheres at various impact velocities

Cratering experiments have been conducted with 0.5-mm diameter AISI 52100 steel spherical projectiles and 30-mm diameter, 15-mm long graphite targets. The latter were made of a commercial grade of polycrystalline and porous graphite named EDM3 whose behavior is known as macroscopically isotropic. A two-stage light-gas gun launched the steel projectiles at velocities between 1.1 and 4.5 km s 1. In most cases, post-mortem tomographies revealed that the projectile was trapped, fragmented or not, inside the target. It showed that the apparent crater size and depth increase with the impact velocity. This is also the case of the crater volume which appears to follow a power law significantly different from those constructed in previous works for similar impact conditions and materials. Meanwhile, the projectile depth of penetration starts to decrease at velocities beyond 2.2 km s 1. This is firstly because of its plastic deformation and then, beyond 3.2 km s 1, because of its fragmentation. In addition to these three regimes of penetration behavior already described by a few authors, we suggest a fourth regime in which the projectile melting plays a significant role at velocities above 4.1 km s 1. A discussion of these four regimes is provided and indicates that each phenomenon may account for the local evolution of the depth of penetration