Investigation of internal erosion processes using a coupled DEM-fluid method

The evolution of granular beds subjected to upward seepage flow is investigated using a coupled DEM-fluid model implemented by Catalano et al. in the open-source software Yade-DEM. Firstly, filtration properties of a coarse narrowly graded material are analyzed by simulating the transport of smaller particles from a base layer through the coarse filter by gravitational loading or downward flow with uniform pressure gradient. The results are analysed on the basis of the constriction size distribution (CSD) of the filter which describes statistically the sizes of throats between pores in the material. Secondly, we examine the results obtained when, instead of two different layers, the coarse and fine materials are initially mixed in one unique layer and subjected to gravity. Thirdly, this mixture of coarse and fine particles is subjected to both gravity and a non-uniform pressure gradient, by injecting the fluid in one point below the layer, as inspired by previous experiments. Similar channeling patterns are obtained in both experiments and simulations when the boundary condition at the injection point is an imposed flux. This boundary condition results in a recirculation mechanism that remains confined in a finite zone around the injection point as long as the flux is below a threshold value. By simulating an imposed pressure condition, we finally show that instabilities can be triggered by the transport of small particles away from the injection point. This segregation process results in a lower porosity and an increased pressure gradient above the eroded zone, so that the instability-triggering pressure gradient in bi-dispersed mixtures is lower than in mono-dispersed mixtures

Numerical modeling of particle migration in granular soils

Suffusion is the process of internal erosion where fine particles migrate under water seepage through a coarser soil matrix. Relevant models of suffusive phenomena must reproduce the poromechanical effects that result from the two-way coupling between the deformation of the solid matrix, the fluid pressure and the flow. In this work, an advanced computational method is used to study the particle migration in granular soils. The so called coupled Discrete Element Method - Pore scale Finite Volume (DEM-PFV) is based on a microscopic hydromechanical approach. It couples the discrete element method that solves the equations of motion for the solid fraction, with a PFV method that solves the fluid flow equations. We use this method to study particle transport through coarser granular assemblies that do not evolve with time. These simulations allow us to obtain, for different cases, the parameters to include in a general advection-dispersion equation (ADE). We paid particular attention to the role played by the intermittent formation of blockages of transported particles in the constrictions of the granular assembly. These temporary and collective trapping events change local fluid flows and affect the particle transport on short time or length scales. As the transport time between consecutive blockages and the duration of blockages have exponential decays, sink and source terms can be added to the ADE

Descending Perineum Associated With Pelvic Organ Prolapse Treated by Sacral Colpoperineopexy and Retrorectal Mesh Fixation: Preliminary Results.

<b>Introduction and hypothesis:</b> Descending Perineum Syndrome (DPS) is a coloproctologic disease and the best treatment for it is yet to be defined. DPS is frequently associated with pelvic organ prolapse (POP) and it is reasonable to postulate, that treatment of POP will also have an impact on DPS. We aimed to evaluate the subjective satisfaction and improvement of DPS for patients who have undergone a sacral colpoperineopexy associated with retrorectal mesh for concomitant POP. <b>Methods:</b> This retrospective cohort study, conducted between February 2010 and May 2016 included all women who had undergone surgery to treat POP and DPS. Improvement of POP was assessed clinically and subjective satisfaction was assessed with a survey. <b>Results:</b> Among the 37 operated patients, 31 responded to the questionnaire and 77.4% were satisfied with this surgical procedure. 94.6% were objectively cured for POP. There was a 60% improvement rate for constipation, 63.5 and 68% were cured or improved for ODS and the need for digital maneuvers respectively. <b>Conclusion:</b> Sacral colpoperineopexy associated with retrorectal dorsal mesh appears to objectively and subjectively improve POP associated with DPS

Asymmetric response of forest and grassy biomes to climate variability across the African Humid Period : influenced by anthropogenic disturbance?

A comprehensive understanding of the relationship between land cover, climate change and disturbance dynamics is needed to inform scenarios of vegetation change on the African continent. Although significant advances have been made, large uncertainties exist in projections of future biodiversity and ecosystem change for the world's largest tropical landmass. To better illustrate the effects of climate–disturbance–ecosystem interactions on continental‐scale vegetation change, we apply a novel statistical multivariate envelope approach to subfossil pollen data and climate model outputs (TraCE‐21ka). We target paleoenvironmental records across continental Africa, from the African Humid Period (AHP: ca 14 700–5500 yr BP) – an interval of spatially and temporally variable hydroclimatic conditions – until recent times, to improve our understanding of overarching vegetation trends and to compare changes between forest and grassy biomes (savanna and grassland). Our results suggest that although climate variability was the dominant driver of change, forest and grassy biomes responded asymmetrically: 1) the climatic envelope of grassy biomes expanded, or persisted in increasingly diverse climatic conditions, during the second half of the AHP whilst that of forest did not; 2) forest retreat occurred much more slowly during the mid to late Holocene compared to the early AHP forest expansion; and 3) as forest and grassy biomes diverged during the second half of the AHP, their ecological relationship (envelope overlap) fundamentally changed. Based on these asymmetries and associated changes in human land use, we propose and discuss three hypotheses about the influence of anthropogenic disturbance on continental‐scale vegetation change

Historical sampling reveals dramatic demographic changes in western gorilla populations

Background: Today many large mammals live in small, fragmented populations, but it is often unclear whether this subdivision is the result of long-term or recent events. Demographic modeling using genetic data can estimate changes in long-term population sizes while temporal sampling provides a way to compare genetic variation present today with that sampled in the past. In order to better understand the dynamics associated with the divergences of great ape populations, these analytical approaches were applied to western gorillas (Gorilla gorilla) and in particular to the isolated and Critically Endangered Cross River gorilla subspecies (G. g. diehli).Results: We used microsatellite genotypes from museum specimens and contemporary samples of Cross River gorillas to infer both the long-term and recent population history. We find that Cross River gorillas diverged from the ancestral western gorilla population ~17,800 years ago (95% HDI: 760, 63,245 years). However, gene flow ceased only ~420 years ago (95% HDI: 200, 16,256 years), followed by a bottleneck beginning ~320 years ago (95% HDI: 200, 2,825 years) that caused a 60-fold decrease in the effective population size of Cross River gorillas. Direct comparison of heterozygosity estimates from museum and contemporary samples suggests a loss of genetic variation over the last 100 years.Conclusions: The composite history of western gorillas could plausibly be explained by climatic oscillations inducing environmental changes in western equatorial Africa that would have allowed gorilla populations to expand over time but ultimately isolate the Cross River gorillas, which thereafter exhibited a dramatic population size reduction. The recent decrease in the Cross River population is accordingly most likely attributable to increasing anthropogenic pressure over the last several hundred years. Isolation of diverging populations with prolonged concomitant gene flow, but not secondary admixture, appears to be a typical characteristic of the population histories of African great apes, including gorillas, chimpanzees and bonobos