88 research outputs found
Jump at the onset of saltation
We reveal a discontinuous transition in the saturated flux for aeolian
saltation by simulating explicitly particle motion in turbulent flow. The
discontinuity is followed by a coexistence interval with two metastable
solutions. The modification of the wind profile due to momentum exchange
exhibits a second maximum at high shear strength. The saturated flux depends on
the strength of the wind as
Numerical simulation of turbulent sediment transport, from bed load to saltation
Sediment transport is studied as a function of the grain to fluid density
ratio using two phase numerical sim- ulations based on a discrete element
method (DEM) for particles coupled to a continuum Reynolds averaged description
of hydrodynamics. At a density ratio close to unity (typically under water),
vertical velocities are so small that sediment transport occurs in a thin layer
at the surface of the static bed, and is called bed load. Steady, or
'saturated' transport is reached when the fluid borne shear stress at the
interface between the mobile grains and the static grains is reduced to its
threshold value. The number of grains transported per unit surface is therefore
limited by the flux of horizontal momentum towards the surface. However, the
fluid velocity in the transport layer remains almost undisturbed so that the
mean grain velocity scales with the shear velocity u\ast. At large density
ratio (typically in air), the vertical velocities are large enough to make the
transport layer wide and dilute. Sediment transport is then called saltation.
In this case, particles are able to eject others when they collide with the
granular bed, a process called splash. The number of grains transported per
unit surface is selected by the balance between erosion and deposition and
saturation is reached when one grain is statistically replaced by exactly one
grain after a collision, which has the consequence that the mean grain velocity
remains independent of u\ast. The influence of the density ratio is
systematically studied to reveal the transition between these two transport
regimes. Based on the mechanisms identified in the steady case, we discuss the
transient of saturation of sediment transport and in particular the saturation
time and length. Finally, we investigate the exchange of particles between the
mobile and static phases and we determine the exchange time of particles.Comment: 17 pages, 14 figures, submitted to Physics of Fluid
Assessing changes in wind erosion climatic erosivity in China’s dryland region during 1961–2012
Measuring and modeling the effect of surface moisture on the spectral reflectance of coastal beach sand
Surface moisture is an important supply limiting factor for aeolian sand transport, which is the primary driver of coastal dune development. As such, it is critical to account for the control of surface moisture on available sand for dune building. Optical remote sensing has the potential to measure surface moisture at a high spatio-temporal resolution. It is based on the principle that wet sand appears darker than dry sand: it is less reflective. The goals of this study are (1) to measure and model reflectance under controlled laboratory conditions as function of wavelength () and surface moisture () over the optical domain of 350–2500 nm, and (2) to explore the implications of our laboratory findings for accurately mapping the distribution of surface moisture under natural conditions. A laboratory spectroscopy experiment was conducted to measure spectral reflectance (1 nm interval) under different surface moisture conditions using beach sand. A non-linear increase of reflectance upon drying was observed over the full range of wavelengths. Two models were developed and tested. The first model is grounded in optics and describes the proportional contribution of scattering and absorption of light by pore water in an unsaturated sand matrix. The second model is grounded in soil physics and links the hydraulic behaviour of pore water in an unsaturated sand matrix to its optical properties. The optical model performed well for volumetric moisture content 24% ( 0.97), but underestimated reflectance for between 24–30% ( 0.92), most notable around the 1940 nm water absorption peak. The soil-physical model performed very well ( 0.99) but is limited to 4% 24%. Results from a field experiment show that a short-wave infrared terrestrial laser scanner ( = 1550 nm) can accurately relate surface moisture to reflectance (standard error 2.6%), demonstrating its potential to derive spatially extensive surface moisture maps of a natural coastal beach
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