Keld Rasmussen

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Ludwig-Maximilians-Universität München

Marc Laemmel

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Laboratory studies of aeolian sediment transport processes on planetary surfaces

Geomorphology, 2015

We review selected experimental saltation studies performed in laboratory wind tunnels and collis... more We review selected experimental saltation studies performed in laboratory wind tunnels and collision experiments performed in (splash-) laboratory facilities that allow detailed observations between impinging particles on a stationary bed. We also discuss progress in understanding aeolian transport in nonterrestrial environments.

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Using orthophotos to map the groundwater/surface water interaction type on an outwash plain

ABSTRACT The mechanisms that govern groundwater seepage to streams on glacial outwash plains are ... more ABSTRACT The mechanisms that govern groundwater seepage to streams on glacial outwash plains are far from understood. In particular, little is known on how to quantify the variability of groundwater / surface water interaction (GSI) on catchment scale. In the present study, groundwater seepage to two tributary streams was examined at six field sites using hydrological, geophysical, and geomorphological data. Along both streams the gain in stream flow coming from groundwater was estimated using synchronous stream flow measurements, and seepage through the streambed was estimated using seepage meters. At both streams the data indicate, that seepage enters the stream either (i) directly through the streambed; (ii) as nearly-superficial flow from large diffuse discharge areas within the riparian zone; or (iii) as a combination of the two. Moreover, it was found that the ratio between the width of the riparian zone and the effective stream width (RS-ratio) could be used as an indicator of the magnitude of water entering the stream directly through the streambed. When the RS-ratio is small the streambed seepage is large. When the RS-ratio is large the streambed seepage is small and groundwater mainly enters the stream as nearly-superficial flow from large diffuse discharge areas within the riparian zone. In the RS-ratio the effective stream width is defined as the product of the stream sinuosity and the stream mean width, and the width of the riparian zone is defined as the width of the zone where peat deposits are found along the stream. Orthophotos were used to map the boundary between the riparian zone and the surroundings. Field reconnaissance was used to verify the position of the boundary, resulting in minor corrections. The RS-ratio was found to be fairly constant over long reaches. This indicates that the type (i, ii, iii) of groundwater seepage to the stream does not vary much within the individual reach. However, it may be very different between reaches. Whether the RS-ratio on other stream scales also can be used to indicate the type of GSI is yet to be examined.

Wind Driven Grain Transport on Mars

Properties of the Martian Dust Aerosol

Mars is a test bed for the study of dust aerosols since, lacking the active circulation of water ... more

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Wind Tunnels for the Study of Particle Transport

Wind Tunnels and Experimental Fluid Dynamics Research, 2011

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Enhancement in wind-driven sand transport by electric fields

Planetary and Space Science, 2009

Aeolian grain transport is a powerful erosion mechanism and a significant factor affecting atmosp... more Aeolian grain transport is a powerful erosion mechanism and a significant factor affecting atmospheric dynamics by the creation of particulate aerosols. Electric fields, both natural and man-made, occur widely at the Earths surface in environments where granular material is found. Such electric fields may induce electrification of granular material and affect the transport dynamics of the grains. In this laboratory study we show that the influence of such electric fields is significant at field strengths well below that at which breakdown occurs and present a simple semi-empirical expression which allows this mechanism to be quantified.

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Barchan dune mobility in Mauritania related to dune and interdune sand fluxes

Journal of Geophysical Research, 2007

We present a 1 year study of 50 dunes in a small field of barchan dunes in Mauritania. We documen... more We present a 1 year study of 50 dunes in a small field of barchan dunes in Mauritania. We documented the morphological evolution of the dunes and their migration rates and measured at 10 min intervals the interdune sand transport, the wind strength, and its direction for the same interval of time. The dune heights H 0 range between 2 and 5 m, and their celerity c is found to be well approximated by the standard migration law: c = Q 0 /H 0 , with Q 0 % 50 m 3 /m yr. From both the interdune sand flux and the migration rate of the dunes we were able to estimate the spatially averaged sand flux at the dune crest as well as the bulk sand flux associated with the mass of sand transported by the dune. We found that the sand flux at the crest was about 3 times greater than the interdune mass transport rate, whereas the bulk sand flux was surprisingly of the same order as the interdune flux. Moreover, we analyzed carefully the interdune sand transport data, which can be well described by the Sorensen law. The cumulative mass of sand transported during moderate wind events within 1 year was much greater than that transported during strong wind events.

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Evaluating the salinity distribution of a shallow coastal aquifer by vertical multielectrode profiling (Denmark)

Hydrogeology Journal, 2009

... of the porewater. Archie (1942) provides a linear relation between formation and porewater re... more

When do pyroclastic particles move? Wind tunnel experiments on saltation threshold and surface roughness

ABSTRACT Our understanding of the dynamics of pyroclastic density currents (PDCs) is largely base... more ABSTRACT Our understanding of the dynamics of pyroclastic density currents (PDCs) is largely based on the study of their deposits. Pyroclasts have strongly deviating properties in density and angularity compared to the wind blown and fluvial sand usually studied in clastic sedimentology. In order to test whether these specificities have an impact on the sedimentation processes and dynamic behavior of dilute PDCs, wind tunnel experiments were carried out in order to characterize the onset of saltation and surface roughness induced by pyroclastic material. Saltation is the major transport process occurring at the boundary between a flow and the sediment, and corresponds to alternate, sub-planar jumps and landing of particles on the ground. The surface roughness is a measure of how rough a bed is seen by a wind, and is a property of the bed depending on grains&amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; size and shape. The static saltation threshold corresponds to the minimum shearing necessary for particles to be lifted off the ground and begin to bounce. The dynamic saltation threshold corresponds to the minimum shearing necessary to maintain an already saltating bed in that state. Experiments were done in a 6 m long, inclinable, wind tunnel in Aarhus (Denmark). Two sample types (pumices and scoriaceous particles) were investigated individually at 1 Phi grain-size intervals between 0.125 and 16 mm. The surface roughness was measured over a quiescent bed of particles. The static saltation threshold was derived from wind profiles reaching the onset of particle transport and was measured for bed slopes between -20° and 25° (every 5°). The surface roughness measured are of the order expected for rounded particles of similar grain sizes (c.a. 1/30th of the grain diameter). However, a slight deviation to smaller surface roughness is observed for particles &amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 1 mm, possibly due to the occurrence of a laminar sub-layer near the bed, and a deviation to higher surface roughness values for particles &amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; 1 mm, possibly showing the increasing influence of shape for larger grains. The static saltation thresholds (given as a shear velocity value) for horizontal surfaces show that the shearing necessary to initiate transport is smaller for fines than for coarse grains, thus only macro-scale forces are acting over the whole range of grain sizes. The shear velocity necessary to erode scoria is 2 to 3 times higher than for low-density pumices. The results agree remarkably well with standard curves for rounded particles of similar densities for both pumice and scoria, suggesting that shape is not a relevant factor in these experiments. Additionally, a downslope wind lowers the threshold shear velocity by a factor 0.6 to 0.9, whereas it increases the value when blowing upslope by a factor up to 1.2. Dynamic saltation threshold is predicted (15-20% lower than static) derived from the static saltation threshold and the ratio of dynamic over static repose angles for grain piles. Our experimental results contribute to a better understanding of the genesis of dune bedforms produced by dilute pyroclastic density currents, provide inputs for models, and serve as analogue for other sedimentary environments (other planets, nuclear base surges). They also permit to give a lower estimate of near bed velocities for field-observed erosion-planes truncating stratification.

Activities at a European Planetary Simulation Facility

IFA Aarhus University operates a unique environmental simulation facility capable of recreating e... more IFA Aarhus University operates a unique environmental simulation facility capable of recreating extreme terrestrial and other planetary environments. It is supported by ESA and EU activities. Specifically it is capable of recreating the key physical parameters such as temperature, pressure (composition), wind flow and importantly the suspension/transport of dust or sand particulates. This facility is also available to the scientific community for collaborative research. Recent research and testing/calibration activities will be presented.

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Laboratory studies of aeolian sediment transport processes on planetary surfaces

Geomorphology, 2015

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Using orthophotos to map the groundwater/surface water interaction type on an outwash plain

Wind Driven Grain Transport on Mars

Properties of the Martian Dust Aerosol

Mars is a test bed for the study of dust aerosols since, lacking the active circulation of water ... more

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Wind Tunnels for the Study of Particle Transport

Wind Tunnels and Experimental Fluid Dynamics Research, 2011

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Enhancement in wind-driven sand transport by electric fields

Planetary and Space Science, 2009

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Barchan dune mobility in Mauritania related to dune and interdune sand fluxes

Journal of Geophysical Research, 2007

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Evaluating the salinity distribution of a shallow coastal aquifer by vertical multielectrode profiling (Denmark)

Hydrogeology Journal, 2009

... of the porewater. Archie (1942) provides a linear relation between formation and porewater re... more

When do pyroclastic particles move? Wind tunnel experiments on saltation threshold and surface roughness

Activities at a European Planetary Simulation Facility

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