The initiation of grain movement by wind

When air blows across the surface of dry, loose sand, a critical shear velocity must be achieved to initiate motion. Since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot really be defined by a finite value but should be viewed as a threshold range which is a function of the mean size, sorting, and packing of the sediment. In order to investigate the initiation of particle movement by wind, a series of wind tunnel tests were carried out on a range of screened sands and commercially available glass beads of differing size, sorting, and shape characteristics. In addition, individual samples of the glass beads were mixed to produce rather poorly sorted bimodal distributions. Test results suggest the when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first engrained by the fluid drag of the air either in surface creep or in saltation. As velocity continues to rise, the larger more protected grains may also be moved by fluid drag. The data also indicate that predicted values based on the modified Bagnold equation fall within the range of threshold values defined by the transition section of the grain movement/shear velocity plots. Moreover, the predicted values are very similar to the threshold values derived for the point maximum inflection on the curves

Physics of windblown particles

A laboratory facility proposed for the Space Station to investigate fundamental aspects of windblown particles is described. The experiments would take advantage of the environment afforded in earth orbit and would be an extension of research currently being conducted on the geology and physics of windblown sediments on earth, Mars, and Venus. Aeolian (wind) processes are reviewed in the planetary context, the scientific rational is given for specific experiments to be conducted, the experiment apparatus (the Carousel Wind Tunnel, or CWT) is described, and a plan presented for implementing the proposed research program

Geomorphological and vegetation interaction and its relationship to slope stability on the Niagara Escarpment, Bruce Peninsula, Ontario

The stability of the face of the Niagara Escarpment is a critical issue in the possible development of this important natural resource. In an attempt to provide background data for future resource management strategies a pilot study was initiated in the Hope and Barrow Bay section of the Bruce Peninsula. Results indicate that most of the initial erosion and steepening of the slopes resulted from the movement of glacial ice over the upper Escarpment. These steep slopes were maintained by low water stages of Lake Algonquin which resulted in the concentration of erosion in the relatively weak Fossil Hill Formation. At present instability is localized to certain parts of the Escarpment and appears to be both spatially random and sporadic, but is generally associated with certain conditions. Instability is usually found where outcrops of more resistant beds are being presently undercut by the weathering and mass wasting of weaker shale and heavily jointed dolomite beds. This activity is recognizable by lack of vegetation or by the dominance of Thuja occidentalis.La stabilité du front de l’escarpement du Niagara est un élément de première importance dans le cadre du développement éventuel de cette ressource naturelle. Afin de fournir les données de base à un projet de planification de l’aménagement, on a entrepris une étude pilote dans la région des baies de Hope et de Barrow de la péninsule de Bruce. Les résultats montrent que la plus grande partie de l’érosion initiale et l’augmentation de la déclivité des pentes est le résultat de mouvements glaciaires qui se sont produits au-dessus de la partie supérieure de l’escarpement. Les versants ont conservé la raideur de leur pente, alors que le niveau du lac Algonquin était bas, ce qui a eu pour effet de concentrer l’érosion dans la partie relativement plus fragile de la formation de Fossil Hill. À l’heure actuelle, seuls certains secteurs de l’escarpement sont instables. Bien qu’ils semblent dispersés au hasard, ils sont en fait associés à certaines conditions. L’instabilité est plus grande là où les couches plus résistantes reposent sur des couches plus fragiles de shiste altéré et affecté par des mouvements de masse et de dolomie fortement fracturées. La pauvreté de la végétation et la dominance du Thuja occidentalis nous permettent de repérer facilement ces zones de plus grande activité.Die Stabilität des Niagara Steilhanges ist von besonderer Bedeutung fur eine mögliche Entwicklung dieser wichtigen Naturquelle. In dem Versuch Hintergrunddaten für zukünftige Verwaltungsstrategien zu beschaffen, wurde in der Hope und Barrow Bucht, Teil der Bruce Halbinsel eine Vorarbeitsforschung begonnen. Die Resultate zeigen, dass der grösste Teil der ursprünglichen Erosion und die Steilung des Abhanges durch die Bewegung vom Gletschereis über den oberen Abhang verursacht wurde. Diese Steilhänge wurden durch tiefe Wasserstände des Algonquin Sees erhalten, welche auch die Ursache der Erosionskonzentration in der verhältnissmässig schwachen «Fossil Hill» Formation ist. Zur Zeit ist die Unbeständigkeit auf bestimmte Teile des Steilhanges lokalisiert und scheint sowohl räumlich begrenzt und sporadisch zu sein, ist aber im allgemeinen mit bestimmten Verhältnissen verbunden. Unbeständigkeit ist gewöhnlich dort zu finden, wo Vorsprünge von widerstandsfähigem Gestein jetzt durch die Verwitterung unterschnitten werden und schwere Zerstörung von schwächeren Schalen und sehr zerklüftetem Dolomitgestein eintritt. Diese Tätigkeit ist durch das Fehlen jeglicher Vegetation oder durch das Vorherrschen von Thuja occidentalis gekennzeichnet

Representation of Vegetation and Other Nonerodible Elements in Aeolian Shear Stress Partitioning Models for Predicting Transport Threshold

The presence of nonerodible elements is well understood to be a reducing factor for soil erosion by wind, but the limits of its protection of the surface and erosion threshold prediction are complicated by the varying geometry, spatial organization, and density of the elements. The predictive capabilities of the most recent models for estimating wind driven particle fluxes are reduced because of the poor representation of the effectiveness of vegetation to reduce wind erosion. Two approaches have been taken to account for roughness effects on sediment transport thresholds. Marticorena and Bergametti (1995) in their dust emission model parameterize the effect of roughness on threshold with the assumption that there is a relationship between roughness density and the aerodynamic roughness length of a surface. Raupach et al. (1993) offer a different approach based on physical modeling of wake development behind individual roughness elements and the partition of the surface stress and the total stress over a roughened surface. A comparison between the models shows the partitioning approach to be a good framework to explain the effect of roughness on entrainment of sediment by wind. Both models provided very good agreement for wind tunnel experiments using solid objects on a nonerodible surface. However, the Marticorena and Bergametti (1995) approach displays a scaling dependency when the difference between the roughness length of the surface and the overall roughness length is too great, while the Raupach et al. (1993) model's predictions perform better owing to the incorporation of the roughness geometry and the alterations to the flow they can cause

Wind speed and sediment transport recovery in the lee of a vegetated and denuded nebkha within a nebkha dune field

Field observations of scaled wind speed and sand transport recovery in the lee of a nebka within a field of nebkhas and then subsequently for the nebkha denuded of its vegetation cover were collected. The measurements of wind speed at 0.4 times the element height indicate that for both conditions wind speed recovery in the lee is exponential. The porous vegetation cover influences the rate of this recovery being more gradual for the vegetated form. The return to equilibrium wind speed occurs in both cases at approximately eight element heights. For either case the recovery of shear stress and the return to a constant value occurs much closer to the bluff body form than has been described for porous fences. The recovery of sand transport in the lee appears to be more rapid for the un-vegetated condition, which corresponds to the observed faster rate of wind speed increase. The observations did not show a continual increase in saltation flux with increasing downwind distance due to the increasing shear stress downwind and the increase that may be expected due to the fetch effect. The change in saltation flux with downwind distance was controlled by the sediment supply, which diminished with downwind distance. The interaction of a changing shear stress and the zone of influence created by the wind as it interacts with the roughness dimensions, along with the distribution of sediment available for transport bring increased complexity to modeling sand flux for this type of environment over different temporal scales

Shear Stress Partitioning in Large Patches of Roughness in the Atmospheric Inertial Sublayer

Drag partition measurements were made in the atmospheric inertial sublayer for six roughness configurations made up of solid elements in staggered arrays of different roughness densities. The roughness was in the form of a patch within a large open area and in the shape of an equilateral triangle with 60 m long sides. Measurements were obtained of the total shear stress (tau) acting on the surfaces, the surface shear stress on the ground between the elements (tau(sub S)) and the drag force on the elements for each roughness array. The measurements indicated that tau(sub S) quickly reduced near the leading edge of the roughness compared with tau, and a tau(sub S) minimum occurs at a normalized distance (x/h, where h is element height) of approx. -42 (downwind of the roughness leading edge is negative), then recovers to a relatively stable value. The location of the minimum appears to scale with element height and not roughness density. The force on the elements decreases exponentially with normalized downwind distance and this rate of change scales with the roughness density, with the rate of change increasing as roughness density increases. Average tau(sub S): tau values for the six roughness surfaces scale predictably as a function of roughness density and in accordance with a shear stress partitioning model. The shear stress partitioning model performed very well in predicting the amount of surface shear stress, given knowledge of the stated input parameters for these patches of roughness. As the shear stress partitioning relationship within the roughness appears to come into equilibrium faster for smaller roughness element sizes it would also appear the shear stress partitioning model can be applied with confidence for smaller patches of smaller roughness elements than those used in this experiment