Aeolian Sand Transport Research Articles

Measurements of Drag Coefficients and Roughness Length at a Sea-Beach Interface

Abstract During unstable conditions the drag coefficient of the surf zone and the roughness length of a sand beach have been measured. It is concluded from ∼100 half-hour profiles that the drag coefficient under unstable conditions over the surf zone is the same as over the ocean. For wind speeds 8 m s−1 the roughness length of a sand beach is ∼10 times higher than the value measured below 6 m s−1. This jump is related to aeolian sand transport at high wind speeds.

The Formation of Shadow Dunes

ABSTRACT An investigation of the modification of airflow by a discrete, semi-circular roughness element (in this case, vegetation), the resulting flow structure, and the formation of pyramidal-shaped shadow dunes is presented. Two erect pioneer sand dune grasses, Festuca littoralis and Ammophila arenaria, were used in the experiments which were conducted in both the field and a wind tunnel for a range of wind velocities (4-20 m sec-1). The three-dimensional flow structure about the vegetation is dominated by horizontal separation characterized by symmetrically opposed reversing vortices which form within a triangular wake. A vertical separation envelope is also formed over the vegetation. Pyramidal shadow dunes are formed within this flow structure when aeolian sand transport occurs. Shadow dune height is dependent on element basal width and repose angle of the sand. Shadow dune length is dependent on element basal width and wind velocity. The results are extrapolated to include all semi-porous to non-porous, semicircular, discrete roughness elements. The role of shadow dune formation in the development of foredunes and related environments is examined.

Beach Salcrete and Eolian Sand Transport: Evidence from North Queensland

ABSTRACT The environmental conditions associated with the development of a salt crust on a marine beach in North Queensland are described. The crust reached a maximum thickness of 1.5 cm close to the berm and resisted sand transport by winds of up to 12 m/s. Chloride content of the crust averaged 0.188%. Salt crust formation reduces eolian deflation from beach surfaces, but is less important than rate of sand supply and exposure to strong onshore winds as a limiting factor in coastal dune development in North Queensland.

A review of selected literature on sand stabilisation

The fluid mechanics and particle mechanics of uniform eolian sand transport are very briefly surveyed with attention to the work of R.A. Bagnold and subsequent contributors. The review then turns to work on the production and maintenance of sand dunes and places emphasis on the use of sand fences for these purposes. In both areas reference is made to earlier reviews, as well as to original sources. The final part of the paper consists of an extensive list of references which does not, however, seek to be exhaustive, but confines itself to items which have been found significant in studying the mechanics of dune growth.

Environmental management of the Colorado River within the Grand Canyon

Irreversible environmental changes are occurring along the Colorado River in the Grand Canyon as a result of regulation of the river flow by the Glen Canyon Dam. The questions of primary importance in managing this great natural resource are 1) in what manner and how rapidly are the physical and ecological adjustments taking place, and 2) is the increased use of the river for recreational boating contributing to the degradation? Human use along the Colorado River is limited, for the most part, to the relic, pre-dam fluvial deposits colloquially called “beaches.” With the new river regime these deposits are positioned well above the present high-water stage, 27,000 cubic feet/second (cfs), or 765 cubic meters/second (cms), so they are not replenished periodically as they were prior to construction of the dam in 1963. The dominant natural processes now are aeolian sand transport and mass wasting. The float-trip passengers use the river beaches for hiking, camping, and. lunch stops. At the most desirable sites thirty to forty people camp on the beaches each night over a four to five month season. Human impact includes incorporation of campsite litter, burial of chemically treated waste, and the direct stress associated with people walking on the vegetation and unstable sedimentary deposits. Results of our investigations indicate that the rate of degradation at the most heavily used sites exceeds the capacity of aeolian processes to reestablish natural landscapes. Therefore, careful management of float trjps is needed if these environments are to be maintained in a natural state rather than a “sand-box” state.

Computing Eolian Sand Transport from Shear Velocity Measurements

Atmospheric shear velocity measurements for eolian transport of some common sand sizes were made on beaches, dry tidal flats, and small dune fields in Florida, Texas, the Alaskan Arctic, and Ecuador. The linear relationship between shear and wind velocities was further verified by these field data incorporated with Bagnold's Libyan Desert data. From these measurements, a simple method is given for computing eolian transport of the most common sand particle sizes which occur on coasts and deserts.

Wind stress criteria in eolian sand transport

The linear relationship between shear velocities and wind velocities was verified from field data collected on Ecuadorian beaches during periods of sand transport. From these data a Froude number was established that expresses the relationship among wind stress, gravity acceleration, and mean grain size of sand particles. The rate of eolian sand transport by wind can be scaled by using this number. The concept was verified by available field and wind tunnel data on the subject. The equation is expressed as follows: q=K (Fr)3 where q is the rate of eolian sand transport, K is the eolian sand transport coefficient with the same dimension as q, and Fr is the Froude number.

Aerodynamic roughness criteria in aeolian sand transport

It is widely stated in wind erosion studies that the boundary layer flow of surfaces of sand is aerodynamically rough at the inception of particle motion. At comparable roughness Reynolds numbers in water, however, the roughness state is considered as transitional. Clarification of the matter is important if the nature of fluid force acting on the surface particles is to be understood. In this study the boundary layer velocity profiles of published wind tunnel results are reanalyzed in terms of the relation between the roughness length z0 and the roughness Reynolds number. For aerodynamically rough flow, the ratio z0/k is expected to be constant as the Reynolds number (V*k/ν) varies. Most of the evidence indicates that the boundary flow is transitional rather than rough. Furthermore, the results do not follow the classical work of Nikuradse for uniform sand roughness. This may be due to the uncertainties in defining the reference datum (z = 0) and the effective roughness element length k. It is concluded that there is insufficient experimental evidence for the assertion of fully developed roughness.