Eolian sorting and roundness of beach and dune sands

Abstract

Measurements of sand transport on the beach near Noordwijkerhout (Netherlands) showed, that the wind picks up from the beach more of the angular grains than of the rounder grains. This should lead to the formation of dune sands that are more angular than the adjoining beach sands. However, Shepard and Young (1961) found that in the fine sand grades dune sands are rounder than the nearby beach sands, while Kuenen (1963) showed, that this is also true for the coarse sand grades, but that the medium sized dune sands are slightly more angular than the beach sands from which they derive. An explanation of these contradictory observations is tentatively given here, taking into account the different mechanism of sorting that may occur when sand is being transported by the wind. Roundness-selection may occur when the sand is creeping, when it is in saltation and when it is in suspension. Transport during creep and during saltation is favoured by roundness, because the rounder grain roll and bounce better. They also rotate more easily, thereby reaching a greater height above the bottom (Magnus-effect). As the wind velocity increases with height, they are transported faster and further. Transport during suspension is favoured by angularity, as the more angular grains have a lower fall-velocity. It was observed that creeping sand cannot move upslope, even during storms, when the slope is 10°–15° or more. The dunes, therefore, will consist mainly of sand transported in saltation and suspension. The observations at Noordwijkerhout showed that at low wind velocities (5–9 m/sec at 1 m height) actually dune sands are formed that are more angular than the nearby beach sands. It is assumed, therefore, that during the less frequent storm winds, which are at least as effective in building dunes as the more frequent gentle winds, conditions are reversed, leading to the formation of dune sands that are rounder than the corresponding beach sands. As the relative importance of the mechanisms of sorting will depend mainly on grain size and wind velocity, alternative deposition of sand transported by gentle winds and by storm winds, and removal of the more angular fine sand grades in suspension, may explain the observed differences in roundness between beach and dune sands.