Skimming Flow and Sand Transport Within and Above Ammophila (Marram) Grass on a Foredune

Abstract

AbstractAn investigation of wind flow and sand transport over a low to high density Ammophila dominated foredune on the Łeba barrier, Poland, utilizing multiple anemometers and sand traps is presented. The study provides the first high resolution data on modified saltation and suspended sand transport over a foredune under conditions of both canopy and skimming flow. Topographic acceleration of flow and topographic steering over the foredune were observed during oblique incident above threshold flow conditions. The concentration of sand in aeolian transport decreases with height to a minimum that is reached at different elevations above the ground, depending on Ammophila grass density. Of the three Ammophila (marram) grass densities investigated, nearly half (46%) of the total sand‐transport rate occurred above the lower density Ammophila grass, whereas up to 70% of the total transport occurred above the higher density marram grass. This likely results from both elastic collisions of sand grains with grass leaves over the Ammophila canopy and the different positions of the canopy rebounding surface, which is sensitive to even slight changes in wind speed. In addition, as plant density increases, there is less flow penetration into the canopy, vegetative forcing (similar to topographic forcing) of vertical flow is greater, and sand in both modified saltation and suspension is forced higher above the canopy in skimming flow. The maximum sand transport above the vegetation canopy increased in elevation from 32 to 48 to 53 cm as the vegetation density increased from low to moderate to high, respectively. The study also demonstrates that in skimming flow, relatively more sand is transported at higher elevations above a canopy surface than above a bare sand surface.