Aeolian sand transport and deposition patterns within a large woody debris matrix fronting a foredune

Abstract

Sediment transport pathways and resulting erosion-deposition patterns across beach-foredune systems can be complex. Although a great deal is known about the effects of wind fetch, surface moisture, topographic forcing, and vegetation cover, the role of large woody debris (LWD) as a control on sediment redistribution across beaches is relatively understudied. Pieces of LWD act as non-porous roughness elements that induce secondary flow circulation, thereby creating unique sedimentation patterns that differ markedly from those over a flat beach. Large accumulations of LWD collectively have a bulk porosity that provides substantial sand trapping volume, yet, no studies to date have quantified the effect of LWD on aeolian sand transport. Results from a field study on a macrotidal beach on Calvert Island, British Columbia, Canada, show that the LWD matrix alters the character of the turbulent boundary layer in a way that reduces sediment flux by 99%. Sand is trapped within the LWD matrix, thereby interrupting sediment delivery from the nearshore to the foredune. As such, LWD has the potential to modulate rates of foredune recovery, growth, and evolution. The relative importance of this effect depends on the density and arrangement of LWD as a fundamental control on aeolian sediment transport, as well as on the magnitude and frequency of events that erode the beach periodically and re-organize the LWD matrix.