Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event Bernard O. Bauer a, ⁎, Patrick A. Hesp b , Ian J. Walker c , Robin G.D. Davidson-Arnott d a Earth and Environmental Sciences & Geography, University of British Columbia Okanagan, Kelowna, BC V1V 1V7, Canada b School of the Environment, Faculty of Science and Engineering, Flinders University, Bedford Park, South Australia 5042, Australia c Dept of Geography, University of Victoria, PO Box 1700 STN CSC, Victoria, BC V8W 2Y2, Canada d Dept of Geography, University of Guelph, Guelph, ON N1G 2W1, Canada abstract article info Article history: Received 17 February 2015 Received in revised form 29 April 2015 Accepted 10 May 2015 Available online 16 May 2015 Keywords: Foredune change Sediment transport Flux divergence Wind steering Lee-side eddy recirculation Flow dynamics and sediment transport responses over a large, vegetated foredune at Prince Edward Island, Canada, during an offshore wind event are examined. Data were collected along an instrumented transect that extended from the dune crest, down the lee-side (seaward) slope of the dune, across a wave-cut scarp, and on to the back-beach. When the wind direction at the dune crest was approximately crest-normal (less than about 15° deviation), the mean near-surface flow directions along the dune slope and on the back beach were generally onshore, indicating reversed (onshore) flow relative to the regional (offshore) wind direction. Although flow patterns were consistent with a lee-side recirculation eddy, large excursions in flow direction were also prevalent, suggesting that the eddy was unstable and alternated with highly turbulent wake flow. As wind direction at the crest veered to greater than 20° from crest-normal, lee-side winds shifted toward strongly alongshore flow with minimal directionally variability. On the dune slope, the wind vectors were slightly offshore whereas on the back-beach they were slightly onshore. Wind speeds and sediment transport were greatest at the foredune crest and declined rapidly downslope due to flow expansion and deceleration in the wake zone as well as to the influence of a sparse vegetation layer. Mean particle counts (averaged over a 15-min interval) derived from laser sensors positioned at the crest were large (7.76 per second) in comparison to those measured in the immediate lee of the crest (0.52 per second) and farther down the dune slope (b 0.13 per second). In contrast, the values were as large as 25.62 per second on the middle of the back-beach, declining rapidly to a value of only 0.24 per second at the dune toe. Transport intensity was highly variable with the largest Activity Parameter (AP = 0.5) values at the dune crest and on the back-beach, and with the smallest values (AP b 0.1) on the lee-side dune slope down to the top of the scarp. Calculations of sediment (particle) flux divergence between instrument stations show that deposition was significant immediately downwind of the dune crest but negligible across most of the lower dune slope. Deposition was also prevalent on the dune ramp below the scarp. These results demonstrate that sediment transport across the beach–dune system was spatially discontinuous during this offshore wind event. Rebuilding of the dune ramp at the toe of the scarp occurred quite independently of, and with a different sediment source than, the broadening of the dune crest, which was fed with sediment from the landward side of the foredune. Such process ‘decoupling’ is an example of the complexity by which foredunes evolve or are maintained, and as with previous research reinforces the importance of offshore and alongshore wind events to beach–dune morphodynamics. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Considerable effort has been devoted to the understanding of aeolian dunes as equilibrium features that arise intrinsically from the steady state interaction of a moving fluid passing over a deformable bed comprised of non-cohesive sediments (Andreotti et al., 2009; Durán et al., 2010). The characteristics of the fluid (e.g., viscosity, density) and of the flow field (e.g., speed, direction, depth) in combination with the characteristics of the sediment surface (e.g., grain size, mineral density, sorting, roundness, roughness, cohesion) determine the rate of sediment transport and thereby the scale and geometry of the bedform that is likely to exist for a specific set of state parameters. Supporting evidence comes from studies in wind tunnels (e.g., Walker and Nickling, 2002, 2003; Dong et al., 2007) as well as a host of increasingly sophisticated numerical model simulations (e.g., Parsons et al., 2004; Geomorphology 245 (2015) 135–148 ⁎ Corresponding author. E-mail addresses: bernard.bauer@ubc.ca (B.O. Bauer), Patrick.hesp@flinders.edu.au (P.A. Hesp), ijwalker@uvic.ca (I.J. Walker), rdarnott@uoguelph.ca (R.G.D. Davidson-Arnott). http://dx.doi.org/10.1016/j.geomorph.2015.05.004 0169-555X/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph