Sediment transport (dis)continuity across a beach–dune profile during an offshore wind event

Abstract

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 o deviation), 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 o 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-minute 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 (< 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 < 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 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 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.

Sediment budget controls on foredune height: Comparing simulation model results with field data

“…Zhou et al . () focus on assessing morphodynamic equilibrium in terms of sediment flux equilibria which can be expressed using a form of the Exner equation (Paola and Voller, ; Bauer et al ., ): where ƞ is elevation of the bed, t is time, ρ is sediment porosity, q s is sediment (volume) flux, and σ is an undefined sediment source or sink. Using this approach, they recognize three forms of morphodynamic equilibrium.…”

Offshore aeolian sediment transport across a human‐modified foredune

“…Lynch et al ., , and Bauer et al ., indicate that crest‐normal offshore winds can reverse to onshore directions on the seaward side of high sharp crested dunes consistent with lee‐side eddy formation, while more oblique winds tend toward more longshore flow directions. Bauer et al .…”

“…Wind speed can be greatest at the foredune crest and decline rapidly downslope, resulting in minimal transport at the base of the dune; wind speed and transport intensity then increase with distance from the dune (Walker and Nickling, ; Bauer et al ., ). In the absence of reversed flows, as monitored here, this increase in wind speed may be better termed reestablishment than reattachment.…”

“…Bauer et al . () report reversed flows with a vertical drop of 7 m in 20 m horizontal. The vertical drop at our site is 4.3 m in 20 m for a 12.1 deg slope.…”

“…Understanding the processes and rates of offshore transport will facilitate calculating coastal sediment budgets and assessing the effect of human actions on dune evolution. Previous studies have examined wind flow and rates of offshore aeolian transport on undeveloped coasts (Gares, ; Wal and McManus, ; Nordstrom et al ., ; Lynch et al ., and numerous studies identified in Walker and Nickling, and Bauer et al ., ), but comparative data on human altered coasts are limited (Nordstrom et al ., ).…”