Onshore flow steering and sand transport over a linear foredune during high energy oblique incident winds

Mike Hilton, Scott Hatcher
University of Otago, Dunedin, New Zealand



Incident winds strike many coasts at oblique angles.  These winds are then deflected onshore as the flow approaches and crosses the seaward slope of the foredune. To date there have been few observations of flow deflection and few measurements of beach-foredune sand flux during strongly oblique incident winds. The current paper examines these processes during two high-energy wind events (incident wind speeds of 15-32 ms-1), where the incident winds vary from oblique offshore (124o), foredune crest parallel (90o), to oblique onshore (31o) - where 0o is directly onshore.

Wind flow across the back beach and foredune was measured using sonic anemometers (1Hz) on two occasions during 2015 (Experiments 1 & 2). The anemometers were deployed parallel to the bed at 0.30m in Experiment 1. During the second experiment anemometers were placed at 0.35, 0.66 and 1.66m above the bed, allowing comparisons with recent empirical work and CFD simulations. Sand flux across the back-beach and stoss face of the foredune was measured using a new self-orientating sand trap (see Hilton et al., this conference). Sand deposition on the lee slope of the foredune was estimated using pot traps. Incident wind (IW) speed and direction were measured on a 4m beach tower.

We observed flow deflection across the stoss face in onshore oblique wind conditions. Flow deflection varied between 33o (for an IW of 58o) and 41o (for an IW of 63o). Onshore flow deflection decreases with increasing incident wind obliquity. Most deflection occurred across the steeper upper stoss face (57o). Wind speed decreased across the stoss face of the foredune, however, deceleration was greater for alongshore winds (90o, 165% loss) compared to less oblique winds (73o, 13% loss). Deceleration for highly oblique incident winds results from increased fetch/surface friction as these winds cross a greater length of vegetated foredune. During Experiment 2 oblique flows were observed to be deflected less at the 0.66m and 1.66m anemometer heights and during more oblique incident winds and acceleration was measured across the stoss face at these heights.

Highly oblique and alongshore winds are geomorphically important. We trapped at three elevations at 6 locations across the face of the foredune during Experiment 1. Oblique alongshore winds (55o - 67o) exceeding 10ms-1 transported sand along and across the stoss face, but very little crossed the crest. Short periods of higher speed and less oblique winds are needed to generate significant cross-crest sand flux.