TY - JOUR
T1 - Responses of three-dimensional flow to variations in the angle of incident wind and profile form of dunes
T2 - Greenwich Dunes, Prince Edward Island, Canada
AU - Walker, Ian J.
AU - Hesp, Patrick A.
AU - Davidson-Arnott, Robin G.D.
AU - Bauer, Bernard O.
AU - Namikas, Steven L.
AU - Ollerhead, Jeff
N1 - Funding Information:
This research was supported by an NSERC Special Opportunities Research Grant to the co-authors, NSERC operating grants to RDA and IJW, and an LSU Faculty Research Grant to PAH and SLN. Additional research infrastructure support was provided by the Canada Foundation for Innovation and British Columbia Knowledge Development Fund to IJW. Permission and generous logistical support for this research was granted from Parks Canada — Prince Edward Island National Park Reserve at Greenwich Dunes and special thanks are extended to Park Ecologist Denyse Lajeunesse and Warden Roger Steadman. Valuable field assistance was also provided by J. Anderson and L. Olsen.
PY - 2009/4/1
Y1 - 2009/4/1
N2 - This study reports the responses of three-dimensional near-surface airflow over a vegetated foredune to variations in the conditions of incident flow during an 8-h experiment. Two parallel measurement transects were established on morphologically different dune profiles: i) a taller, concave-convex West foredune transect with 0.5-m high, densely vegetated (45%), seaward incipient foredune, and ii) a shorter, concave-straight East foredune transect with lower, sparsely vegetated (14%) seaward incipient foredune. Five stations on each transect from the incipient dune to the crest were equipped with ultrasonic anemometers at 0.6 and 1.65 m height and logged at 1 Hz. Incident conditions were recorded from a 4-m tower over a flat beach. Winds increased from 6 m s- 1 to > 20 m s- 1 and were generally obliquely onshore (ENE, 73°). Three sub-events and the population of 10-minute averages of key properties of flow (U, W, S, CVU) from all sample locations on the East transect (n = 235) are examined to identify location- and profile-specific responses over 52° of the incident direction of flow (from 11 to 63° onshore). Topographic steering and forcing cause major deviations in the properties and vectors of near-surface flow from the regional wind. Topographic forcing on the concave-straight dune profile increases wind speed and steadiness toward the crest, with speed-up values to 65% in the backshore. Wind speed and steadiness of flow are least responsive to changes in incident angle in the backshore because of stagnation of flow and are most responsive at the lower stoss under pronounced streamline compression. On the steeper concave-convex profile, speed and steadiness decrease toward the crest because of stagnation of flow at the toe and flow expansion at the slope inflection point on the lower stoss. Net downward vertical velocity occurs over both profiles, increases toward the crest, and reflects enhanced turbulent momentum conveyance toward the surface. All of these flow responses are enhanced with faster speeds of incident flow and/or more onshore winds. Significant onshore steering of near-surface vectors of flow (to 37°) occurs and is greatest closer to the surface and during highly oblique winds (~ 15° onshore). Therefore, even subtle effects of streamline compression and amplification of flow under alongshore conditions effectively steer flow and sand transport toward the dune. As topographic forcing and steering cause significant, three-dimensional deviations in near-surface properties of flow, most regional-scale and/or two-dimensional models of dune process-response dynamics are insufficient for characterizing coastal and desert dune sediment budgets and morphodynamics. In particular, deflection of sand transport vectors with greater fetch distances than those derived from regional winds may occur. Coincident flow, transport and morphological response data are required to better quantitatively model these processes.
AB - This study reports the responses of three-dimensional near-surface airflow over a vegetated foredune to variations in the conditions of incident flow during an 8-h experiment. Two parallel measurement transects were established on morphologically different dune profiles: i) a taller, concave-convex West foredune transect with 0.5-m high, densely vegetated (45%), seaward incipient foredune, and ii) a shorter, concave-straight East foredune transect with lower, sparsely vegetated (14%) seaward incipient foredune. Five stations on each transect from the incipient dune to the crest were equipped with ultrasonic anemometers at 0.6 and 1.65 m height and logged at 1 Hz. Incident conditions were recorded from a 4-m tower over a flat beach. Winds increased from 6 m s- 1 to > 20 m s- 1 and were generally obliquely onshore (ENE, 73°). Three sub-events and the population of 10-minute averages of key properties of flow (U, W, S, CVU) from all sample locations on the East transect (n = 235) are examined to identify location- and profile-specific responses over 52° of the incident direction of flow (from 11 to 63° onshore). Topographic steering and forcing cause major deviations in the properties and vectors of near-surface flow from the regional wind. Topographic forcing on the concave-straight dune profile increases wind speed and steadiness toward the crest, with speed-up values to 65% in the backshore. Wind speed and steadiness of flow are least responsive to changes in incident angle in the backshore because of stagnation of flow and are most responsive at the lower stoss under pronounced streamline compression. On the steeper concave-convex profile, speed and steadiness decrease toward the crest because of stagnation of flow at the toe and flow expansion at the slope inflection point on the lower stoss. Net downward vertical velocity occurs over both profiles, increases toward the crest, and reflects enhanced turbulent momentum conveyance toward the surface. All of these flow responses are enhanced with faster speeds of incident flow and/or more onshore winds. Significant onshore steering of near-surface vectors of flow (to 37°) occurs and is greatest closer to the surface and during highly oblique winds (~ 15° onshore). Therefore, even subtle effects of streamline compression and amplification of flow under alongshore conditions effectively steer flow and sand transport toward the dune. As topographic forcing and steering cause significant, three-dimensional deviations in near-surface properties of flow, most regional-scale and/or two-dimensional models of dune process-response dynamics are insufficient for characterizing coastal and desert dune sediment budgets and morphodynamics. In particular, deflection of sand transport vectors with greater fetch distances than those derived from regional winds may occur. Coincident flow, transport and morphological response data are required to better quantitatively model these processes.
KW - Aeolian
KW - Airflow
KW - Foredune
KW - Greenwich Dunes
KW - Prince Edward Island
KW - Ultrasonic anemometer
UR - http://www.scopus.com/inward/record.url?scp=59349101568&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=59349101568&partnerID=8YFLogxK
U2 - 10.1016/j.geomorph.2007.12.019
DO - 10.1016/j.geomorph.2007.12.019
M3 - Article
AN - SCOPUS:59349101568
SN - 0169-555X
VL - 105
SP - 127
EP - 138
JO - Geomorphology
JF - Geomorphology
IS - 1-2
ER -