Elsevier

Geomorphology

Volume 105, Issues 1–2, 1 April 2009, Pages 127-138
Geomorphology

Responses of three-dimensional flow to variations in the angle of incident wind and profile form of dunes: Greenwich Dunes, Prince Edward Island, Canada

https://doi.org/10.1016/j.geomorph.2007.12.019Get rights and content

Abstract

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.

Introduction

In boundary layer flow over complex dune topography, alterations in the magnitude and direction of near-surface winds (i.e., z < 10 m) are common and can be significant. Resulting patterns of secondary flow define local vectors of sand transport that, in turn, control dune morphodynamics at spatial scales smaller than the landform itself. As a result, relatively simple regional models of sand transport, used to classify dune form — wind regime relations (e.g., Fryberger, 1979), or two-dimensional sediment budget models of coastal dunes (e.g., Arens, 1996a), are limited in modeling the process–response dynamics of dunes. Such models do not consider inherent, topographically-induced three-dimensional variations in near-surface flow and sand transport that occur within the landform system under otherwise “steady” conditions of incident flow.

Recent research has shown that dune morphodynamics in coastal settings are controlled by several key factors including: i) direction of the incident wind and resulting beach fetch and sediment supply (e.g., Nordstrom and Jackson, 1993, Davidson-Arnott and Law, 1996, van der Wal, 1998, Jackson and Cooper, 1999, Bauer and Davidson-Arnott, 2002, Hesp et al., 2005), ii) type and density of vegetation (e.g., Buckley, 1987, Hesp, 1989, Hesp, 1991, Arens, 1996a, Hesp, 2002), and iii) moisture content of the sand (e.g., Belly, 1964, Sarre, 1989, Namikas and Sherman, 1995, Arens, 1996b, Jackson and Nordstrom, 1998, Wiggs et al., 2004, Davidson-Arnott et al., 2005). In contrast, less research exists on the interactions between beach–dune topography and near-surface airflow as they control sand transport and dune morphodynamics. In particular, examination of patterns of secondary flows, such as topographic steering, forcing (e.g., stagnation and acceleration of flow), and (semi)coherent flow structures (e.g., flow separation and reversal cells), has received less attention than some of the key controls listed above, largely because of pre-existent limitations in instrumentation (e.g., Walker, 2005). These patterns are well-recognized within desert and coastal dune systems (e.g., Svasek and Terwindt, 1974, Jackson, 1977, Rasmussen, 1989, Nickling and Davidson-Arnott, 1991, Arens et al., 1995, Hesp and Hyde, 1996, Lancaster et al., 1996, Wiggs et al., 1996, McKenna Neuman et al., 1997, Hesp and Pringle, 2001, Walker and Nickling, 2002, Hesp et al., 2005, Walker et al., 2006). Understanding the three-dimensional properties of flow and implications for sand transport and morphodynamics of coastal dune systems, however, remains limited. To date, the combined effects of these interactions with the effect of beach fetch (i.e., the available transportable sand surface controlled by the direction of incident flow and beach moisture content) have limited the ability to quantitatively model beach–foredune sediment budgets and morphodynamics (Bauer and Davidson-Arnott, 2002, Davidson-Arnott et al., 2003).

This study examines the response of near-surface, three-dimensional airflow properties over two morphologically distinct profiles of a vegetated foredune during an intense fall storm. The implications of observed patterns and properties of flow over the dune to a 52° range in onshore flow are discussed. This experiment follows previous work in 2002 (Hesp et al., 2005, Walker et al., 2006) and is part of a larger collaborative study on the airflow and sedimentary dynamics within this beach–dune complex conducted in October 2004 (Bauer et al., 2009-this volume, Davidson-Arnott and Bauer, 2009-this volume).

Section snippets

Study site and event conditions

The study site is located on a stretch of foredune within the Greenwich Dunes, Prince Edward Island National Park on the northeastern shore of Prince Edward Island (PEI), Canada (Fig. 1). Two parallel shore-normal measurement transects were established on morphologically different profiles (Fig. 2, Fig. 3, Fig. 4). The west transect was located on a densely vegetated (17–45%) 9.5-m high foredune with a 0.5-m high incipient seaward foredune. The seaward slope of the foredune consisted of a steep

Methods

Turbulent wind speed and direction were measured at five stations on each transect from the incipient dune (station 1) to the crest (station 5) (Fig. 2, Fig. 3). Stations were equipped with Gill WindSonic 2-d anemometers at 1.65 m and Gill WindMaster 3-d sonic anemometers at 0.6 m above the surface inverted to measure polar wind speed (U in the horizontal plane), direction, and vertical velocity (W) at the height of the vegetation canopy (Fig. 3). All instruments were aligned to true north and

Results

Results are presented from the total duration of the experiment (from 0900 to 1700 h), as well as for three sub-events (Fig. 6). High frequency time series (1 Hz with 60 s running mean) of near-surface (0.6 m) conditions of flow from station E1 on the incipient foredune are shown for Runs 1–3 in Fig. 7. Fig. 8, Fig. 9 show trends in streamwise velocity (U) and steadiness of flow (CVU) and in vertical velocity (W) and steadiness (CVW) in response to changing conditions of incident flow (speed

Discussion

Results presented here demonstrate several key responses of the properties of flow to topographic forcing and steering effects over foredunes that, in some cases, depend on the speed and direction of incident flow and/or on the form of the dune profile (i.e., concave–straight vs. concave–convex).

Conclusions

This study examines three-dimensional variations in key properties of flow measured from ultrasonic anemometry over two morphologically different foredune profiles. Results show that topographic steering and forcing effects cause the local properties and vectors of flow to deviate significantly from the regional wind in response to changes in over 50° of direction of incident flow and because of the effects of the form of the dune profile on near-surface flow. The main conclusions are:

  • (1)

    The

Acknowledgements

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

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