Responses of three-dimensional flow to variations in the angle of incident wind and profile form of dunes: Greenwich Dunes, Prince Edward Island, Canada
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
References (39)
Patterns of sand transport on vegetated foredunes
Geomorphology
(1996)Rates of aeolian transport on a beach in a temperate humid climate
Geomorphology
(1996)- et al.
Aeolian sediment transport on a beach: surface moisture, wind fetch, and mean transport
Geomorphology
(2009) - et al.
Geomorphic response to late Holocene climate variation and anthropogenic pressure, northeastern Prince Edward Island, Canada
Quaternary International
(2002) - et al.
Aeolian sediment transport on a beach: thresholds, intermittency, and high frequency variability
Geomorphology
(2009) - et al.
The effect of wind gusts, moisture content and fetch length on sand transport on a beach
Geomorphology
(2005) Ecological processes and plant adaptations on coastal dunes
Journal of Arid Environments
(1991)Foredunes and blowouts: initiation, geomorphology and dynamics
Geomorphology
(2002)- et al.
Flow dynamics over a vegetated foredune at Prince Edward Island, Canada
Geomorphology
(2005) - et al.
Aeolian transport of sediment on a beach during and after rainfall, Wildwood, NJ, USA
Geomporphology
(1998)
Sediment flux and airflow on the stoss slope of a barchan dune
Geomorphology
Numerical modelling of flow structures over idealised transverse aeolian dunes of varying geometry
Geomorphology
Effects of fetch and surface texture on aeolian sand transport on two nourished beaches
Journal of Arid Environments
Physical and logistical considerations of using ultrasonic anemometers in aeolian sediment transport research
Geomorphology
The role of streamline curvature in sand dune dynamics: evidence from field and wind tunnel measurements
Geomorphology
The dynamic effects of moisture on the entrainment and transport of sand by wind
Geomorphology
Airflow over foredunes and implications for sand transport
Earth Surface Processes and Landforms
Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport intensity
Geomorphology
A general framework for modeling sediment supply to coastal dunes including wind angle, beach geometry, and fetch effects
Geomorphology
Cited by (84)
Flow dynamics over a high, steep, erosional coastal dune slope
2022, GeomorphologyCoastal Dunes
2022, Treatise on GeomorphologyDune Morphology and Dynamics
2022, Treatise on GeomorphologyProcess-Based Beach and Dune Systems
2022, Treatise on GeomorphologyAirflow Dynamics Over Unvegetated and Vegetated Dunes
2022, Treatise on Geomorphology