Sponsored by the Geomorphology Specialty Group
Association of American Geographers 102nd Annual Meeting
Chicago, Illinois, March 7-11.
Aeolian Geomorphology II
Paper Session 4217
Friday, 3/10/06, from 10:00 AM - 11:40 AM
Geomorphology Specialty Group
Jean Taylor Ellis - Texas A&M University
Paul A. Gares
Jean Taylor Ellis - Texas A&M University
10:00 AM Author(s):
*Jennifer L. Booth, Professor - Louisiana State University
Abstract Title: Wind Flow Characteristics: A Comparison Between Artificial and Natural Vegetation
Several studies have been conducted regarding flow dynamics around vegetation and how it interacts with the obstruction to promote sediment deposition. Much of this research however, has been conducted using solid elements including wooden dowels or reeds, which does little to account for the flexibility and porosity of natural vegetation. This experiment is designed to compare previously developed models with sediment trapping around natural vegetation. Studies were conducted over an eight day period at Padre Island Nation Seashore during the month of July when winds were directly onshore. Vegetation density was calculated for both the artificial and the natural vegetation by measuring the total exposed frontal area as well as the surface area of each leaf. The ratio of surface coverage between the artificial and the natural vegetation was roughly a four to one relationship respectively. Measures were taken at 5% density cover increments up to a maximum of 70% vegetation density. It was found that although artificial vegetation reduced wind velocity at the bed more substantially than the natural vegetation, the natural vegetation created a larger momentum-sink overall. By reducing velocities higher in the boundary layer flow, the potential for trapping sediment is much greater than proposed by the original models.
Keywords: Vegetation, density, deposition
- 10:20 AM Author(s):
*Paul A. Gares - East Carolina University
Patrick Pease - East Carolina University
Abstract Title: Influence of topography on wind flow and sediment transport over a blowout at Jockey’s Ridge State Park, North Carolina
Numerous researchers have examined the influence of topography on air flow and sediment transport over coastal foredunes. In general, masts with vertical arrays of anemometers are placed along transects over the dune. These layouts show that wind speed accelerates upward over the dune, and sediment transport can be expected to increase proportionally as long as the slope remains gentle. Few studies have examined a broader spatial distribution of air flow over topography. This project focuses on a blowout located in Jockey’s Ridge State Park, on the Outer Banks of North Carolina. Thirteen anemometers and wind vanes were placed on 0.5 m posts at various locations in and adjacent to the blowout. A 4 m mast with 5 anemometers located upwind of the blowout provided a vertical representation of the air flow. Sixteen wedge-shaped sand traps were positioned next to the wind sensors. Air flow over the topography was sampled during three wind events with different directional components. The blowout, as expected, acts as a funnel for air flow and sediment transport during wind events whose direction is aligned with the orientation of the blowout. When winds blow at an oblique angle to the blowout, large amounts of sand are transported into the mouth of the blowout where they are deposited. When winds blow perpendicular or at an oblique angle in the offshore direction, sediment is moved into the blowout from landward end and small amounts are carried through the blowout to the seaward end of the feature.
Keywords: Air flow; sediment transport; blowout
- 10:40 AM Author(s):
*Patrick Pease - East Carolina University
Paul A. Gares - East Carolina University
Abstract Title: The influence of topography and approach angles on local deflections of the wind field in a coastal blowout, Jockey’s Ridge State Park, North Carolina, USA
The spatial variability of air flow through complex topographies is not fully understood. This study examines the wind field and associated sediment transport through a linear blowout in coastal dunes at Jockey’s Ridge State Park, on the Outer Banks of North Carolina. A spatial array of single-height anemometers and wind vanes were placed within the blowout. Topography exerted a significant control on deflection of local wind directions which in turn either enhanced or inhibited the development of sand transport pathways depending on the relation between topography and approach angle. During winds that were within 50 degrees of the blowout axis, topography exerted the primary control on local airflow direction and sediment transport through the blowout. Wind flow in the trough aligned to the blowout axis regardless of approach angle which maximized erosion and transport of sand. In other locations local aspect variations caused deflections both proportional and disproportional to changes in the approach angle. Dominant wind flow approaching from greater than 50 degrees to the blowout axis was unable to deflect into the blowout topography and flow separation prevailed. Wind directions and speeds within the blowout became erratic as eddies dominated the flow. Net sand transport was minimized and deposition in the blowout occurred. Surrounding topography also played a key role in wind flow and sand transport. Winds blowing onshore were mostly unobstructed and effective at developing continuous transporting through the blowout. Winds from other angles were largely disrupted by upwind dune features and were more likely to cause deposition.
Keywords: Air flow; sediment transport; blowout; coastal dunes
- 11:00 AM Author(s):
*Graziela Miot da Silva - Louisiana State University
Patrick A. Hesp, Dr. - Louisiana State University,
Sergio Dillenburg - Universidade Federal do Rio Grande do Sul
Luiz Emilio de Sa Brito de Almeida - Universidade Federal do Rio Grande do Sul
Abstract Title: Coastline orientation and aeolian sediment transport on a headland bay beach in Southern Brazil
The orientation of the coastline in relation to the dominant or onshore component of the wind is important in determining the degree of aeolian sand transport into or along the foredune. On a headland bay beach with 60o of curvature, this influence can be very pronounced. This paper aims to investigate the magnitude of aeolian sand transport from onshore winds in different sectors of a headland bay beach in southern Brazil. To investigate this, a 30 year record of wind speed and direction was used and the onshore wind directions were determined according to the coastline orientation. The results show that the prevailing winds are from north (19% of the observations) and south (18%). The winds blowing from the north are onshore at the southern portion of the beach and offshore in the northern portion of the beach, and vice versa. In the center of the beach, these prevailing winds are alongshore, the onshore winds are from east and east-southeast, which comprise 4% and 1% of the observations, respectively. These winds also have lower speeds (maximum speeds around 10 m/ s, higher frequencies lower than 3 m/s) than the north and south winds which can reach speeds higher than 20 m/s. These results would suggest that a well developed foredune should develop in both extremities of the beach, but this is only true for the northern end. Here sediment supply is also critical, and modifies the influence of coastline orientation.
Keywords: coastline orientation, aeolian sediment transport, Brazil-southern
Session Description: This series of three special paper sessions are co-sponsored by the Coastal and Marine Geography (CoMa) and Geomorphology specialty groups. The research presented in the paper sessions cover a broad spectrum of spatial and temporal scales within aeolian geomorphology.