Sponsored by the Geomorphology Specialty Group
Association of American Geographers 103rd Annual Meeting
San Francisco, California, April 17-21.
Aeolian Geomorphology I
Paper Session 1344
Tuesday, 4/17/07, from 12:00 PM - 1:40 PM
Coastal and Marine Specialty Group
Geomorphology Specialty Group
Jean Taylor Ellis - Texas A&M University
Paul A. Gares - East Carolina University
Paul A. Gares - East Carolina University
12:00 PM Author(s):
*Patrick P Pease - University of Northern Iowa
Paul Gares - East Carolina University
Abstract Title: Air flow transformations associated with the onshore movement of wind: a field experiment at Jockey's Ridge State Park, North Carolina, USA.
The influence of water waves on the vertical profiles of wind has been well documented. However, the transformation of airflow conditions during the transition from water to land is less understood. This study presents preliminary data from field experiments designed to examine the wind profile and shear stress associated with air flow moving from over water with small waves onto a relatively flat, sandy shore. The experiment was conducted at Jockey's Ridge State Park, North Carolina. Three masts with 5 anemometers each were used in the study. Mast 1 was located in 0.5 m of water, approximately 50 m from the shore. Mast 2 was located about 2 meters from the strandline, oriented normal to the beach relative to the offshore mast. Mast 3 was located 12 m further inland from mast 2, again, aligned shore-normal relative to the other masts. Each mast was also fitted with temperature and humidity sensors. Two pressure transducers, one located at mast 1 and the second located about 20 m from shore, were used to characterize the wave conditions during experimental runs. Additionally, 16 sand traps were placed in a transect between masts 2 and 3 to characterize variations in transport. Significant differences in the wind profiles at the three masts were seen during the transformation from water to beach. The roughness characteristics measured at mast one also showed significant variation with different wave regimes. The wind characteristics and transport potential are discussed for several wind regimes.
Keywords: Air flow; land-sea interactions, sediment transport; coastal dunes
- 12:20 PM Author(s):
*Irene Delgado-Fernandez - University of Guelph
Robin Davidson-Arnott - University of Guelph
Abstract Title: Vertical Distribution of Aeolian Sand Transport on Beaches.
Sand transport studies indicate that, among the three processes of wind blown sand drift, saltation is the principal mode of movement, representing three quarters of the transport. The total distribution of transport rates is a function of height. Our understanding of vertical flux profiles thanks to wind tunnel studies is improving, and we know that the sand flux over a sandy surface increases with height in the very near surface layer, but then decays exponentially. Several theoretical models of saltation have been published, in particular on the higher portion of the sand trajectory, but none of them have proven to be applicable at a wide range of sites. We do not have much data on field measurements of the distribution of transport with height above the bed. More experimental work is needed if we are to refine our predictions of total sediment transport rates. This study reports on the results of the measurement of vertical aeolian transport in the field. We analyze the temporal and spatial variability of the saltating cloud over several wind events, along the line of the main wind direction. A tower of Safires, properly calibrated and distributed at specific heights, is located at different points on the beach. The data is combined with the measurements of total sand transport and wind characteristics. Results allow us to critically assess the measurements of saltating grains taken with Safires, and the convenience of locating them at certain heights in order to obtain a better representation of the saltating cloud.
Keywords: saltation, field study, aeolian transport, flux profile
- 12:40 PM Author(s):
*Graziela Miot da Silva - Louisiana State University
Abstract Title: Foredune Vegetation Patterns and Longshore Environmental Gradients.
This paper examines the spatial variation in foredune vegetation along Moçambique beach, a headland bay beach in southern Brazil. This embayment extends through 600 of curvature, and displays significant alongshore variations in exposure to the prevailing winds and waves, beach/surfzone morphodynamic type, type and dimensions of the dune systems, foredune vegetation cover and diversity. In order to assess the characteristics of the foredune vegetation, two surveys were carried out in contiguous quadrats of 1m2 extending across 6 foredune profiles. The results show that the vegetation cover decreases from south to north, possibly reflecting the increasing exposure to wind and wave energy. Distinct patterns of species distributions occur along Moçambique beach, such that different plant species are dominant in the southern, middle and northern ends of the beach. A cluster analysis demonstrated two associations: the first one is represented by the profiles located in the lower energy zone of the beach and the second association is represented by the profiles more exposed to wind and wave energy, sediment deposition and salt spray. There is a relatively higher diversity of species in the middle of the beach, and the lowest diversity occurred in the northernmost end of the beach, possibly influenced by the surfzone type, number of breaking waves and highest aeolian transport.
Keywords: foredune vegetation, coastline orientation, Brazil-southern
- 1:00 PM Author(s):
*Valdez Andrew - National Park Service
Abstract Title: Physical Processes that Control the Development of the Great Sand Dunes Aeolian System, Colorado, USA and their role in the management of Great Sand Dunes National Park.
Great Sand Dunes, Colorado (GRSA) is a geographic wonderland with varied aeolian deposits that respond to modern processes and critical land management issues generated by the desire to protect natural processes. GRSA is the site of an aeolian system that transitions along a topographic gradient from a sabkha, to a sandsheet, to a dunefield, and ends in sand ramps juxtaposition to fault block mountains. Dune types also vary along that gradient. The variation in sand deposits and dune types result from the interaction of 5 geologic processes that are fundamental to the development of the aeolian system. These processes are: crustal rifting, sand transport related to wind regime, sand transport by stream flow, sand stabilization by vegetation growth and sand cementation by evaporate minerals. Rifting creates a depositional environment that allows sand to accumulate. Wind regime controls dune type and behavior. Streams modify the perimeter of the dunefield. Vegetation has stabilized surfaces. Other areas have been hardened by evaporite minerals. The National Park Service (NPS) has managed GRSA since 1932. Initially, the management focus was on visitor services, but a proposed water development project adjacent to the dunes exposed the lack of scientific knowledge. That began an effort to better understand the area's natural system, leading to the geologic process model described above. The NPS has used the geologic process model to interpret the dunes to visitors, evaluate potential threats, protect park resources, and to justify a boundary expansion to manage a larger portion of the aeolian system.
Keywords: Aeolian Geomorphology
- 1:20 PM Author(s):
*Haim Tsoar - Ben Gurion University of The Negev
Noam Levin - Ben Gurion University of The Negev
Abstract Title: The Effect of Climate Change on the Mobility and Stability of Coastal Sand Dunes in NE Brazil.
The coastal zone of NE Brazil consists of sandy beaches and is backed by extensive dune fields. In Lencois Maranhenses, the largest area of coastal dunes in NE Brazil, barchan dunes cover an area of 1,550 square km and penetrates more than 25km inland. Vegetated parabolic dunes are found there at a distance of more than 150km inland from the present coastline. The area is tropical with 1500-2500 mm of annual average rainfall. Our model is based on several fact findings. The mobility of sand dunes is related to wind power. Wind erosion is the limiting factor for vegetation in sand dunes because seedlings are eroded at high wind power. The amount of rainfall is not a significant factor for vegetation growth on dune sand. A dynamical model, based on the ITCZ migration predicts lower wind speeds during wet periods in NE Brazil, when the ITCZ shifted more to the south than what exists today. That ought to favor the stabilization of sand dunes. Impediment of the southern migration of the Atlantic ITCZ is caused when a north-south temperature gradient drives a low level atmospheric circulation that strengthens the trade winds in NE Brazil, together with a reduction in rainfall. Wind power and rainfall are negatively correlated, both on a monthly and a yearly scale, as shown from the seasonal wind and rainfall variation that accompanies the yearly displacement of the ITCZ.
Keywords: aeolian processes; climate change; coastal sand dunes; NE Brazil
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.