Sponsored by the Geomorphology Specialty Group
Association of American Geographers 102nd Annual Meeting
Chicago, Illinois, March 7-11.
Aeolian Geomorphology I
Paper Session 4117
Friday, 3/10/06, from 8:00 AM - 9:40 AM
Geomorphology Specialty Group
Jean Taylor Ellis - Texas A&M University
Patrick A. Hesp - Louisiana State University
Paul A. Gares
8:00 AM Author(s):
*Douglas J. Sherman, Professor - Texas A&M University
Abstract Title: Error Analytics and Wind Blown Sand
Understanding sediment transport processes is fundamental to interpretation and prediction of the evolution of geomorphological systems comprising unconsolidated sediment deposits such as beaches and dunes. Despite decades of efforts to develop deterministic and stochastic models of aeolian sand transport, we continue to find generally poor agreement between predictions and measurements of sand flux. Explanations of the poor correspondence, or error, usually invoke problematic field methods or impacts of unanticipated (often highly-localized) environmental factors. This paper examines the apparent failure of our theoretical/experimental endeavor from the perspective of error analytics. Allchin (2001) identifies four types of error: material, observational, conceptual, and discoursive. Particular attention will be paid to one form of conceptual error - misspecified assumptions or boundary conditions. Errors of this sort have also been termed bias errors by Muste (2002). The effects of bias errors are pernicious in modeling efforts, because due diligence in their consideration requires placing error bands around all results. The range encompassed by such error bands may be substantial relative to any predicted value. For predicting aeolian sand transport, three sources of bias error are associated with most transport models: the specification of the value of von Karman’s constant for the derivation of shear velocity estimates; the specification of wind speeds used to derive velocity profiles; and the specification of the “A” constant in the threshold shear velocity equation. Small (e.g. 10%) bias errors from each of these sources, when compounded, may lead to estimation errors greatly exceeding 100% of predicted values.
Keywords: geomorphology, aeolian processes, errors
- 8:20 AM Author(s):
*Jean Taylor Ellis - Texas A&M University
Abstract Title: Quantifying Unsteadiness in Aeolian Saltation
The role of turbulence in shaping landforms has been one focus of geographical research for almost a century. The importance of turbulence is manifested in its impacts on sediment transport, whether in air or water. Although this importance has been widely recognized and studied in several disciplines, direct correspondence between turbulent events and pulses in sediment transport has been difficult to establish. Previous field- and laboratory-based aeolian investigations are contradictory whether saltation intermittency is related to wind speed fluctuations. Those that found a correspondence primarily established this relationship based on qualitative evidence. However, one deficiency with the previous works is that few measure wind velocity and saltation with precisely and tightly controlled time and space synchronization. The purpose of this study is to establish and quantify correspondence for the transport of sand by wind using novel instrumentation. Fieldwork was conducted on a beach in Shoalhaven Heads, New South Wales, Australia. Co-located thermal anemometer and high-frequency, microphone-based, saltation sensors were deployed 0.02 m above the bed. Individual grain impacts were discerned from the 6000 Hz sample-rate microphone time series and compared to the anemometer time series. Qualitative analysis indicates saltation responds to wind speed fluctuations. This correspondence was also established using statistical methods: linear regression analysis, principle components analysis, and variable interval time estimation method (VITA).
Keywords: coastal geomorphology, wind blown sand, sediment transport
- 8:40 AM Author(s):
*Steven Namikas - Louisiana State University
Yuanda Zhu - Louisiana State University
Abstract Title: Spatial and Temporal Variability in the Surface Moisture Content of a Fine-Grained Beach
It is widely recognized that surface moisture can be a key control on rates of aeolian transport in coastal environments. Although several empirical and physically-based models have been proposed to represent this influence, application of such models to real world systems is hampered by an almost complete lack of knowledge regarding actual spatial and temporal patterns of surface moisture content. The present study begins to address this need. To identify spatial and temporal patterns, surface moisture contents were measured across a 20m by 60m grid seven times per day over a two week spring-neap tidal cycle. To provide a process framework for interpretation of the observed patterns, simultaneous measurements of key parameters thought to influence moisture content were collected (e.g., tide and water table elevations, evaporation, condensation, humidity, temperature, wind speed and direction, and precipitation). Preliminary results and interpretations will be presented.
Keywords: aeolian transport, sediment transport, surface moisture content, beaches
- 9:00 AM Author(s):
*Bernard O. Bauer - University of British Columbia Okanagan
Robin G.D. Davidson-Arnott - University of Guelph
Abstract Title: Moisture and Temperature Trends on an Active Aeolian Beach Surface
Temporal and spatial changes in surface moisture content are especially prevalent across sandy coastal beaches due to precipitation, wave spray, tidal fluctuations, relative humidity gradients, solar radiation, and sediment stripping or accumulation. These realities confound our ability to model and predict sediment transport volumes from the beach into the adjacent dunes. Laboratory experiments are unable to simulate the complex relationships experienced on a natural beach, therefore mandating the deployment of careful measurement techniques in the field. As part of a broader experiment investigating air flow and sediment delivery from the beach and into the foredune system at Greenwich Dunes, Prince Edward Island, Canada, measurements of surface moisture and temperature were made extending from the upper limits of the swash zone to the base of the foredune. The results demonstrate that there are both strong temporal and spatial elements to beach surface temperature patterns where the former are controlled by the effectiveness of solar radiation and air temperature in heating or cooling the surface locally whereas the spatial trends are primarily dependent on proximity to the swash zone. In contrast, the trends in surface moisture content are dominantly spatial. This has significant implications for the ability of the wind to entrain and transport sediment to the dunes largely because of decreased fetch length during high tide or storm surge events.
Keywords: aeolian transport, beaches, moisture effects, temperature
- 9:20 AM Author(s):
*Patrick A. Hesp, Dr. - Louisiana State University,
Sergio Dillenburg, Dr. - CECO, UFRGS
Abstract Title: Evolution and Dynamics of a Prograded Transgressive Dunefield Barrier in Southern Brazil
This paper outlines the first detailed study of a southern Brazilian coastal barrier system in the Itapeva to Xangri-lá region in Rio Grande do Sul State. The barrier is a prograded, transgressive dunefield barrier, created by the multiple formation of overlapping transgressive dunefields. The majority of the transgressive dunefields are now fully vegetated, and the modern active transgressive dunefield is merely the latest phase of a suite of phases which have occurred since the beginning of barrier progradation. C14, TL and OSL dating of the barrier indicate the barrier began forming by 7,000 to 8,000 years ago and it continues to prograde to the present day. Sea-level curves for the region indicate that sea level reached around 2 meters above its present level around 5,000 years B.P., and then fell to the present level. The barrier system prograded throughoutthis period of sea level rise, stability and fall indicating that sediment supply can completely dominate barrier dynamics and negate coastal erosion due to sea level rise.
Keywords: Barrier evolution; sea level rise; prograded transgressive dunefield barrier
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.