Sponsored by the Geomorphology Specialty Group
Association of American Geographers 103rd Annual Meeting
San Francisco, California, April 17-21.
Aeolian Geomorphology III
Paper Session 1544
Tuesday, 4/17/07, from 4:00 PM - 5:40 PM
Coastal and Marine Specialty Group
Geomorphology Specialty Group
Jean Taylor Ellis - Texas A&M University
Paul A. Gares - East Carolina University
Jean Taylor Ellis - Texas A&M University
- 4:00 PM Author(s):
*Douglas J. Sherman - Texas A&M University
Helena Granja - University of Minho
Jean Ellis - Texas A&M University
Eugene Farrell - Texas A&M University
Bailiang Li - Texas A&M University
Abstract Title: Vertical Mass-Flux Gradients in Aeolian Saltation.
The purpose of this research was to obtain measurements of the distribution of saltating grains above a movable bed. Field work was conducted in a coastal dune system at Esposende, Portugal, in May and June, 2006. Mass-flux gradients were measured with vertical stacks of 4-7 hose traps with vertical openings ranging in size from 25 to 100 mm. Wind speed was measured with a vertical arrays of 4 Gill-type anemometers (at z = 0.25, 0.50, 0.75, and 1.00 m) sampled at 10 Hz. Twenty-one sets of mass-flux measurements were collected. Grain sizes ranged from 0.25 to 0.40 mm, with a mean of 0.30 mm. We evaluate appropriate curve fitting functions and test the maximum and mean saltation height models of Zingg (1953) and Owen (1964). We also compare logarithmic, power, and exponential functions for best descriptor of the flux gradient (Namikas 2003), plot the data as Rouse-type profiles for normalization, and calculate Rouse Numbers so that the behavior of the aeolian system can be compared to that of hydrodynamical systems. Preliminary analysis for normalized mass-flux gradients indicates that the exponential function best represents the measured distribution.
Keywords: geomorphology, Rouse Profile, coastal, Portugal
- 4:20 PM Author(s):
*Hosahng Rhew - Seoul National University
Robin Davidson-Arnott - University of Guelph
Abstract Title: A Stochastic Approach to Modeling the Dynamic Effect of Moisture upon Aeolian Sand Transport on a Beach.
The dynamic effect of surface moisture is one of the urgent issues in understanding aeolian sand transport on a beach. A number of laboratory and field observations indicate that spatial and temporal variability in the drying or wetting process and in deflation together produce spatially and temporally varying conditions of the sand surface and thus influence the spatial and temporal pattern of sediment entrainment. These all combine to induce intermittent saltation. Based upon the assumption that the above three processes cause the temporal and spatial variability of the threshold velocity on a moist surface, we constructed a simplified stochastic model, and explore how well the model reproduces the magnitude and frequency of aeolian sand transport patterns on a beach by comparing the model output with the measured data. According to the results, the model reproduces the stochastic properties of the measure data on the foreshore area. The model, however, has the tendency to underestimate the frequency of saltation events in the middle and upper beach. The reason may be that our model does not consider the effect of impact by transported sand grains on the saltation event. Another disadvantage is that there is much arbitrariness in the parameterization of the surface drying processes. The further research is directed to explore the possibility of integrating a standard evaporation model into this model, and to find the proper strategies to include the influence of grain impacts on particle entrainment from a damp surface so that the model can cover the whole beach.
Keywords: surface moisture, aeolian sand transport, spatially-uneven drying, stochastic approach, intermittent saltation
- 4:40 PM Author(s):
*Eugene Farrell - Texas A&M University
Douglas J. Sherman - Texas A&M University
Abstract Title: Wind tunnel simulations of saltation: Is close enough, good enough?.
This paper examines the scaling up process of aeolian sand transport models from wind tunnels to beach and desert environments. Most experimental work is carried out at small spatial scales due to reductionist approaches and/or logistical and financial constraints. However, the application of these small scale experimental results to real world problems has been largely unsatisfactory. This paper demonstrates that airflow - surface relationships and subsequent transport models derived from wind tunnel studies should not be applied to natural systems without scaling corrections. Specifically, the scaling constraints of wind tunnels impact aeolian processes in two ways: (1) wind tunnel geometry inhibits the development of turbulent coherent flow structures and, thus, particle trajectories are reduced in height and length, and (2) the saltation enhanced roughness is much smaller in wind tunnels; probably for the same reason. Until these process scaling issues are explicitly addressed in wind blown sand studies, the accuracy of transport models will continue to be poor except by accident. The verification and validation of empirical and theoretical methods for correcting the results of small scale process studies to predictions at larger scales remains an important consideration for the discipline of aeolian research.
Keywords: saltation, sand transport, scale, surface roughness, particle trajectory
- 5:00 PM Author(s):
*Paul A. Gares - East Carolina University
Patrick Pease - Northern Iowa University
Abstract Title: The influence of topography and approach angles on local wind speed in a coastal blowout, Jockey's Ridge State Park, North Carolina, USA.
This study examines the wind field and associated sediment transport through a 3-5 m deep blowout at Jockey's Ridge, North Carolina. The blowout has an orientation of 240o. Thirteen wind vane/anemometers were placed 0.5 m above the surface throughout the blowout. A 4-meter mast, located upwind of the blowout, provided a reference for the blowout sensors. Wind speed records at each sampling location were grouped according to the mast wind direction. Wind direction data were classified into 10o groups ranging from 250 to 310o. Wind speed varied significantly with wind direction. In the blowout, wind speed increased gradually (100-120%) relative to the mast speed as the direction aligned with the blowout axis. However, locations on the blowout floor generally showed a reduction in wind speed relative to the reference. This reduction was greatest (50-80%) for winds approaching the blowout from azimuths of 300o and greater. Significant acceleration (100-150%) relative to the reference was recorded at locations at the landward ends of the blowout. On the dune crest north of the blowout, acceleration was very high and consistent (160%) regardless of direction, whereas on the dune crest to the south, wind speeds were lower than the reference (60-110%) except when wind approach was nearly normal to the crest. Sand transport patterns mimicked the wind speed patterns. The most effective sand transport occurred when wind direction was oriented along the blowout axis. 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
- 5:20 PM Author(s):
*Chris Houser - University of West Florida
Abstract Title: Linking Dune Morphology on a Barrier Island to Transverse Bars on the Inner Shelf.
The morphological response of a barrier island to the elevated water levels and storm waves of a hurricane partly depends on the height and extent of the foredune, and the presence of secondary dunes. Prior to Hurricane Ivan, the height of the dunes along Santa Rosa Island in Northwest Florida exhibited considerable variability alongshore. Empirical orthogonal function (EOF) analysis was used to reduce pre-storm LiDAR data into characteristic cross-island profiles. Cross-correlation analysis between the eigenmodes of the pre-storm morphology and the island width revealed a statistically significant but lagged correlation. The largest dunes tended to be found at and just east of the backbarrier cuspate headlands, while the smallest dunes were found at the narrowest sections of the island. The pre-hurricane height of the foredunes was also found to be correlated with the offshore bathymetry. The largest dunes tended to be associated with a dissipative profile, while the smallest foredunes (and the absence of secondary dunes) tended to be associated with a reflective profile. The variation in offshore bathymetry is shown to be associated with transverse bars on the inner shelf and it is argued that the resulting bathymetric highs and lows create alongshore variation in storm energy. Areas with a reflective profile are prone to breaching, which lowers the elevation of the island to the watertable and leaves a heavy shell lag. This creates a supply-limited environment and slows the rate of post-hurricane dune recovery compared to sections of the island with a dissipative profile.
Keywords: Dune Morphology, Geomorphology, LiDAR, Barrier Island
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.