Sponsored by the Geomorphology Specialty Group
Association of American Geographers 103rd Annual Meeting
San Francisco, California, April 17-21.
Aeolian Geomorphology II
Paper Session 1444
Tuesday, 4/17/07, from 2:00 PM - 3:40 PM
Coastal and Marine Specialty Group
Geomorphology Specialty Group
Jean Taylor Ellis - Texas A&M University
Paul A. Gares - East Carolina University
Steven Namikas - Louisiana State University
- 2:00 PM Author(s):
*Jean Taylor Ellis - Texas A&M University
Abstract Title: Characterizing Saltation Events.
This research investigates unsteadiness in a turbulent wind field and pulses in a sand transport field on a beach near Shoalhaven Heads, New South Wales, Australia. Microphone-based saltation sensors, "miniphones," and thermal anemometers were co-located (0.02 m separation on center) and deployed between 0.01 and 0.0225 m above the bed, and sampled at 6000 Hz. Five runs totaling 2050 seconds of wind and saltation data were analyzed. The first goal of this research was to identify wind and saltation events. The continuous wavelet transform, using the Morlet wavelet base, was used to analyze the wind and saltation records. Wind and saltation events were discerned by selecting wavelet power coefficients between wavelet scales of 0.4 and 3.0 seconds and with coefficients exceeding the 95% confidence interval. The wavelet method allow for wind and saltation event duration and event spacing to be determined. The second goal of this research was to characterize the coherent structures in the wind field. The average event spacing measured in this research was compared to the empirical-based model presented by Rao, Narashimha, and Narayanan (1971). The correspondence between the model and this research strongly suggests that bursting-type coherent structures were present. Integral time scales, calculated using normalized auto correlation and power spectral density analysis, were approximately two seconds for the wind and saltation systems. The temporal coincidence of the integral time scale estimations and the event durations for the wind and saltation system strongly suggests that wind events are driving sand transport events.
Keywords: aeolian geomorphology, beaches, coastal
- 2:20 PM Author(s):
Ian Darke - University of Guelph
Robin Davidson-Arnott - University of Guelph
Abstract Title: Remote Monitoring of Beach Surface Moisture.
Surface moisture is known to be one of the most significant constraints on aeolian sand transport into the coastal foredune. However, acquiring accurate measurements of surface moisture that capture its spatial and temporal variability during a transport event is difficult and time-consuming. Using readily available digital photography and common software a new method for mapping beach surface has been tested in the field. The method, which is non-destructive of the beach surface, is based on the tonal change of beach sand with increasing moisture. Two digital video cameras were mounted at the top of a 5 metre tower at Greenwich Dunes, Prince Edward Island, Canada, and continually take photographs of the beach surface. The brightness of the resulting digital images are corrected for ambient light conditions by comparison with a white card and the resulting corrected images have been calibrated against simultaneous measurements of gravimetric moisture content. It is anticipated that by combining measurements of wind speed and direction with simultaneous data on surface moisture content improved modelling of sediment transport from the beach surface into the foredune can be achieved. This paper describes the monitoring system and calibration in detail and presents results from an application over a one month period.
Keywords: digital photography, beach surface moisture, aeolian sediment transport
- 2:40 PM Author(s):
*Bailiang Li - Texas A&M University
Douglas J. Sherman - Texas A&M University
Abstract Title: Temperature and pressure corrections for aeolian sediment transport experiments.
Air density and kinematic viscosity are commonly treated as constants in aeolian sand-transport experiments and modeling. However, they are notuniversal constants, and will change gradually with air temperature and pressure. The typical range of temperatures that we find in nature or in wind tunnels is large enough that these effects can substantially impact results. For example, if the air temperature rises from 0ºC to 40ºC and the atmospheric pressure is fixed at 1 atm, the air density decreases by 12.8% and kinematic viscosity increases by 25.8%. On the other hand, density increases when pressure rises, but decreases with increased pressure. Specifically, if the pressure drops from 1 atm to 0.7 atm (associated with high altitude arid regions, for example) and the temperature is fixed, the density will decrease by 30.0%, kinematic viscosity will increase by 42.9%. As air density and kinematic viscosity are significantly changed under different conditions, two equivalent dimensionless-parameters (free stream velocity and grain size) are recommended to standardize the reporting of aeolian sediment dynamics under different temperature, pressure. Here,we use dimensionless mean flow velocity and dimensionless sediment grain size. From the above two parameters, the free stream velocity and grain size under different conditions can be converted to an arbitrary reference condition (here we use dry air at 1atm and 10 ºC ). Dimensionless analysis can help to correct the temperature and pressure influence, which may reduce the errors.
Keywords: equivalent density, equivalent kinematic viscosity, air temperature, air pressure, air humidity, aeolian sediment
- 3:00 PM Author(s):
*Ian James Walker - University of Victoria
Becs Cumming - University of Victoria
Abstract Title: Airflow behaviour over a driftwood-laden macrotidal beach-dune system.
Coastal dune morphodynamics on high-energy, macrotidal beaches are not well understood given complex interactions between near-surface winds, tides, surface roughness (e.g., driftwood, vegetation), and dune topography. These interactions control the effective fetch (f) and resulting rates of sand transport to coastal dunes. This study examines airflow behaviour over a macrotidal, forest-backed dune system with a distinct backshore driftwood jam (DWJ). Measurements from an 8+hr wind event from 2D and 3D ultrasonic anemometers were sampled at seven locations from the beach to the forest. Incident wind speeds at 5-m increased from 10 to >12m/s from 284° (52° onshore). Three subevents are examined: oblique (f=265m), oblique-onshore (f=250m), and shifting (f=600 to 285m). The population of 10 minute averages (n=55) for key flow properties (U, W, CVU) are also analysed for location-specific responses to over 50° change in flow direction. Near-surface flow over the backshore deviates by 55° from the incident wind due to DWJ roughness effects and flow stagnation from the dune and forest. Turbulence increases in the DWJ and negative relationship exists between turbulence and fetch due to the enhanced boundary layer development distance. Flow accelerates up the dune due to streamline compression effects then expands and decelerates in the lee. The colonizing tree stand causes a flow stagnation of 30-40% and significant flow steering along the treeline. due to a longer boundary layer development distance. Downward velocities are greatest in the lee and at the treeline. From this, implications for sand transport and dune dynamics is discussed.
Keywords: aeolian, dune, coastal, driftwood, geomorphology
- 3:20 PM Author(s):
*Patrick A. Hesp - Louisiana State University
Graziela Miot da Silva - Louisiana State University
Abstract Title: Winds, Coastline Orientation and Barrier and Dunefield Development, Southern Brazil.
This paper examines the spatial variation in barrier and dune development along Moçambique beach, a headland bay beach in southern Brazil, and embayments to the south of Santa Catarina Island. Moçambique extends through 600 of curvature, and displays a significant south to north increase in littoral sediment supply, in exposure to prevailing winds and waves, and changes in beach/ surfzone morphodynamic type from low energy dissipative, through moderatehigh energy intermediate, to high energy intermediate-dissipative. The barrier system extends from a small foredune-blowout complex (protected southern end), to a blowout/parabolic dune complex, to a narrow transgressive dunefield (mid-embayment), to a large-scale transgressive dunefield in the exposed northern end. The system behaves according to the wave-beach-dune model (Short and Hesp, 1982) where the largest dunes are found in the northern, most exposed, dissipative section. Sediment supply is at a maximum here also, but the dune systems do not correspond with Psuty's scheme (2004). The southern beaches display the same northwards wave, surfzone and littoral drift trends, but opposite trends in prevailing winds and dune development. Neither model above applies. We show that regional prevailing wind direction is critical in determining barrier and dunefield development, but tuned very much by coastline orientation, since the degree of exposure to winds and waves also controls dunefield development.
Keywords: Coastline orientation, winds, barrier development, Brazil-south
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.