Eolian processes in a coastal dune environment, Indiana Dunes National Lakeshore

In an effort to investigate the nature of eolian processes in a temperate continental coastal environment a field program that included repeated transit surveys and both long- and short-term measurements of wind and sand transport in four subenvironments of the Indiana Dunes National Lakeshore was undertaken between October, 1990 and October, 1993. Continuous measurements of wind conditions and sand transport at hourly intervals were achieved using electronic monitoring devices, providing a long-term data record of nearly eight months in duration during the fall, winter, and spring of 1993. Automated sand traps, anemometers, and wind vanes were deployed simultaneously on the backshore, on a small blowout, on the crest of a vegetation-covered foredune, and on a large unvegetated dune. These data were supplemented by detailed measurements of near-surface wind profiles and sand transport.; Measurements from repeated transit surveys and bulk sand transport data indicate that the blowout dune experiences periods net erosion and net deposition, whereas the vegetated foredune primarily experiences net deposition in the form of accretion on the lakeward dune flank. Most measured wind profiles match theoretical logarithmic profiles despite the non-uniform topography within the area. Major deviations from the logarithmic profile occur in the blowout during southerly winds, when the wind speed decreases between heights of 0.4 m and 1.6 m. Measurements of short-term sand transport rates display a great deal of variability within and between subenvironments. Ratios of measured rates to those computed using Bagnold's (1941) standard equation for eolian transport ranged between about 0.3 and 1.7. Because sand transport increases exponentially with shear velocity, differences between the actual and computed mass flux of sand can be particularly large at high shear velocities within this coastal dune environment. The greatest observed transport rates were generally recorded on the backshore (up to 0.33 g cm{dollar}sp{lcub}-1{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}) and on the lakeward slope of Mt. Baldy (up to 0.43 g cm{dollar}sp{lcub}-1{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}). Data collected by the automated sand traps indicate that transport rates can vary by as much as two orders of magnitude between subenvironments during a given wind storm. Rates of transport over the vegetated foredune and Mt. Baldy were consistently the lowest and highest of the four study subenvironments, respectively. Eolian transport rates along the backshore exceeded those in the blowout by up to a factor of ten during shore-parallel winds. However, backshore rates were up to ten times lower than blowout rates when winds were directed perpendicular to the shore. The results of this study indicate that the majority of the net annual influx of eolian sand along the southern Lake Michigan shoreline occurs through blowouts and over unvegetated dunes during large northerly wind storms that are common during autumn, winter, and early spring months.