Wind, Sand, and Dunes
Douglas J. Sherman
Department of Geography, University of Alabama (sherman@ua.edu)
Abstract
Our ability to understand and predict the behavior of coupled wind and sand systems has
improved substantially over the last several decades. These advances, however, remain limited in their
application to the simplest of aeolian systems and over relatively short time scales and small spatial
scales. Under near-ideal conditions, the rate of wind-blown sand transport depends only upon the wind
speed, the composition and size of the sand, and the value of one or two empirical constants
(depending on the model used). For such conditions, the model of Lettau and Lettau, for example, can
explain, statistically, 79% of the variability of observed transport rates.
Coastal environments seldom present the simple circumstances represented by the
deterministic models. Aeolian transport from beaches to dunes is complicated by several factors,
including: variable source width caused by fluctuating waterlines associated tides and storms; variable
granular cohesion associated with moisture or the development of salt crusts; unknown fetch effects;
variable effects of beach and dune slopes; the presence (absence) of wrack; the effects of vegetation;
and broadly, anthropogenic influences such as sand fences.
The situation complicates rapidly when considering aeolian environments and time and space
scales of interest to coastal managers. Even for near-ideal conditions, the model performance
deteriorates as we move away from detailed wind measurements, for example, to time averaged data
(typically three or more hour means) that are reporting products from weather stations or reanalysis
models. Similarly, details of the sediment system are lost as larger spatial and temporal scale
representations are required. At annual to decadal (or longer) scales and over spatial scales appropriate
for dune management, approaches to forecasting aeolian transport and dune evolution switch from
deterministic models to empirically-based, probabilistic models, such as the DUBEVEG (Dune, Beach,
and VEGetation) model developed for The Netherlands. These models require site specific information
for calibration and validation.
This presentation will review briefly the basics of wind-blown sand as relevant to dune
development with particular attention to treatment of some of the complicating factors common to
coastal environments. Maximizing or minimizing transport rate predictions, contingent on the
management scenario, will be discussed, as well as the role for empiricism and large scale model
development for natural and human altered systems.