FAROUK EL-BAZ AWARD FOR DESERT RESEARCH: DYNAMICS OF LINEAR SAND DUNES: PRESENT, PAST, AND FUTURE

Dunes of linear form (also known as longitudinal or seif dunes) are the most widespread desert dune type today, covering approximately 60% of the area of desert sand seas. A wealth of field research over the past two decades has provided a general understanding of the present day dynamics of this dune type, but major questions as to their origins and long- term development (past dynamics) still remain (future research).

Field studies of dune processes show that all dunes are characterized by flow acceleration on their stoss (windward) slopes, leading to an exponential increase in sand transport rates toward the dune crest. On the lee side, flow separation and diversion dominate in near-crest regions. On linear dunes, seasonal changes in wind direction mean that these processes affect each side of the dune, leading to switching of lee-face orientations and a seasonal oscillation of the crest position, but very little change in lower areas of the dune. Lee-side flow diversion promotes extension of the dune at its downwind tip. Such processes are able to maintain linear dunes in present-day climatic regimes.

Linear dunes appear to be very conservative landforms and therefore preserve a history of their development. Some of the best records of periods of Quaternary dune construction have come from linear dunes. These records suggest that many linear dunes were formed (or completely reworked) during the last glacial period and therefore are a product of glacial wind regimes. Support for this hypothesis comes from the widespread existence of: (1) dunes with trends that are not in equilibrium with modern wind regimes; and (2) superimposed small dunes that appear to be modifying the original linear dune form.

Despite great progress in understanding many aspects of linear dune dynamics, we still do not have clear answers to several important questions. These include the fundamental nature of the sedimentary structure of linear dunes and the related issue of lateral migration of these bedforms. Finally, we are still uncertain of the way(s) in which a linear dune pattern is initiated. Such issues will provide major challenges for dune research in the future.