Dynamic processes acting on a longitudinal (seif) sand dune

Abstract

Field measurements were made on a longitudinal dune in the Sinai Desert in order to understand its morphology and dynamics. The field measurements contradicted the wind structure indicated by the helicoidal flow theory. Rather, it was found that winds coming from two basically different directions at different times and striking the dune obliquely were responsible for sand transport and erosion or deposition along the lee flank. The essence of this mechanism is the deflection of the wind airflow on the lee flank of the dune to a direction parallel to the crest line. The occurrence of erosion or deposition depends upon the angle of incidence between the wind and the crest line. When this angle is < 40° the velocity of the deflected wind is higher than on the crest line or the windward flank and longitudinal sand transport occurs. When the angle is less acute (> 40°) the velocity of the deflected wind drops and deposition takes place on the lee flank. The angle of incidence in each wind storm is changed intermittently between 30° and 100° along the dune because the dune meanders and because of the sinuous outline of the crest line. In this manner sand transport and erosion or deposition occurs along the lee flank depending on the angle of incidence between the wind and the crest line. As a result of the deflection of the wind the dune elongates at an average rate of more than 1 m per month. Peaks and saddles along the crest line advance at an average rate of 0.7 m per month. The lack of uniformity in the effects of the wind on both sides of the dune creates a lack of uniformity in the rate of erosion and deposition. This can explain the formation of peaks along the crest line of the dune.

Types of Aeolian Sand Dunes and Their Formation

“…Each wind is diverted along the lee slope, after reattachment of the separated flow, to blow parallel to the crest-line in a down-dune direction. This process is referred to as the flow diversion model [51,52]. It follows that two different processes act upon the lee slope of a dune.…”

“…• to the crest-line, the flow separates obliquely to the crest-line and the reattachment flow on the surface of the lee slope is deflected to a direction parallel to the crest-line at a magnitude that is above the threshold speed [51]. Therefore, there is less deposition and more erosion and transport of sand in this segment of the lee slope (Fig.…”

Foredune accretion under offshore winds

“…Further field research into airflow patterns in dune systems (e.g. Arens et al, 1995;Frank and Kocurek, 1996;Hesp, 2002;Kocurek et al, 1992;Tsoar, 1983;Walker, 1999;Walker and Nickling, 2002) as well as wind tunnel studies and numerical modelling (Parsons et al, 2004a,b;Schatz and Herrmann, 2006 ;van Boxel et al, 1999;Walker and Nickling, 2003) has yielded a more detailed understanding of lee-side air flow. The three broad categories--attached and undeflected flow, attached and deflected flow, and separated flow--determined by Sweet and Kocurek (1990) and modified by Walker and Nickling (2002) are dependent primarily on dune topography and the approach angle of the wind.…”

Temporal and spatial variability in dune reactivation across the Nebraska Sand Hills, USA

“…This regular spacing gave rise to the original theory for the formation of linear dunes, in which counter-rotating parcels of air were thought to converge at the surface, pushing sand up into dunes in the spaces between (Hanna, 1969). This theory has since been dismissed, with wind and flume studies showing that linear dunes typically form under bimodal flows (Rubin and Ikeda, 1990;Tsoar, 1983;Tsoar and Yaalon, 1983), with winds from two directions approaching the crest at an angle (Ping et al, 2014;Tsoar and Yaalon, 1983). Morphodynamically, linear dunes can be grouped into oblique and longitudinal dunes; longitudinal dune crests are oriented nearly parallel (0°-15°) to the resultant sand transport direction while oblique dune crests are oriented at an angle (15°-75°; Hunter et al, 1983).…”