Corridors of barchan dunes: Stability and size selection

Abstract

Barchans are crescentic dunes propagating on a solid ground. They form dune fields in the shape of elongated corridors in which the size and spacing between dunes are rather well selected. We show that even very realistic models for solitary dunes do not reproduce these corridors. Instead, two instabilities take place. First, barchans receive a sand flux at their back proportional to their width while the sand escapes only from their horns. Large dunes proportionally capture more sand than they lose, while the situation is reversed for small ones: therefore, solitary dunes cannot remain in a steady state. Second, the propagation speed of dunes decreases with the size of the dune: this leads, through the collision process, to a coarsening of barchan fields. We show that these phenomena are not specific to the model, but result from general and robust mechanisms. The length scales needed for these instabilities to develop are derived and discussed. They turn out to be much smaller than the dune field length. As a conclusion, there should exist further, yet unknown, mechanisms regulating and selecting the size of dunes.

Remote sensing and spatial analysis of aeolian sand dunes: A review and outlook

“…Similarly for linear dunes, allometric relations revealed linear relations between dune height and crest-to-crest spacing (Thomas, 1986;Lancaster, 1988b). When coupled with advances in numerical modeling, this type of approach also clarified some important morphological details for barchan dunes, especially the observation that the ratio of horn width to barchan width decreases with increasing dune width, which is important for the representation of barchan dune form in numerical models (Hersen et al, 2004). Thus, in the absence of collisions, the net effect is that small barchans get smaller as they migrate, while large barchans get larger.…”

“…The importance of dune form evolution has emerged as a top research priority in light of recent progress in numerical modeling and flume experiments (e.g., Schwämmle and Herrmann, 2003;Endo et al, 2004;Hersen et al, 2004;Durán et al, 2005;Hersen, 2005;Hersen and Douady, 2005;Durán et al, 2011;Katsuki et al, 2011). These approaches afford insight into dune dynamics and interactions and can be used to supplement the paucity of field observations.…”

Migration of Barchan Dunes in Qatar–Controls of the Shamal, Teleconnections, Sea-Level Changes and Human Impact

“…The multitemporal analysis of satellite and aerial imagery revealed the well-established pattern of size-dependent migration rates of barchan dunes [14,15]. Even though average dune size in our sample varied only marginally ( Figure 7) and seems not to have affected variability in migration rates, it may vary significantly over larger time scales [4,13], potentially leading to uncertainty when extrapolating short-term rates.…”

“…Interpolating migration rates from multi-year or even decadal observations is challenging since small dunes may separate from the horns of larger ones [13] and the size of barchans fluctuates over time [4]. Size changes are determined by the difference between upwind sand supply and efflux from the horns, which is a function of horn width [14]. However, on a dune-field scale and in the long term, the size of barchans is considered to stay rather stable [2,15], and therefore, fluctuations in dune swarm migration over larger time-scales may be closely related to external factors, mostly changing wind patterns or changes in vegetation cover.…”

Analysis of barchan dune dynamics using numerical simulations and flume experiments

“…barchan Bagnold, 1941;Pye and Tsoar, 1990;Cooke et al, 1993Hersen et al, 2002 (Jackson and Hunt, 1975) and Bagnold (Bagnold, 1941) techniques, are required to compute each phase of the process. Niño andBarahona, 1997 Hersen et al 2002 …”