Minimal Model for Sand Dunes

We propose a minimal model for aeolian sand dunes. It combines an analytical description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The model provides a qualitative understanding of important features of real dunes, such as their longitudinal shape and aspect ratio, the formation of a slip face, the breaking of scale invariance, and the existence of a minimum dune size.Comment: 4 pages, 4 figures, replaced with publishd versio

Templated Quasicrystalline Molecular Ordering

Quasicrystals are materials with long-range ordering but no periodicity. We report scanning tunneling microscopy (STM) observations of quasicrystalline molecular layers on 5-fold quasicrystal surfaces. The molecules adopt positions and orientations on the surface consistent with the quasicrystalline ordering of the substrate. Carbon-60 adsorbs atop sufficiently separated Fe atoms on icosahedral Al−Cu−Fe to form a unique quasicrystalline lattice, whereas further C60 molecules decorate remaining surface Fe atoms in a quasi-degenerate fashion. Pentacene (Pn) adsorbs at 10-fold symmetric points around surface-bisected rhombic triacontahedral clusters in icosahedral Ag−In−Yb. These systems constitute the first demonstrations of quasicrystalline molecular ordering on a templat

Correction to: Assessing the impact of hurricane Fiona on the coast of PEI National Park and implications for the effectiveness of beach‑dune management policies

This article has been published with incorrect captions for Figs. 7, 8 and 9. The correct figures and captions are presented below. (Figure presented.) (Figure presented.) (Figure presented.) ERA5 hindcast significant wave height data offshore of Brackley Beach, for intense storms such as Hurricanes Dorian and Fiona, as well as recent Nor'easter storms in January of 2022 Significant wave height and water level data during Fiona. ERA5 is the plot of hourly estimates of wave height for the offshore hindcast point closest to Brackley Beach; Buoy Swh is the significant wave height (Hs) measured by Fisheries and Oceans Canada’s AZMP Viking wave buoy located between PEI and Îsles de la Madeleine; Gauge WL is water level measured by a still water gauge located on Lennox Island and operated by the PEI Storm Surge Early Warning System; and the tide predictions are provided by the Canadian Hydrographic Service for nearby Rustico Harbor, which is located approximately 4 km west of Brackley and 12 km east of Cavendish Examples of beach and dune management at Cavendish Beach: A) View looking east of the main beach at Cavendish in June 2016, two years after restoration began on a large blowout resulting from trampling next to the boardwalk access point. Repurposed Christmas trees are used as a substitute for sand fences to trap sand, and marram grass is planted in the area between the trees. Fencing was used to keep visitors off the regeneration area; B) View looking east of the beach in May 2022 showing the restored foredune on the right and virtual fencing along the high tide line to keep visitors away from the vegetation forming embryo dunes along the base of the stoss slope of the foredune. Signage, visible to the right of the buried tree trunk on the stoss slope, is used to inform visitors of the need to keep off the foredune The original article has been corrected

Corridors of barchan dunes: stability and size selection

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 than they loose sand, 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.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E. Pictures of better quality available on reques