Phase diagrams of dune shape and orientation depending on sand availability

Abstract:

New evidence indicates that sand availability does not only control dune type but also
the underlying dune growth mechanism and the subsequent dune orientation. Here we
numerically investigate the development of bedforms in bidirectional wind regimes for
two different conditions of sand availability: an erodible sand bed or a localized sand
source on a non-erodible ground. These two conditions of sand availability are associated
with two independent dune growth mechanisms and, for both of them, we present
the complete phase diagrams of dune shape and orientation. On an erodible sand bed,
linear dunes are observed over the entire parameter space. Then, the divergent angle
between the two winds and the ratio of their transport capacity control dune orientation
and dynamics. For a localized sand source, different dune morphologies are observed
depending on the wind regime. There are systematic transitions in dune shape from
barchans to linear dunes extending away from the localized sand source, and vice-versa.
These transitions are captured fairly by a new dimensionless parameter, which compares
the ability of winds to build the dune topography in the two modes of dune orientation.