Morphodynamics and sedimentary structures of bedforms under supercritical-flow conditions: New insights from flume experiments

Supercritical-flow phenomena are fairly common in modern sedimentary environments,
yet their recognition and analysis remain difficult in the stratigraphic
record. This fact is commonly ascribed to the poor preservation
potential of deposits from high-energy supercritical flows. However, the number
of flume data sets on supercritical-flow dynamics and sedimentary structures
is very limited in comparison with available data for subcritical flows,
which hampers the recognition and interpretation of such deposits. The
results of systematic flume experiments spanning a broad range of supercritical-
flow bedforms (antidunes, chutes-and-pools and cyclic steps) developed in
mobile sand beds of variable grain sizes are presented. Flow character and
related bedform patterns are constrained through time-series measurements of
bed configurations, flow depths, flow velocities and Froude numbers. The
results allow the refinement and extension of some widely used bedform stability
diagrams in the supercritical-flow domain, clarifying in particular the
morphodynamic relations between antidunes and cyclic steps. The onset of
antidunes is controlled by flows exceeding a threshold Froude number. The
transition from antidunes to cyclic steps in fine to medium-grained sand
occurs at a threshold mobility parameter. Sedimentary structures associated
with supercritical bedforms developed under variable aggradation rates are
revealed by means of combining flume results and synthetic stratigraphy. The
sedimentary structures are compared with examples from field and other
flume studies. Aggradation rate is seen to exert an important control on the
geometry of supercritical-flow structures and should be considered when
identifying supercritical bedforms in the sedimentary record.