Laboratory experiments were conducted to investigate the formation of river bedforms under sediment supply‐limited conditions, i.e. when a motionless substratum is bared by the dynamics of the mobile sediments. Three series of experiments were organized in a laboratory flume by fixing all the hydrodynamic and morphodynamic parameters but varying the thickness normalΔ of the initial layer of mobile sediments which covers the rigid bottom of the flume. At the end of all the experiments, which lasted for the same amount of time, the formation of transverse sand dunes was observed. For decreasing normalΔ, the rigid bottom of the flume was bared progressively earlier during the experiment and the measurements showed a clear tendency of the bedforms to lengthen, i.e. to increase their crest‐to‐crest distance. Moreover, under strong supply limitation, the two‐dimensional transverse dunes turned into three‐dimensional barchanoid forms and into isolated barchan dunes characterized by an abrupt reduction in bedform heights. A two‐dimensional Fourier analysis of the bottom profile was performed, providing the amplitude of the main streamwise and spanwise harmonic components of the bottom morphology as a function of normalΔ. © 2019 John Wiley & Sons, Ltd.

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Tropical cyclones impose stresses on narrow and shallow continental shelves. The interaction of strong wind- and wave-induced currents with the local topography near the shore gives rise to complex flow and sediment transport patterns. Considerable uncertainty remains on the initiation mechanisms of turbidity currents, particularly in coastal oceanic settings subject to extreme weather events. Here we use state-of-the-art numerical models to investigate the implications of tropical cyclone-induced coastal circulation patterns for the generation of turbidity currents. In our simulations tropical cyclones induce megarip currents associated with shoreline curvature and rotation of incoming wave directions. These currents flush water and sediment towards submarine canyons, ultimately triggering turbidity currents into deep waters. Evidence of sediment-laden underflows, which resulted in subsea pipeline displacements, supports our hypothesis that tropical cyclone-induced megarip currents can trigger turbidity currents offshore from tropical river deltas.