Modeling the Genesis of Sand‐Starved Dunes in Steady Currents.

The formation of fluvial dunes is usually studied by investigating the time development of a small amplitude bottom perturbation of a uniform stream and considering that the dunes originate by the growth of the bottom mode characterized by the largest amplification rate under the assumption of an infinite availability of the mobile sediment (linear stability analysis). Here we undertake the stability analysis investigating the formation of sand dunes in steady currents by accounting for the nonlinear effects of sediment starvation on the formative mechanisms of the bedforms and comparing the theoretical results with laboratory experiments, and an application of a fully nonlinear commercial model of finite amplitude dunes, thus enabling an improved understanding of the genesis of starved fluvial dunes. As the growth of the dunes progressively exposes the motionless substratum, both the stability‐based and the numerical models predict starved dunes characterized by increasing crest‐to‐crest distances. The increase of the crest‐to‐crest distance corresponds to a decrease of the length of individual dunes as well as a growing irregularity in their spacing and morphology. These findings conform with the outcome of physical experiments performed earlier in a laboratory flume and existing measurements of starved fluvial dunes in the field. Plain Language Summary: The genesis of fluvial dunes is a topic of considerable interest to river engineering as dunes are a primary source of flow resistance, regulating water levels and transport processes. The modeling of fluvial dunes is usually performed assuming an infinite availability of the sediment that can be eroded and deposited by the water flow. Field observations and laboratory experiments nevertheless indicate that the supply of sediment affects dune growth and the resulting morphology of the river bed. Here we present a new theoretical model able to reproduce the effects that the lack of sediment has on the growth of dunes and compare it against laboratory experiments, and a numerical commercial model. Our results show that a progressively decrease in sediment supply leads to an increase in dune spacing and a decrease in dune length corresponding to steeper longitudinal profiles. These findings agree with the results of laboratory experiments and existing measurements in the field. As such, this study paves the way to the modeling of fluvial dunes in sediment starved environments and the prediction of their effects on the river flow. Key Points: Our model results indicate that sediment starvation affects fluvial dunes, whose spacing increases as sediment availability decreasesA quasi‐linear and a fully nonlinear model predict larger wavelengths for sediment starved dunes than for their contiguous counterpartsModeling outcome conforms with those of previous flume experiments and existing measurements of starved fluvial dunes in the field [ABSTRACT FROM AUTHOR]