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Tide‐dominated sandy shelf seas, such as the Dutch North Sea, are covered by sand waves. Yet, basin‐scale hydrodynamic models do not include any sand wave information because their grid sizes are too coarse to resolve sand waves individually. We explore the possibility of parametrizing the effects of sand waves on the larger‐scale tidal flow by means of a form roughness. Specifically, our aim is to see to what extent the flow over a sand wave field can be reproduced by that over a flat seabed with an increased effective roughness (accounting for both grain and form roughness). To do so, we use two process‐based hydrodynamic models: a second order perturbation approach, and Delft3D. Both models demonstrate that the presence of sand waves causes amplitude decrease and phase shift of the tidal flow. We explore the dependencies of form roughness on different sand wave characteristics (wavelength, height and asymmetry). Shorter and higher sand waves cause a higher form roughness, while our analysis does not reveal any dependency on sand wave asymmetry. Notably, the consideration of a tidal flow, characterized by several tidal constituents, each represented by an amplitude and a phase, results in a more complex form roughness analysis than in a fluvial setting, where the flow is unidirectional and steady. We thus obtain an amplitude‐based form roughness and a phase‐based form roughness, each yielding a different value, yet displaying the same qualitative dependencies. Plain Language Summary: Tide‐dominated seas, such as the Dutch North Sea, are often covered by tidal sand waves, that is, seabed undulations characterized by wavelengths of hundreds of meters, heights of several meters and migration rates of several meters per year. The presence of sand waves causes an increase in the roughness experienced by the tidal flow. As a result, the tidal flow may for example, slow down or lower in amplitude. Yet, the effects of sand waves are generally not considered in larger‐scale hydrodynamic models. We explore the possibility of representing the effects of sand waves through an increase in bed roughness on a flat seabed, that is, a form roughness. To do so, we use two process‐based hydrodynamic models. Results show that higher and shorter sand waves result in a higher form roughness. Furthermore, the unsteady nature of the tidal flow gives rise to multiple criteria to determine form roughness. These criteria yield different values for form roughness, highlighting the complexities of considering form roughness in a tidal setting. Key Points: The characteristics of tidal flow (unsteady, multiple tidal constituents) make studying form roughness more complex than in a river settingThe presence of sand waves causes amplitude decrease and phase shift of the tidal flow through an increased effective roughnessHigher and shorter sand waves result in an increased form roughness [ABSTRACT FROM AUTHOR]

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]