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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]

Tidal sand waves form a dynamic bed pattern, widely occurring in shallow shelf seas such as the North Sea. Their importance to coastal engineering has inspired many advances in process-based sand wave modelling, aimed at explaining physical mechanisms in the formation stage ('linear regime') and capturing the finite amplitude evolution to equilibrium states ('nonlinear regime'). However, systematic validation of particularly the nonlinear sand wave models is still lacking. Here, we perform a two-step calibration and validation study of a sand wave model (specifically, their linear and nonlinear model versions) against field data from the North Sea. In the first step, the linear model is calibrated by seeking overall values of two uncertain input parameters (slip parameter, wave period) for which the modeled and observed wavelengths show the best agreement. In the second step, using the calibrated input parameters and preferred wavelengths from the linear model, equilibrium heights from the nonlinear sand wave model are validated against the observed sand wave heights. Our results show satisfactory agreement between observed and modeled sand wave lengths (from the linear sand wave model) and a systematic overprediction of sand wave heights (using the nonlinear model). Regression analysis can be used to rescale the nonlinear model results to obtain realistic predictions of sand wave heights. [ABSTRACT FROM AUTHOR]

A new method to calculate flow in compound channels is proposed: the interacting divided channel method (IDCM), based on a new parametrization of the interface stress between adjacent flow compartments, typically between the main channel and floodplain of a two-stage channel. This expression is motivated by scaling arguments and allows for a simple analytical solution of the average flow velocities in different compartments. Good agreement is found between the analytical model results and laboratory data from the literature. DOI: 10.1061/(ASCE)0733-9429(2008) 134:8(1158) CE Database subject headings: Flood plains; Hydraulic models; Flow resistance; Hydraulic models; Lateral stress.

Tidal sand waves are dynamic bedforms found in coastal shelf seas. Moreover, these areas are inhabited by numerous benthic species, of which the spatial distribution is linked to the morphological structure of sand waves. In particular, the tube‐building worm Lanice conchilegais of interest as this organism forms small mounds on the seabed, which provide shelter to other organisms. We investigate how the interactions between small‐scale mounds (height ∼⃒dm) and large‐scale sand waves (height ∼⃒m) shape the bed of the marine environment. To this end, we present a two‐way coupled process‐based model of sand waves and tube‐building worm patches in Delft3D. The population density evolves according to a general law of logistic growth, with the bed shear stress controlling the carrying capacity. Worm patches are randomly seeded and the tubes are mimicked by small cylinders that influence flow and turbulence, thereby altering sediment dynamics. Model results relate the patches with the highest worm densities to the sand wave troughs, which qualitatively agrees with field observations. Furthermore, the L. conchilegatubes trigger the formation of sandy mounds on the seabed. Because of the population density distribution, the mounds in the troughs can be several centimetres higher than on the crests. Regarding sand wave morphology, the combination of patches and mounds are found to shorten the time‐to‐equilibrium. Also, if the initial bed comprised small sinusoidal sand waves, the equilibrium wave height decreased with a few decimetres compared to the situation without worm patches. As the timescale of mound formation (years) is shorter than that of sand wave evolution (decades), the mounds induce (and accelerate) sand wave growth on a similar spatial scale to the mounds. Initially, this leads to shorter sand waves than they would be in an abiotic environment. However, near equilibrium the wavelengths tend towards their abiotic counterparts again. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd [ABSTRACT FROM AUTHOR]