Combined-flows, which involve a combination of unidirectional and wave-induced oscillatory flow, are omnipresent in coastal and lacustrine environments. Despite the extensive progress made in understanding the bedforms generated by such flows in past studies, there still remains a wide range of unexplored flow conditions where the bedform geometry, and hence consequent sedimentary structures, have not been explored, especially for strong unidirectional flows (greater than 0.30 m s 21 ) and intermediate oscillation periods (between 2 and 8 seconds). To address this gap in our knowledge, the stability of bedforms in a 0.25 mm diameter sand bed was studied under pure unidirectional, pure-oscillatory, and combined-flow conditions with oscillation periods of 4, 5, and 6 s. The maximum orbital velocity (U o ) was varied from 0.00 to 1.00 m s 21 while the unidirectional component (U u ) was varied from 0 to 0.50 m s 21 . The experimental data collected under unidirectional flows stronger than 0.30 m s 21 allows expansion of current understanding of the bed configurations for current-dominated combinedflows, where the phase boundary between combined-flow bedforms and current ripples was uncertain. Under these flow conditions, ten distinctive bedform states can be recognized: no motion (NM), 2D symmetric ripples (2D SR), 3D symmetric ripples (3D SR), 3D symmetric dunes (3D SR), 3D asymmetric ripples (3D AR), 3D quasi-asymmetric ripples (3D QAR), 3D asymmetric dunes (3D AD), 3D current ripples (3D CR), 3D current dunes (3D CD), and upper-stage plane bed (USPB). Each of these bedform stages is described, quantified, and characterized in dimensional phase diagrams.A complete re-evaluation of the nomenclature for combined-flow bedforms is proposed, which includes their planform and crosssectional geometry, in order to better represent the bed morphologies. This new nomenclature also unifies past research on bedforms in both unidirectional and oscillatory flows and thus presents a new synopsis of bedforms developed under such flows. One of the main changes proposed that allows integration with the nomenclature used in unidirectional flows is the reclassification of large ripples as dunes. Furthermore, the introduction of the planform and cross-sectional geometries as properties by which to classify bedforms leads to the definition of a stable phase space for two-dimensional symmetrical ripples and three-dimensional quasi-asymmetrical ripples. These new data and analysis allow proposition of a new unified phase diagram for combined-flows.

The development of bedforms under unidirectional, oscillatory and combined-flows results from temporal changes in sediment transport, flow and morphological response. In such flows, the bedform characteristics (for example, height, wavelength and shape) change over time, from their initiation to equilibrium with the imposed conditions, even if the flow conditions remain unchanged. These variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record. Hence, understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record. This article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional, purely oscillatory and combined-flow conditions, which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples. The present results yield a unified model for bedform development and equilibrium under unidirectional, oscillatory and combined-flows. The experimental results show that the processes of bedform genesis and growth are common to all types of flows, and can be characterized into four stages: (i) incipient bedforms; (ii) growing bedforms; (iii) stabilizing bedforms; and (iv) fully developed bedforms. Furthermore, the development path of bedform; growth exhibits the same general trend for different flow types (for example, unidirectional, oscillatory and combined-flows), bedform size (for example, small versus large ripples), bedform shape (for example, symmetrical or rounded), bedform planform geometry (for example, two-dimensional versus three-dimensional), flow velocities and sediment grain sizes. The equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition.

The Trinity River system provides a natural laboratory for linking fluvial morphodynamics to stratigraphy produced by sea-level rise, because the sediments occupying the Trinity incised valley are well constrained in terms of timing of deposition and facies distribution. Herein, the Trinity River is modeled for a range of base-level rise rates, avulsion thresholds, and water discharges to explore the effects of backwater-induced in-channel sedimentation on channel avulsion. The findings are compared to observed sediment facies to evaluate the capability of a morphodynamic model to reproduce sediment deposition patterns. Base-level rise produces mobile locations of in-channel sedimentation and deltaic channel avulsions. For scenarios characteristic of early Holocene sea-level rise (4.3 mm yr À1 ), the Trinity fluvial-deltaic system progrades 13 m yr À1 , followed by backstepping of 27 m yr À1 . Avulsion is reached at the position of maximum sediment deposition (located 108 km upstream of the outlet) after 3,548 model years, based on sedimentation filling 30% of the channel. Under scenarios of greater base-level rise, avulsion is impeded because the channel fill threshold is never achieved. Accounting for partitioning of bed-material sediment between the channel and floodplain influences the timing and location of avulsion over millennial time scales: the time to avulsion is greatly increased. Sedimentation patterns within the valley, modeled and measured, indicate preference toward sandy bed material, and the rates of deposition are substantiated by previous measurements. Although the results here are specific to the Trinity River, the analysis provides a framework that is adaptable to other lowland fluvial-deltaic systems. appropriate over the late Holocene and time scales of a few centuries Nittrouer et al., 2012;Ganti et al., 2014]. However, during the early and middle Holocene, fluvial-deltaic systems would have been MORAN ET AL.MODELING OF CHANNEL FILL AND AVULSIONS 215Key Points:• Fluvial channel sedimentation and avulsion location backstep during sea-level rise • Time to avulsion scales with increasing rate of sea-level rise • Floodplain capture of sediment is necessary to model influence of base-level rise on stratigraphy (2017), Morphodynamic modeling of fluvial channel fill and avulsion time scales during early Holocene transgression, as substantiated by the incised valley stratigraphy of the Trinity River, Texas,

We present results of coupled direct numerical simulations between flow and a deformable bed in a horizontally periodic, turbulent open channel at a shear Reynolds number of Reτ = 180. The feedback between the temporally and spatially evolving bed and the flow is enforced via the immersed boundary method. Using the near‐bed flow field, we provide evidence on the role of locally intense near‐bed vortical structures during the early stages of bed formation, from the emergence of quasi‐streamwise streaks to the formation of incipient bedform crestlines. Additionally, we take a new look at a number of defect‐related bedform interactions, including lateral linking, defect and bedform repulsion, merging, and defect creation, and show that the underlying mechanisms, in these flow‐aligned interactions, are very similar to each other. Consequently, the interactions are labeled differently depending on the geometry of interacting structures and the outcome of the interaction. In the companion paper, we compare our results to published experimental data and provide an extensive quantitative analysis of the bed, where we demonstrate the importance of neighboring structures, especially upstream neighbors, on bedform dynamics (growth/decay and speed) and wave coarsening. Video files of bed evolution are available in the supporting information.

Turbidity currents are one of the most significant means by which sediment is moved from the continents into the deep ocean; their properties are interesting both as elements of the global sediment cycle and due to their role in contributing to the formation of deep water oil and gas reservoirs. One of the simplest models of the dynamics of turbidity current flow was introduced three decades ago, and is based on depth‐averaging of the fluid mechanical equations governing the turbulent gravity‐driven flow of relatively dilute turbidity currents. We examine the sedimentological regimes of a simplified version of this model, focusing on the role of the Richardson number Ri [dimensionless inertia] and Rouse number Ro [dimensionless sedimentation velocity] in determining whether a current is net depositional or net erosional. We find that for large Rouse numbers, the currents are strongly net depositional due to the disappearance of local equilibria between erosion and deposition. At lower Rouse numbers, the Richardson number also plays a role in determining the degree of erosion versus deposition. The currents become more erosive at lower values of the product Ro × Ri, due to the effect of clear water entrainment. At higher values of this product, the turbulence becomes insufficient to maintain the sediment in suspension, as first pointed out by Knapp and Bagnold. We speculate on the potential for two‐layer solutions in this insufficiently turbulent regime, which would comprise substantial bedload flow with an overlying turbidity current.