A New Phase Diagram for Combined-Flow Bedforms

Abstract

Abstract 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−1) 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 (Uo) was varied from 0.00 to 1.00 m s−1 while the unidirectional component (Uu) was varied from 0 to 0.50 m s−1. The experimental data collected under unidirectional flows stronger than 0.30 m s−1 allows expansion of current understanding of the bed configurations for current-dominated combined-flows, 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 cross-sectional 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.