Experiments on Oscillatory-Flow and Combined-Flow Bed Forms: Implications for Interpreting Parts of the Shallow-Marine Sedimentary Record

Abstract

Analog laboratory experiments were conducted to investigate subaqueous bed forms generated under storm-like oscillatory and combined flow. Experiments were carried out in a large wave tunnel and used a range of sand sizes (fine and very fine), wave periods (10.5 and 8 s), oscillatory velocities (0 to 125 cm/s), and unidirectional velocities (0 to 25 cm/s).At low unidirectional velocities (Յ10 cm/s), addition of an increasing collinear oscillatory flow caused the bed to evolve from small-scale (wavelength Ͻ 20 cm), symmetric, anorbital ripples, to large-scale (wavelength Ͼ 100 cm), symmetric, orbital ripples, to plane bed. At higher unidirectional velocities (Ͼ10 cm/s), a similar trend was noted, but ripples were more asymmetric. Three phase diagrams are presented to summarize the observed relationships of bed configurations to flow conditions. These diagrams are intended to assist in the interpretation of shallow marine sedimentary environments where oscillatory and combined flow are thought to be omnipresent.Distinctive features of small-scale asymmetric combined-flow ripples generated are: a 3D planform, round crest, and convex-up sigmoidal profile with local pronounced scour at the toe of the stoss side giving the ripple profile a ''boxy'' appearance. Similarly, large-scale asymmetric combined-flow ripples had broad and round crests, convex-up stoss sides, and ''compressed profiles'' due to scouring at the toe of the stoss side.Hummocky bed forms were generated under moderate to high oscillatory velocities and low unidirectional velocities. Hummocks were not observed as a distinct bed state but rather appeared to mark transitions in bed-form scale and symmetry. Hummocks were more prevalent at longer oscillatory periods and in finer-grained sediment. Stratification produced by ''synthetically'' aggrading hummocky bed profiles closely resembles hummocky cross-stratification. With the introduction of only a small unidirectional-flow component, hummocks evolve into downstream-migrating large-scale asymmetric ripples, and the resultant cross-stratification becomes similar to that produced by unidirectional-flow dunes. Accordingly, these experiments suggest that much of the hummocky cross-stratification observed in the stratigraphic record is produced by storm-generated long-period oscillatory-dominant combined flows. Inasmuch as long-period, high-energy waves require deep, wide basins to form, hummocky cross-stratification may therefore serve as a useful indicator of deposition in unrestricted, open-water conditions.

Predicting bedforms and primary current stratification in cohesive mixtures of mud and sand

“…The primary stratification formed by bedforms in combined flow may be difficult to distinguish from the stratification formed by pure current-and wave-generated bedforms (Dumas et al 2005). This interpretative process is further complicated by the fact that the angles of approach of the current and the waves may be different (Lacy et al 2007).…”

“…4c) are controlled by a larger number of variables than current-and wave-generated bedforms, which hampers their classification in 2D phase space. The bedform phase diagrams for combined flows proposed by Arnott & Southard (1990), Dumas et al (2005) and Perillo et al (2014b) use maximum near-bed orbital velocity on the abscissa and mean flow velocity on the ordinate, whereas the bedform phase diagram of Kleinhans (2005) uses the non-dimensional equivalents θ′ w and θ′ c (Fig. 6a).…”

Large eddy simulation of the velocity-intermittency structure for flow over a field of symmetric dunes

“…Dunes are observed, and are deemed of significance to the dynamics of near-surface boundary layers in various planetary environments [4]. As a consequence, there have been extensive field studies of flow over such features in aeolian [5,6] and fluvial or marine environments [7][8][9][10] as well as many experimental studies [11][12][13][14].…”

Bed forms created by simulated waves and currents in a large flume

“…In addition, combinations of colinear waves and currents have been investigated in wave tunnels [e.g., Dumas et al, 2005;Cataño Lopera and Garcia, 2006]. Bagnold [1946] pioneered the use of an oscillating plate to simulate the influence of waves on the bed, reasoning that, in the reference frame of the plate, the fluid force on the grains on the plate is very similar to that produced by harmonic waves.…”