G432 - Unidirectonal Flow Bedforms

I.) Relationship of unidirectional current flow to bedforms:

Sediment movement is accompanied by the organization of grains into morphologic elements called bedforms. Experiment has shown that a number of bedforms exist between certain values of flow strength, thus defining various bedform states.

The three main parameters that determine the stable bedform in unidirectional flow conditions are:

1) grain size
2) flow velocity
3) flow depth

In addition, as we have learned, several other parameters are equally important, though for most pure fluid flows on Earth, these parameters can be assumed to be constant. They include:

m = fluid viscosity
rf = fluid density
rs = grain density
g = gravitational constant

A variety of ‘bedform phase diagrams’ exist. Most of these are 2-dimensional. That is, they plot median grain size vs. mean flow velocity (e.g., Figures 3.11 and 3.13, Boggs). Sometimes, all three of the main parameters that govern bedform stability are plotted on a 3-D diagram (e.g., Figure 3.14, Boggs).

II) Unidirectional Ripples:

How are unidirectional ipples formed? Above threshold of movement on artificially smoothed bed unidirectional flow ripples are formed at relatively low flow strengths. They may also form from initial bed irregularities on bed surface. Unidirectional flow ripples are sometimes also called "current ripples".

The inititial formation of ripples is not well understood, but probably has something to do with burst and sweep processes which disrupt the smooth surface (sweeps) and deposit a small pile of grains as a result of decelleration (bursts).

Small piles of grains a few grain diameters high can begin to create flow separation and form a small back eddy downstream.

Increased erosion associated with flow re-attachment tends to entrain grains which then move up the stoss side of the downstream pile of grains until they reach the next point of flow separation. Grains accumulate high on the steep lee ripple face.

Periodically, grains become unstable and exceed angle of repose (angle of initial yield) and a grain avalanche occurs down the lee face. Thus ripples form which eventually shift or move downstream.

Structure of ripples: Grain Avalanches down lee slopes result in small scale cross lamination. Sections normal to flow may be horizontal, defining planar cross lamination (2-D ripples) or may be trough-shaped, defining trough cross lamination (3-D ripples).

Sketch of two dimensional (straight-crested) ripples in a unidirectional flow. Note the planar (also called tabular) cross-bedding.

Sketch of three dimensional (sinuous-crested) ripples in a unidirectional flow. Note the trough cross-bedding on the cut that is perpendicular to flow (i.e. the v-w cut)

This figure shows flow along the bed across a set of sinuous-crested bedforms. Note that this bottom-hugging flow converges into a scour point (i.e., forms an attachment point). Note also the geometry of the cross beds along different cuts relative to the flow direction and that these sinuous-crested bedforms result in trough cross-beds in the cut perpendicular to flow.

Net deposition during ripple formation: produces an element of vertical motion of ripple crests as well as an element of horizontal motion.

Sets of cross lamination may be formed, bounded by erosive surfaces. Climbing ripples are formed as a result; require net deposition, as in decelerating flows associated with river floods or turbidity currents.

Depending on the relative magnitude of the climb angle vs. the stoss angle, climbing ripples can be classified as subcritically-climbing, critically-climbing, or supercritically-climbing.

Some examples of current ripples:

Climbing ripples in sandstone. Note coin for scale. Flow was right to left. Some of these ripple sets are critically-climbing, whereas others are supercriticallly-climbing.

Here’s a shot of sme critically(?)-climbing ripples from an core that Exxon recovered from one of its oil fields in Texas. Flow was left to right.

Here are some subcritically-climbing ripples in a modern river sand bar. Note the translatent surfaces that truncate the ripple foresets. Flow was left to right.

Here are a series of critically-climbing to supercritically-climbing ripples from a core Exxon recovered. Flow was right to left.

Some subcritically-climbing ripples recovered by Exxon from an oil-soaked reservoir. Note the translatent bounding surfaces. Flow was left to right.

A good example of a translatent surface in a set of ripple drift strata from a core recovered by Exxon. Flow was right to left.

III) Subaqueous Dunes: If flow strength is increased beyond the ripple field for medium and coarse sand, dunes result. Dunes are similar to ripples, but dynamically distinct. Dune wavelengths commonly range from 0.6 m to hundreds of meters; heights range from 0.05 -10.0 m.

Dunes commonly show correlation of spacing and height with flow depth y, whereas current ripples do not. From experiment and field measurements,

L = 1.16y1.55


H = 0.086y1.19

A word about Bedform Hierarchies: Commonly, ripples are preserved on the stoss side of dunes. Uncertainty exists as to whether this is a result of equilibrium flow or changing flow with time.

With large-scale trough cross stratification, there is commonly a tangential contact between the ‘toe’ of the foreset and bounding surface below, due to weak development of lee-side eddy and high fallout rate of sediment on the lee side of a dune. If back eddy is well developed, it is possible to develop counterflow ripples as grains are swept back up the lee side by near bed flow in the separation bubble.

Differences in flow conditions throughout the history of dune (e.g. rising and falling stage flows) may result in reactivation surfaces. A reactivation surface is produced when the lee side of a dune is partially eroded by low stage flow. When normal flow resumes, avalanching process begins again and dune migrates.

Here are a series of large, exposed subaqueous dunes from a river in southern Louisiana. Note the group of people standing in the upper right hand part of the photo. Flow was from lower left to upper right. Most of these dunes have well-developed sinuous-crested current ripples on their stoss sides.

Here’s a cross-sectional cut, parallel to flow direction, through the tops of one of the dunes. The object in the upper left corner is the tip of our trenching shovel. Note the small ripple drift sets in the bottom half of the exposure, the prominent dune foresets in the top half of the exposure, and the one set of current ripples at the very top of the exposure.

Tabular dune cross-beds in a Sinian sandstone from northwestern China. Flow was left to right.

Multiple sets of dune foresets forming tabular cross-stratification. The outcrop is about 4 meters high. Flow was right to left. Note that all sets are subcritically-climbing as is typical for dunes, and that some of the dune foresets are overturned.

IV) Upper plane bed: Upper plane bed flow occurs in fine and very fine sand when the flow velocity is increased above that needed for ripple formation (very fine sand) or dune formation (fine sand). Upper plane bed flow occurs in the upper flow regime (Fd>1). Basically, upper plane bed flow is intense sediment transport over a flat bed.

The bed surface in upper plane bed flow is marked in detail by system of low linear ridges, a few grain diameters high, which align parallel to flow direction. These ridges are separated by low linear furrows; both form by disruption of the viscous sub-layer. Incoming sweeps push grains aside to form the small ridges and furrows.

Upper plane bed results in planar stratification. If lithified rock containing upper plane bed lamination is split parallel to bedding, parting lineation is commonly observed.

Here’s a cartoon showing the development of upper plane bed stratification. Note the current lineations shown on the bed surface and the parting lineations shown on the parted surface (top of the second white layer).

This shot was taken in a small tidal channel on the central California coast. You can see the edge of the channel in the upper left corner and a quarter for scale in the right-central part of the photograph. Flow is from the top of the photo to the bottom of the photo. Most of the lower and central part of this photo is in the upper plane bed field. There is a small standing wave train at the top of the photo, where antidunes are being produced.

Here’s a close up shot of the upper plane bed portion of the tidal channel shown above, looking straight down. Flow is right to left. Note the quarter in the upper right part of the photograph. Note also the streaking of grains parallel to flow.

Upper plane bed stratification in a modern beach deposit. Note that the 3-D nature of this cut demonstrates the horizontal and planar nature of this structure.

Here’s an example of upper plane bed stratification in a Cretaceous fluvial deposit in northwestern China. The black object in the upper central part of the photograph is a lens cap for scale.

Parting lineation in a Jurassic sandstone from northwestern China. Lens cap (lower right) for scale. Flow was either top to bottom or bottom to top.

V) Antidunes: Antidunes are produced by in-phase, shallow flow at Fr > 0.8 or so.

Spacing is roughly dependent upon the square of the mean flow velocity

L = U2g/2P

Antidunes migrate upstream, giving rise to low angle cross lamination that is dipping upstream but is faint (no grain sorting by avalanching.). Significantly, antidunes to not migrate due to grain avalanching (unlike ripples and dunes), but due to grain accretion on the upstream side of the bedform.

Here are some antidunes forming in a small tidal channel on the California coast. The antidunes are forming under the train of prominent waves in the middle of the photograph. Flow in the tidal channel is from lower right to upper left (i.e. towards the surf).

Here’s a shot of a standing wave train with associated antidunes in another tidal channel on the California coast. The bottom of the photograph is about 4 meters across. Note that the wave train is only about 1 meter in width; on either side of it other bedforms (mostly upper plane bed flow) is the stable structure.