Page 64 - Sedimentology and Stratigraphy

Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
Final Proof page 51
26.2.2009 8:16pm Compositor Name: ARaju
Flows, Sediment and Bedforms 51
irregularity such as a step in the bed caused by an
accumulation of grains, the streamlines converge and
there is an increased transport rate. At the top of the
step, a streamline separates from the bed surface and
( +
0 a region of boundary layer separation forms
/ between the flow separation point and the flow
attachment point downstream (Fig. 4.8). Beneath
this streamline lies a region called the separation
bubble or separation zone. Expansion of flow over
the step results in an increase in pressure (the Ber-

8 * / & + *
' noulli effect, 4.2.3) and the sediment transport rate

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is reduced, resulting in deposition on the lee side of
the step.
Current ripples (Figs 4.9 & 4.10) are small bed-
forms formed by the effects of boundary layer separa-
tion on a bed of sand (Baas 1999). The small cluster of
grains grows to form the crest of a ripple and separa-
( +
tion occurs near this point. Sand grains roll or saltate
0
/ up to the crest on the upstream stoss side of the
ripple. Avalanching of grains occurs down the down-
stream or lee side of the ripple as accumulated grains
become unstable at the crest. Grains that avalanche
on the lee slope tend to come to rest at an angle close
to the maximum critical slope angle for sand at
* / & *
' around 308. At the flow attachment point there are

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increased stresses on the bed, which result in erosion
Fig. 4.7 Layers within a flow and flow surface roughness: and the formation of a small scour, the trough of the
the viscous sublayer, the boundary layer within the flow and ripple.
the flow depth.
Current ripples and cross-lamination
The following sections are concerned mainly with A ripple migrates downstream as sand is added to the
the formation of bedforms in flowing water in rivers crest and accretes on the lee slope. This moves the
and seas, but many of the fluid dynamic principles crest and hence the separation point downstream,
also apply to aeolian (wind-blown) deposits: these are which in turn moves the attachment point and
considered in more detail in Chapter 8. trough downstream as well. Scour in the trough and
on the base of the stoss side supplies the sand, which
moves up the gentle slope of the stoss side of the next
4.3.1 Current ripples ripple and so a whole train of ripple troughs and crests
advance downstream. The sand that avalanches on
Flow within the viscous sublayer is subject to irregu- the lee slope during this migration forms a series of
larities known as turbulent sweeps, which move layers at the angle of the slope. These thin, inclined
grains by rolling or saltation and create local clusters layers of sand are called cross-laminae, which build
of grains. These clusters are only a few grains high up to form the sedimentary structure referred to as
but once they have formed they create steps or defects cross-lamination (Fig. 4.9).
that influence the flow close to the bed surface. Flow When viewed from above current ripples show a
can be visualised in terms of streamlines in the fluid, variety of forms (Fig. 4.11). They may have relatively
imaginary lines that indicate the direction of flow continuous straight to sinuous crests (straight rip-
(Fig. 4.8). Streamlines lie parallel to a flat bed or the ples or sinuous ripples) or form a pattern of uncon-
sides of a cylindrical pipe, but where there is an nected arcuate forms called linguoid ripples. The