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Fig. 5.13 ( a ) Field of ebb-oriented 3D dunes on the surface of
an elongate sand bar, Cobequid Bay. ( b ) Trench through a flood-
asymmetric dune, with an ebb cap and two internal reactivation
surfaces that define a tidal bundle; the dune migrated a distance
of approximately 1 m during one tidal cycle. The surface at the
right side of the dune will be buried when the flood current
resumes and the ebb cap is eroded
in detail by Dalrymple and Rhodes ( 1995 ) and only the
main points are summarized here (see also Chap. 13).
In estuaries, tidal dunes commonly scale with water
depth (height approximately 20% of the depth; wave-
length approximately five times the depth, where the
depth is that which corresponds with the maximum
current speed, and not the depth at high tide; Dalrymple
et al. 1978 ), such that the largest dunes occur in the
bottom of channels. In these channels, dunes can reach
several meters in height. However, dune size is influ-
enced by factors other than water depth, including cur-
rent speed, grain size and sediment availability;
consequently, there can be deviations from this gener-
alization. Bedforms that are less than about 10 m in
wavelength tend to be simple dunes ( sensu Ashley
1990 ), whereas larger dunes are generally 'compound',
with smaller, simple dunes covering all or part of their
stoss and lee sides. The smaller, simple dunes can be
either 2D or 3D, whereas the larger compound dunes
are typically 2D and lack scour pits. Dunes tend to be
approximately perpendicular to the main flow, but an
oblique orientation is possible in cases where the flood
and ebb currents are not 180° apart, or because of lat-
eral gradients in the dune migration rate. As a result,
caution is required when using the crestline orientation
to deduce sediment-transport directions in detail.
Almost all dunes are asymmetric, but the significance
of a given asymmetry is strongly dependent on the size
of the dune, because the lag time (the time required for
the bedform to equilibrate with the flow) increases
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