Geology Reference
In-Depth Information
essentially rectilinear, and reverse by 180° between the
flood and ebb tides (Fig.
5.5
). The longitudinal varia-
tion in the peak tidal-current speeds mimics the
distribution of tidal range, increasing landward to some
maximum value (Dalrymple et al.
1991
), termed the
tidal maximum
by Dalrymple and Choi (
2007
)
(Fig.
5.3b
), before decreasing to zero at the tidal limit.
In general terms, the incoming tidal wave is typically
asymmetric because the crest migrates onshore more
quickly that the trough, a feature that is analogous to the
behavior of wind waves as they approach the beach
(Dyer
1995, 1997
). The shorter duration of the flood
tide causes the flood currents to be faster than the ebb
currents (e.g. Li and O'Donnell
1997
; Moore et al.
2009
), which, in turn, creates a flood dominance and a
net onshore movement of bed material (i.e. sand and/or
gravel), at least in the seaward part of estuaries
(Dalrymple et al.
1990
). This occurs because the amount
of bed material that can be moved is a power function of
the current speed, so that the direction of net sediment
movement is determined more by an inequality in the
peak speeds than by differences in the durations of the
flood and ebb currents (Chap. 2; Dalrymple and Choi
2003
). The inner part of estuaries, by contrast, experi-
ences an ebb dominance as a result of the superposition
of river currents on the tides. As a result of these oppos-
ing directions of net bedload movement, tide-dominated
estuaries contain a
bedload convergence
(Johnson et al.
1982
; Dalrymple and Choi
2007
), a location toward
which bedload migrates from both directions when
averaged over a period of years. This process, supple-
mented by the trapping of suspended sediment (see
more below), is responsible for filling the accommoda-
tion (i.e. unfilled space) that is created by flooding and
transgression of the river mouth. In general, filling of an
estuary is most rapid in the inner part, and progresses in
a seaward direction. Thus, as the space fills, the bedload
convergence migrates seaward until river-dominated
seaward transport of bed material extends all the way to
the main coast. At this point, the estuary has been filled,
river-supplied sediment is exported to the ocean, and the
system is considered to be a delta. Here, this transitional
phase is referred to as the
progradational phase
of estu-
ary evolution, as opposed to the
transgressive phase
when the estuary is created.
The time-velocity asymmetry between the flood
and ebb currents, and the resulting patterns of net sedi-
ment transport described above, are accentuated by the
longitudinal variation in the cross-sectional shape of
the channels (Friedrichs and Aubrey
1988
; Friedrichs
Fig. 5.6
Contrasting channel cross-sectional shapes for (
a
) an
unfilled part of the estuary near the mouth, and (
b
) a more com-
pletely filled part of the estuary near the head. The shape in (
a
)
promotes flood dominance because the tidal-wave crest (i.e.,
high water) migrates faster than the trough (i.e., low water),
whereas the shape in (
b
) promotes ebb dominance because the
progression of the tidal-wave crest is retarded because of the
broad shallow tidal flats
et al.
1990
; Pethick
1996
). In situations with relatively
small intertidal areas, the average water depth (across
the entire channel) is less at low tide than at high tide
(Fig.
5.6a
). However, in situations with broad intertidal
areas, the water depth averaged across the entire width
of the channel and flats is actually less at high tide
(Fig.
5.6b
) because of the inundation of the wide, shal-
low tidal flats. In the first case, the crest of the tidal
wave moves more quickly than the trough, because of
the greater water depth at high water, causing the flood
tide to be shorter than the ebb, which then creates flood
dominance. By contrast, in the second case, the tidal-
wave crest moves into the estuary more slowly than the
trough, generating a shorter ebb tide and ebb domi-
nance. In most estuaries, the latter situation tends to
occur in the inner part because this is where infilling
occurs first. Consequently, there is a tendency for the
inner part to be ebb dominated, independent of the
river current, whereas the outer part tends to be flood
dominated. As the estuary fills, more and more of the
system has the cross-channel morphology (Fig.
5.6b
)
that promotes ebb dominance, and, eventually, the sys-
tem becomes a sediment-exporting delta. (For a dis-
cussion of the factors controlling tidal-flat morphology
see Chaps. 9 and 10, and Roberts et al.
2000
).
