Geology Reference
In-Depth Information
Fig. 5.2
Simplified map view of a tide-dominated estuary
showing the spatial distribution of processes:
W
D
= wave dom-
inated;
T
D
= tide dominated;
T
D
R
I
= tide dominated, river influ-
enced; and
R
D
T
I
= river dominated, tide influenced. Large
black arrows
indicate the directions of predominant sediment
transport: note the presence of two sediment sources and of a
bedload convergence (
BLC
) within the estuary. As the relative
importance of waves increases, the seaward extent of tidal
dominance decreases until the entire front and mouth of the
estuary becomes wave dominated, with the production of a
barrier island—tidal inlet system (see Chap. 12). Many estuar-
ies close to the tide-dominated end of the spectrum have one
or two small spits that extend a short distance into the
estuary
tide-dominated but wave-influenced conditions. Even
here, however, intense wave action during storms can
exert a strong influence on sediment movement, and
might promote rapid morphological change. As one
moves into the estuary, wave action is attenuated by
friction (Pethick
1996
) and sedimentation becomes
tide dominated, except along the high-tide margins of
the outer estuary where wave-dominated conditions
exist because the tidal currents are weak and the fetch
is large (e.g. Pye
1996
; Tessier et al.
2006
).
Tidal domination persists inland along the axis of the
estuary but with a progressively larger influence of river
currents (Fig.
5.3b
). Moving landward, one encounters
first tide-dominated, river-influenced, and then river-
dominated, tide-influenced conditions (Fig.
5.2
). The
landward limit of the estuary is taken where tidal influ-
ence is no longer evident, a position that can be many
tens to hundreds of kilometers inland from the main
coast (cf. Van den Berg et al.
2007
). This
tidal limit
can
be defined easily over a short time, but is a diffuse zone
over longer time periods for two reasons.
1. The gradual weakening of the tides in a landward
direction causes flow patterns to evolve gradually
from reversing flow with a seaward residual move-
ment because of the river current, to seaward-directed
flow that stops periodically, and then to continuous
seaward flow that slows down and speeds up periodi-
cally in response to the tidal backwater effect
(cf. Dalrymple and Choi
2007
, Fig. 14).
2. All of these zones can migrate up and down river
over long distances as a result of variations in the
intensity of river flow. Thus, during periods of low
flow, tidal influence penetrates further up the river
than it does during river floods (Fig.
5.4
; Allen et al.
1980
; Uncles et al.
2006
; Kravatsova et al.
2009
).
Changes in the intensity of the tides, because of
neap-spring and longer-term astronomic cyclicity,
have a similar but smaller effect, with the tidal influ-
ence penetrating further into the estuary during
spring tides, for example.
Because of the funnel shape of tide-dominated estu-
aries (Fig.
5.1
), the energy of the incoming tidal wave
is concentrated into an ever-decreasing cross-sectional
area as it propagates up the estuary. This tendency is
not initially offset fully by friction, so the tidal range
increases into the estuary, reaching a maximum value
some distance landward of the coast (cf. Dalrymple
and Choi
2007
, their Fig. 5; Li et al.
2006
, their Fig. 4).
Beyond a certain point in the estuary, however, the
decreasing water depth causes friction to become more
important than convergence, and the tidal range
decreases toward the tidal limit. Such a hydrodynamic
pattern (i.e. a landward increase in the intensity of the
tides) has been termed
hypersynchronous
(Salomon
and Allen
1983
; Nichols and Biggs
1985
; Dyer
1997
).
Within tide-dominated estuaries, the tidal wave
adopts the characteristics of a standing wave (cf. Dyer
1997
) with the fastest currents occurring approxi-
mately at mid-tide, and little or no water movement at
both high and low water, creating two
slack-water
periods (Fig.
5.5
). Because of the lateral constraint
provided by the estuary margins, the currents are
