Geology Reference
In-Depth Information
Fig. 10.3
The back-barrier energy gradient, as reflected in
the progressively shoreward decreasing mean settling velocity
of the sediment (Modified after Flemming
2002
). Note the
pronounced spatial displacement between the summer (high
water temperature, low kinematic viscosity) and winter (low water
temperature, high kinematic viscosity) gradient
10.3
Morphology, Sedimentology
and Mass Physical Properties
Tidal flow patterns and suspended matter transport in
back-barrier tidal basins have to date been successfully
simulated over a number of tidal cycles (Stanev et al.
2007, 2009
; Lettman et al.
2009
). In addition, prom-
ising advances in morphodynamic modeling on
decadal to millennial time scales have been made in
recent years (Fortunato and Oliveira
2004
; Dastgheib
et al.
2008
; van der Wegen and Roelvink
2008
;
Dissanayake et al.
2009
; Ganju et al.
2009
; van der
Wegen et al.
2010
).
As mentioned above, wave action is an important
secondary hydrodynamic factor on tidal flats. Indeed,
it is doubtful whether barrier islands, supratidal flats
and a number of other coastal environments located
above the spring high-tide level would exist at all with-
out the influence of waves. In the case of back-barrier
tidal flats, longer-period open ocean swells (T > 8 s)
and wind waves (T = 4-7 s) penetrating tidal inlets
loose as much as 95% of their energy through friction
and breaking when crossing the ebb-deltas (Lettmann
et al.
2009
). As a consequence, back-barrier sedimen-
tary processes are more strongly influenced by locally
generated short-period wind waves (T = 2-3 s). This is
reflected by the preponderance of small-scale wave
ripples in both intertidal sand and mud deposits (Davis
and Flemming
1995
).
10.3.1 Morphological Characteristics
Back-barrier tidal basins are typically bounded by
barrier islands on the seaward side and the mainland
coast on the landward side. Laterally they are sepa-
rated from neighbouring tidal basins by slightly elevated
watersheds (tidal divides). The location of the water-
sheds depends on the angle of approach of the tidal
wave relative to the orientation of the coast. If the tide
approaches normal to the coast, the watersheds are
located midway between the two heads of the islands.
However, the more acute the angle of approach, the
stronger the displacement of the watersheds in the
direction of tidal wave propagation. In the case of
Fig.
10.4
, for example, the tidal wave approaches from
the left (west), as a consequence of which the water-
sheds are displaced toward the right (east). As a rule of
thumb, the watersheds are located where two separat-
ing flow paths at the head of the tidal wave meet at
high tide behind the islands after having travelled
roughly equal distances through adjacent inlets.
Individual back-barrier tidal basins are composed
of a number of characteristic morphological elements.
