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is the wave climate. Thus, sandy barriers are today
restricted to coasts exposed to mean wave heights of
less than about 2.2 m (Hayes 1979 ). In terms of wave
climate and tidal range, the stability field of modern
sand barriers essentially occupies the mixed wave- and
tide-dominated energy regimes (Fig. 10.2 ). In this
interacting, relative energy constellation the Gulf coast
of north-west Florida (NWF), for example, represents
a low wave/low tidal energy endmember, the German
Bight (GB) an intermediate wave/high tidal energy
endmember, and the barrier coast of south-eastern
Iceland (ICE) a high wave/intermediate tidal energy
endmember. To date it is not clear whether the bound-
aries of this stability field, especially the one between
the GB and ICE endmembers, are definitive or purely
fortuitous, the occurrence of gravelly barriers in
macrotidal environments suggesting that grain size
may play an important additional role (Jennings and
Coventry 1973 ; Hayes 1994 ).
In contrast to the processes along the open coast,
the tidal basins on the landward side of barriers are
predominantly controlled by tidal energy fluxes, although
wave action is an important secondary factor, as
emphasised by the ubiquitous occurrence of wave-
generated sedimentary structures. The high correla-
tions between physical parameters such as the surface
area of a tidal basin, tidal prism, tidal discharge,
inlet width, inlet cross-section, inlet depth, channel
depth, and ebb-delta area and volume document the
overriding control by the tides (Walther 1972 ; Jarrett
1976 ; Walton and Adams 1976 ; Hume and Herdendorf
1992 ; Flemming and Davis 1994 ; van Dongeren and
de Vriend 1994 ; Biegel and Hoekstra 1995 ; van der
Spek 1995 ; Williams et al. 2002 ). With respect to
back-barrier tidal flats, important hydrological factors
are the time/distance velocity asymmetries between
flood and ebb currents, tidal flats being generally
flood dominated, whereas deeper channels are ebb
dominated (Groen 1967 ; Boon and Byrne 1981 ;
Aubrey and Speer 1985 ; Speer and Aubrey 1985 ;
Dronkers 1986 ; Ridderinkhof 1988 ; Friedrichs and
Aubrey 1988 ; Friedrichs et al. 1992 ; Stanev et al.
2007 ). This has two important implications. First, the
residual current over intertidal shoals (tide-induced
drift) results in a net shoreward transport of sus-
pended sediment, a process that may be enhanced or
retarded by wind stress and wave action. An additional
factor may be the development of horizontal density
gradients over tidal flats, as recently proposed by
Burchard et al. ( 2008 ). Suspended particulate matter
(SPM) eventually settles out in places where the
settling velocity exceeds the erosion velocity. This
process acts in conjunction with the settling lag/scour
lag mechanism (van Straaten and Kuenen 1957 ;
Postma 1961 ) which is responsible for an overall
stepwise net displacement of resuspended particles in
the direction of the flood current. By this mechanism
suspended particles settle out at high water slack tide
before being resuspended in the course of the sub-
sequent ebbing tide. As the particles require higher
velocities to be resuspended than to settle out, the
time-velocity asymmetry between the ebb and flood
phase produces a net landward transport. This mecha-
nism proceeds until a balance between settling
velocity and erosion velocity is reached. The resulting
shoreward decrease in grain size is one of the main
consequences and hence a fundamental diagnostic
criteria for intertidal deposits (e.g., van Straaten and
Kuenen 1957, 1958 ; Nyandwi and Flemming 1995 ;
Chang and Flemming 2006 ).
Both mechanisms outlined above may be strongly
enhanced by seasonal changes in water temperature
which, at higher latitudes, may differ by >20°C.
The higher kinematic viscosities of the seawater in
winter result in lower settling velocities of equivalent
particles, i.e. the same particles behave as coarser sedi-
ment in summer and finer sediment in winter (Anderson
1983 ; Krögel and Flemming 1998 ; Chang et al. 2006a ).
That this effect is significant is demonstrated by the
fact that, for example in the Wadden Sea (>55°N),
particles with equivalent settling velocities in winter
(T <5°C) and summer (T >20°C) are spatially sepa-
rated by as much as 3 km (Fig. 10.3 ).
A second implication is that the channel systems of
back-barrier tidal basins are not landward-facing
flooding systems, but rather seaward-facing drainage
systems analogous to terrestrial drainage networks
(Flemming and Davis 1994 ). However, as the flow is
bidirectional, there are some morphological modifica-
tions associated with flow separation between the dom-
inant ebb and the subordinate flood current (Jakobsen
1962 ; van Straaten 1964 ). This flow separation is mod-
ulated by the Coriolis effect, which deflects the flow to
the right in the Northern and to the left in the Southern
Hemisphere. As a consequence, tidal channels are
frequently split longitudinally into ebb- and flood-
dominated sections that can, for example, be identified
by the corresponding orientation of larger bedforms.
 
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