Geology Reference
In-Depth Information
typical longshore migration instead of cross-shore
evolution (Wells and Coleman
1981
; Froidefond et al.
1988
; Eisma et al.
1991
; Augustinus
2004
). There are
totally 20-25 mudbanks along the coast of the Guianas
(Fig.
9.4
), and each is generally 10-40 km in length
and several tens of kilometers in width extending gently
from the shore to nearly 20-m isobaths (Fig.
9.10
). The
mudbanks migrate westward, driven by the alongshore
current and northwest wind waves generated by trade
winds. The mudbank migration is completed by depo-
sition at the leading (western) edge and erosion at the
trailing (eastern) edge, which is highly related to the
attenuation of the incoming waves significantly by
presence of fluid mud at the leading edge or less by
presence of the compacted clay deposits at the trailing
edge (Fig.
9.10
, Wells and Coleman
1981
; Allison and
Lee
2004
). The mudbanks migrate downdrift at mean
rates of 0.9-1.5 km year
-1
(Froidefond et al.
1988
;
Eisma et al.
1991
). Over an annual interval, the windy
season from January to April accounts for major sedi-
ment exchange between the trailing and the leading
edges, producing the most rapid mudbank migration.
The annual sediment exchange from the trailing to
leading edges along the entire 1,400-km-long Guiana
coast adds up to an amazing figure, approximately
equal to the average input of new sediment from the
Amazon (Allison and Lee
2004
).
and erosion at the lower flat (below −2-m)” in the
net-deposition periods, especially during 1864-1911,
whereas a reversed pattern of “erosion at the higher flat
and accretion at the lower flat” occurred in the net-
erosion periods, especially during 1842-1864
(Fig.
9.21
). The alternations of erosion and deposition
phases between the higher and lower flats were pre-
sumably linked to multi-decadal alternations of the
frequency of intense tropical storms. In other words,
the coastal morphology tends to preserve the storm-
induced profile in terms of “erosion at the higher flat
and accretion at the lower flat” in the stormy decades,
whereas the normal-wave morphology with “the higher
flat accretion vs. the lower flat erosion” is cumulatively
present in the relative calm decades (Huo et al.
2010
).
The multi-decadal variations in the Guiana coast-
line are produced by its typical alongshore migration
of series of the mudbank and the interbank units, which
continuously pass through a given location (Fig.
9.22
).
The mudbanks migrate with an average rate of
1.5 km year
-1
along the Surinam coast, and the average
alongshore width is 45 km for each geomorphologic
unit including a mudbank and the neighboring inter-
bank area (Fig.
9.10
). It is therefore estimated a roughly
30-year cycle of erosion and deposition along the
Surinam coast (Augustinus
2004
). Due to huge sedi-
ment input from the Amazon, a net coastal-plain
growth is generally produced after each mudbank-
interbank cycle (Fig.
9.22
, Allison and Lee
2004
). It
was noteworthy that the comparison studies using his-
torical maps and Holocene core data demonstrated the
difference between the present and Holocene sedimen-
tary processes of the mudbanks. The present, roughly
30-year cycle of high rates of accretion and erosion
associated with the mudbank migration extended at
least as far back as the last two and a half centuries, but
the beginning of this cycle is still uncertain (Plaziat
and Augustinus
2004
).
There was reported to exist another multi-decadal
cycle of erosion and deposition along the Guiana
coast, driven by secular change of ocean wind and
wave climate (Eisma et al.
1991
; Allison et al.
2000
;
Augustinus
2004
). A comparison study of different
ages of air photographs showed that the Surinam
coast changed as a whole from net erosion during the
period 1947-1966 to net deposition in the period
1966-1981 (Fig.
9.23
, Table
9.2
). The change was
found coincidently with an increase in mean wind
velocity and a general shift of wind direction from
9.4.1.3 Long-Term Cycles
(A Few Years to Decades)
Long-term cycles of tidal-flat developments are com-
monly envisioned along the deltaic plains/coastal
plains, where the shoreline can change rapidly and
vastly with a few kilometers or more in a few decades,
driven by different mechanisms. Coastal development
of the Nanhui Mudbank in the Changjiang Delta was
analyzed by using time series of nautical charts from
1842 to 2004 (Fig.
9.21
). It was shown that net erosion
occurred during the two periods of 1842-1864 and
1911-1958, alternating with two deposition periods of
1864-1911 and 1958-2004. A general erosion/deposi-
tion pattern of the mudbank can be summarized as
“accretion at the eastern flat with erosion at the south-
ern flat (smaller area)” in the net-deposition periods,
and vice versa in the net-erosion periods (Fig.
9.21
). It
is presumed to mainly result from the coincident shifting
of the Changjiang's main channel away (erosion) or
toward (deposition) the mudbank. Another pattern was
also noted as “accretion at the higher flat (above −2-m)
