Geoscience Reference
In-Depth Information
Case study 9.1 Cyclical phases of mangrove shoreline accretion and erosion, French Guiana
The nearshore environment along the coast of French Guiana is characterized by the develop-
ment of highly mobile mangrove-colonized intertidal mudflats. The sediments that accumulate
in these mudflats are derived from the Amazon, which discharges an estimated 1200
10
6
t of
sediment per year. Around 20% of this sediment is transported north-west alongshore by the
Guiana Current (Case Fig. 9.1A), which at its peak (January-April) reaches 5 knots and extends
up to 40 km offshore. Sediment is moved both in suspended form (
c
. 150
×
10
6
m
3
yr
−1
) and
temporarily stored in the mobile mudbanks that develop along the coast (
c
. 100
×
10
6
m
3
yr
−1
).
This zone of sediment transport thus creates a mobile nearshore setting, with individual sections
of the coast alternating between phases of active accretion and erosion.
The distribution of accretionary and erosional sectors is not, however, uniform. Lengths
of coastline undergoing active accretion range from 20 to 40 km, are up to 5 km wide at low
tide, and comprise on average an estimated 3
×
10
12
m
3
of sediment. The mudbanks have a
maximum thickness of 10 m and typically exhibit an asymmetric morphology, being steeper
on the leeward side. Erosional sectors extend for 8 -26 km along the coast and are character-
ized by numerous smaller embayments (each up to 500 m wide) giving the eroding sectors a
'sawtooth' profile. Mudbank migration and associated degradation of the colonizing mangrove
forests occurs primarily as a result of ongoing erosion of the windward (up-current) side of
the mudbanks and sediment accretion on the lee side (Case Fig. 9.1A). Migration is attributed
primarily to longshore currents and an oblique wave approach. This process, however, may be
enhanced in some areas by particularly rapid rates of sediment accretion causing smothering of
mangrove pneumatophores and subsequent mortality.
×
(a)
(b)
Magnan Bank
Organabo Bank
Iracoubo Bank
ATLANTIC OCEAN
Kourou Bank
nt
N
Guyana
Venezuala
Cayenne -
Mahury Bank
Approuague -
Behague Bank
Surinam
French
Guiana
Coastal plain
French Guiana
Brazil
0 km 50
100 km
Amazon
Submarine section of mudflat
(c)
Accretion
Case Fig. 9.1A
(a) Location map showing section of
coast between the Amazon and Orinoco rivers influenced
by the Guiana Current. (b) Detail of inset in (a) showing
the location of intertidal mudbanks along the French
Guiana coast. (c) Schematic plan view of an intertidal
mudbank showing areas of erosion and accretion, and the
development of mangrove communities across the upper
intertidal area. (Adapted from Froidefond et al.1988.)
Erosion
Erosion
Pioneer mangrove
Young mangrove
Mature mangrove
