Geoscience Reference
In-Depth Information
9.3
PROCESSES AND IMPACTS OF NATURAL
DISTURBANCE EVENTS
prograded (Wolanski & Chappell 1996). This
continues in areas that receive abundant allo-
chthonous sediment inputs. Examples include
the Ganges-Brahmaputra delta where seaward
accretion rates of 5.5-16 km
2
yr
−1
have been
estimated (Allison et al. 2003), and the Mekong
delta where some 62,500 km
2
of deltaic sedi-
ment has accumulated over the past 4500 years
(Nguyen et al. 2000).
These rapidly prograding mangrove-colonized
shorelines represent one end-member in a range
of mangrove settings which exhibit variable shore-
line dynamics. This reflects not only spatial and
temporal variability in rates of sediment supply,
but also the unconsolidated nature of the sediment
substrates and their susceptibility to nearshore
dynamics. Alternating phases of progradation and
retreat have, for example, been described along
mangrove-fringed coastlines in French Guiana
(Froidefrond et al. 1988). This results from long-
shore sediment transport, but although indi-
vidual sections of this coast alternate between
phases of accretion and erosion, there remains
a net balance in the sediment budget along the
wider coastal section (Case Study 9.1).
Along the north-east coast of Australia,
mangrove-colonized chenier ridge sequences
(see section 9.3.2) have shifted from similar
phases of relative shoreline stability (punctuated
by periodic accretion and erosion) to phases of
rapid progradation (Chappell & Grindrod 1984).
Prior to 1200 yr BP, coastal evolution was char-
acterized by periods of 'cut-and-recover'. Erosion
occurred during chenier ridge migration, but was
followed by renewed small-scale progradation
(Fig. 9.15). This period was associated with
the development of narrow (
Although sediment accumulation within both
coral reef and mangrove environments is influ-
enced by a range of physical and biogenic factors,
these sedimentary environments are also sub-
ject to natural changes in shoreline morphology
associated with changing patterns of sediment
transport and accumulation. They also respond
to larger scale and higher magnitude events such
as cyclones and tsunami, and to periodic shifts
in oceanographic conditions such as those linked
to the El Niño-Southern Oscillation. These events
may influence the biological components of
the respective environments and thus sediment
accumulation.
9.3.1 Natural dynamics of sediment transport
and accumulation
Long-term (Holocene) evolutionary trends along
coral reef and mangrove-colonized coastlines
reflect complex interactions between eustatic and
relative sea-level change, and a host of external
factors including, climate, marine environmental
parameters, sediment supply and shoreline geo-
morphology. Extensive discussion of these longer
term controls is beyond the scope of this section,
but in reef environments a range of evolutionary
models have been identified, for example, from
studies of Holocene reef systems (Neumann &
Macintyre 1985). In particular, these illustrate
growth response to relative sea-level change.
Where reefs initiate at depth, vertical accretion
is typical, whereas reefs initiating close to (or
reaching) sea-level have tended to prograde
laterally. These simple models are often modified
by local substrate, tectonic or hydrodynamic
conditions (Kennedy & Woodroffe 2002).
Mangrove shorelines have also undergone
both progradation (seaward advance) and trans-
gression (shift landward) in response to past sea-
level fluctuations (Woodroffe 1992). In general,
the post-glacial Holocene sea-level rise resulted
in transgression of mangrove shorelines. As sea-
levels stabilized during the past 5000 years, areas
receiving sufficient sediment supply subsequently
150 m) mangrove
fringes colonizing relatively steep shorelines
(slope angle 1:200). Since 1200 yr BP, the coast-
line has undergone rapid progradation. Wide
mangrove fringes have developed along lower
angle shorelines (1:1000) and promote continued
sediment accretion (Fig. 9.15).
In contrast, in north-western Australia there is
widespread net erosion of mangrove-colonized
tidal flats (Semeniuk 1981). Three types of erosion
influence local and regional intertidal geomor-
phology: cliff erosion, sheet erosion and tidal
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