Geoscience Reference
In-Depth Information
Fig. 9.16
Severe sheet erosion leading to removal of the upper layers of mangrove sediment and the exposure of root networks.
Gordon Creek, Australia. (Photograph courtesy of Piers Larcombe.)
conditions major sedimentological changes can
occur - see section 9.3.2). Coral communities
that occur within some nearshore, turbid settings
may, however, be susceptible to episodic sedi-
ment mobilization. At Inhaca Island in southern
Mozambique, coral communities (but not frame-
work reefs) are patchily developed along the
margins of shallow intertidal channels (Perry
2003). These communities undergo episodic
mortality due to smothering and burial of coral
colonies associated with longshore sediment
transport. In turn, new areas of bedrock and
coral rubble may be exposed and subsequently
colonized (Perry 2005). The natural character-
istics of coastal sediment dynamics thus create
an ephemeral suite of coral communities.
significant changes also occur in response to
low-frequency, high-magnitude events such as
cyclones and tsunamis. Tropical cyclones (also
termed hurricanes and typhoons, depending
upon geographical location) are characterized by
cyclonic surface winds formed around centres
of low pressure (McGregor & Nieuwolt 1998).
These weather systems may be up to 800 km in
diameter and are characterized by strong winds
(often
30 m s
−1
). Impacts on nearshore sedi-
ments, and on reef and mangrove communities,
result from strong wind-driven currents, high
wave heights (5 -15 m), and elevated coastal
sea-levels.
Physical disturbances to both coral reef and
mangrove communities are well documented
consequences of cyclones. In coral reefs, wave
damage may result in widespread breakage, top-
pling and abrasion of shallow water (especially
branching) corals (Rogers 1993), while high wind
speeds and storm-wave surges into intertidal areas
cause mangrove uprooting and damage (McCoy
>
9.3.2 Physical disturbance (cyclones and tsunami)
Although background or seasonal fluctuations
in sediment transport processes exert a clear
influence upon reef and mangrove systems,
