Geoscience Reference
In-Depth Information
1992 (Swiadeck 1997), although this is likely to
have been mitigated by the presence of nearby
protective reef flats, and by the relatively restricted
fetch of the approaching waves. Well-documented
depositional features of cyclone-influenced tropi-
cal shorelines are chenier ridges. These comprise
sands and shells winnowed from adjacent mud-
flats and transported through the mangroves
by waves during cyclone-related storm surges.
Individual cyclones may result either in the
emplacement of chenier ridges or the migration
of ridges across chenier plains (Chappell &
Grindrod 1984; Woodroffe & Grime 1999).
Cyclone-related sedimentation, driven by storm-
surges, can also result in widespread mangrove
mortality due to root burial (Ellison 1998).
High-energy conditions are not confined to
cyclones, with significant physical disturbance also
associated with tsunami. The long-wavelength
waves associated with tsunami are generated by
earthquakes, volcanic eruptions or submarine
slides and can move at speeds in excess of 700 km
h
−1
in the open ocean. As water depth decreases
nearshore wave height increases dramatically,
resulting in significant nearshore damage and
sediment and rubble transport (Solomon & Forbes
1999). The high-magnitude (Richter scale 9.0)
earthquake-generated Indian Ocean tsunami
of 26 December 2004 provided dramatic evid-
ence of both the major geomorphological and
sedimentary impacts that these events can have
(Fig. 9.19), as well as the high human and socio-
economic cost. The impact of this event on coral
reef communities appears, as is commonly the
case with cyclone damage, to have been spatially
very variable, with major coral damage reported
at some sites, whereas other areas survived un-
scathed. At the time of writing a number of post-
event geomorphological and geological surveys
were being conducted, and these may provide a
useful insight into the role (or not) of reefs and
mangroves in affording protection to affected
shorelines. Common sedimentological indicators
of past tsunami events (tsunamites) include the
occurrence of large limestone megaclasts on reef
flats, along with the occurrence of allochthon-
ous reworked sands and shells (Noormets et al.
2002; van den Bergh et al. 2003).
9.3.3 Large-scale oceanographic changes
In addition to the physical disturbance associated
with high-energy events, major changes in near-
shore environmental conditions periodically occur
due to shifts in oceanographic conditions. The
most common example of this is associated with
the El Niño-Southern Oscillation (El Niño) which
causes major changes in sea-surface temperatures
and climate in the Pacific Basin (McGregor &
Nieuwolt 1998). During normal (non-El Niño)
years, warm water in the equatorial Pacific is
driven westwards by prevailing trade winds,
resulting in higher and warmer sea-surface con-
ditions in the west. The westward flow is com-
pensated for in the eastern Pacific (i.e. the western
seaboard of central America) by upwelling of
cool, nutrient-rich waters. These waters limit
extensive reef development (see section 9.1.3).
During El Niño years, however, the trade winds
weaken and warm water sloughs back eastwards,
restricting upwelling and bringing warm waters
into the nearshore areas of the eastern Pacific.
This further influences the coral communities,
via coral bleaching and mortality as sea-surface
temperatures increase. There are, however, also
significant implications for reef carbonate budgets.
Studies in Panama (Eakin 1996) documented
major reductions in reef carbonate production
following the 1982-83 El Niño. Before 1982, reefs
around Uva Island were characterized by net
carbonate production rates of 0.34 kg m
2
yr
−1
,
but after 1983 a 50% reduction in coral cover,
along with continued high rates of bioerosion, led
to net reef erosion (average of
0.19 kg m
2
yr
−1
).
Sea-level fluctuations of up to 0.5 m between El
Niño years can also result in significant remobi-
lization of nearshore sediments and shoreline
erosion (Solomon & Forbes 1999), as changes in
climatic conditions are marked by significantly
increased rainfall and fluvial runoff.
−
9.4
PROCESSES AND IMPACTS OF ANTHROPOGENIC
DISTURBANCE
Although most reef and mangrove environments
are subject to natural physical reworking and
