Geoscience Reference
In-Depth Information
Cut and recover mode
Rapid prograding mode
Old chenier
ridge
T1 - Progradation
T1 - Rapid progradation
Shell beds
T2 - Cyclone-driven shoreline
erosion and chenier ridge emplacement
T2 - Cyclone-driven emplacement of chenier
ridge, but little/no shoreline erosion
New chenier
ridge
Cyclone
Cyclone
Mangrove damage
T3 - Continued rapid progradation
T3 - Progradation restores shoreline
Fig. 9.15
Contrasting models of shoreline evolution at Princess Charlotte Bay, northern Australia. (Adapted from Chappell &
Grindrod 1984.)
creek erosion. Cliff erosion is caused by tidal
scour and subsequent mass slumping of channel
banks and occurs primarily along the seaward
fringes of mangroves. Rates of retreat average
2myr
−1
, but in places reach 30 -50 m yr
−1
. Sheet
erosion occurs due to sediment desiccation and
burrowing and results in vertical stripping of
surface sediments, typically removing a few
millimetres at a time (Fig. 9.16). This produces
highly erodible sediment which is removed
during spring tides, and rates of vertical loss
average 1-3 cm yr
−1
. Tidal creek erosion occurs
as a result of channel widening and deepening,
with lateral erosion of up to 3 m yr
−1
. Strati-
graphical studies indicate that this net erosional
regime has been active over the past 5000 years
(Semeniuk 1981).
Smaller scale changes in sediment accumula-
tion also occur as a result of local changes in
fluvial sediment discharge or nearshore sedi-
ment dynamics, and may result in direct erosion
or increased sedimentation. At Portuguese Island
in southern Mozambique, changes in coastline
configuration have caused both direct erosion
of mangrove substrates and progressive tidal
restriction, leading to a 75% reduction in man-
grove extent between 1958 and 1989 (Hatton &
Couto 1992). Conversely, sediment mobilization
along mangrove channels in Brunei, Malaysia
has caused channel damming and increased
mangrove flooding, leading to localized mortal-
ity of mangrove species and intertidal fauna
(Choy & Booth 1994). A common impact of
changing sediment dynamics is modified sedi-
ment accumulation rates. Although many man-
grove species can tolerate accretion rates of
5-10 mm yr
−1
, higher rates may result in man-
grove mortality due to root smothering (Ellison
1998). Where this occurs, intertidal substrates
may become unstable and prone to reworking.
Mangrove shorelines are often, therefore, highly
dynamic in areas of either high allochthonous
sediment input, or where intertidal sediments
are subject to frequent mobilization.
In reef environments, rapid geomorphological
change, in terms of the position of the reef body
and its shoreline geometry, are often less evident.
This probably reflects the more rigid structure of
many framework-dominated reefs, which require
significantly elevated energy regimes, such as
those associated with cyclone activity, to facilit-
ate sedimentary responses (although under such
