Geoscience Reference
In-Depth Information
Contaminant remobilization is often triggered
by natural (climate) or anthropogenic (land use)
changes that cause modifications, first in sediment
load and delivery, and eventually in erosion and
deposition. Macklin (1996) warned that flood-
plain contaminant remobilization is increasing
as a result of the hydrological changes associ-
ated with global warming, and stressed that the
long-term stability of contaminant metals with
respect to changes in physical (river bank and
bed erosion, land drainage and development) and
chemical conditions (redox and pH) is poorly
understood. The remobilization of contaminants
through the physical erosion of contaminated
saltmarsh sediment can also be a significant source
of contaminants to estuaries and coastal waters
(see Chapter 7).
dramatically increase discharge rates through
the catchment (Fig. 1.12a), resulting in increased
suspended (Fig. 1.12b) and bedload sediment
transport (Amos et al. 2004). These high-energy
events also influence spatial variability in sedi-
ment transport and storage within fluvial catch-
ments. For example, in the Rajang River Delta
of Sarawak, eastern Malaysia, sediments are
stored on the delta plain during the 'dry' season,
but undergo rapid offshore transport during
the 'wet' season when discharge rates increase
(Staub et al. 2000).
In coastal environments, storm waves are
directly responsible for extensive reworking of
unconsolidated sedimentary deposits and this is
manifested in changes in beach profiles and the
breaching of coastal barriers (see Chapter 8), and
the on- and offshore transport of sediment and
rubble (see Chapter 9). In the tropics, high wind
speeds and storm waves associated with cyclones
can lead to tree damage and mortality within
mangroves. This, in turn, can facilitate substrate
destabilization and erosion of intertidal sedi-
ment substrates. These events can thus result
in significant localized ecological damage and
often marked short-term changes in patterns
of nearshore sediment accumulation. However,
cyclones and other high-energy episodic events
are also important controls on the longer term
distribution and development of sedimentary
environments. On the Great Barrier Reef shelf,
for example, cyclones generate northward flow-
ing alongshore currents, which result not only in
significant along-shelf sediment transport, but
also a marked partitioning of sediment across the
shelf (Larcombe & Carter 2004; see Chapter 9).
1.6
SEDIMENTARY RESPONSES TO ENVIRONMENTAL CHANGE
1.6.1 Sedimentary responses to natural
disturbance events
Although daily or ongoing processes of fluvial
or tidal flow and water/wind velocity influence
background levels of sediment transport and
accumulation, the amount of sediment transport
that occurs during 'normal' conditions is rela-
tively low. Most sediment transport and, as a
result, much of the morphological change that
occurs within sedimentary environments takes
place during low-frequency but high-magnitude
events. These may be associated with storms or
high (seasonal) rainfall episodes, or with episodic
high-energy events such as cyclones and tsunami.
At these times, sediment transport rates can dra-
matically increase and hence a high proportion
of annual sediment movement may occur over a
period of only a few days. This is particularly the
case in many arid and semi-arid environments,
which are characterized by highly 'flashy' dis-
charge events and where short-lived but high-
intensity rainfall events lead to very high-energy
flows (see Chapter 5). High-magnitude discharge
events also characterize many seasonally influ-
enced fluvial systems. In the Burdekin River
catchment, North Queensland, tropical cyclones
1.6.2 Anthropogenic modifications of sediments
and sedimentary systems
Although seasonal and natural (i.e. storm-
induced) changes in energy levels can lead to
changes in sediment dynamics and accumulation
rates, anthropogenic activities also have poten-
tial to modify rates of sediment input, sediment
transport pathways and the composition of the
accumulating sedimentary materials. Clear links
between anthropogenic activity and sedimentary
