Environmental Engineering Reference
In-Depth Information
the Somerset Levels, Romney Marsh, and Fenland systems in the United
Kingdom). Former marshes in northern Europe have been extensively
diked over the past 2,000 years, and the few surviving areas are reduced
in size and strongly modifi ed. On the German coasts of the Wadden Sea,
approximately 1,000 km
2
of former coastal marshes have been diked over
the last millennium. Continuous embankments of newly accreted land
have shaped the mainland coasts of the northern Netherlands, where
anthropogenic salt marshes extend over 190 km
2
, and virtually no natural
tidal marshes remain (Lotze 2004). In the Severn Estuary, about 840 km
2
of marshes have been impounded since the end of the Roman occupation,
whereas only 14 km
2
of active marsh remains (Allen and Duffy 1998) and
similar relations between active and embanked marshes would also apply
for France and Denmark (Allen 2000).
In Eastern Asia, several human activities, such as dredging and
deepening of navigation channels, water diversions to northern China
and large-scale land reclamation are thought to remarkably change the
Yangtze Delta environment in the next few decades (Xiqing 1998). The Three
Gorges Dam is the largest hydroelectric dam in the world. Its construction
began in 1994, and the reservoir began to fi ll in 2003. Since cultivation in
this region dates back to more than 7,000 years, human activities have
played a major role in shaping present landscapes. With more than 50% of
the world's population living in Asia, and most of the world's ricefi elds
occurring in Asian deltas (Galloway and Melillo 1998), human pressures on
the contributing watersheds have greatly infl uenced the past and present
coastal processes. The future development of big Asian deltas may be driven,
to a great extent, by large scale engineering projects.
Prior to European settlement in the Bay of Fundy, marshes covered wide
areas of the Minas Basin and the upper reaches of Chignecto Bay, where
large amounts of fi ne sediments accumulate. However, during the past 400
years lowlands have been intensely diked and reclaimed, with an estimated
reduction of about 70% of the former marsh area (Gordon and Cranford
1994). Agricultural expansion is the major cause of wetland losses. Since the
early 1800s, wetland conversion to agriculture is estimated at over 20 million
hectares, including 65% of the coastal marshes of Atlantic Canada.
The temperate coasts of Australia have also been greatly affected
by human activities. Although losses have been small, compared to the
original wetland area (Adam 1990), reclamation for industrial and human
development has been concentrated on the southeastern coasts. Besides
reclaimed areas, the main threats to salt marshes are ecosystem degradation
due to alterations in hydrologic regimes, pollution, and weed invasion. The
presence of large wetland complexes, comparatively little affected by human
alterations, is an attribute that distinguishes South America (Pratolongo et
al. 2009). Salt marshes have commonly been used as pasture for livestock
