Environmental Engineering Reference
In-Depth Information
function is a combination of tidal inundation frequency, depth, and duration,
known as hydroperiod (French 1993). Wetland environments below the
highest astronomical tide experience direct tidal inundation, with decreasing
frequency and duration as a function of increasing elevation within the tidal
frame. For coastal wetlands landward, the water table is linked to the sea
level infl uence, which is an important control on the groundwater position
that provides the waterlogged conditions necessary for their development
(Hageman 1969). The result of the interaction between hydrodynamics and
elevation is a shore-parallel zonation of plants, where each zone tend to
move both vertically and horizontally in response to changing sea level
and associated stressors (Hayden et al. 1995).
Coastal wetlands developing under rising relative sea level during
the past 10,000 years (the Holocene) have been largely studied along the
eastern coast of North America, as well as marshes in northern Europe. The
process of wetlands migration under a rising sea level was earlier described
by Dutch geologists. Hageman (1969) termed the area where freshwater
wetlands persist under the control of relative sea-level as the perimarine
zone and studied the evolution of freshwater swamps in the western Rhine/
Meuse delta, in response to the rise in sea level during the Holocene. There
are examples of sedimentary records (Kirby 2001, Waller 1994) showing
that peat-forming perimarine wetlands accumulated deep layers of organic
matter between around 6,000 and 2,000 years BP, and palynological analysis
of these peat deposits showed sequences of salt marshes, reed swamps,
fens and woodland carr communities developing under a rising sea-level,
which maintained a near surface watertable (Waller et al. 1999).
In contrast to these well studied examples of continuous rising in relative
sea level, little is known about wetland response in coastal environments
that developed under different conditions after the last glacial age. While the
globally averaged sea level has been rising from the Last Glacial Maximum
(LGM) to the present, the relative height of the sea with respect to land
(relative sea level) can vary from place to place due to local tectonic and
hydrographic effects (Fig. 1). As the mass of the continental ice melted, a
huge weight was released from continental shelves, which rose by isostatic
rebound of the land. In those areas where the ice load was the greatest and
the largest rebound occurred, the land rose faster than the sea, the relative
sea level decreased, the coast prograded, and new land emerged over the
last 10,000 years (relative sea level curve type A in Fig. 2). In other areas, the
coast initially receded from a rising sea until the relative sea level reached a
maximum about 5,000 years ago. After the transgressive maximum the coast
prograded, as the relative sea level decreased to its present elevation (relative
sea level curve type C in Fig. 2). Where the relative sea fell rapidly, new land
constantly emerged, the coastal wetlands continuum migrated seaward,
and the Holocene estuarine environments became part of the terrestrial
