Environmental Engineering Reference
In-Depth Information
Indus, Nile, and Ebro deltas (Day et al. 2006, Ibañez et al. 1999, Milliman et
al. 1989, Pont et al. 2002, Snedaker 1984, Stanley and Warne 1993), and Venice
Lagoon (Day et al. 1999, Pirazzoli 1987). Deltaic regions are particularly
vulnerable to a relative sea level rise because of a rapid subsidence. Under
this scenario, river sediment supply and human-induced changes in
sedimentary fl uxes are critical agents in shaping deltaic evolution. Dam
construction and the increase in water demand for agriculture, industry,
and tourist development have dramatically reduced the sediment load of
rivers, and are thought to be a major cause of deltaic degradation when
coupled with subsidence (Stanley and Warne 1993).
Vertical accretion and progradation resulting from sedimentation are
not the only processes supporting wetland persistence within the zone of
hydrologic infl uence of sea level. For coastal areas experiencing a relative
rise in sea level, the different plant associations within the coastal wetlands
continuum are expected to migrate landward, and the future extent of the
wetland zone will depend upon the combined effect of seaward vertical
accretion, disturbance, and landward transgression (Christian et al.
2000).
There are good examples of coastal marshes and mangroves throughout
the world, set against the land as a fringe parallel to the shore that seem
capable of responding to sea level rise by moving inland, but there are also
some exceptions. Wetlands growing on islands within estuaries have no
land to migrate (Kearney and Stevenson 1991, Wray et al. 1995). Similarly,
the migration of wetlands inland may also be prevented in places where
the landward slope is too steep, or where people have built hard barriers
landward of the wetlands. In these cases where transgression stalls, low
sediment supply results in an eroding seaward margin, and wetland
communities may disappear by erosion over time (Brinson et al. 1995).
Erosion of salt marsh sediments as a response to higher tidal fl ow
velocities and wave energy also raises concerns. Although eroded sediments
supply mud to the tidal sediment budget, these resuspended muds contain
pollutants accumulated over the past years, many of which are now
banned or strictly regulated due to their detrimental environmental and
health impacts (Valette-Silver 1993, Williams et al. 1994). For example, the
erosion of sediment from the Mersey Estuary will release DDT, mercury,
lead, and radionuclides into suspension (Fox et al. 1999). As a side effect of
sea level rise and marsh erosion, pollutants that are currently locked away
in sedimentary reserves may become a major source of environmental
pollution in the near future.
