Environmental Engineering Reference
In-Depth Information
Figure 17.14
(a) Global pattern of microtidal, mesotidal and
macrotidal ranges, (b) the variation of range during monthly
tidal cycles and (c) the occurrence of coastal landforms
associated with tidal range. High frequency is indicated by
solid lines.
MANAGED RETREAT
new developments
Managed retreat
- or
rolling easement
in the eastern United States - is the least
expensive, most sustainable response to rising sea levels. It accepts the inevitability of
inundation but attempts to harness natural defence measures. Managed retreat may be a
preferred option in areas of marginal economic value but also attracts interest in
industrialized regions as part of integrated
coastal cell
management schemes. Muddy
estuaries such as the Severn and Thames estuaries in Britain, Chesapeake Bay in the
United States and throughout south-east Asia, for example, support salt marshes or
mangroves on the landward side of mud flats in the intertidal zone. They provide some of
the most effective wave-breaking systems and highly diverse habitats which migrate in
response to sea-level change. Landward migration of these biogeomorphic systems
occurs if sediment accretion rates match sea-level rise. If sea-level rise exceeds accretion
rates, the marsh front can erode and lead to its eventual removal. However, their
particular value as natural, sustainable and inexpensive sea defences is often
compromised by hard defences at their inland margin. The marsh eventually disappears,
squeezed between rising sea level and inner sea walls, if migration is prevented (Plate 1).
Moreover, the inland barrier may induce the formation of a sedimentary ramp from the
disappearing marsh which eases the path of tidal and storm surges. Experimental removal
of hard defences, in areas of low economic vulnerability, permits inundation but
conserves the marsh. This creates opportunities to assess the effectiveness of managed
retreat as part of a comprehensive coastal defence system.
