Environmental Engineering Reference
In-Depth Information
4
Managed Realignment: A Coastal
Flood Management Strategy
IAN TOWNEND, COLIN SCOTT AND MARK DIXON
Introduction
from taking place, it is often the case that the
more seaward part of the shore profile continues
to move landward but, because the upper part
of the profile is held by the sea defences, the
intertidal, in front of the defences, gets narrower
(Taylor et al. 2004).
Conservation and restoration of deteriorating
coastal habitats, alongside project-specific com-
pensation measures, have been the dominant mo-
tives for managed realignment. Across northern
Europe some 76 realignment schemes have been
completed over the last 20 years, and in this time
there have been a further 18 Regulated Tidal Ex-
change (RTE) projects, where the tidal flows are
managed through control structures such as
sluices or weirs rather than full openings of the
sea wall. While many of these 94 schemes had
more than one objective, their primary motives
have been conservation (33 schemes), compensa-
tion (32 schemes), flood defence improvement or
cost reduction (16 schemes) or the development of
a more natural shoreline (four schemes) with the
primary driver for the remaining nine schemes
being unknown (Rupp-Armstrong et al. 2008,
Rupp-Armstrong and Nicholls 2010). In the USA
almost all the managed realignments (alternative-
ly referred to as 'restoring tidal inundation' or
'dike/levee breaching') have been undertaken for
conservation or compensation reasons (Rupp
2009). Outwith these primary aims for managed
realignment, there are, as we shall discuss later,
many socioeconomic benefits of undertaking
habitat restoration work that go beyond these key
aims and, equally, many such socioeconomic im-
pacts from the ongoing habitat deterioration. As
The removal of existing flood defences has been
variously referred to as managed retreat, managed
realignment and habitat creation or restoration,
depending on the underlying objectives of the par-
ticular scheme. For the purposes of this review,
managed realignment is the form of coastal adapta-
tion that removes a part, or all, of a sea wall in order
toallowsomeadditionallandareatobesubjectto
tidal action. This may, or may not, require the
provision of modified defences, or defences set back
on a new line, to protect local assets.
The primary motives for carrying out managed
realignment are to adapt to sea level rise, to en-
hance coastal protection levels, to create a more
cost-effective defence alignment and/or to create
new coastal habitats. The habitat creation objec-
tives are, in turn, driven by the need to compensate
for losses following coastal developments or to
restore and conserve habitats that have been, and
are, subject to more general deterioration. This
deterioration of coastal habitat is often attribut-
able to 'coastal squeeze', and is a process that has
been exacerbated by past land reclamations in
many instances. On unprotected coasts, as sea
level rises, the shore is able to move landwards,
giving rise to what is called marine transgression.
However, around many coasts and estuaries, the
existing defences fix the interface between land
and sea. By preventing the process of transgression
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