Environmental Engineering Reference
In-Depth Information
2007; Mudd et al. 2004; Morris 2006; Hood 2007;
Kirwan and Murray 2007), and detailed process
models are being extended to include biological
processes such as biostabilizers, bioturbators
and vegetation such as saltmarsh (Widdows
et al. 2000).
Finally, in developing scheme proposals it is
important to be aware of the uncertainties inher-
ent in the design process, particularly relating to
habitat rate of development and extent, tomanage
expectations and to be pragmatic about the targets
set. In particular, net sediment accretion and the
resulting increase in bed levels is a common oc-
currence in realignment sites although the rates
vary depending on the sediment supply and the
hydrodynamic conditions. For instance, in the
very turbid Humber Estuary, accretion has led to
clear changes in habitat frommudflat to saltmarsh
over a relatively short timeframe (e.g. at Paul
Holme Strays and Welwick). In those instances
where habitat delivery targets are necessary or
requested then they can readily be framed to an-
ticipate and accept the likelihood of such changes
(ABPmer 2004b). Similarly, there is an increasing
recognition that the best solution is the one that
uses the natural morphology to best effect and that
physical manipulation of the landform to achieve
a predetermined habitat mosaic is usually not
warranted (beyond that required to achieve a hy-
drodynamically stable system). Such intervention
is not only unlikely to achieve a fixed habitat
composition but also might not be feasible before
realignment because of cost and sediment man-
agement implications, and is almost certainly
impossible after realignment due to safety and
access issues. Indeed, some useful features can
often be formed, without directed intervention,
from the site's natural morphological complexity.
At theWallasea North Bank realignment site both
mudflat and saltmarsh were created as compen-
sation targets, and these habitats currently appear
to be functioning well. However, there are also
areas of intermediate elevation (e.g. areas of exca-
vated clay spoil deposition on the mud) that are
poorly vegetated because they are too low in the
tidal frame and have a paucity of invertebrates
because of their elevation and sediment type.
need to be worked through in order to develop a
robust design. The sort of questions that have to be
addressed include:
. How will the site change the hydraulic and
sedimentary regime in the area adjacent to the
site?
. Is there sufficient flushing of the site?
. Will the breach cause channel incisions in the
existing mudflat?
. Will the flows in and out of the site disturb other
interests (navigation, recreation, shell fisheries,
etc.)?
. Is there any contamination in any of the sedi-
ments that are likely to be disturbed?
. What is the balance of saline and freshwater
in the site and is this appropriate for the target
habitats?
. What is the potential impact on offsite flood
hazards and drainage?
. Is there any requirement to include public access
or other community benefits on the site?
. Is there any threat from invasive species?
The full range of issues to be considered is ad-
dressed in the Habitat Review (ABP Research
1998, available at www.estuary-guide.net) and
in various publications (Zedler 2000; Zedler &
Callaway 2001; Williams and Faber 2004; Leggett
et al. 2004, and a number of websites - see 'Online
resources' below).
Once an outline layout has been established,
this can be modelled and the layout refined to
achieve the desired performance. This will usually
entail the use of well-established models to exam-
ine water flows, sediment transport and morpho-
logical change (Abbott and Price 1994; Reeve
et al. 2004). To make an assessment of the long-
term development of the site and its impact on
the surrounding area a hybrid model (Huthnance
et al. 2007) such as the regime model (Wright and
Townend, 2006; Spearman 2007) or simplified box
models such as ASMITA (Aggregated Scale Mor-
phological Interaction between a Tidal Basin and
the Adjacent coast; Stive et al. 1998) can be used.
As already noted, a number of models are now
becoming available that can represent the evolu-
tion of creek networks, saltmarshes and mudflats
(Marani et al. 2003, 2007; D'Alpaos et al. 2005,
Search WWH ::




Custom Search