Environmental Engineering Reference
In-Depth Information
8 sensitivity and risk assessment;
9 construction programme, costings and tender
documents; and
10 design and
a menu from which future studies can select the
criteria that most effectively address the project
needs.
The extent towhich screening is required varies
between projects in response to the availability
of the appropriate strategic information. Where a
coastal or estuarine strategy is already in place
then much of the screening may well have been
done and only the scoping phase needs to be carried
out. Thus the process of searching for individual
sites can be greatly accelerated, and the number of
iterations reduced. This is because the strategy
should provide the rationale for site selection,
so that only the more detailed work to identify
site-specific issues and constraints needs to be
undertaken. Equally, the availability of a strategy
can itself facilitate the consultation process be-
cause it should set out the reasonswhy a particular
location has been selected.
specification of monitoring
programme.
These various aspects of the design process
are presented in the form of a flow diagram in
Figure 4.1.
Identifying design constraints
A number of physical and ecological factors will
limit or constrain what can be achieved at a given
location. The relevant design constraints must
therefore be identified at the outset of the design
process. For a managed realignment the con-
straints might typically include various physical
and habitat constraints:
Physical constraints
.
maximum area,
length and width of the
site;
.
existing level of site in tidal frame;
.
potential to remove sea wall;
.
surrounding bathymetry, extent of adjacent
intertidal and distance to main channel;
.
underlying geology and lithology;
.
availability of suitable surficial sediments;
.
freshwater input.
Habitat constraints
.
habitat value of existing site and adjacent
sites;
.
target species and habitats (if a compensation
or mitigation scheme);
.
exposure to waves (storms and ship wake);
.
water quality and sediment quality;
.
Designing the Scheme
There are usually a number of options as to how a
particular site is developed, although the most
common is to simply breach the existing wall and
create a new area of mudflat and saltmarsh to the
rear. Alternatives include tidal creeks, breached
sea wall or complete removal of the sea wall,
managed breaches with sluices to control flow,
saline/brackish lagoons, and the provision of flats
and islands. The choice will depend on which
option, or combination of options, best meets the
objectives for the site and the constraints that
are invariably posed by the specific setting of the
site.
Typically the design process will encompass
the following steps:
1 confirmation of design objectives;
2 identification of design constraints;
3 selection of site (if not predetermined);
4 characterization of the site;
5 determination of target habitats and species
(if required);
6 field investigations;
7 develop site layout and design any features to be
constructed;
levels of disturbance.
Characterizing the existing site conditions
The success of the created habitat will be depen-
dent on the existing physical, chemical and bio-
logical characteristics of the site and adjacent
environment. Much of this information will need
to be obtained through field surveys, literature
review or the analysis of existing data sources.
Investigations will usually need to address:
.
sediment and substrate characteristics;
