Environmental Engineering Reference
In-Depth Information
Table 4.3
Indicative scope for site characterization
Sediment and substrate
Hydrodynamic and hydrological
Habitats and species
Type of strata and sediment
characteristics including:
Tidal characteristics (levels, asymmetry,
discharge/prism)
Nutrient and organic carbon accretion
rates seasonally
.
particle size analysis
.
bedrock/boulder clay levels
.
organic carbon
Channel and intertidal
ow velocities
Morphological parameters (hydraulic geometry,
hypsometry)
Monitoring of existing usage and
identi
cation of requirements for:
Benthic microbes
.
shear strength for cohesive soils
.
sediment density tests
Location of aquifers and discharges, and volumes
of freshwater input
Phytoplankton
Benthos
Resuspension parameters for nearby
intertidal mud
Groundwater levels
Water chemistry
Fish (adult, juveniles, spawning grounds)
Vegetation
ats
Sediment consolidation
Sediment quality
Type and quantity of sediment material
deposited seasonally
Water quality including nutrients, bacteria,
heavy metals, polyaromatic hydrocarbons and
pesticides
Weed species identi
cation
Overwintering birds
Bird feeding/migration
Patterns of feeding
Levels of suspended sediments in the
adjacent areas
Need for buffer zones
Imported material characteristics
(if required)
Developing the design layout
alternative is to use managed realignment as a
means of improving the resilience of an estuary.
This can be done by selecting the location of the
site(s) to enhance the dissipation of the tidal
wave and by increasing the accommodation space
(the area the estuary occupies as it migrates land-
wards in response to sea level rise) by removing
rather than simply breaching the sea walls
(Townend and Pethick 2002). In some cases the
latter may need to be done in stages; first breaching
the walls to allow the site to accrete to a level com-
patible with the external mudflat or saltmarsh and
then, some time later, removing the remaining sea
wall (or allowing it to simply collapse over time).
The nature of the openings to the site will
depend on the type of habitat to be created. This
can range from the complete removal of the exist-
ing sea wall, as just noted, through the formation
of one, or more, breaches, to the use of some form
of structure such as culverts, weirs and sluices.
Standard texts on hydraulic engineering provide
details on the design of culverts, weirs and sluices
(Chow 1959), and a method for designing a breach
in a sea wall is given in Townend (2008).
Within the site, there is invariably a need to
provide some means for the tide to propagate into
the area. Where the chosen option requiresmarine
The preliminary design is about exploring the art
of the possible, seeking to find a suitable combi-
nation of components to deliver the desired habi-
tats and related scheme objectives, whilstworking
within the identified constraints. This invariably
gives rise to conflicting requirements; some of
these can be addressed by optimizing the options
but others will need to be resolved by suitable
compromise. So, for example, whilst it may
be desirable to maximize the tidal prism within
the site to achieve the desired range of habitats,
there may be a conflicting requirement to con-
strain the prism to a certain magnitude in order to
avoid undue impacts outside the site. Determin-
ing the optimum balance invariably involves con-
sideration of a range of options and a process of
iteration to develop a scheme that is acceptable.
There are a number of options that form the basis
of the design, either individually or in some suit-
able combination. These can be described in terms
of their form, habitat type or both. A number of
options are summarized in Table 4.4.
Managed realignment can be used in one of
two ways. The most common use, as has been
described previously, is as a means of creating, or
reinstating, habitat within a particular site. The
