Environmental Engineering Reference
In-Depth Information
Table 4.4
Summary of design options
Feature
Concept
Suitable for
Intertidal
mud
Low-lying sites that can be opened up to a high degree of
exposure will tend to formmud
Benthic fauna and feeding resources for
overwintering bird communities and
at or
at or sand
at depending on
sh species
sand
at
the sedimentary environment
Saltmarsh
Can form at the back of a mud
at, or in a sheltered breached
enclosure with an elevation of MHWN and above. A suitable
dendritic channel network will be required to provide tidal
inundation
Saltmarsh and possibly grazing marsh. Also
sh
feeding areas
Saline lagoon
Use some form of regulated exchange to allow site to be
inundated with salt water and limit the release so that water
is retained in the lagoon
Specialist BAP invertebrate habitat and wildfowl
roosting/feeding area
High marsh
lagoon
Shallow 'pans' created in upper saltmarsh that act like saline
lagoons
Specialist BAP invertebrate habitat and wildfowl
roosting/feeding area
Tidal creek
Large-scale channels, with associated mud
at and saltmarsh,
and possibly a dendritic network of tributary channels
Subtidal, intertidal and saltmarsh fauna. Also
sh
feeding areas
Intertidal
islands
Areas of high ground (above high water) within the intertidal
provide safe areas for bird roosts
Bird roosting and breeding sites as well as locations
for plant and terrestrial invertebrate species
Brackish marsh
and lagoon
Similar to the saline lagoon but using freshwater sources
Mitigation habitat for losses incurred to invertebrates
and water voles on the island
Freshwater
marsh
Can be created above the tidal frame with a suitable freshwater
supply
Freshwater marsh and grazing marsh
Regulated tidal
exchange
A combination of weirs to allow the top of the tide to enter a
section of the site, with one-way sluices to release the water
at low tide
Mixed brackish wetland habitat dependent on land
morphology and water exchange volumes
BAP, biodiversity action plan; MHWN, mean high water neaps.
inundation, it is often necessary to provide a series
of channels or ditches to promote both influx and
drainage of the tidal waters. In many instances it
may be appropriate to simply modify the existing
field drains to establish a coherent network.
This may not be ideal from a morphological point
of view but, by using what is there, the amount
of earthworks and hence costs are minimized
(Figs. 4.2 and 4.3). In sites without extensive field
drains, it is necessary to develop a dendritic chan-
nel network that fits within the planned topogra-
phy for the site. For sites that have been
historically reclaimed and are being reinstated, it
is often possible to identify the original channel
network on aerial photographs. This is because the
infilling and subsequent differential settlement of
the original channels produces minor differences
in level and drainage across the surface that are
visible in vertical aerial photographs. These can be
used as a template for the channels in the rein-
stated site.
However, in some sites it is necessary to devel-
op a channel layout based on the site topography
and the habitats that are to be supported (Fig. 4.4;
see also Fig. 4.2). Various guidance is available on
the hydraulic and morphological geometry re-
quired for successful habitat restoration schemes
(Krone 1993; French 1996; Haltiner et al. 1997;
Friedrichs and Perry 2001; Williams et al. 2002). In
addition, a number of models are beginning to be
developed that are able to represent the evolution
of saltmarshes and mudflats and provide a more
process-based means of establishing how creek
networks (Marani et al. 2003; D'Alpaos et al.
2005; Temmerman et al. 2005; Hood 2007) and
saltmarshes (Mudd et al. 2004; Morris 2006;
D'Alpaos et al. 2007; Marani et al. 2007; Kirwan
and Murray 2007; Temmerman et al. 2007) are
likely to develop.
For preliminary design of the site, some simple
rules can be used to explore the mix of marsh,
mudflat and channel that may be possible on a
