Environmental Engineering Reference
In-Depth Information
associated sediment demand. Traditionally, salt-
marsh extent has been defined using empirical
equations relating the presence of particular spe-
cies to the tidal range, hydroperiod (duration of
submersion) and exposure (see paper by Grey in
Allen and Pye 1992). More recent developments
relate the behaviour to the available sediment
supply, tidal conditions and biomass density
(Morris et al. 2002; Temmerman et al. 2004). For
preliminary design it is sufficient to know the
likely species, the optimum depth below high
water of the marsh community, and the maxi-
mum depth below high water that any of the
species present is able to colonize.
The unvegetated slope for a mudflat can be
developed using some form of idealized intertidal
profile (Lee and Mehta 1995; Friedrichs and
Aubrey 1996). A developed marsh will tend to
intersect this profile around the maximum colo-
nization depth and then form a step, rapidly grad-
ing up to the optimummarsh elevation. If the site
is left to develop this transition naturally, it may
give rise to a significant sediment demand.
The basis for developing a channel layout will
be constrained by the size of the site, its length and
breadth, the number of breaches, and the require-
ments that:
.
the channel network meets the constraint de-
fined by Equation 4.1; and
.
that no point on themarsh ismore than a defined
distance from a channel.
The relative level of the intertidal alongside the
tidal channels and its slope away from the chan-
nels, determines site hypsometry and hence the
potential for saltmarsh colonization, allowing the
extent of mudflat and saltmarsh to bemapped out.
If the site is simply opened up to tidal influence,
the hypsometry of the site will largely determine
the distribution of habitats.Where specific habitat
objectives have been set, it may be necessary to
move material around the site, or import/export
material to achieve the desired topographic varia-
tions and hence habitats.
A simple way of developing a layout, based on
the foregoing requirements, is to place a simple
grid over a plan of the site, with the grid spacing
defined by the maximum distance to a channel
this may not be possible, or may take a very long
time. In considering the movement of sediments
in and out of the site and the extent to which the
site is likely to be a source or a sink, it is important
to consider the impact on the external estuary,
although this may be most readily done as part of
the detailed modelling phase. Modelling of the
layout can also be used to check that the flow
speeds are reasonably uniform across the site (and
through the breaches). Typically these need to be
in the range 0.5-0.7ms
1
to sustain mudflat hab-
itat - being sufficient to prevent accretion and
potential for saltmarsh colonization but insuffi-
cient to erode the channel or flat.
Where possible, meanders should be introduced
into the main creeks, as this reduces the available
fetch and hence the influence of waves (Allen and
Pye 1992). Subdivision of the area by a series of
branching creeks further helps with dissipation of
tidal energy and the reduction of wave propaga-
tion. It is also the most efficient way to both drain
the site and provide a supply of water and sediment
across the whole of the intertidal surface.
The appropriate junction angles can be based on
the ratio of the tidal discharges within the
branches (French 1996), or for preliminary design
purposes it may be sufficient to set them at 120
where the branches are of equal size and at 90
where a minor tributary feeds into the main chan-
nel (Haltiner and Williams 1987). For the latter,
there may also be a need to grade the bed of the
channel into the main channel to avoid ponding.
Within saltmarsh, the additional friction due to
the vegetation means that flow velocities slow
and sediment is deposited. The ability of the flow
to transport sediment across the marsh therefore
decays with distance from the channels. Observa-
tion of the space-filling nature of dendritic creeks
as well as work on the transport and deposition
of sediment through the marsh canopy suggest
that no point on the marsh should be more than
30-70m from a creek channel (Haltiner and Wil-
liams 1987; Pethick 1994; Marani et al. 2003).
Recent developments in the understanding of
saltmarsh behaviour and its interaction with the
hydraulics and morphology make it possible to
estimate the likely extent of saltmarsh and the
