Environmental Engineering Reference
In-Depth Information
Box 4.1 Worked example
A 186-ha site is enclosed by a fronting sea wall with high ground to landward. The sea wall is a clay
embankment with 1:2 side slopes. The site itself slopes from a low point of 0.7mODN (ODN ¼
Ordnance Datum Newlyn) up to 3.5mODN at the rear. The borrow ditch used to form the sea wall
has an invert at 1.4mODN.
The optimumdepth for themarsh is 0.4mbelowMHW (mean highwater), i.e. about þ 2.0mODN.
The maximum depth of colonization is 0.6m below MHW, i.e. þ 1.8mODN.
Site data
MHWS ¼ 2.85mODN
Area at HW
¼
186 ha
MHWN ¼ 1.90mODN
Lowest level of site ¼ 1.4mODN
Bed density of mudflat ¼ 1300 kgm 3
MLWN ¼ 1.35mODN
MLWS ¼ 2.20mODN
Target Habitats
Marsh-dominated dendritic network
Mudflat and marsh creeks
Following the graphical method outlined
above, an indicative network can be
drawn as shown in Figure 4.5c. The prism
at MHWS is simply the depth over the
marsh the area, i.e. 1.6Mm 3
(Mm 3
If an area ofmudflat is to be createdwithin the site
this will need to be lower in the tidal frame.
In Figure 4.5d, some 44 ha of the site is lower
than þ 1.8mODN and therefore likely to be
mud. In this case, if we define 10 ha of the site
as the area at the lowest elevation in the site
(i.e. 1.4mODN), there is a reduced prism over
the marsh of 1.2Mm 3 and an additional volume
over the mudflat, which we assume slopes from
the 10 ha at 1.4mODN up to the 44 ha at
þ 1.8mODN. This results in an additional
volume of about 1.3Mm 3 , so that the total tidal
prism increases to some 2.5Mm 3 .
¼ million m 3 ). Using Equation 4.1
to determine the section close to the
breach, the width will be 16-22m and the
depth 2.5-3.0m, depending on the values
of k and b that are assumed. The upstream
area can be used to determine the
dimensions at any other point in the
network.
Given the existing site topography and the very different design topographies of the two schemes
(cf. prisms of 1.6 and 2.5Mm 3 ) there is likely to be very different cut and fill requirements and quite
possibly the need to import material for the marsh-dominated scheme. The exact earthwork
requirements will need more detailed work but these initial estimates do allow habitat aspirations
to be quickly related to construction implications.
The size of the breach (or breaches) can be estimated using the method outlined in Townend (2008)
and the resultant breach sections are illustrated in Figure 4.6. The 'constructed' line in the figures
shows how this section is likely to be constructed, with a series of working platforms, allowing the
tidal flow to do the final trimming. However, it must be remembered that whilst some preliminary
lowering can take place, the final opening generally has to take place over a single tide. This in itself
limits the size of breach that can be constructed and, in many cases, leads to a preference for several
smaller breaches than the single large breach shown here.
(MHWS, mean high water springs; MHWN, mean high water neaps; MLWS, mean lowwater springs;
MLWN, mean low water neaps.)
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