Environmental Engineering Reference
In-Depth Information
High
Water
High
Water
(a)
(b)
Rear defence
or contour of
high ground
Sea
wall
Proposed
site
Breach in
sea wall
Low
water
Low
water
High
Water
High
Water
(c)
(d)
Fig. 4.5
Illustration of site layout design:
(a) site plan; (b) base grid and main channel
configuration; (c) marsh-dominated
dendritic network; (d) mudflat and marsh
creeks.
Low
water
Low
water
(Fig. 4.5). The main channels are then sketched on
to reflect the shape of the site and any topography
that might determine flow directions, or cut and
fill volumes. Meanders and channel branches can
be based on the simple guidance at this stage (see
Fig. 4.5b). For a predominantly saltmarsh system,
the dendritic network can then be developed
around the main channels so that they provide
water and sediment to every part of the marsh -
with this graphicalmethod any point on themarsh
should be within a grid square (two grid squares at
most) from a channel (Fig. 4.5c). The widths and
depths of the channels can then be worked out
using Equation 4.1. Where the site is lower in the
tidal frame, part of the site will be mudflat. In this
case, themarsh creeks will migrate into themarsh
from the marsh edge (Fig. 4.5d), and a similar basis
for spacing of the creeks can be adopted. The
position of the marsh edge is determined by map-
ping the contour of the maximum colonization
depth and assuming a step (or cliff) up to the
optimummarsh elevation. As the area of mudflat
increases so the site becomes more like a tidal
inlet, and the sizing of the channels will be gov-
erned by the tidal prism, rather than the surface
area as expressed in Equation 4.1. In this case, the
size of the channels can be estimated using
well-established prism or discharge relationships
(O'Brien 1931; Myrick and Leopold 1963; Town-
end 2005). The worked example in Box 4.1 serves
to further illustrate the process.
Whilst the foregoing focuses on how to estab-
lish site layouts to achieve the desired habitats,
there are a number of other considerations that
