Environmental Engineering Reference
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Fig. 4.4 Example of dendritic network design for a more extensive realignment on the Wallasea site. (See the Colour
Version of this figure in Colour Plate section.)
assuming that the marsh has an approximately
constant depth relative to high water and that the
velocities are approximately uniform, such that:
(exposure, sediment characteristics, flow veloci-
ties, etc.). There are not a lot of published data
available to define this coefficient and an analysis
of other marshes in the locality is the best way
of obtaining a suitable value. Data presented by
Lawrence et al. (2004) suggest a value of the order
of k
p
b
W
h
S a
W
¼
k
where
b ¼
and
a 1
ð 4 : 1 Þ
0.01, and data for Venice lagoon suggest a
value of the order of k
0.005 (Rinaldo et al. 1999).
Tidal asymmetry is an important consideration
(Dronkers 2005), with ebb dominant channels
tending to be foundwhere the channels are narrow
and there are extensivemudflats that are relatively
high in the tidal frame. In contrast, flood domi-
nance occurs when the channels are deep and
wide and the flats are low in the tidal frame. It is
generally considered better to create a flood-dom-
inant system and allow some siltation of the
channels and flats, rather than make the channels
too small or the flats too high, as this requires the
tidal action to erode the system and, depending
on the consolidated nature of the site sediments,
In this expression W is the channel width, S is the
surface drainage area and h is the average, or
hydraulic, depth of the channel (D'Alpaos
et al. 2005). Data from Venice lagoon suggest the
aspect ratio, b , has a value of around 6 within
saltmarshes, whereas much higher values are ob-
served for tidal flat channels (Marani et al. 2002).
In contrast, data for Stiffkey Marsh in Norfolk,
UK, exhibit a variation in aspect ratio as a function
of streamorder, withvalues between2 in the upper
marsh, increasing to about 8 towards the marsh
edge (Lawrence et al. 2004). The constant, k,
reflects the particular characteristics of the area
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