Geoscience Reference
In-Depth Information
determined by water circulation patterns. These
circulation patterns are, in turn, a reflection of
both the local hydrology (determined by factors
such as freshwater runoff and evapotranspira-
tion) and tidal regime. Mechanisms of sediment
transport therefore vary widely depending on
the local climate (which may be highly seasonal)
and tidal regime (which varies over both diurnal
and monthly spring-neap cycles). In addition,
very different mechanisms and rates of sedi-
ment transport occur in the tidal creeks and on
the mangrove flats. The tidal creeks often form
long, branched networks and represent the main
conduits of water (and sediment movement).
In contrast, the mangrove flats are often wide
and heavily vegetated and, because of the high
frictional effects of the vegetation, represent
sites of sediment trapping and accumulation
(see section 9.2.5.2). Ratios of creek to flat area
are in the order of 2-10 in many mangroves
(Wolanski et al. 1992) and thus the mangrove
flat represents a significant proportion of the tidal
prism (especially during the spring-tide phase).
Consequently, the processes of sediment trans-
port in mangroves exhibit marked temporal and
spatial variability.
The tidal cycle represents a particularly
important influence on sediment transport by
dictating current speeds and the magnitude and
frequency of tidal inundation. Tidal circula-
tion is the primary cause of water movement
through mangrove creeks and there is often
a strong asymmetry between ebb- and flood-
tide current velocities (see Chapter 1). The
ebb-phase is typically shorter and character-
ized by stronger current speeds. These may be
as much as one-third higher than peak flood
currents and exceed rates of 1 m s
−1
(Wolanski
et al. 1980). There are also marked differences
between the creek networks and the mangrove
flats, with current speeds on the flats often less
than 0.1 m s
−1
. As a result, significant variations
occur both in suspended and bedload sediment
transport (see section 9.2.4.1), as well as in the
flux of sediment through tidal creeks and onto
the mangrove flats.
An additional influence on sediment trans-
port in the mangrove creeks is the degree of
freshwater versus tidal influence, because this
dictates the extent of saline incursion. Where
there is strong fluvial outflow, complete flush-
ing of saltwater from creeks commonly occurs
and results in a net outflow of sediment. In areas
of restricted or seasonal outflow, sediment is
often retained in upper parts of creek systems
(Wolanski et al. 1992). Tidal incursions also
influence the areas over which processes such
as secondary circulation, saltwater flocculation,
baroclinic circulation and tidal pumping occur.
These, in turn, influence rates of sediment trans-
port through the tidal creeks (Wolanski et al.
1992). Secondary circulation occurs within creek
networks, where marked vertical differences in
either salinity or suspended sediment loads create
density gradients (Wolanski 1995; Ridd et al.
1998). These are commonly established around
meander bends. On the inside of meanders the
interface between density layers may be raised
resulting in differential transport and accumula-
tion between the upper and lower parts of the
channel. Under these conditions, finer sediments
may accumulate on the upper, inside parts of
the bank, and coarser (bedload) material on the
channel floor (Fig. 9.11).
Saline incursions into mangrove creek networks
also influence sediment transport through grain
flocculation (fine sediment aggregation). In fresh-
water reaches of creeks, fine sediments rarely
flocculate except where organic particles pro-
mote grain adhesion and/or suspended sediment
concentrations exceed 1 g L
−1
. As a result, sedi-
ment settling velocities are influenced by grain
size and density (Wolanski 1995). In saline-
influenced creeks (salinity levels
1), however,
saltwater flocculation of fine clay/silt particles
occurs (Fig. 9.13), with the metallic and organic
coatings on grains promoting grain aggregation.
This leads to the formation of large flocs up to
200
>
m across. Although both clay and calcare-
ous flocs have a low cohesive strength, the larger
grain sizes increase settling velocities, which may
be up to 100 times those of unflocculated particles
(Furakawa & Wolanski 1996). It is likely that
without this process most fine sediment would
travel through the mangrove as a 'wash load'
(Wolanski 1995).
μ
