Geoscience Reference
In-Depth Information
across the mangrove. This occurs because man-
groves reduce flow velocities through the marsh.
As a result, flood-tide water levels rise and fall
much faster in creek and creek margin areas than
on the mangrove flat itself and can create water
gradients of up to 1:1000. Where this occurs the
tide will be falling at the creek mouth while high
tide waters will be ponded on the mangrove flat,
producing strong ebb-tide velocities. At Ross
Creek, northern Australia, threshold velocities
for the fine-grained channel substrates are around
0.4 m s
−1
, and flood and ebb-tide current velo-
cities are 0.4 and 0.8 m s
−1
respectively. As a
result, bedload transport is more prominent dur-
ing the ebb-tide phase (Larcombe & Ridd 1995).
Under such conditions, sediment will be trapped
by vegetation on the mangrove flats during
spring-tide phases, but scoured from creek areas
and exported (Wolanski et al. 1992).
Landw
a
rd S
eaw
ard
Salinity
0
10 20
30
3
1
6
2
4
5
Return flow
Turbidity-maximum zone
Fig. 9.13
Schematic diagram illustrating the processes of
grain flocculation and transport associated with baroclinic
circulation in a partially stratified estuary: 1, fluvial transport of
unflocculated sediments; 2, saltwater flocculation - large flocs
settle out; 3, unflocculated fines remain in suspension; 4,
settling of finer grained flocs to the bed; 5, baroclinic circulation
transports settled flocs back upstream; 6, disaggregation of
reworked flocs and re-entrainment in the water column. The
turbidity-maximum zone forms close to the limits of saline
intrusion. (Adapted from Wolanski 1995.)
The accumulation of flocculated sediments
within mangrove creeks is influenced by two
additional factors: (i) baroclinic circulation and
(ii) tidal pumping. Baroclinic circulation occurs
as a result of the landward movement of denser,
saline waters along the bottom of tidal creeks
(Fig. 9.13). These currents entrain flocculated
sediments as they settle out and return them
upstream towards the limit of saline intrusion
(Wolanski 1995). This process is strongest in
areas with a pronounced salinity gradient, and
results in most fine sediment remaining in sus-
pension near the creek floor. Tidal pumping can
occur in mangrove creeks where there is an
asymmetry between the peak flood and ebb-
tidal current velocities. The relative strength of
the flood- or ebb-tide current varies depending
upon tidal regime and the seasonality of the
mangrove system (see section 9.2.4.1), but in
sites where tidal currents are stronger during the
flood- rather than the ebb-tide phase the result is
a net upstream movement of sediment. In com-
bination these two processes form a turbidity
maximum zone where fine sediment accumula-
tion is concentrated.
As outlined previously not all sediment that
enters mangrove creeks is retained within the
creek network and many mangroves are charac-
terized by net sediment export. Sediment export
from creek systems will be promoted in systems
where the frictional effects of high vegetation
cover produce a marked surface-water gradient
9.2.5.2 Mangrove sediment trapping and stabilization
Although tidal and fluvial currents exert an
important influence on sediment transport,
sediment accumulation is strongly influenced
by mangrove root type and pneumatophore
density, which modifies current velocities and
flow regimes (Woodroffe 1992). As currents pass
through the dense mangrove root networks on
the mangrove flats, the vegetation induces micro-
turbulent flow (eddies, jets, stagnation zones),
which maintains sediment in suspension. This
material is typically transported landward and
settles out around slack high tide as flow turbu-
lence reduces (Furukawa & Wolanski 1996).
On the mangrove flats this process is restricted
to periods of high spring tide, and material is
prevented from re-entrainment during the ebb-
tide phase by vegetation-induced friction. As a
result, tidal currents can act as a 'pump', trans-
porting sediment landward, so that the highest
sedimentation rates occur close to the high tide
limit. This has been demonstrated at Middle
Creek, Australia (Furukawa et al. 1997) where
preferential sediment trapping occurs in areas of
vegetation-induced flow stagnation and around
80% of suspended sediment is trapped in the
mangroves. This equates to 10 -12 kg of sediment
