14.2 Tides and Waves in Mangrove Forests
425
waves recorded approximately 150 m in front of the forest. Between this site
and the seaward boundary of the forest, there is a gradually sloping mudflat
resulting in some wave transformation in front of the forest. This could explain
normalized energy slightly greater than unity at, and near, the sea/mangrove
boundary.
At any location within the forest we would expect Enorm (x) to be a function
of the water depth, given the changing numbers of mangrove trunks and roots,
and their diameters, with vertical distance from the sea bottom. In general, the
amount of obstruction caused by trunks, prop roots, and pneumatophores decreases with distance upwards from the forest floor until the canopy is reached.
This characteristic makes the drag coefficient, Cd, a function of both distance
from substrate and the Reynolds number, Re.
14.2.3 Sedimentation in Mangrove Forests
Most coastal mangrove forests are connected to the sea via tidal creeks. Sediment transport in these creeks has been investigated in detail (for example,
Wolanski, 1995). Asymmetry of the tidal current (mentioned earlier), baroclinic circulation and shear-induced destruction of flocs control the amount of
sediment transported in tidal creeks.
Water flow in mangrove forests is impeded by the friction caused by the high
vegetation density. Interaction of the water flow with mangrove roots creates
complex systems of eddies, jets and stagnation zones. This micro-turbulence
environment maintains sediment in suspension at flood tidal currents which
transport fine sediment particles into the forest. Turbulent intensities are
largest in the complex matrix of roots, such as Rhizophora species, and are
much smaller for single trees such as Ceriops species (Furukawa and Wolanski,
1996).
Some data indicate that the sediment movement threshold velocities for sediments with modal diameter 0.2 mm are about 0.4 m/s. It has also been inferred
that accumulation of fine sediment takes place on the mangrove forest flats by
settling at high water spring tides. In some creeks, a net flux is mostly seaward
because of higher ebb flows.
The sedimentation of suspended sediment occurs when turbulence vanishes
at slack high tide. During ebb tide, there is a substantial attenuation of current
speed which is not sufficient to resuspend the sediment. Furukawa et ai. (1997)
showed that about 80% of the suspended sediment brought into the mangroves
from the coastal waters at spring flood tide became trapped in the mangroves.
The maximum suspended sediment concentration in the mangroves at the creek
edge was about 150 mg/l, which is a typical value for Queensland mangrove
creeks at spring tides. Therefore mangroves are an important sink for fine
sediment from rivers and coastal waters.
Although tidal currents through mangrove forests are weak, these currents
influence water quality in terms of dissolved organic carbon (DOC). A study
by Furukawa et ai. (1997) shows that the import of DOC at flood tide, and
425
waves recorded approximately 150 m in front of the forest. Between this site
and the seaward boundary of the forest, there is a gradually sloping mudflat
resulting in some wave transformation in front of the forest. This could explain
normalized energy slightly greater than unity at, and near, the sea/mangrove
boundary.
At any location within the forest we would expect Enorm (x) to be a function
of the water depth, given the changing numbers of mangrove trunks and roots,
and their diameters, with vertical distance from the sea bottom. In general, the
amount of obstruction caused by trunks, prop roots, and pneumatophores decreases with distance upwards from the forest floor until the canopy is reached.
This characteristic makes the drag coefficient, Cd, a function of both distance
from substrate and the Reynolds number, Re.
14.2.3 Sedimentation in Mangrove Forests
Most coastal mangrove forests are connected to the sea via tidal creeks. Sediment transport in these creeks has been investigated in detail (for example,
Wolanski, 1995). Asymmetry of the tidal current (mentioned earlier), baroclinic circulation and shear-induced destruction of flocs control the amount of
sediment transported in tidal creeks.
Water flow in mangrove forests is impeded by the friction caused by the high
vegetation density. Interaction of the water flow with mangrove roots creates
complex systems of eddies, jets and stagnation zones. This micro-turbulence
environment maintains sediment in suspension at flood tidal currents which
transport fine sediment particles into the forest. Turbulent intensities are
largest in the complex matrix of roots, such as Rhizophora species, and are
much smaller for single trees such as Ceriops species (Furukawa and Wolanski,
1996).
Some data indicate that the sediment movement threshold velocities for sediments with modal diameter 0.2 mm are about 0.4 m/s. It has also been inferred
that accumulation of fine sediment takes place on the mangrove forest flats by
settling at high water spring tides. In some creeks, a net flux is mostly seaward
because of higher ebb flows.
The sedimentation of suspended sediment occurs when turbulence vanishes
at slack high tide. During ebb tide, there is a substantial attenuation of current
speed which is not sufficient to resuspend the sediment. Furukawa et ai. (1997)
showed that about 80% of the suspended sediment brought into the mangroves
from the coastal waters at spring flood tide became trapped in the mangroves.
The maximum suspended sediment concentration in the mangroves at the creek
edge was about 150 mg/l, which is a typical value for Queensland mangrove
creeks at spring tides. Therefore mangroves are an important sink for fine
sediment from rivers and coastal waters.
Although tidal currents through mangrove forests are weak, these currents
influence water quality in terms of dissolved organic carbon (DOC). A study
by Furukawa et ai. (1997) shows that the import of DOC at flood tide, and