14.2 Tides and Waves in Mangrove Forests
419
structure and growth, are intimately linked to water movement. However, studies of physical processes in tropical mangrove swamps and mangrove-fringed
estuaries are few, and far behind compared to those of temperate estuaries.
Water circulation in riverine mangrove forests, which comprises tidal creeks
and shallow mangrove swamps, has been studied somewhat more than other
types of mangrove forests (Wolanski et ai., 1992; Furukawa and Wolanski, 1996;
Furukawa et ai., 1997). Long waves, namely tidal waves, are the dominant cause
of water movement and sedimentation in mangrove systems. Tidal currents in
creeks often exceed 1 mis, however, velocities within swamps rarely reach 0.1
m/s. Modelling of tidally induced water motion in mangrove creeks is based
on vertically averaged, barotropic equations of motion for unsteady flow in an
open channel with lateral storage in mangrove swamps (Wolanski et ai., 1992).
Circulation within the creeks is characterized by a pronounced asymmetry between the ebb and flood tides. This asymmetry is caused by the phase changes
that occur between the mouth and the head of the creek. When the water level
reaches high tide at the head of the creek, it is already falling at the mouth.
This provides the necessary water slope to accelerate the water back towards
the mouth as the ebb phase commences. The ebb-flood tide asymmetry is crucial in the maintenance of the geometry of the system. The larger ebb currents
tend to export sediments from the creek and maintain a deep, navigable tidal
channel. Reduction in the size of the swamp reduces the tidal asymmetry and
the peak ebb tidal currents. As a result the creek silts rapidly. This increased
siltation and reduced navigability is a common occurrence in the South-East
Asia region where prawn farms have been constructed in the mangrove forests.
The complex circulation through mangrove vegetation is friction dominated,
with jets, eddies, stagnation regions and vegetation-scale turbulence. Observations in a mangrove area close to Cairns (the eastern coast of Australia), and
computer modelling indicate that at small velocities (0.005 m/s), non-separated
laminar flows prevail, while at high flow speed (0.2 m/s), eddies and wakes are
generated around individual roots (Furukawa and Wolanski, 1996).
14.2.2 Storm or Cyclone-Induced Waves in Mangrove Forests
Numerical Modelling. During storms or tropical cyclones, the energy of
surface waves substantially exceeds tidal energy. Due to the complexity of
mangrove structures, the transmission of cyclone induced waves through mangrove areas is poorly understood. Assuming that the diameter of mangrove
trunks is very small in comparison with wavelength, wave energy is dissipated
mostly due to drag forces induced on trunks by waves.
Only recently, a series of laboratory and field experiments, as well as theoretical analysis, have been carried out to investigate surface wave energy dissipation in mangrove forests (Brinkman et ai., 1997; Mazda et ai., 1997a,b;
Massel et ai., 1998). There are two main energy dissipation mechanisms in
mangrove forests: multiple interactions of wave motion with mangrove trunks
and roots, and bottom friction. Bottom friction can be accommodated through
419
structure and growth, are intimately linked to water movement. However, studies of physical processes in tropical mangrove swamps and mangrove-fringed
estuaries are few, and far behind compared to those of temperate estuaries.
Water circulation in riverine mangrove forests, which comprises tidal creeks
and shallow mangrove swamps, has been studied somewhat more than other
types of mangrove forests (Wolanski et ai., 1992; Furukawa and Wolanski, 1996;
Furukawa et ai., 1997). Long waves, namely tidal waves, are the dominant cause
of water movement and sedimentation in mangrove systems. Tidal currents in
creeks often exceed 1 mis, however, velocities within swamps rarely reach 0.1
m/s. Modelling of tidally induced water motion in mangrove creeks is based
on vertically averaged, barotropic equations of motion for unsteady flow in an
open channel with lateral storage in mangrove swamps (Wolanski et ai., 1992).
Circulation within the creeks is characterized by a pronounced asymmetry between the ebb and flood tides. This asymmetry is caused by the phase changes
that occur between the mouth and the head of the creek. When the water level
reaches high tide at the head of the creek, it is already falling at the mouth.
This provides the necessary water slope to accelerate the water back towards
the mouth as the ebb phase commences. The ebb-flood tide asymmetry is crucial in the maintenance of the geometry of the system. The larger ebb currents
tend to export sediments from the creek and maintain a deep, navigable tidal
channel. Reduction in the size of the swamp reduces the tidal asymmetry and
the peak ebb tidal currents. As a result the creek silts rapidly. This increased
siltation and reduced navigability is a common occurrence in the South-East
Asia region where prawn farms have been constructed in the mangrove forests.
The complex circulation through mangrove vegetation is friction dominated,
with jets, eddies, stagnation regions and vegetation-scale turbulence. Observations in a mangrove area close to Cairns (the eastern coast of Australia), and
computer modelling indicate that at small velocities (0.005 m/s), non-separated
laminar flows prevail, while at high flow speed (0.2 m/s), eddies and wakes are
generated around individual roots (Furukawa and Wolanski, 1996).
14.2.2 Storm or Cyclone-Induced Waves in Mangrove Forests
Numerical Modelling. During storms or tropical cyclones, the energy of
surface waves substantially exceeds tidal energy. Due to the complexity of
mangrove structures, the transmission of cyclone induced waves through mangrove areas is poorly understood. Assuming that the diameter of mangrove
trunks is very small in comparison with wavelength, wave energy is dissipated
mostly due to drag forces induced on trunks by waves.
Only recently, a series of laboratory and field experiments, as well as theoretical analysis, have been carried out to investigate surface wave energy dissipation in mangrove forests (Brinkman et ai., 1997; Mazda et ai., 1997a,b;
Massel et ai., 1998). There are two main energy dissipation mechanisms in
mangrove forests: multiple interactions of wave motion with mangrove trunks
and roots, and bottom friction. Bottom friction can be accommodated through