Estuary Processes and Managment - River Dynamics and Integrated River Management - page 528

Fig. 8.71 Tides of the Indian Ocean tsunami battering the coast of Sri Lanka (from Marris, 2005b)
The consequence of removing coral wetland habitats can be illustrated in laboratory experiments on
wave propagation. Fernando et al. (2008) carried out a series of experiments in a long wave tank (32 m ×
0.8 m × 1.8 m); wave-generated currents are measured in the presence of corals simulated using an array
of submerged cylinders. A strip of solid vertical cylinders (1.25 cm diameter and 20 cm high) was
attached to a bottom plate (Fig. 8.72) and placed on a sloping bed at a mean water depth of 30 cm. This
array of cylinders acted as a submerged porous barrier to the oncoming waves. Two packing densities of
rods were used, having porosities 20% and 50%. In some cases, the porous rod structure covered the
entire width of the tank whereas in others the rods were removed to create a low resistance (i.e., low
bottom drag) gap of width W = 6.5 cm in the middle of the canopy. This mimicked the local removal of
corals in an otherwise uniform coral cover.
Fig. 8.72 Corals simulated by vertical cylinder arrays in laboratory experiment: left (high density, 20% porosity), and
right (low density, 50% porosity)
Figure 8.73 shows mean velocity traces measured at the mid-height of the reef in the direction of wave
propagation: without the reef, with the reef and with a gap in the reef. There is a clear reduction of
velocity in the presence of the reef, indicating increased drag on wave motion; by comparison, the flow
velocity is enhanced in the presence of the gap, with the magnitude of this enhancement decreasing with
increasing porosity. Therefore, it can be concluded that: ķ the flow velocity is substantially decreased
in coral-laden areas due to the larger bottom drag coefficient, which is a strong function of the canopy
porosity; ĸ if a stream-wise gap is created in the reef to create a low resistance path, then the approaching