The protection of mangroves in the coastal areas is now an endeavour which is wholeheartedly supported by all parties in the world to provide protection from future disasters. As a result the restoration and rehabilitation of mangroves in coastal areas is now being extensively promoted to enhance sustainable natural regeneration. It
involves renewing natural mangroves that have been lost or degraded and reclaiming their functions and values as a vital component of the ecosystem, to protect and increase their habitat areas and which the natural ecosystem will return to what the mangrove region was like before (Westwater, 2001).
Consequently in the presence of extreme natural events, this process of natural mangrove re-colonisation is compromised and becomes more complex (Jimenez et al., 1985). Clear cutting by human activity has often led to a significant impact on the ecological system in mangrove swamps and possibly also exposed the site to erosion, which makes subsequent reforestation difficult; as a result seedlings have often been washed away, where this is also noted by Wu et al. (2001). Mangroves have to withstand these actions, especially on exposed sites and where they colonise cleared or newly established mudflats. Tidal currents and wave energy will hardly be so large as to cause problems to mature mangroves, but the establishment of seedlings and young mangroves may be difficult in the presence of such wave environments (Ong, 1982; Jimenez et al., 1985; Ong et al., 1991). Both human-induced and natural stresses make the restoration and rehabilitation of mangroves more complicated.
Other problems encountered also include barnacle infestation, inappropriate site selection, attacks by crabs, dying out anchorage of the sediments and deep inundation, which may adversely result in seedlings being washed out.
In general, the restoration and rehabilitation of mangroves can only be undertaken in areas that are suitable for them, for example in estuaries, areas sheltered by a stretch of coastline or in protected bays, embayments and offshore islands protected by reefs and shoals (Chatenoux and Peduzzi, 2007). However, besides the selection of ideal sites for mangroves, the establishment of seedlings and young
mangroves may be easier in the shelter of other mature mangroves as these sites also help to rehabilitate damaged mangrove ecosystems and to protect existing mangrove stands. At the exposed site where there is no shelter provided by other mature mangroves, there is a need to duplicate the real natural environment of this shelter for sustainable restoration and rehabilitation. Hence, an innovative and environmentally friendly system, namely the Artificial Mangrove Shelter (AMS), has been first introduced and discussed in more detail in this chapter. Also, to confirm that the AMS can have the same effect as a natural mangrove shelter for coastal protection, the hydrodynamic effects of the AMS have been studied and modelled.
As noted by Struve and Falconer (2001), “Mangroves are woody and mature trees are rigid, but young mangroves can bend with the flow. Mature mangroves and stems will always protrude above the water surface, whereas young mangroves and stilt roots may be submerged, in which case their effects are depth dependent and stems are similar to straight cylinders.” The AMS system is designed by replicas of the mangrove stem configuration (as shown in Figure 4.15). The system consists of wooden circular cylinders of different sizes, which are easy to construct and simple for installation on site, though the vertical profile of natural mangroves is very complex. Figure 4.15 illustrates a setting up design for the AMS system.
The AMS would be the frontier in this system to play an important role as a coastal defence and preventing seedlings or young mangroves from being washed away by strong waves and currents, or even tsunamis, by the sea or river. In this system, mature mangroves would remain at the same location to play the same roles as before, and even backup the role of an AMS as a coastal defence. The sustainable innovation of the AMS will provide 4 As and 4 Es as follows:
1. Artificial defences against natural disasters by absorbing and obstructing the energy o f tidal currents, waves and even tsunamis.
2. Attenuation and mitigation o f floods by increasing the storage capacity and dissipating wave energy propagation.
3. Artificial shelter to encourage restoration and rehabilitation o f mangroves.
4. Artificial sediment traps for soil formation and stabilizing the coastline.
5. Erosion control and protection from strong waves and currents at exposed sites.
6. Environmentally friendly with little change to the environment and habitat.
7. Enhancement o f the natural environment’s aesthetic aspects for recreation.
8. Enrichment and enhancement o f the natural habitat for natural regeneration.
Sea/River Artificial Mangrove Shelter Seedling Mature M angroves
Figure 4.15 Setting up design for sustainable innovation o f the AMS system
4.7.2 Hydrodynamic Modelling of Artificial Mangrove Shelter
Although research on hydro-environmental impact o f mangroves has made good progress in recent years, there are still major uncertainties concerning the
establishment of seedlings and innovative solutions for sustainable restoration and rehabilitation of mangroves. An improved understanding of such problems and solutions is crucial for better management procedures of the long term sustainable use of mangroves as natural resources. For that reason, numerical model are now employed to provide some of the useful findings to these problems.
Thus, this section will use the same numerical model as described previously, to study the hydrodynamics of AMS along the floodplains, particularly on: (i) the influence regard to the AMS on the tidal flow structure, and (ii) the consequential impacts of these changes on tsunami currents. In order to investigate these effects of AMS along the floodplains, an idealised case study was considered with circular cylinders being distributed along the whole length of a channel, of rectangular shape (see Figure 4.7 for details). The idealised channel width was set to 550 m, with a length of 2,450 m. In the model a mesh of 22 x 94 grid squares was used with a uniform grid size of 25 m.
Firstly, the model was set up and applied to simulate the hydrodynamic processes of the tidal flow structure in the AMS. At the open seaward boundary a sinusoidal wave was assumed, with the tidal range being set to 1.5 m, with low water at 0.8 m and high water at 2.4 m and a time step being set to 12 s. Simulations were undertaken for this tidal wave condition for the cases with and without the AMS.
Comparisons of the velocity profiles and water elevations for both of these cases were undertaken to provide a good knowledge of the effects of the AMS on the tidal flow structure. Time series plots of velocities and water elevations at two selected locations of the idealised channel are observed as shown in Figures 4.16 and 4.17.
(a)
Figure 4.16 Time series of velocities at selected locations of (a) the downstream and (b) upstream of the channel
Water Elevation(m) &Water Elevation(m)
(a)
2.5 n
With AMS Without AMS
0.5
0 2 4 6 8 10 12 14
Time (Hr)
2.5
With AMS Without AMS
(b) Upstream
0.5
0 2 4 6 3 10 12 14
Time (Hr)
Figure 4.17 Time series of water elevations at selected locations of (a) the downstream and (b) upstream of the channel
Based on the observations, it was found that the AMS had a significant impact on the flow patterns as shown in Figures 4.16 and 4.17. It can be seen that the tidal wave induced velocities and water elevations were reduced for the case with the AMS, for both selected locations. The results also showed that the AMS system played an important role in acting as a defence and preventing seedlings and young mangroves from being washed away by strong waves and currents by the sea or river.
Thus, the establishment of seedlings and young mangroves could be easier with such a system in place.
Simulations were also undertaken for various test cases with different tidal wave conditions for the cases with and without the AMS. Comparisons of velocity profiles and water elevations for all of these cases were undertaken. Based on the findings for test cases with different tidal wave conditions, generally, the tendency of the results for all of the test cases considered is almost the same, as shown in Figures 4.16 and 4.17. Thus, Figures 4.16 and 4.17 provide a good representation of the results to show the impact of the AMS on the tidal flow structure.
Secondly, with a similar model set-up, the study focused on investigating the influence of the consequential impact on tsunami currents. Based on observations from the last major tsunami, at one of the coastal sites near Pulau Pinang, Malaysia, the near shore height of the tsunami was reportedly 2 to 4 m (Abdullah et al., 2005).
Hence, the model included: an idealised wave, with a period of 1 hour, low water at 0.5 m and high water at 4.5 m, and a time step set to 12 s, to drive the tsunami currents. Simulations have been undertaken for various conditions of the AMS with different porosities (i.e. different diameters and densities), and compared to the case without the AMS.
Comparisons of velocity profiles and water elevations for all of these cases were undertaken to study the capability of the AMS, particularly with influence regard to the AMS on tsunami currents. Figure 4.18 illustrates time series plots of the velocity profiles and water elevations for various cases with different porosities of the AMS. From these results, it can be seen that the tsunami induced velocities and water elevations were significantly reduced for the case with the AMS. For the condition with the AMS, the velocities were reduced by up to 1.2 m/s from the condition without the AMS, as shown in Figure 4.18 (a).
These results showed that the drag force induced by the AMS greatly affected the tsunami induced flow structure. The findings also implied that the blockage effect of the AMS would make a contribution to the velocity profile due to a considerable fraction of the flow volume being taken up by the AMS. It was found that the AMS had a significant impact on the hydrodynamic characteristics of the tsunami currents.
The study has thus shown that the AMS also play a key role as natural mangrove shelter in wave attenuation and in slowing down tsunami currents.
It can be seen in Figure 4.18 (a), the smaller the porosity of the AMS, or the closer the trees were located to one another (i.e. the higher density) and the bigger the trees (i.e. the bigger the diameter), then the greater was the reduction in the wave energy. The findings again confirm that the AMS can play the same roles as natural mangrove shelters for coastal protection and preventing young mangroves from being washed away by strong waves and currents, or even tsunamis. Thus, the AMS should be widely promoted as an innovative solution to provide better management practices for the long term sustainable use of mangroves as natural resources, eventually for the effective management of floodplains.
(a)
Figure 4.18 Time series of (a) velocities and (b) water elevations for the AMS with different porosities