Mangrove forests Plays an importantrole in protecting coastlines, reduced coastal water quality. They also provide vital habitat for fishand wildlife. Many species new to science have recently been documentedin mangrove forest areas (thompson2008).The trunks and over ground roots ofmangrove forests have a considerable influence on the hydrodynamics andsediment transport within forest (Quartel et al 2007). Mangrove forests arethought to play and reducing erosion rates (Hong&son 1993 Wu et al. 2001).
Coastal forests can reduce long waves,even tsunami. By observing the casualties of the tsunami of 26 December2004, Kathiresan & Rajendran (2005) highlighted the effectiveness of mangroveforest in reducing the impact of waves. Human mortality and loss of wealth in areas of dense mangroves were less.A review by Alongi (2008) concludedthat the pressure of the tsunami wave of war was significantly reduced when theforests of the forest mango were 100 meters wide. The wave energyspectrum and wave power are dissipated within a mangrove forest even over smalldistance (Vo-Luong&Massel2008).
The magnitude of the energy absorbeddepends strongly on the mangrove structures (e.g. density,stem and rootdiameter, shore slop) and the spectral characteristics of incident waves(Masselet al.
1999,Alongi 2008) . The dissipation of wave energy inside mangroveforests is caused mostly by wave-trunk interactions and wave breaking(Vo-Luong&Massel 2006).However, problems in the interaction of the trunk wave as well as sedimentin mangroves forest are very complicated. Wave interaction and plants causeconfusion and turmoil. The interaction of sediment disturbance is mainlydue to the suspension and coherence of sediments in the mangroves. There are many studies on hydrodynamicsand sedimentation in Mangrove forests, cycles, model-based processes,vegetation trapping and turbulence have been studied quantitatively. The flows through mangrove forests are sluggish as a result ofthe high vegetation increasing friction (mazda et al.
, 1997). Suspendedsediments in mangrove forest are cohesive and inflocs as a result of theturbulence created by the flow around the vegetation (furukawa and wolanski,1996).Mangroves slow the flow of wateras the surge moves inland and reduce the waves riding on top of the surge,Lowering water levels and reducing damage behind the mangroves. our current understanding ofthe effect of mangroves on storm surges comes from relatively few studies.
These studies measured reductions in peak water levels of 5 to 50 cmper kilometer of mangrove. This implies that a mangrove belt several kilometerswide is needed to significantly reduce storm surge water levels. Such largeareas of mangroves are still present in many parts of the tropics that areaffected by cyclones and storm surges, including mexico, the caribbean, Florida…
.In these locations, the conservation and restoration of mangroves cancontribute to a risk reduction strategy against storm surge inundation anddamage.While mangroves can only reducestorm surges when they are present over large areas, the wind and swell waveson top of the storm surge may be reduced over much shorter distances(waveheight reduction is expected to be greater than 75% over 1 Km of mangrove; thisis based on studies of smaller waves). By reducing wave height, mangroves areexpected to reduce wave set-up and run-up, which contribute to the raised waterlevels, inundation and damage caused by storm surges. Mangroves also buffer thewater surface from winds that would otherwise cause larger wind waves to fromon the surge water surface. There is considerable variabilityin the recorded levels of storm surge reduction by mangroves. Storm surgereduction is influenced by the characteristics of individual storm surgeevents, the local physical setting, and the characteristics of the mangrovecommunities. The relationship between storm surge reduction, bathymetry,topography, distance from shore and width of mangrove vegetation is highlycomplex; numerical models based on the underlying physics of wind forcing andwater movement are best able to represent the behavior of storm surges (resioand westerink, 2008).
Such models are needed to explore the effects of mangrovecharacteristics on storm surge reduction: for example, Zhang et al. (2012) usednumerical models to explore the effect of changing the width of the mangrovebelt. They showed that peak water levels are expected to decline non-linearlywith distance, with the greatest reduction in peak water level per unitdistance occurring at the seaward margin. Therefore an increase in the width ofthe mangrove belt may not provide a proportional increase in water levelreduction.The ability of mangroves to reducestorm surges also depends on the storm surge forward speed, the height of thestorm surge and the cyclone intensity. Numerical models suggest that mangroveswill be more efficient at reducing surge height for fast moving surges.
Extremeevents, with very strong winds or surges many metres high, may damage ordestroy mangroves, reducing their ability to reduce surge height. The thresholdat which such damage occurs is likely to depend on mangrove species and height(lacambra et al,. 2008). Such damage is usually localized to areas that arerelatively close to storm track.One limitation of the currentnumerical models is their inability to include spatial variation in mangrovecharacteristics, such as mangrove density. It is very likely that the abilityof fragmented or channelized areas reducing storm surge water levels lesseffectively than dense mangrove vegetation. Currently, mangroves arerepresented in numerical models as an increase in surface roughness, and asingle value for the roughness coefficient is used for all mangroves areas (Xuet al,.2010;Zhang et al,.
20012). Including mangrove variation would probablyimprove the prediction of storm surge height, and would therefore aid inplanning the use of mangroves in coastal defence.Where extensive areas of mangrovescurrently exist, reducing the threats they face from development, sea levelrise and other anthropogenic factors will help to maintain the coastal defencefunctions that they currently provide against storm surges. In other areas,large-scale restoration or afforestation of mangroves may provide increasedlevels of protection from storm surges. In such settings, numerical storm surgemodels will generally be required to calculate the potential benefits ofmangroves, based on the known frequency and magnitude of surges in the region,and the physical characteristics of the mangroves(—) and the coast(—).
Where mangrove planting is proposed as a means of reducing risk from stormsurges , many other considerations should also be taken into account, includingthe chances of successful mangrove planting, which is dependent both on themethods employed(Lewis,2005;Lewis and Perillo,2009;Twilley andRivera-Moroy,2005) and on the social and legal frameworks, which may greatly influence future use andstability of tenure ( Primavera and Esteban,2008).The most appropriate use ofmangroves in coastal defence is likely to be in combination with other riskreduction measure. For example, sea walls and levees place on the landward sideof mangrove forests are likely to experience reduced water levels and waveenergy during storm surges, greatly reducing the likelihood of the wall beingovertopped or damaged during a storm surge; this could significantly reduce thedesign specifications and therefore the cost of the sea wall (such combinationsare sometimes referred to as ‘hybrid engineering’).
Another example discussedby Das and Vincent(2009) demonstrated how early warning systems and evacuationcentres had the greatest effect on reducing the death toll during a cyclone inIndia, but mangroves further reduced the death toll among those people who didnot evacuate.As William et al. (2007) pointout, it is not just the presence of mangroves which is required to providecoastal defence services, but good coastal planning. This can ensure thatevacuation plans and procedures are in place, that people are informed aboutthese plans and procedures, and that they are willing to comply. When CycloneLarry hit Australia in 2006, commercial, recreational and naval vessels in theport of Cairns sheltered in deep mangrove creek.
The protection given to themangrove forests, and the careful planning that ensured that all vesseloperation knew where and when to go, resulted in all vessels riding out thestorm safely with no loss of life (Williams et al., 2007).