Mangrove forests Plays an important
role in protecting coastlines, reduced coastal water quality. They also provide vital habitat for fish
and wildlife. Many species new to science have recently been documented
in mangrove forest areas (thompson2008).The trunks and over ground roots of
mangrove forests have a considerable influence on the hydrodynamics and
sediment transport within forest (Quartel et al 2007). Mangrove forests are
thought 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 December
2004, Kathiresan & Rajendran (2005) highlighted the effectiveness of mangrove
forest in reducing the impact of waves. Human mortality and loss of wealth in areas of dense mangroves were less.
A review by Alongi (2008) concluded
that the pressure of the tsunami wave of war was significantly reduced when the
forests of the forest mango were 100 meters wide. The wave energy
spectrum and wave power are dissipated within a mangrove forest even over small
distance (Vo-Luong&Massel2008). The magnitude of the energy absorbed
depends strongly on the mangrove structures (e.g. density,stem and root
diameter, shore slop) and the spectral characteristics of incident waves(Massel
et al. 1999,Alongi 2008) . The dissipation of wave energy inside mangrove
forests 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 sediment
in mangroves forest are very complicated. Wave interaction and plants cause
confusion and turmoil. The interaction of sediment disturbance is mainly
due to the suspension and coherence of sediments in the mangroves. There are many studies on hydrodynamics
and 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 of
the high vegetation increasing friction (mazda et al., 1997). Suspended
sediments in mangrove forest are cohesive and inflocs as a result of the
turbulence created by the flow around the vegetation (furukawa and wolanski,
1996).

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Mangroves slow the flow of water
as 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 of
the effect of mangroves on storm surges comes from relatively few studies
. These studies measured reductions in peak water levels of 5 to 50 cm
per kilometer of mangrove. This implies that a mangrove belt several kilometers
wide is needed to significantly reduce storm surge water levels. Such large
areas of mangroves are still present in many parts of the tropics that are
affected by cyclones and storm surges, including mexico, the caribbean, Florida….
In these locations, the conservation and restoration of mangroves can
contribute to a risk reduction strategy against storm surge inundation and
damage.

While mangroves can only reduce
storm surges when they are present over large areas, the wind and swell waves
on top of the storm surge may be reduced over much shorter distances(wave
height reduction is expected to be greater than 75% over 1 Km of mangrove; this
is based on studies of smaller waves). By reducing wave height, mangroves are
expected to reduce wave set-up and run-up, which contribute to the raised water
levels, inundation and damage caused by storm surges. Mangroves also buffer the
water surface from winds that would otherwise cause larger wind waves to from
on the surge water surface.

There is considerable variability
in the recorded levels of storm surge reduction by mangroves. Storm surge
reduction is influenced by the characteristics of individual storm surge
events, the local physical setting, and the characteristics of the mangrove
communities. The relationship between storm surge reduction, bathymetry,
topography, distance from shore and width of mangrove vegetation is highly
complex; numerical models based on the underlying physics of wind forcing and
water movement are best able to represent the behavior of storm surges (resio
and westerink, 2008). Such models are needed to explore the effects of mangrove
characteristics on storm surge reduction: for example, Zhang et al. (2012) used
numerical models to explore the effect of changing the width of the mangrove
belt. They showed that peak water levels are expected to decline non-linearly
with distance, with the greatest reduction in peak water level per unit
distance occurring at the seaward margin. Therefore an increase in the width of
the mangrove belt may not provide a proportional increase in water level
reduction.

The ability of mangroves to reduce
storm surges also depends on the storm surge forward speed, the height of the
storm surge and the cyclone intensity. Numerical models suggest that mangroves
will be more efficient at reducing surge height for fast moving surges. Extreme
events, with very strong winds or surges many metres high, may damage or
destroy mangroves, reducing their ability to reduce surge height. The threshold
at 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 are
relatively close to storm track.

One limitation of the current
numerical models is their inability to include spatial variation in mangrove
characteristics, such as mangrove density. It is very likely that the ability
of fragmented or channelized areas reducing storm surge water levels less
effectively than dense mangrove vegetation. Currently, mangroves are
represented in numerical models as an increase in surface roughness, and a
single value for the roughness coefficient is used for all mangroves areas (Xu
et al,.2010;Zhang et al,.20012). Including mangrove variation would probably
improve the prediction of storm surge height, and would therefore aid in
planning the use of mangroves in coastal defence.

Where extensive areas of mangroves
currently exist, reducing the threats they face from development, sea level
rise and other anthropogenic factors will help to maintain the coastal defence
functions that they currently provide against storm surges. In other areas,
large-scale restoration or afforestation of mangroves may provide increased
levels of protection from storm surges. In such settings, numerical storm surge
models will generally be required to calculate the potential benefits of
mangroves, 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 storm
surges , many other considerations should also be taken into account, including
the chances of successful mangrove planting, which is dependent both on the
methods employed(Lewis,2005;Lewis and Perillo,2009;Twilley and
Rivera-Moroy,2005) and on the social and legal frameworks,  which may greatly influence future use and
stability of tenure ( Primavera and Esteban,2008).

The most appropriate use of
mangroves in coastal defence is likely to be in combination with other risk
reduction measure. For example, sea walls and levees place on the landward side
of mangrove forests are likely to experience reduced water levels and wave
energy during storm surges, greatly reducing the likelihood of the wall being
overtopped or damaged during a storm surge; this could significantly reduce the
design specifications and therefore the cost of the sea wall (such combinations
are sometimes referred to as ‘hybrid engineering’). Another example discussed
by Das and Vincent(2009) demonstrated how early warning systems and evacuation
centres had the greatest effect on reducing the death toll during a cyclone in
India, but mangroves further reduced the death toll among those people who did
not evacuate.

As William et al. (2007) point
out, it is not just the presence of mangroves which is required to provide
coastal defence services, but good coastal planning. This can ensure that
evacuation plans and procedures are in place, that people are informed about
these plans and procedures, and that they are willing to comply. When Cyclone
Larry hit Australia in 2006, commercial, recreational and naval vessels in the
port of Cairns sheltered in deep mangrove creek. The protection given to the
mangrove forests, and the careful planning that ensured that all vessel
operation knew where and when to go, resulted in all vessels riding out the
storm safely with no loss of life (Williams et al., 2007).