Combined with growing
population, a large section of
which remains dependent on
agriculture and fishing, the east
coast of India is categorized as
moderate to high disaster risk
region. These natural hazards
cause loss of life, injury or other
health impacts, property damage,
loss of livelihoods and services,
social and economic disruption
and severe environmental damage.
However, it has been reported
that in areas where coastal zone
is characterized by the presence
of coastal tree vegetation such as
mangrove and non-mangrove forest
and sand dunes with vegetation,
loss of life and damage to property
and environment due to cyclonic
disturbances and tsunami is
relatively less. These coastal tree
vegetations along with sand dunes
are called as coastal bioshields.
Considering the effectiveness of
bioshield the National Disaster
Management Authority of Indian
recommended raising mangrove
and non-mangrove bioshield as an
important measure in cyclone and
tsunami management (NDMA,
2008)
Effectiveness of coastal bioshields
in reducing the impact of natural
disasters
For a long time, local communities
living around mangroves and other
coastal vegetation have been aware
of the effectiveness of this vegetation
as shields against cyclones, storm
surges and tsunami. For example,
the people of Tamil Nadu state,
have been calling mangrove
forest as “aalayaathi kaadu” for
thousands of years (“Aalai” means
waves and “aathi” means mitigate
and “kaadu” means forest). This
traditional wisdom is supported by
experimental studies in the field
and laboratory. Theoretical studies
52
on wave forces and modelling of
fluid dynamics suggest that tree
vegetation may shield coastlines
form tsunami damage by reducing
wave amplitude and energy.
Analytical models show that 30
trees - either mangrove or nonmangrove - per 100 m2 in a 100
m wide may reduce the tsunami
flow pressure by more than 90%
(Hiraishi et al, 2003). Effectiveness
of a coastal forest in mitigating
the impact of natural hazards
depends on the width, density
and structure of the forest and the
tree characteristics (height and
diameter at breast height). A study
indicates that for a tsunami wave
height of 3 m, the effective forest
width – mangrove or non-mangrove
- is about 20 m and for 6 m high
tsunami wave the effective width
of forest is about 100 m. It has
also been estimated that trees with
10 cm diameter at breast height is
effective against 4.6 m tsunami
waves and 35 cm diameter for 7 m
( FAO, 2006). However, empirical
and field based evidences were not
available for long time. The super
cyclone of Orissa in October 1999
and tsunami in December 2004
provided opportunities to collect
field based evidences on the role of
coastal vegetation in reducing the
impact of such natural disasters.
Three villages with three different
physical settings, i) a village within
the shadow of mangrove forest, ii)
a village which is not in the shadow
of mangroves and also not protected
by embankment that is constructed
to avoid seawater intrusion and iii) a
village not protected by mangroves
but protected by embankment was
selected for the study. All these
three villages were located at
equidistance from the sea and the
socio-economic conditions were
also same. Eleven variables such
as the cyclone damage to houses,
livestock, fisheries, trees and other
assets owned by the people and
the level and duration of flooding
etc were used to compare the
damage in these three villages. In
the mangrove protected villages,
variables had the lowest values for
adverse factors such as damage to
houses. The economic loss incurred
per household was higher in the
village that was not protected by
mangroves but had an embankment.
Attitude survey conducted among
the people of the villages nearby
the Bhitarkanika areas showed that
they were aware of the role played
by the mangroves in protecting
their lives and properties during the
super cyclone and also appreciated
the protective functions performed
by the mangroves.
Mangroves and Super Cyclone
Mangrove and Non-mangrove
Bioshield and Tsunami
A super cyclone with wind
speed of about 258 km per hour
struck the coastal areas of Orissa
on 29 th October 1999 causing
extensive damage to life and
properties. Immediately after
the super cyclone, a study was
conducted to evaluate the extent of
damage caused in areas that were
under the umbrella of Bhitarkanika
mangrove forest and areas that
were not sheltered by mangroves
(Badola. and Hussain, 2005).
Similarly, after 26th December
2004 tsunami the role of mangroves
and other coastal vegetation in
mitigating the impact of tsunami was
evaluated scientifically following
two kinds of approach.
Approach 1: In the first approach
tsunami mitigating role of coastal
vegetation was studied at macro
level using pre and post tsunami
high resolution remote sensing
imageries. This study was carried
YOJANA March 2012