Mangroves as coastal defence

1. MANGROVES As COASTAL DEFENCE

2. Sea level rising

3. • Today's warning comes from US researchers at
Rutgers University.
• The analysis showed that during the past 5,000
years, sea levels rise at a rate of around 1mm each
year . But in the past 150 years, sea levels are
rising at 2mm a year.
• The rising tide is expected to make oceans 40cm
higher by 2100.
IPCC REPORT 2013

4. Causes of sea level rising
Natural effects Human activities
The normal melting
of ice sheets
Burning of fossil fuels
and producing CO2
The development
of industry

6. Mangroves
• Tropical plants that are adapted to
loose, wet soil, salt water, and being
periodically submerged by tides
• 80 species of mangroves found
throughout world
• Fast-growing trees that can reach
up to 25 meters
• Unique because they can thrive in
all types of water:
• Poor in oxygen content
• Salt water
• Brackish water
• Fresh water

7. • They are fast growers. Worldwide, mangroves cover approximately 152,000 km2 of
the earth’s surface. However, about 35% of this was lost during the last several years
decades
Subodh and abhiroop 2013

8. (Source: Forest Survey India).

9. Mangroves
Red Mangroves Black Mangroves White Mangroves

10. Red Mangroves
• Red Mangrove (Rhizophora
mangle)
• The red mangrove is easily
recognised by its exposed prop
and aerial root system which
stabilises the tree, and also by the
long seedlings (propagules) that
hang from the branches.

11. Black Mangrove
• Black Mangrove(Avicenniagerminans)
Also known as Courida.
• Black mangroves, like red mangroves
are easily identified by their roots.
However, it is not the exposed prop
roots that make the black mangroves
stand out.
• Instead it is the numerous breathing
tubes, called pneumatophores, which
stick vertically out of the ground.
These pneumatophores trap oxygen
for the plant.

12. White Mangrove(Laguncularia racemosa)
• This type grows shorter than
the red or black mangroves
and is found in the back
portion of the mangrove forest
• The white mangrove’s leaves
have special openings/glands
which allow salt to pass from
inside the plant to the outside
• This causes a coating of white
salt crystals to remain on the
leaves, hence the name white
mangrove.

13. Types of roots
Three types of roots.
Aerial roots(breathing roots) Prop roots Kneed roots

14. Characteristics of Mangroves
• Evergreen plants with thick leathery leaves designed to minimize
transpiration.
• Viviparous germination where the seed germinates on the tree and falls
down in the germinating condition with a long radicle

15. MANGROVES REDUCE WAVE DAMAGE
• Mangroves reduce energy of wind and
waves passing through them, reducing their
ability to erode sediments
• waves lose energy as they pass through
the tangled above-ground roots and
branches and their height is rapidly
diminished, by between 13 and 66% over
100 m of mangroves
Mazda et al. (2006) and Quartel et al. (2007)

16. Waves passing through mangrove forests show an exponential
reduction in height with distance (Bao, 2011 and Massel et al., 1999)

17. Tsunamis are caused by earthquakes and
landslides disturbing large masses of water
Mangroves can help to reduce tsunami Damage
Mangrove belts several hundred meters
wide have been shown to reduce
tsunami height by between 5 and 30%.
There is growing evidence, particularly from the
devastating Indian Ocean tsunami of 2004 that
mangroves reduced tsunami impacts by reducing the
destructive energy of water flowing inland
Post tsunami—In an area of S.E. India
there was significantly less damage where
mangroves had been conserved (Danielsen
et al., 2005; Science)

19. Super cyclone in Orissa on east coast of India October
1999 @ 310 km/h speed Killed 10,000 people made
7.5 million homeless
Only one area not affected: Bhitharkanika with dense
mangroves not even a single tree uprooted in the area
S. Das and J.R. Vincent 2009

20. • 1970 typhoon in Bangladesh 0.3 million human died due
to 1000s ha replaced with paddy fields
• 1983 cyclone in the Kachchh, Gujarat, India heavy human
loss
• 1991 flood disaster in Bangladesh due to 300 sq km
mangrove area replaced with shrimp farming and rice
cultivation

21. Mangroves reduce erosion
• On every coastline waves and currents create change.
• The mangrove vegetation reduces wave energy and slows the flow of
water over the soil surface, reducing the water’s capacity to dislodge
sediments and carry them out of the mangrove area

22. Mangroves may keep up with sea level rise
• Mangrove soils are actively growing in many places
• The fine mangrove roots also help to trap and bind the
particles.
• Due to a lack of oxygen in the waterlogged soil, organic
matter is not broken down by soil organisms. This allows the
organic matter to build up over time, producing the deep
peaty soils that under lie mangroves in some areas.
• Some mangroves sit on top of deep layers of mangrove peat
that may be 6 meters deep or more, that were built up over
thousands of years as sea levels rised.
• Mangrove soils grew vertically at rates of up to 10 mm per
year
In numerous cases, there has been proof of annual sedimentation rate, ranging between
1 and 8 mm, in mangrove areas with expansion of land (Bird & Barson, l977).

24. Mangroves protect other marine ecosystems
(seaweeds, coral & sea grass)

26. IMPORTANCE OF MANGROVES

27. IMPORTANCE OF MANGROVES
• Trap and cycle various organic materials, chemical elements,
and important nutrients in the coastal ecosystem.
• Provide food for marine organisms.
• Provide physical habitat and nursery grounds for a wide
variety of marine organisms, such as fish, shrimp and crabs.
• Serve as roosting and nesting sites for many of our birds.
• Serve as storm buffers by reducing wind and wave action in
shallow shoreline areas.
• Assist in protecting water quality and clarity by filtering runoff
and trapping sediments and debris from adjacent uplands.

29. Wave reduction in a mangrove forest dominated
by Sonneratia sp.
Yoshihiro Mazda, Michimasa Magi, Yoshichika Ikeda, Tadayuki Kurokawa and Tetsumi Asano
Wetlands Ecology and Management (2006) 14:365–378

30. • The study site is in the coast along Vinh Quang village in northern
Vietnam

31. • This belt is around 100 m wide. Vertically erect pneumatophores
emerge densely from the bottom substrate up to a height of around
0.25 m, and thick leaves spread out from a height of around 0.8 m
above the bottom substrate
• At Stns.1–6 the water elevations were measured by RMD-type wave
recorders

33. Vegetation characteristics of Sonneratia sp. at the Vinh Quang
coast

34. The rate of reduction per unit length wave in
the direction of propagation is defined as:-
where H is the significant wave height at an arbitrary sea side station
∆H is an amount of decrease of the wave height at an inshore station with a distance ∆x away from the
sea side station along x-axis with the direction of the wave propagation.

36. Time series plots of the wave height, the rate of wave
reduction and water depth

37. Relationships between the rate of wave reduction and water
depth in areas with or without mangroves
The rate of wave reduction in an area with mangroves is as large
as approximately 0.006 m-1 at shallow water depths.
The great change in the rate of wave reduction with water depth
in a shallower range is considered to be due to the resistance
caused by vertically erect pneumatophores
When the water depths are 0.6 and 0.2 m in the mangrove area,
the rates of wave reduction are 0.003 and 0.006 m-1

38. Relationships between the rate of wave reduction and
wave height in areas with or without mangroves
The area with mangroves the rate of wave reduction
depends in linear fashion on the wave height.

39. Conclusion
• The reduction rate of sea waves in this area changes substantially
with tidal phase due to the unique vertical configuration of
Sonneratia sp
• when the water level reaches the height of the thickly spread
branches and leaves of these trees, the rate of wave reduction
increases again with the increase in the water level
• It should be recognized that the wave reduction depends on the
mangrove species, vegetation condition, water depth and the incident
wave condition.

40. How do mangrove forests induce sedimentation
K. Kathiresan
Rev. biol. trop vol.51

41. • The study area is located along the Vellar estuary (11°29 N; 79°46 E),
south east coast of India
• Along the estuary, nine sampling points were fixed with a distance of 5 m
between sampling points at the mangrove zones(A,B,C) and with a
distance of 20 m between sampling points (D-H) at the esturine proper.
• A refers to landward Avicennia zone, B refers to Avicennia Rhizophera
interphase, C water front Rhizophera zone

43. Average height of water column during low and high rides across the
Vellar estuary

44. • The water column height ranged from 25 to 48 cm in the mangrove-lined
bank.
• The water column height ranged from 128 to 390 cm in the estuarine
proper
• The water column height varied from 128 to 150 cm. in the non-mangrove
bank
• Among the mangrove sites, the water level was low in the landward
Avicennia zone and high in the low-lying Rhizophora zone of the intertidal
slope.

45. Average velocity during low and high tides across the Vellar estuary

46. • The velocity of water flow was found high in the estuarine proper,
ranging from 16-23 cm/sec in the estuarine proper, and 18-20 cm/sec
in the non-mangrove bank.
• velocity was low in the mangrove-lined bank, varying from 0-9 cm/sec
• The difference in velocity between low and high tides was more
significant in the mangrove areas than the others.

47. Average concentration of suspended sediment during low and
high tide waters across the Vellar estuary

48. • The concentration of particles suspended in water column ranged
from 0.09 to 0.15 g/l in the mangrove-lined bank.
• The concentration of particles suspended in water column ranged
from 0.008 to 0.01 g/l in both the estuarine proper and the non-
mangrove area.
• Sediment particles are carried in suspension into mangrove forests at
high tide, and they are maintained in suspension.
• This maintenance is due to the turbulence caused by mangrove
structures

49. • The efficiency of sediment trapping varies with mangrove zones.
• Avicennia-Rhizophora interphase is much efficient in trapping 30% of
total suspended sediment received at high tide.
• Avicennia zone is efficient by 25%
• Rhizophora can trap only 20% of sediment.

50. Conclusion
• Mangroves can contribute to the reduction of coastal hazards,
including wind and swell waves, storm surges, and tsunamis.
• It is important to note that hazards are usually only partially reduced
by mangroves. However, even small changes in water levels can result
in reduced flood extents and, therefore, reduced damage to property
and loss of life.
• Dense forests are expected to be most effective at reducing hazards,
and mangrove species with aerial roots are better able to reduce
wind and swell waves and also to withstand inundation by tsunamis

51. THANK YOU

52. REFERENCES
• Subodh, K. M. and Abhiroop chowdary, 2013, Effects of Anthropogenic
Pollution on Mangrove Biodiversity: A Review. Journal of Environmental
Protection, Vol.4 No.12
• Mazda, Y., M. Magi, Y. Ikeda, T. Kurokawa, and T. Asano. 2006. “Wave
Reduction in a Mangrove Forest Dominated by Sonneratia Sp.” Wetlands
Ecology and Management 14(4), 365–378.
• Quartel, S., A. Kroon, P. Augustinus, P. Van Santen, and N. H. Tri. 2007. “Wave
Attenuation in Coastal Mangroves in the Red River Delta, Vietnam.”
Journal of Asian Earth Sciences 29(4), 576–584.