mangroves as coastal defence

MANGROVES As COASTAL DEFENCE

Rahul PatilDept. Soil and Water Engineering

Ph.D I YearPHD16AEG6047

Sea level rising

• 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

Causes of sea level rising

Natural effects Human activities

The normal melting of ice sheets

Burning of fossil fuelsand producing CO2

The development of industry

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

•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

(Source: Forest Survey India).

Mangroves

Red Mangroves Black Mangroves White Mangroves

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.

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.

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.

Types of rootsThree types of roots.

Aerial roots(breathing roots) Prop roots Kneed roots

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

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)

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

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)

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

• 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

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

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).

Mangroves protect other marine ecosystems (seaweeds, coral &

sea grass)

IMPORTANCE OF MANGROVES

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.

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

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

• 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

Vegetation characteristics of Sonneratia sp. at the Vinh Quang coast

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.

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

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

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.

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.

How do mangrove forests induce sedimentation

K. Kathiresan

Rev. biol. trop vol.51

• 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

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

• 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.

Average velocity during low and high tides across the Vellar estuary

• 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.

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

• 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

• 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.

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

THANK YOU

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.