66
greenhouse gas emissions, and decreases net primary productivity (Delaune et al.
1990).
Porteresia coarctata, commonly known as salt marsh grass is abundantly available in Indian Sundarban region. It acts as a pioneer species in the process of ecological succession in an island ecosystem (level 1 species in the steps of ecological
succession). The species has unique sediment holding capacity and gradually invites
other floral species to settle as a part of ecological succession (Table 3.1).
The property of mangroves to impart stability to the shores was successfully
used to stabilize the boulders placed to protect the Nayachar Island (Fig. 3.8) in the
upper stretch of the Hugli estuary. The island is located about 56 nautical miles
south of Kolkata, directly facing the port-cum-industrial complex of Haldia. The
island is bounded by latitudes 21
o
54′41″ N – 22
o
01′28″ N and longitudes 88
o
03′02″
E – 88
o
08′43″ E. The island has an area of about 47 km
2
and a maximum elevation
of 6.5 m above mean sea level (MSL). Length of the Island centrally measured from
North to South is 15,861 m and from East to West is 4340 m.
The land topography is generally flat, with elevation ranging from a low of
1.08 m MSL to as high as 3.68 m MSL along the coastal region of the island.
Average ground level of the island is 1.366 m MSL. The island is convex upward
with gently sloping eastern and western flanks. The eastern flank is undergoing erosion at present while the western edge is accreting. According to recent estimation,
the present accretion rate is about 1 km
2
/yr on the Haldia port area and the erosion
rate is about 0.05 km
2
/yr on the eastern part of Nayachar. The island is exposed to
strong scouring action of the running Hugli River and hence a programme was
undertaken by Calcutta Port Trust in early nineties to stabilize the island and
increase the draft of the adjacent navigational channel. 11 true mangrove species
were planted inside and along the border of the island on geo-jute matrix and within
a span of 2 years excellent growth of the species (particularly Sonneratia apetala)
stabilized the boulders with considerable accretion of silt particles. No damage
occurred on the island except tidal inundation and uprooting of few trees during the
AILA of 25th May, 2009. The biodiversity of the island has also increased gradually over a period of time (Table 3.2).
Table 3.1 Process of ecological succession in the islands of Indian Sundarbans
Species
Level of succession
Porteresia coarctata
Level 1 (No. of
species = 1)
Aegiceras corniculatum, Avicennia alba, Avicennia officinalis,
Sonneratia apetala
Level 2 (No. of
species = 4)
Bruguiera sp.
Level 3 (No. of
species = 1)
Heritiera fomes, Rhizophora sp., Xylocarpus sp.
Level 4 (No. of
species = 3)
Ceriops decandra, Excoecaria agallocha
Level 5 (No. of
species = 2)
3 Mangroves: A Barrier Against Erosion
greenhouse gas emissions, and decreases net primary productivity (Delaune et al.
1990).
Porteresia coarctata, commonly known as salt marsh grass is abundantly available in Indian Sundarban region. It acts as a pioneer species in the process of ecological succession in an island ecosystem (level 1 species in the steps of ecological
succession). The species has unique sediment holding capacity and gradually invites
other floral species to settle as a part of ecological succession (Table 3.1).
The property of mangroves to impart stability to the shores was successfully
used to stabilize the boulders placed to protect the Nayachar Island (Fig. 3.8) in the
upper stretch of the Hugli estuary. The island is located about 56 nautical miles
south of Kolkata, directly facing the port-cum-industrial complex of Haldia. The
island is bounded by latitudes 21
o
54′41″ N – 22
o
01′28″ N and longitudes 88
o
03′02″
E – 88
o
08′43″ E. The island has an area of about 47 km
2
and a maximum elevation
of 6.5 m above mean sea level (MSL). Length of the Island centrally measured from
North to South is 15,861 m and from East to West is 4340 m.
The land topography is generally flat, with elevation ranging from a low of
1.08 m MSL to as high as 3.68 m MSL along the coastal region of the island.
Average ground level of the island is 1.366 m MSL. The island is convex upward
with gently sloping eastern and western flanks. The eastern flank is undergoing erosion at present while the western edge is accreting. According to recent estimation,
the present accretion rate is about 1 km
2
/yr on the Haldia port area and the erosion
rate is about 0.05 km
2
/yr on the eastern part of Nayachar. The island is exposed to
strong scouring action of the running Hugli River and hence a programme was
undertaken by Calcutta Port Trust in early nineties to stabilize the island and
increase the draft of the adjacent navigational channel. 11 true mangrove species
were planted inside and along the border of the island on geo-jute matrix and within
a span of 2 years excellent growth of the species (particularly Sonneratia apetala)
stabilized the boulders with considerable accretion of silt particles. No damage
occurred on the island except tidal inundation and uprooting of few trees during the
AILA of 25th May, 2009. The biodiversity of the island has also increased gradually over a period of time (Table 3.2).
Table 3.1 Process of ecological succession in the islands of Indian Sundarbans
Species
Level of succession
Porteresia coarctata
Level 1 (No. of
species = 1)
Aegiceras corniculatum, Avicennia alba, Avicennia officinalis,
Sonneratia apetala
Level 2 (No. of
species = 4)
Bruguiera sp.
Level 3 (No. of
species = 1)
Heritiera fomes, Rhizophora sp., Xylocarpus sp.
Level 4 (No. of
species = 3)
Ceriops decandra, Excoecaria agallocha
Level 5 (No. of
species = 2)
3 Mangroves: A Barrier Against Erosion