16 Embankments and Their Ecologicallmpacts
173
Wave heights can exceed 2.5 m during cyclones, and wave periods are >14 s
(Banerjee 1972). In contrast, wave heights measure 0-0.6 m, and wave periods are 5-7
secs during the calm winter season. These values increase to 1.8-2.4 m and 12-14 s, respectively, during the rough summer months. The average annual rainfall is 17501800 mm, 80% of which occurs from May to September. Average monthly air temperatures vary between 34-36° C in April-June and 21-25° C in December-January. Relative
humidity is >80% in June-September, and >75% in October-May.
The major economic activities in the Sunderbans region involve fishing, forestry,
industry, navigation, urbanisation, agriculture and recreation, many of these being
threatened by erosion in the coastal zone. Erosion can occur as either a gradual denudation or a catastrophic event. Gradual erosion reflects the effects of 1) strong tidal
currents at times of high tidal ranges (>6 m), and 2) a moderate to strong littoral drift.
Catastrophic erosion events are linked with 1) cyclonic storms and hurricanes, i) tidal
bores, and 3) tectonic and neotectonic movements (Bhattacharya and Das 1994;
Bhattacharya 1997a).
16.3
Embankment Function and Design
In the Sunderbans region embankments mainly serve to:
• Prevent the erosion of river banks
• Protect the hinterlands from saltwater, particularly the agricultural and inhabited
areas situated inlmediately above the intertidal zone
• Control water runoff in tidal channels for flood alleviation
• Guard against pirates as well as wild animals such as the Royal Bengal tiger
• Protect low-lying areas from tropical cyclones.
Embankments are large-scale constructions commonly made of bricks, concrete,
boulders (mostly laterites) and mud with or without revetments and rip raps. Facing
the sea or rivers they are often reinforced at their bases by boulder mounds which
thereby provide so-called toe protection. Four main types of embankments are described below.
16.3.1
Boulder Embankments
The most common construction design of boulder embankment makes use of laterite
boulders from the Middnapore district. The rough surfaces of these boulders impart
additional safety by effectively dissipating wave energy (Petts and Calow 1996). Banks
of earth about 15 m wide are constructed along the seashore and riverbanks (Fig. 16.2).
Facing the river the earthworks have slopes measuring 25-30 degrees. They are subsequently covered by 8 to lO-cm-thick fllter beds of crushed brick bat overlain by regularly positioned laterite boulders. The boulder layers are a 75 cm thick. The constructions are 4-5 m high in all.
173
Wave heights can exceed 2.5 m during cyclones, and wave periods are >14 s
(Banerjee 1972). In contrast, wave heights measure 0-0.6 m, and wave periods are 5-7
secs during the calm winter season. These values increase to 1.8-2.4 m and 12-14 s, respectively, during the rough summer months. The average annual rainfall is 17501800 mm, 80% of which occurs from May to September. Average monthly air temperatures vary between 34-36° C in April-June and 21-25° C in December-January. Relative
humidity is >80% in June-September, and >75% in October-May.
The major economic activities in the Sunderbans region involve fishing, forestry,
industry, navigation, urbanisation, agriculture and recreation, many of these being
threatened by erosion in the coastal zone. Erosion can occur as either a gradual denudation or a catastrophic event. Gradual erosion reflects the effects of 1) strong tidal
currents at times of high tidal ranges (>6 m), and 2) a moderate to strong littoral drift.
Catastrophic erosion events are linked with 1) cyclonic storms and hurricanes, i) tidal
bores, and 3) tectonic and neotectonic movements (Bhattacharya and Das 1994;
Bhattacharya 1997a).
16.3
Embankment Function and Design
In the Sunderbans region embankments mainly serve to:
• Prevent the erosion of river banks
• Protect the hinterlands from saltwater, particularly the agricultural and inhabited
areas situated inlmediately above the intertidal zone
• Control water runoff in tidal channels for flood alleviation
• Guard against pirates as well as wild animals such as the Royal Bengal tiger
• Protect low-lying areas from tropical cyclones.
Embankments are large-scale constructions commonly made of bricks, concrete,
boulders (mostly laterites) and mud with or without revetments and rip raps. Facing
the sea or rivers they are often reinforced at their bases by boulder mounds which
thereby provide so-called toe protection. Four main types of embankments are described below.
16.3.1
Boulder Embankments
The most common construction design of boulder embankment makes use of laterite
boulders from the Middnapore district. The rough surfaces of these boulders impart
additional safety by effectively dissipating wave energy (Petts and Calow 1996). Banks
of earth about 15 m wide are constructed along the seashore and riverbanks (Fig. 16.2).
Facing the river the earthworks have slopes measuring 25-30 degrees. They are subsequently covered by 8 to lO-cm-thick fllter beds of crushed brick bat overlain by regularly positioned laterite boulders. The boulder layers are a 75 cm thick. The constructions are 4-5 m high in all.