176
A. Bhattacharya
often too low, their inefficiency results largely from improper design. Thus, frequent
and large-scale overflowing during storm surges and tidal bores is probably the single
most important consequence of the improper design of embankments in the Sunderbans
region.
Mechanical defects resulting from the stalling of construction schedules can also
pose large problems. After the completion of the earthworks, construction activities
are often interrupted, sometimes for several years, because of a paucity of funds or
even plain negligence. Without protective layers of boulders or brick blocks, mud banks
in particular develop desiccation cracks whereby leakage can result in the disruption
of the uncompleted constructions at later stages (Fig. 16.5a). In the Bay of Bengal simple tunnels/channels and sluices are widely used for sewage disposal and the drainage
of saltwater from inland ponds. Because these passages are often inadequately fortified, tidal scouring can severely weaken and ultimately even destroy the embankments
in the vicinity.
16.4.2
Natural Destabilisation
Growth of the roots and pneumatophores of mangroves also decreases the resistance
and durability of embankments by creating interconnected openings and passages
through which leakage occurs (Fig. 16.5b). The burrowing activities of the mud lobster
or ghost shrimp Thalassina anomala have similar negative effects on bank stability.
Indeed, these organisms excavate passages as much as 2 m deep in the upper intertidal
and supratidal zones of rivers (Mandal and Ghosh 1989).
The effects of various factors reducing the stability and, therefore, the performance
of embankments are severely compounded by the liquefaction of the subsoil below the
water-table. Liquefaction can lead to 1) the tilting of the embankments (Fig. 16.5C), 2)
the cracking of mud below the embankments (Fig. 16.5d), and 3) the lateral spreading
of the subsoil along gently sloping riverbanks (Fig. 16.5e); Youd and Keefer 1981;
Youd 1984; Johnson and de Graff 1988).
16.4.3
Inadequate Maintenance
Last but not least, the failure of embankments to provide effective protection is also
explained by the oft tardy and piecemeal nature of repair operations. These can generally be carried out only after the monsoon season because persistently hostile weather
conditions render repair work largely impossible before. Rapid repair work can be severely impaired also by logistic problems in procuring boulders for construction from
distant regions.
16;4.4
Embankment Breaching
During severe cyclonic storms strong wave action can cause the breaching of embankments in the Sunderbans region. Consequently, tidal waters flood the low-lying villages
and adjacent farmland, causing large-scale devastation on the coastal plains. The vil-
A. Bhattacharya
often too low, their inefficiency results largely from improper design. Thus, frequent
and large-scale overflowing during storm surges and tidal bores is probably the single
most important consequence of the improper design of embankments in the Sunderbans
region.
Mechanical defects resulting from the stalling of construction schedules can also
pose large problems. After the completion of the earthworks, construction activities
are often interrupted, sometimes for several years, because of a paucity of funds or
even plain negligence. Without protective layers of boulders or brick blocks, mud banks
in particular develop desiccation cracks whereby leakage can result in the disruption
of the uncompleted constructions at later stages (Fig. 16.5a). In the Bay of Bengal simple tunnels/channels and sluices are widely used for sewage disposal and the drainage
of saltwater from inland ponds. Because these passages are often inadequately fortified, tidal scouring can severely weaken and ultimately even destroy the embankments
in the vicinity.
16.4.2
Natural Destabilisation
Growth of the roots and pneumatophores of mangroves also decreases the resistance
and durability of embankments by creating interconnected openings and passages
through which leakage occurs (Fig. 16.5b). The burrowing activities of the mud lobster
or ghost shrimp Thalassina anomala have similar negative effects on bank stability.
Indeed, these organisms excavate passages as much as 2 m deep in the upper intertidal
and supratidal zones of rivers (Mandal and Ghosh 1989).
The effects of various factors reducing the stability and, therefore, the performance
of embankments are severely compounded by the liquefaction of the subsoil below the
water-table. Liquefaction can lead to 1) the tilting of the embankments (Fig. 16.5C), 2)
the cracking of mud below the embankments (Fig. 16.5d), and 3) the lateral spreading
of the subsoil along gently sloping riverbanks (Fig. 16.5e); Youd and Keefer 1981;
Youd 1984; Johnson and de Graff 1988).
16.4.3
Inadequate Maintenance
Last but not least, the failure of embankments to provide effective protection is also
explained by the oft tardy and piecemeal nature of repair operations. These can generally be carried out only after the monsoon season because persistently hostile weather
conditions render repair work largely impossible before. Rapid repair work can be severely impaired also by logistic problems in procuring boulders for construction from
distant regions.
16;4.4
Embankment Breaching
During severe cyclonic storms strong wave action can cause the breaching of embankments in the Sunderbans region. Consequently, tidal waters flood the low-lying villages
and adjacent farmland, causing large-scale devastation on the coastal plains. The vil-