178
A. Bhattacharya
the revetment boulders and concrete slabs at the foot of the embankments, thereby
exposing the mud earthworks at the core of the constructions. Breaches tens of metres
long can occur repeatedly along the sandy beaches and muddy tidal flats.
Along the margins of the embankments strong wave action occasionally causes the
formation of to 3-4-m-deep ditches whose surface areas can reach 10 m 2 in each case.
These ditches promote the undercutting oflocal sediments, thereby facilitating the collapse of embankments during subsequent floods. The catastrophic breaching of the embankments and the ensuing devastation in the hinterlands incur enormous economic
costs in the Sunderbans region. In addition, much man-power and money is spent every
year on the maintenance and repair of these large-scale protective constructions.
16.4.5
Ecological Impacts of Embankments
In combination with other large-scale constructions, man-made embankments cause
both short-term and long-term perturbations of the ecological equilibrium of the
Sunderbans deltaic plains. These negative effects can be grouped into three main categories.
1. As large-scale constructions embankments substantially hinder natural siltation in
floodplains. Consequently, excessive siltation has caused the marked shallowing of
rivers, inlets and creeks. The shallowing of almost all rivers in the Sunder bans region has currently reached alarming proportions. This state of affairs is particularly
noticeable in the rivers Ganges, Saptamukhi and Thakuran (Fig. 16.1). In turn, the
shallowing of rivers enhances the sinuosity and migration of downstream tributaries and channels.
2. The migration of waterways and frequent flooding occur mainly during periods of
high precipitation in the July-September monsoon period. It has recently been documented that the tidal network and drainage systems have increased in length in the
region, a trend accentuated by intermittent tectonic subsidence of the Bengal Basin.
This is reflected in the intricately braided patterns of the rivers on the Sunderbans
coastal plains. The breaching of the embankments during the monsoon season can
greatly damage reclaimed arable land which then reverts to being marshland
(Chatterjee 1972). Thus, temporal and spatial shifts in the flora and fauna reflect
differing sets of pre-embankment and post -embankment environmental conditions.
3. The man-induced loss of valuable mangrove wetlands is a direct result of embankment construction policy. These large-scale constructions often cause the abrupt
truncation of the wetlands because they are situated in intertidal habitats favoured
by mangrove forests. Invasion by marsh vegetation and land reclamation then hinder
the survival of mangroves landwards (Fig. 16.6).
It has been demonstrated that the loss of mangrove ecosystems imparts a serious
ecological imbalance in tropical coastal regions (Woodroffe 1990). Most importantly in
the present context, the buffering capacity of the mangroves is seriously impaired
(e.g. Mazda et al. 1997). The hydrodynamic forces acting on the embankments are thus
stronger than they would be if these constructions were built further inland beyond an
intact mangrove belt.
A. Bhattacharya
the revetment boulders and concrete slabs at the foot of the embankments, thereby
exposing the mud earthworks at the core of the constructions. Breaches tens of metres
long can occur repeatedly along the sandy beaches and muddy tidal flats.
Along the margins of the embankments strong wave action occasionally causes the
formation of to 3-4-m-deep ditches whose surface areas can reach 10 m 2 in each case.
These ditches promote the undercutting oflocal sediments, thereby facilitating the collapse of embankments during subsequent floods. The catastrophic breaching of the embankments and the ensuing devastation in the hinterlands incur enormous economic
costs in the Sunderbans region. In addition, much man-power and money is spent every
year on the maintenance and repair of these large-scale protective constructions.
16.4.5
Ecological Impacts of Embankments
In combination with other large-scale constructions, man-made embankments cause
both short-term and long-term perturbations of the ecological equilibrium of the
Sunderbans deltaic plains. These negative effects can be grouped into three main categories.
1. As large-scale constructions embankments substantially hinder natural siltation in
floodplains. Consequently, excessive siltation has caused the marked shallowing of
rivers, inlets and creeks. The shallowing of almost all rivers in the Sunder bans region has currently reached alarming proportions. This state of affairs is particularly
noticeable in the rivers Ganges, Saptamukhi and Thakuran (Fig. 16.1). In turn, the
shallowing of rivers enhances the sinuosity and migration of downstream tributaries and channels.
2. The migration of waterways and frequent flooding occur mainly during periods of
high precipitation in the July-September monsoon period. It has recently been documented that the tidal network and drainage systems have increased in length in the
region, a trend accentuated by intermittent tectonic subsidence of the Bengal Basin.
This is reflected in the intricately braided patterns of the rivers on the Sunderbans
coastal plains. The breaching of the embankments during the monsoon season can
greatly damage reclaimed arable land which then reverts to being marshland
(Chatterjee 1972). Thus, temporal and spatial shifts in the flora and fauna reflect
differing sets of pre-embankment and post -embankment environmental conditions.
3. The man-induced loss of valuable mangrove wetlands is a direct result of embankment construction policy. These large-scale constructions often cause the abrupt
truncation of the wetlands because they are situated in intertidal habitats favoured
by mangrove forests. Invasion by marsh vegetation and land reclamation then hinder
the survival of mangroves landwards (Fig. 16.6).
It has been demonstrated that the loss of mangrove ecosystems imparts a serious
ecological imbalance in tropical coastal regions (Woodroffe 1990). Most importantly in
the present context, the buffering capacity of the mangroves is seriously impaired
(e.g. Mazda et al. 1997). The hydrodynamic forces acting on the embankments are thus
stronger than they would be if these constructions were built further inland beyond an
intact mangrove belt.