Cyclone and Storm Surge - Iczmpwb.org

6.5
• to reduce the destructive force of the waves developed during periods of strong wind,
especially along the Southern banks of the wider rivers,
• to create a zone to plant or replant mangroves or other vegetation tolerating saline water, in
order to increase the reduction of the forces of currents and waves,
• to reduce the chance of occurrence of bank failures by lowering the pressure on the edge of
the river; this pressure can be reduced even more by lowering the level of the berm but the
berm level should remain high enough to be planted or replanted successfully with
mangrove.
In many coastal areas in the tropics and also in the Sunderban proof can be found that a zone of
rough vegetation like mangroves is a very effective and very economic protection against wave dash
during storms. In periods the zone is submerged the hydraulic roughness is so high that the velocity
of the flow over the zone and through the mangroves is practically zero. Consequently all the
sediment in the water will settle, even the suspended clays.
The width of the berm which is required to guarantee a solid base for the embankment depends on
the depth and the shape of the river. Often channels are eroded in the bottom of the rivers, due to the
currents. In the outer curves of the river these channels are close to the river banks endangering the
stability of these banks. To prevent that a possible bank failure includes part of the embankment, the
gradient of the imaginary line between the toe of the outer slope of the embankment to the toe of the
river bank should be equal or flatter than 3(horizontal): 1 (vertical).
Another criterion for the required width of the berm is that the zone (re)planted with mangrove should
be at least 10 meters. Considering the criteria as discussed above many of the embankments in the
Sundarban are too close to the rivers and because of this their stability is at least doubtful.
The Forest Department is implementing a programme of replanting mangroves along the rivers in the
estuary. This programme needs to be encouraged. Some coordination between the Forest
Department and the Irrigation Department in order to select the areas where this reforestation is
required most might be useful.
Within the framework of measures to prevent or to reduce the damage of the embankments attention
need to be given to the many boats in the area. The owners drag these boats on the outer slope of
the embankments and sometimes damaging the vegetation on the berm. It might be advisable to
investigate the possibilities to allocate zones for each village where this can be allowed and other
zones where these practices are prohibited.