Environmental Engineering Reference
In-Depth Information
Southeast coastal polders
comprehensive flood and land management
strategy. Embankments of a total length of more
than 8300 kmhave been constructed since 1959 in
the country. According to the Bangladesh Water
Development Board (Ali 2002), based on types of
infrastructure, location, topography and main
flood management issues, the schemes can be
classified into the following categories:
.
haor (floodplain depressions in northeast part of
the country) schemes;
.
southeast coastal polders;
.
southwest coastal polders;
.
beel (floodplain depression) schemes;
.
floodplain schemes.
In order to protect the coastal areas from regular
tidal inundationand salinity intrusion, a total of 48
polders have been built since the 1960s under the
Coastal Embankment Project (CEP). The south-
east coastal polders are located in Chittagong and
Cox's Bazaar, at the foot of the southeastern
hilly regions. These polders are characterized by
sea-facing embankments on one side and minor
embankments for protection from river flooding
from one or two sides. The schemes usually have
relatively small parallel canals perpendicular to
the sea with many cross-dams across the width
of the canals. Numerous minor tidal sluices are
located along the embankment for drainage. The
drainage canals and tidal sluices have to drain out
runoff from adjacent highlands in addition to re-
tained rainwater within the polder area. Salt and
shrimp production along with rice cultivation are
the main economic activities within the polders
(after Ali 2002).
Haor schemes
Haors are large saucer-shaped floodplain depres-
sions located mostly in the northeastern region
and covering about 25%of the entire region. There
are altogether 411 haors comprising an area of
about 8000 km
2
and dispersed in the districts
of Sunamgonj, Sylhet, Moulvibazar, Hobigonj,
Netrokona and Kishoreganj (Paul 1997).
The extreme flashy character of the rivers and
extremely high rainfall causes frequent flash
floods during the pre-monsoon period of April/
May. These types of floods cause damage to stand-
ing boro and infrastructure. However, monsoon
flooding and retention of water is imperative to
sustain the complex hydroecological characteris-
tics of the haor wetlands.
To protect the crops in the haor areas, the
erstwhile zamindars, with the participation of the
local people, used to construct small dykes for
early flood protection and irrigation. Based on
these initiatives, since 1966, the BWDB has fo-
cused on the construction of about 2000 km of
submersible embankments and additional struc-
tures in the haor region (BWDB2008). Submersible
river embankments provide protection from flash
floods in the pre-monsoon, and during monsoon
overtopping is allowed. The depressions can be
inundated by 1.5-6m of floodwater from May to
November/December. After recession of flood-
waters only the deepest parts remain wet.
Southwest coastal polders
The southwest region of Bangladesh is character-
ized by a flat, low-lying alluvial landscape inter-
spersed by an extensive system of tidal rivers and
streams andwater-filled depressions locally called
beels. The rivers are distributaries of the Ganges
and the beels are actually oxbow lakes. The river
system is highly active, carrying large concentra-
tions of sediment, and the river waters are carried
into the depressions with unobstructed high tides
causing saline water intrusion.
Before the 1960s the locals built temporary
embankments called oshtomaisha gher, meaning
'embankment of eight months'. Water was
allowed to enter into the depressions during
the monsoon when salinity in the rivers was low.
The silt carried with the water dispersed in the
depressions and was deposited during ebb tide.
During the 1960s BWDB constructed a series of
polders or closed embankments with numerous
tidal regulators to reclaim elevation lands and
check saline water intrusion. After more than
a decade of increased productivity in agriculture,
