Environmental Engineering Reference
In-Depth Information
Floodplain schemes
drainage congestion began to create serious water-
logging problems. The presence of polder embank-
ments restricted tidal flow into the beel area
preventing sedimentation in the low-lying areas.
Moreover, reduced flows in the Ganges especially
during the dry season reduced the flushing capac-
ities of the rivers and canals. Siltation of drainage
channels began to occur and by the 1980s many
drainage canals became inoperative due to silta-
tion rendering vast tracts of lands waterlogged all
year round.
In the late 1990s, tidal basin management was
identified and studied as an alternative solution
to major regulators, proposed for relieving drain-
age congestion. Supported by public opinion, silt-
laden waters are being allowed into designated
tidal basins, thereby elevating the land by depo-
sition of silt during ebb tide. Themain features of
Tidal River Management (TRM) are:
.
tidal flow is allowed in the basin;
.
tidal basin increases tidal volume;
.
tidal basin stores flood water during flood event;
.
sedimentation takes place during the long stor-
age period acting as sedimentation trap;
.
These schemes are characterized by high-level
flood embankments on one or both sides of a river.
The main source of flooding in the floodplain
areas is bank overflow from the major rivers - the
Ganges, Brahmaputra and Meghna - and their
tributaries and distributaries during June to Sep-
tember. These floods are characterized by slowrise
and fall extended over 10-20 days or more. Flood-
plains adjacent to the rivers are subjected to this
type of flood every year, which affect about 30%of
the country. Furthermore, high-intensity rainfall
events of long duration cause local flooding.
Average annual rainfall for the whole country is
about 2400 mm. About 80% of annual rainfall
occurs during June to September. In addition, dur-
ing these months the main rivers and tributaries
flow at high stage due to huge discharge from
snowmelt and rainfall from catchments outside
the country. High stage in the rivers impedes
drainage, and the excess volume generated from
rainfall causes local flooding.
Many canals run relatively parallel to the main
rivers and are actually tributaries or distributaries
of the rivers, providing drainage for the protected
areas situated in the surrounding the country.
Mostly, gravity drainage is available in these
schemes, while in some cases pumped drainage
is also provided particularly in areas of high eco-
nomic value, such as Dhaka city.
erosion and maintenance of
rivers
takes
place.
Beel (floodplain depression) schemes
Several clusters of natural depressions, or beels,
are situated in the southern part of the north-
west region of the country. There are about
50 beels, having a surface area ranging from
25 to 1500 acres and depth varying from 0.3 to
3 m. These beels act as temporary flood reten-
tion reservoirs and as linkage channels between
the parallel rivers, which are tributaries of the
Jamuna. Since the 1970s a large number of
polders consisting of one main river embank-
ment and minor embankments have been
constructed around these beels to protect rice
from monsoon flooding. These schemes usually
have one main drainage canal with one outfall.
Higher land situated on two or three sides of the
scheme results in runoff into the scheme (after
Ali 2002).
Impacts and Issues
Infrastructure built for flood protection has, in
general, provided enhanced safety and security to
people, crops and livestock. The positive impacts
include protection against early flooding (mainly
submersible embankments), salinity exclusion,
and reductionofmonsoonflood depths. Protection
has resulted in changes in land type, which con-
sequently have brought about changes in land use
patterns (Box 24.2). According to NWMPP (2000),
the major agricultural impact of the hydrological
and land type changes resulting from FCD
schemes was on Kharif cropping practices within
