Abstract:
Flows in the Gorai river have been considerably altered since the commission of
Farakka Barrage on the Ganges river in 1975. Effects of this flow alteration on
livelihood, environment and ecology have been significant. Gorai is the main source
of fresh water flow in the southwest estuarine area. Since Gorai flow is important for
fishery, agriculture, mangrove forest and prevention of salinity intrusion, knowledge
of instream flow is necessary for undertaking adequate river restoration and
resuscitation work.
The objective of this study is to gam experience m investigating instream flow
requirement of Gorai river considering river problems and functional requirements,
quantifying the impact of changing flows and developing techniques for
recommending flow regimes for altcrnative uses. For investigation of salinity
intrusion the entire reach of Gorai from its headwaters at the Ganges upto the
downstream limit and the interconnected channels were co~sidered. For analyzing the
habitat requirement for dominant fish species using Physical Habitat Simulation
(PHABSlM) model, a reach -of about 26 km of the river was selected. Onedimensional
unsteady salinity intrusion in a tidal estuary was based on salt balance
equation while the tidal hydrodynamics was solved using a system of onedimensional
unsteady flow continuity and momentum equations. Taking the salinity
, concentrations observed in the April-May 2002 period when the minimum discharge
was 6.7 m)/s considered as the base condition, salinity concentrations were simulated
for discharge values ranging from 100 m)/s to 250 m3/s to quantify the effect of
changing flows. It was found that discharge of about i00 m3/s and 160 m)Is,
respectively are required to keep salinity level upto Khulna station (about 247 km
downstream of Gorai river headwaters) within allowable limits for irrigation water
and source of drinking water supply. Furthermore a discharge of about 250 m3/s is
needed to maintain salinity level within allowable limits for the part of mangrove
forest influenced by Gorai river. On the other hand PHABSIM simulation for two
selected fish species, Ayeer (Aorichthy oar) and Bacha (Eutropiichthes vacha),
yielded flow requirements of about 256 and 597 m3/s respectively. This shows that
flow needed to provide habitat for the selected target fish species also suffices to maintain salinity concentration within tolerable limits for the specified uses. Since
there is no alJ encompassing method that wilJ provide for alJ needs, it is appropriate to
apply alternative flow assessment methods considering prevailing problems and
functions of a river in order to be able to present results and alternative scenarios to
the decision makers.
