Assessment of instream flow requirement of gorai river considering salinity intrusion and fish habitat

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.