Abstract:
Environmental flow (e-flow) in a river describes the quantity, quality and timing of water flow required to sustain minimum flow/ fresh water, estuarine ecosystem, the livelihoods and well-being of people that depend on these ecosystems. Bangladesh is already facing problem due to upstream withdrawal on Ganges River which caused the Gorai River (main tributary of Ganges in Bangladesh) almost dried up during low flow season. As a result, the demands for irrigation, navigation, industries and habitat suitability of fish species and other water users could not get the minimum water flow during the dry low flow season. Because of the reduced flow of water, the intrusion of saline water in the upstream has progressively made the south west region of the country vulnerable to increase salinity. In view of these river functions and problem, the assessment of environmental flow is essential. This research work has been carried out to assess the e-flow requirement of Gorai-Madhumati-Kaliganga-Balaswar River System which is a dominant river system in south-west region of Bangladesh.
There are different methods have been used for determining e-flow in many countries around the world. To obtain the e-flow of Gorai-Madhumati-Kaliganga-Balaswar River System the methodology covers the hydrological methods such as Tennant Method, Flow Duration Method (FDC), Constant Yield Method (CYM), Hydraulic rating method (Weighted Perimeter Method), Habitat simulation method (PHABSIM), Holistic Method (Building Block Methodology). Fisheries such as Bacha, Ayeer, Golda, Carp Species and Salinity are the indicators for this research work. One-Dimensional mathematical model has also been set up by using HEC-RAS Modelling Tool for the condition of salinity to maintain the required e-flow through Gorai-Madhumati-Kaliganga-Balaswar River System. Historical flow data, Historical Water Level, Cross-sectional data and Salinity Data collected from Bangladesh Water Development Board (BWDB).
The e-flow of Gorai and Madhumati River has been assessed by considering hydrological approaches and dominant fish requirement. Both of the Rivers, Dominant fish requirement is higher than hydrological approaches and the mean monthly flow is less than the required e-low during low flow season. In that case Gorai and Madhumati River must be maintained the minimum computed e-flow during low flow season for keeping the sustainable ecosystem. The flow demand for considering hydrological approaches and Dominant fish requirement in Kaliganga and Balaswar River is so called satisfactory; the mean monthly flow is higher than the required e-flow in both Kaliganga and Balaswar River. The existing salinity condition of Gorai and Madhumati River (upstream of Kamarkhali Bridge) has remained below 0.20 and 0.25 ppt respectively and the salinity level based on required e-flow has been remained the near to same. But Madhumati (downstream of Kamarkhali Bridge), Kaliganga and Balaswar River existing salinity condition has been remained from January to June is 1.2-5.2 ppt, 2.1-7.5 ppt and 5.5 to 8.2 ppt respectively and July to December 0.75-4 ppt, 1.2-6 ppt and 1.2-7 ppt respectively. The salinity level based on required e-flow of Madhumati, Kaliganga and Balaswar River from January to May has been remained around 0.3-1.80 ppt, 0.75-2.0 ppt and 1-2.5 ppt respectively and also from July to December has been remained below 1ppt.
