The purpose of the study is to assess the Environmental Flow Requirement (EFR) of Gorai River in Bangladesh and to evaluate the change in flow characteristics in recent time compared to past. Daily discharge data of Gorai railway bridge station were collected from Bangladesh Water Development Board and analyzed for two periods. Mean Annual Flow (MAF) in G2 period (2000-20166) is found about 13% lower than G1 period (1984-1999). The Mean Monthly Flow (MMF) in low flow season is increased by 99%, and that of high flow is decreased by 20% in G2 period compared to G1. In this study, EFR was determined considering various methods including Tennant, Tessmann, Variable Monthly Flow, Modified Constant Yield, FDCA Q50-Q90, FDCA Q50-Q75 and Smakhtin Method. The average EFR for low, high and intermediate flows were found as 89, 915 and 273 m3/s, which is 9, 61 and 27% of MAF and 96, 23 and 61% of mean seasonal flow, respectively. The overall annual EFR for the river is found as 295 m3/s or 29% of MAF. It is observed that when the EFRs are expressed as percent of mean seasonal flow, the Low-Flow Requirements (LFR) were found higher than High-Flow Requirements (HFR). However, when the EFRs are expressed as percent of MAF, the LFR is lower than HFR. Among all the EFRs predicted by 8 methods, Smakhtin predicts the smallest HFR (6% of MAF) and FDCA Q50-Q90 have the lowest LFR value (1.2% of MAF). The Tennant method is not found to be capable to capture the temporal change of MMF of different seasons. The Average Annual EFR was found to be reduced by 14% in latter period. A deficient flow situation was observed from December to May. The findings can be used for future reference in management of flows in Gorai river.
Keywords: Constant yield; Environmental flow requirement; Flow duration curve; Mean annual flow; Mean monthly flow.
© 2022 The Author(s).