Environmental Flow Requirement and Comparative Study of the Turag River, Bangladesh

Environmental flow is necessary for securing ecological health status of a river. Around the Dhaka city of Bangladesh the Turag, an ecologically critical area, is an important river. Environmental flow assessment of the Turag can help to sustain ecosystem and navigation in it. The aims of the research are to estimate environmental flow of the Turag and to assess current flow condition. Mean daily discharge data recorded by Bangladesh Water Development Board had been collected and analyzed for two periods (1989-2004) and (2005-2009). Mean Annual Flow (MAF), Flow Duration Curve (FDC) and Range of Variability Approach (RVA) methods were used and IHA software had been exercised in calculating environmental flow. According to RVA, FDC and MAF methods estimated environmental flow was found to be 192.72 cms, 206.5 cms and 180.77 cms respectively with the mean of 192.72 cms. It is observed that, in more recent time, the environmental flow is not maintained during the low flow season in the river and overall flow rate is reduced. Mean, maximum and minimum annual flows are reduced about 35%, 42% and 9% respectively from the period (1989-2004) to (2005-2009). Considering environmental flows estimated by RVA and MAF methods, about 40% and 36% of total flow of the period 2005-2009 flowed below the required environmental flow. In FDC method about 35% flow is flowed below the required environmental flow during the low flow season in the same period. These findings may help the policy makers to manage water resources in the catchment area of the Turag.