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

Mirza A.T.M. Tanvir Rahman*
Department of Environmental Sciences, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh

Sanjida Hoque Moly
Department of Environmental Sciences, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh

A H M Saadat
Department of Environmental Sciences, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh

Abstract/ Executive Summary:

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.

Keywords: Flow Duration Curve, Hydrologic Alteration, Indicators of Hydrologic Alteration, Mean Annual Flow, Range of Variability Approach.

Location: Gazipur district and joins the Buriganga at Mirpur in Dhaka district

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Water management

Objectives:

The present study aims to estimate the environmental flow of the Turag River which can be used for future reference in management purposes and to assess the flowing condition of the river through the comparison between past and more recent times.

Methodology:

2.1. Study area and data collection The Turag River is the upper tributary of the Buriganga. The Turag originates from the Bangshi River. The Turag flows through Gazipur district and joins the Buriganga at Mirpur in Dhaka district. The Tongi Khal links the Turag with the Balu River (Choudhury and Choudhury, 2004). The entire regime of the Turag is almost a semi-funnel-shaped basin and its catchment is located on the central and southern part of the Madhupur tract and flows from north to south within the basin and its length is about 40 miles and 15 miles in wide to the maximum. The average width and depth of the river are about 218m and 13.5m, respectively. It has a total area of 386 square miles (Uddin, 2005). From 1989 to 2009 mean daily discharge (cms) data had been collected from Bangladesh Water Development Board (BWDB). Among these years, the supplied data had been missing for 1994, 1995 and 2007.

2.2. Analysis of flow characteristics The historical river data had been analyzed under two periods named P1 (1989-2004) and P2 (2005-2009). The river characteristics and estimated environmental flow of P1 were compared with P2. Results are organized as parametric statistics in which data are characterized by mean or standard deviation. The flow in the Turag varies from season to season. For the convenience in analysis of the flow data, annual flow had been characterized in three separate seasons named low flow season (February to May), high flow season (July to October) and intermediate flow season (November to January and June).

2.3. Estimation of environmental flow An instream flow policy requires clear and measurable goals, ideally defining the goal (e. g. retention of a resource or instream use), the extent to which this is to be achieved (i.e. level of protection) and criteria for evaluating the achievement (Beecher, 1990). In practice, either the proportion of the flow, wetted perimeter or physical habitat that is retained by a minimum flow is used as a measure of the level of protection (Jowett, 1997). There are 207 methods from 44 countries that had been used to assess environmental flow (Tharme, 2003). The different methods of environmental flow assessment used all over the world can be grouped into four main categories: hydrological, hydraulic rating, habitat simulation and holistic methods (Tharme, 2003 and King et al., 2000). Again quantitative instream flow methods are generally divided into three major categories: historic flow regime, hydraulic and habitat (Jowett, 1997). Among these methods hydrological (or historic flow regime) is the easiest to use and the required data is only the historic flow records of the stream concern (Mullick et al., 2010). For the research work, Mean Annual Flow (MAF) method (Tennant, 1976), Flow Duration Curve (FDC) method and Range of Variability Approach (RVA) (Richter et al., 1997) were used to estimate the environmental flow which are under the hydrological method. In MAF method environmental flow requirement is set at different percentage of the mean annual flow that varies from 10% to 200%. The percentage is set considering the desired habitat quality. The FDC is a very useful tool for assessing the overall historical variation in flow (Lamb et al., 2009). The RVA method provides a flow target that resembles the natural flow regime with the primary objective of protecting natural ecosystem (Mullick et al., 2010). The IHA software (version 7.1) had been exercised to acquire required calculation of FDC and RVA to estimate environmental flow of the Turag River.

Source of Information: International Journal of Scientific Research in Environmental Sciences (IJSRES), 1(10), pp. 291-299, 2013

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The estimated environmental flow requirement of the Turag according to RVA, FDC and MAF methods are 206.5 cms, 180.77 cms and 190.88 cms, respectively. By taking average of these values the Turag should have at least 192.72 cms all the year around to meet environmental requirement or instream demand. Mean, maximum and minimum annual flows are reduced about 35%, 42% and 9% respectively from P1 to P2 periods. Considering environmental flows estimated by RVA and MAF methods, about 40% and 36% of total flow of P2 period flow below the estimated required environmental flow. Again in FDC method, about 35% flows are flowed below the required environmental flow during the low flow season of P2. Therefore, in all scenarios the Turag suffers from significant hydrologic alteration that reveals reduction of flow in recent time.

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