Flow Characteristics and Environmental Flow Requirement of Gorai River in Bangladesh

Abstract

Environmental flow requirement certifies natural condition prominence of a river. Due to terrestrial location, rivers in Bangladesh have faces precise high flow in wet season and low flow in dry season. Since the current condition of a river flow characteristics has proven on historic flow data, so estimation of environmental flow requirements for the rivers are censoriously important for Bangladesh. The purpose of the research is to assess environmental flow requirement of Gorai River and to evaluate flow characteristics of the river through the comparison between past and recent time. Two stations are selected to assess the environmental flow circumstance for Gorai River system. The selected stations are Gorai Railway Bridge and Kamarkhali Transit station. There are several methods for calculating the environmental flow requirements of a river system. Three popular methods are used here for estimation of the environmental flow on the selected stations. The three methods are Mean Annual Flow (MAF) method, Flow Duration Curve (FDC) method and Constant Yield (CY) method. These methods are appropriate for hydrological attitude and in use of chronological flow data. Daily discharge data of selected stations recorded by Bangladesh Water Development Board (BWDB) has been collected and analyzed for two periods i.e. G1 period (for the year 1984 to 1999) and G2 period (for the year 2000 to 2016), and IHA software (version 7.1) has been exercised. The analysis has been done according to MAF, FDC and CY methods. It is observed from the analysis that, the Mean annual flow of Gorai Railway bridge station is 1012 Cumec during 1984 to 2016, and Mean annual flow of Kamarkhali transit station is 795.1 Cumec during 1984 to 2016. As low flow season is the main concern, about 202.4 Cumec flow is required to maintain good condition for Gorai Railway bridge station and 159 Cumec flow is required for Kamarkhali transit station in MAF method. The relationship between the magnitude and duration of stream flows is presented by flow duration curve (FDC). FDCs are used mainly to set environmental flow purposes. Flow duration intervals are stated as percentage of exceedance, with zero corresponding to the highest stream discharge in the record (i.e. flood conditions) and 100 to the lowest (i.e. drought conditions). As low flow season is the main concern, the environmental flow requirement based on FDC in LFS is found as 290 Cumec for Gorai Railway bridge station and 167 Cumec flow is required for Kamarkhali transit station in FDC method. During the low flow season the minimum requirement based on FDC method is retained during both intermediate and high flow seasons but not for low flow season which is the main concern. Environmental flow considering CY method for Gorai Railway bridge station is found as 221.4 Cumec and for Kamarkhali transit station it is found as 162.85 Cumec. The flow found in CY method is close enough to environmental flow requirement obtained from MAF and FDC methods. It is found that the estimated environmental flows of Gorai River at Gorai Railway Bridge station are 202.4 Cumec, 290 Cumec and 221.4 Cumec for MAF, FDC & CY method, respectively. By averaging these values, the environmental flow is estimated as 237.93 Cumec. In the Kamarkhali transit station of Gorai River the environmental flows are estimated as 159 Cumec, 167 Cumec and 162.85 Cumec for MAF, FDC & CY method, respectively. Thus the average value of environmental flow is estimated as 162.95 Cumec. It shows that insufficient flow condition remains from December to May in both of the stations according to the estimated environmental flow requirement. August and March are the highest and lowest flowing months respectively for both of the stations. It is observed that, the river condition is good at the high flow season but when the flow comes in low flow season it becomes lower than the environmental flows required for good habitat quality. The flows in the month of January to May are less than the EFR required. The flows of these months are less than the severe degradation flow. It shows severe problems for both the stations. For the Gorai river, it is necessary to maintain the flow values more than the severe degradation throughout the year to sustain the habitat quality for the river. The three methods show different values for environmental flow requirement. The flow requirements in the low flow season for three methods are found lower than the required flow in both stations. It shows that the river is endangered for habitat quality in low flow seasons. In every method it proved that, the Gorai River has flow scarcity because of the low flows from upstream. The chloride concentration is generally found higher in the month of November to June. These are the low flow season that includes post-monsoon and pre-monsoon period. It shows a higher value of chloride concentration. The highest individual one day chloride concentration is found as 511 ppm, and the average monthly highest chloride concentration is found as 152.8 ppm in the month of February. At high flow season the flows are higher in the month of July to October. It shows a lower value of chloride concentration. The lowest individual one day chloride concentration is found as 20 ppm, and the average monthly lowest chloride concentration is found as 37.5 ppm in the month of October. The chloride concentration at low tide and high tide shows no significant difference for the same day. The chloride concentration in high flow season is not much less than the concentration of the low flow season for the exceedance 0% to 30%. For the exceedance probability 30% to 100%, it shows a big difference of chloride concentration among the seasons. The salinity for less than 100 Cumec discharge is higher and the salinity decreases with increase in the fresh water discharge up to 500 Cumec. The salinity shows lower value with high discharge upto 60% of probability of exceedance. After that from 60% to 100% probability of exceedance freshwater discharge value did not shows significant effect on salinity change. Small increase in fresh water discharge cause large change in salinity concentration from 5% to 60% probability of exceedance. The environmental flow highlights the basic need of a river to sustain the assortment of natural status of hydrologic systems in order to defend native biodiversity. Indicators are anticipated to assess the complete ecological health of the river and the degree of hydrologic alteration triggered by a particular functioning policy. The condition of a river systems eventually rest on environmental flow constituents, which may change seasonally.