Assessing environmental flow for the Kobadak river and developing a framework for its maintenance

Abstract

Bangladesh is a riverine country. The rivers of the country are the most important source of freshwater. The ecology of a river catchment depends largely on the fresh water flow of the river. The rivers have been the major sources of fish production, livelihood resources, biodiversity, freshwater, etc. But the natural flows of the rivers are being altered by human interventions. The absence of environmental flow has become one of the major problems in the rivers of Bangladesh. The River Kobadak was once one of the main sources of fresh water in the South-Western region of Bangladesh. Now, the entire catchment of the Kobadak River is in lifeless condition. The users of this river have been facing a number of problems like flooding, water scarcity for irrigation, insufficient depth of water in the river for navigability, salinity intrusion, and shortage of water for the ecosystem services. This study was conducted on the Kobadak River stretching between the Mathabhanga River in the upstream and the Sibsa River in the downstream. The objectives of the study were to assess the environmental flow requirement of the river and to develop a framework for maintenance of such flow in it. A quantitative assessment of the environmental flow requirement was made by following a socio-technical approach. The discharge data was essential for the technical assessment of environmental flow in the river which was collected from Bangladesh Water Development Board. The technical assessment was made using the flow duration exceedance percentile method, which is one type of hydrological flow index method. In social assessment, the history of river flow was gathered by conducting Focus Group Discussions (FGDs) with the local people. The month which is critical in terms of environmental flow was also identified through the FGDs. A total of 15 FGDs were conducted with 15 focused groups and a total of 17 Key Informant Interviews (KIIs) were conducted with 17 key persons (officials, teachers, local elites, etc.) through six field visits to the study area. A framework has been developed for the maintenance of the environmental flow in the river. The framework is based on three criteria (ecology, hydrology and morphology) and 15 indicators. It considers restoration, rehabilitation and remediation as river maintenance options. The findings of the study indicate that the environmental flow requirement of the Kobadak River is about 20.0 m3/s. From the pair-wise ranking of the ecological indicators, the weights of vegetation, fish, dolphin, micro-invertebrate, bird and frog are found to be 14.3%, 11.9%, 7.1%, 9.5%, 2.4% and 4.8%, respectively. Also, the weights of the hydrological indicators are found to be 7.9%, 1.6%, 9.5%, 3.1%, 6.4%, and 4.8% for the river connectivity, encroachment area, discharge, salinity, water quality and water level, respectively. Moreover, the weights of the river width, salinity and river depth of the morphological indicators are found to be 2.8%, 5.6% and 8.3%, respectively. It is found from the combined scores of the river management options that the total scores of the restoration, rehabilitation and remediation options are 50.72, 90.26 and 13.99, respectively. So, the rehabilitation of the river is found to be the best solution from a multi-criteria analysis. A rehabilitation plan for the river is also proposed in this study.