Evaluation of Draft Dhaka Structure Plan (2016-2035) Regarding Drainage and Flood Management: A Case study on Turag River and Its Contributory Area

Abstract

Turag river is an upper tributary of the Buriganga River and flows beside the western side of Dhaka, the capital of Bangladesh. Turag river and its flood plain plays a dominant role in flood management of Dhaka city as it assists to save Dhaka city from flooding by containing excessive flood water during inundation. From the very past, Turag and its tributaries are enriched with biodiversity and ecological resources and have provided enormous ecosystem services and functions to the society. Like other areas of Bangladesh, the settlement and livelihood pattern of people especially the farmers and fishermen community of Turag river floodplain are adjusted with the deltaic conditions of Bangladesh. But the lives and livelihood and the settlement of the local community are about to extinct now. Encroachment and pollution and of Turag river-flood plain ecosystem have damaged the socio-economic and ecological harmony of the region. Recognizing the importance of restoring the original character of Turag river, Department of Environment (DoE) of Bangladesh declared Turag river as „ecologically critical area‟ in 2009. The High court division of Bangladesh in 2019 passed an order by declaring Turag river a „living entity‟ and directed concerned authorities to save it from extinction and revive its original character. Conventional flood management strategies/ practices consider flood protection as their only goal but multiple ecosystem benefits/socio-ecological dimensions of flood management approaches are overlooked. In this context, there was an urgent need to critically evaluate the policies/proposals of Dhaka Structure plan (2016-2035) and Dhaka Detailed Area Plan (2022-2035) regarding drainage and flood management of Turag river and its contributory areas from multiple ecosystem benefit approach. This study attempted to understand the status of socio-ecological considerations of flood management approaches in the study area. This study also performed hydrologic and hydrodynamic analysis through mathematical modeling to understand the hydrologic and hydraulic process of the study area and then flood mapping analysis was done to understand the impact of development pattern on the flooding pattern of the study area. Finally, the policies/proposals of Dhaka Structure plan (2016-2035) and Dhaka Detailed Area Plan (2022-2035) regarding drainage and flood management of the study area were evaluated with the output derived from the socio-ecological, hydrologic and hydrodynamic analysis. The Dhaka Structure Plan (2016-2035), the strategic plan for Dhaka prepared by RAJUK, has been approved by the Inter-ministerial Steering Committee of government dated 20 November, 2016 but gazette notification has not yet been published by the government. So, in the title of the thesis, the term „Draft‟ before Dhaka Structure Plan (2016-2035) has been used. However, vi before publishing gazette notification of the plan, the gazette notification for the next tier of the plan, i.e. the Dhaka Detailed Area Plan (DAP, 2022-2035), a detailed or local plan prepared by RAJUK, has already been published dated 23 August, 2022 following approval of the government of Bangladesh. From the field investigation, it is observed that insecurity in land ownership/ tenure rights due to threat of forced eviction is the root cause for conversion of flood plains/agricultural lands to non-agricultural uses in the region. This study finds that the percentage of amount of main flood flow zone is almost same for flood map of study model and DAP (2022-2035). But, the connectivity network, position and alignment of main flood flow zone in flood map of the study model is different from those in flood map of DAP (2022-2035) and Dhaka Structure Plan (2016-2035). Besides this, for extreme flooding event (1988), the percentage of inundation extent in study model is different from that of the DAP (2022-2035) for different flood depths. There may be some differences because boundary condition and catchment extent of model for hydrodynamic analysis are different for study model and DAP (2022-2035) although existing DEM is same for both DAP (2022-2035) and study model. This study also finds that in the flood map of DAP (2022-2035), the main flood flow zone is not connected in some areas and the position and alignment of main flood zone have been shifted in some areas from Dhaka Structure Plan (2016-2035) and Dhaka Detailed Area Plan (2010-2015). But, both in Dhaka Structure Plan (2016-2035) and Dhaka Detailed Area Plan (2010-2015), the flow path of main flood flow zone is continuous. It is observed that the flood map in DAP (2022 2035) is prepared by hydrodynamic analysis based on the existing/latest available DEM (year 2014, updated in year 2018), but no oldest available DEM is used for analysis. As existing development pattern is unplanned and existing DEM only reflects the existing scenario, so in DAP (2022-2035), flooding pattern in main flood flow zone based on existing DEM reflected the existing unplanned development pattern which is the main reason for discontinuity and shifting of the position and alignment of the main flood flow zone from the previous plan. Disconnected and changing pattern of flood plains act against conservation of flood plains and can‟t deliver their ecosystem services and functions. So, it is high time to restore the original character of Turag river-flood plain ecosystem so that maximum benefits/values can be captured from the restored ecosystem services and functions.