Modelling the drainage system of selected coastal polders

Abstract

This research work has been conducted to assess the water logging problem for selected coastal polders namely 17/1 and 17/2 of district Khulna (Dumuriaupazila) in the South-West region of Bangladesh. This research work has also evaluated the vulnerability of the existing situation of these polders with changing climate scenario of IPCC 2014 fifth assessment report and the effectiveness of re-excavation of internal khals, dredging of Peripheral Rivers, Tidal River Management (TRM) and introducing pumping system against water logging problem applying as different scenarios. To carry out the study google images, bathymetric data of south-western coastal area, water level and discharge data, precipitation and evaporation data have been used in the available South West Regional Model (SWRM) from Institute of Water Modelling (IWM). Field visit was conducted with a view to obtain a better idea on the existing condition of study area. The Polder 17/1 and 17/2 has been experiencing severe water-logging problem over the years because of high rate of sedimentation in the peripheral rivers and internal drainage khals. It severely affects the normal social and economic activities of the people of the study area. The drainage system of the study area is composed of Gangril, Haria, Taltola, Bhadra and Salta Rivers along with a vast network of internal drainage khals. These entire river systems have been severely silted up by the incoming silt from the sea with high tide and lost its drainage capacity severely which leads to no drainage condition through the existing structures of the polders. This investigation is made by preparing flood inundation depth maps for 3-day duration maximum water level considering 5-days cumulative rainfall by showing the area of different land classes (F0, F1, F2, F3 and F4) using model results and available Digital Elevation Model (DEM) of the study area. From the local model of Polder 17/1 it has been found that maximum flood free area increased from base condition 17.26% to 69.96% in the final option and Polder 17/2 from base condition 16% to 84% in final option under critical climate change condition. In this study, TRM is applied as a scientific technique to apply for reducing the water logging problem solution in the selected coastal polders. TRM is not a direct solution for reducing the water logging but a long-term method of solution. It increases the conveyance capacity of the peripheral river. Therefore, the drainage system of the polder achieves high drainage capacity due to increase in the upstream and downstream water level difference at the critical condition. In this research, it is found the tidal prism with TRM is significantly high compared to the required tidal prism, which implies that the river conveyance capacity will be increased and improved drainage conditions will be sustainable. The tidal prism is found in Existing Condition 0.07 (Million-m3), Proposed condition with Dredging 2.78(Million-m3) and Proposed condition with Dredging +TRM 7.76(Million-m3) whereas the required tidal prism is 6.06 (Million-m3). In this research pumping is applied water level up to 1.0 mPWD and lowest at 0.50 mPWD for getting the highest benefit at extreme climate change condition.