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Climate Change Implications on Drainage System in Coastal Polders of Bangladesh

Md. Sohel Masud*
Institute of Water Modelling, House 496, Road 32, Mohakhali DOHS, Dhaka 1212,

Abu Saleh Khan
Institute of Water Modelling, House 496, Road 32, Mohakhali DOHS, Dhaka 1212,

Abstract/ Executive Summary:

Coastal polders are the first large-scale human interventions in the coastal area of Bangladesh mainly for enhance agriculture production. The presence of coastal polders restricts the spreading of tidal flow on the low-lying area and thereby prevents deposition of sediment on the tidal floodplain. Drainage congestion problem will be a major impact of climate change, specially for the coastal area of Bangladesh. According to the IPCC reports rising of global temperature will result sea level rise and changes in precipitation pattern. Moreover the sedimentation process of the major coastal rivers is reducing the discharge carrying capacity of many rivers. The combined effect of sea level rise and increase of precipitation, siltation of estuary rivers, and reduced sedimentation in the flood protected areas will gradually increase drainage and water logging problems. It has been observed that the climate change impact is more in polders situated more downstream (near to sea) compared to polders situated inside. The study identified the vulnerable areas of the polders subject to inundation under climate change scenario. The paper focuses on the results achieved from the study and demonstrates the probable impact to the coastal region, specially on some selected polders (22, 29 & 30) of the coastal area of Bangladesh.

Keywords: Climate change, polder, drainage, coastal area, sea level rise

Location:

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Drainage

Objectives:

The study was conducted with the objective to climate change implications on drainage system in coastal polders of Bangladesh

Methodology:

The study is based on the combination of field measurements and numerical modelling. To support the modelling work a systematic data collection program has been carried out. It included the state-of-the-art hydrographic and topographic measurement program and collection of both primary and secondary data. The data has been utilized for the model development, updating and finally to establish the baseline condition. For surface water/ drainage study MIKE 11 (one dimensional model of DHI, Denmark) and MIKE 11 GIS have been utilized. For sedimentation study and sealevel rise impacts MIKE 21 (two dimensional model) has been utilized. Data collection: A substantial effort has been made for the collection of field data of tide, water flow, cross-section of rivers and internal drainage khals and sediment sample and secondary data from different organisations (BWDB, BIWTA) at key locations along the peripheral rivers of the polders to understand the tidal and sediment characteristics of these rivers as well as for hydrodynamic and sediment transport modelling for the prediction of the risk of sedimentation. The field survey campaign was carried out at two phases: monsoon (Aug~ Oct’07) and dry ( Dec ~ Feb’07) to collect the data on river cross-section, water level, water flow, velocity, fall velocity, river bed materials and suspended sediment concentration. All these data have been checked and validated using standard data quality control procedures before using in the study.

Model development: In order to investigate the hydraulic characteristics such as water level fluctuations, variation river flow with tide, net flow during dry and monsoon seasons a hydrodynamic model of the rivers in the area has been developed. The model includes the peripheral rivers of selected polders (22, 23, 29, and 30). The tidal rivers Bhadra, Gengrail, Kazibacha, Haborkhali, Badurgacha, Monga, Chunkuri, Dhaki, Lower Sholmari, Sibsa and Kerulia are included in the model set up. The hydrodynamic model has been calibrated against measured water levels and discharges both for the dry and monsoon period at different locations. The comparison shows reasonable and satisfactory agreement of model results with the measured water flow. It has already been mentioned that in order to investigate the present and future sediment transport and sedimentation pattern in the peripheral rivers sediment transport model has been developed and calibrated in a parallel study component. Detailed sediment transport modelling is out of scope of this paper, however, the output of that study was utilized to identify the future sedimentation pattern of the peripheral rivers.

Source of Information: 2nd International Conference on Water & Flood Management (ICWFM-2009), BUET, Dhaka, Bangladesh

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The project model has been simulated with climate change conditions i.e. tidal boundaries with the effect of sea level rise and internal rainfall incorporating effect in rainfall changes. Moreover, long term sedimentation pattern of the outfall-rivers were also considered. Comparing the model results with the existing condition the following climate change impact have been identified. Model study indicates that the increase precipitation will increase the drainage volume and water level inside the polders. The changes in inundation pattern of the three selected polders during the peak flooding has been compared for existing condition with the climate change condition. It can be noticed from the table that the increase in inundated area will be the result of the climate change. Climate change impact is maximum in Polder 22 which is situated at the most downstream part compared to the other polders. Moreover, the peripheral rivers of this polder were identified to be mostly affected by the river sedimentation, as a result more than 30% additional area will be inundated due to the climate change phenomenon.

Knowledge Management: Report/Proceedings