Role of roads and water control structures on polder water management

Abstract

Bangladesh is a delta formed by the alluvial deposits of three mighty rivers the Ganges, the Brahmaputra and the Meghna. Bangladesh has been suffering from the twin problem of ‘floods and droughts’ for centuries and it’s also one of the most climate vulnerable countries in the world. From 1960, 19 big cyclones have hit the coast of Bangladesh. To protect the coast from recurrent natural disaster like floods, cyclones and storm surges along with salinity intrusion and sedimentation, the government of Bangladesh constructed 139 polders in the coastal area of Bangladesh. The immediate socio-economic consequences of these polders were very impressive and positive and that positive situation persisted for about two decades. But construction of embankments and unplanned roads within the polder disrupted the drainage system by blocking canals and rivers. This created a massive waterlogging problem within the polders.

This study was carried out mainly to find out the opportunity of roads and drainage structures in polder water management within the coastal polders of Bangladesh. To perform the study, two polders were selected. Polder 26 was selected for its massive waterlogging problem within the Polder. Polder 43/2F was selected because it has uneven land elevation within the polder, some areas are high and some low lands. The specific objective for polder 26 was to reduce waterlogging mainly by using roads and water crossing structures. And for polder 43/2F, the specific objective was to find out how roads and water crossing structures can be used to control water flow and store water in high lands for dry season irrigation.

A physical survey, several (around 60) House Hold Survey, 2 Focus Group Discussions (FGDs) and 2 Key Informant Interviews (KIIs) were carried out in each selected polder areas. The goals of these surveys were to find out the existing condition of the polders, impacts of waterlogging in farming and other activities, problems related to roads and water within the polder and possible solutions of those problems in priorities. A catalogue of issues and solutions related to road - water interactions is developed based on the survey findings and outcomes of FGDs and KIIs.

The study findings show that the overall scenario of road condition and water management within the two selected polders is not good. Most of the roads are earthen or broken. Both polders have waterlogging problem but it has become severe in polder 26. In polder 26, Farmers are facing crop failures (almost every year) and they have to abandon some portions of their cultivable land, in some cases up to 85 percent, because of waterlogging. To solve these problems, this study gives some recommendations such as improve the siting and size of water crossings, re-excavate khals to reconnect drainage ways, use (more) gated water crossings to retain and control water, improve the quality of roads and carpeting, improve collaboration between Bangladesh Water Development Board (BWDB) and Local Government Engineering Department (LGED), improve shelter function of embankments etc. Implementation of those recommendations will free a huge area from waterlogging and will improve the communication system. As a result, there will be a positive change in farming activities. And the increased farmland and improved farming activity will increase amount of yields which will bring a big change in the local economy.

Finally, a simple design analysis (using rational method) was performed to calculate proper dimensions for internal culverts in polder 26. Rainfall data was collected from Bangladesh Metrological Department and Log Pearson Type III distribution was used to calculate rainfall intensity. From the recommended design dimensions (Table 6.3), it was found that three existing culverts (C2, C5, C7) are of inadequate sizes. To facilitate proper drainage in those locations, at least the recommended size (for 20 years return period and 3 days rainfall) should be provided. Moreover, the Polder needs at least 6 more culverts in stated locations to reduce waterlogging. Implementations of those recommended culverts will improve drainage in 5628 acre agricultural land. And if the improved drainage system can decrease 50% of the current cop loss then the total financial benefit will be USD 290, 823.