Flood Control? An Evaluation of the Impacts of Flood Control and Drainage Projects in Bangladesh Final Report Margot Drost Master Thesis of Civil Engineering TU Delft Flood Control? An Evaluation of the Impacts of Flood Control and Drainage Projects in Bangladesh Delft, August 2006 Final report M.Sc. graduation study Author: Margot Drost Graduation Committee: Prof.dr.ir. N.C. van de Giesen Dr.ir. M.W. Ertsen Dr.ir. R.J. Verhaeghe Ir. G. Pichel M.Sc. Thesis Delft University of Technology Faculty of Civil Engineering and Geosciences Section Water Resources Management In collaboration with DHV 2 Preface This is the final report of the M.Sc. graduation study of Margot Drost. The study was carried out at the section Water Resources Management of Civil Engineering at Delft University of Technology. In the context of this study I did an internship at DHV in Bangladesh for three months to set up the research. The report consists of four chapters that describe the research and most results are displayed in detail in the appendices. The readers without much time are referred to the first part of the chapter one and the conclusions in chapter four. Those who wish to know more about the hydrodynamic model that was used should read chapter two. For a thorough perception on the results, chapter three and appendix C are recommended. I would like to thank my graduation committee, Prof.dr.ir. N.C. van de Giesen, Dr.ir. M.W. Ertsen and Dr.ir. R.J. Verhaeghe for their assistance during these last months in Delft. During and prior to my stay in Bangladesh, I received much help from Ir. G. Pichel. I would like to thank Gerard for making it possible for me to work and live in Dhaka, where I learned a lot and met many interesting and helpful people. I would also like to thank the Project Director of the Emergency Flood Damage Rehabilitation Project, Mr. Kabir, for his support. He introduced me to many BWDB employees that helped me with my research. Also Dr. Rezaur Rahman and his staff helped me a lot during the difficult first phase of my study. I am also very grateful to all the people of CEGIS, IWM, SPARRSO and the Bangladesh Water Development Board who helped me with finding and getting my data. Upon my return in the Netherlands I had some difficulties with setting up my model. I received much help from the staff of Neele en Schuurmans, where I could stay for a week while building my hydrodynamic model. I would especially like to thank Olivier Hoes for introducing me there and Coen Nengerman for being as patient as he was. Apart from all the help I received concerning the contents of this report, I would also like to thank Mark Huizer for printing it. My final thanks go out to my parents for supporting me during my studies. 3 Summary Bangladesh is a floodplain country with an extensive river system. Annual monsoons result in flooding over a significant part of the country. This can vary from twenty percent, to sixty-five percent as in extreme cases such as the disastrous floods of 1988. Given Bangladesh’s extremely high population density, there is a high pressure on land. This land scarcity forces the government to take flood mitigation measures. In the past this resulted in the implementation of many Flood Control projects with a total investment cost of about US$3 billion. Given this significant sum of money, this research aims to evaluate the effect of these measures – has the damage actually been reduced? The Lower Atrai basin is selected for a case study area, in order to evaluate the flood control measures. The area is very flat and counts numerous depressions. The yearly floods have led to the implementation of many Flood Control and Drainage (FCD) Projects. The evaluation of their impacts requires a comparison between the situation both with and without the FCD projects. To make this comparison, simulations of flood events are made. Hydrodynamic models provide a suitable method for the simulation of a flood. The software that was used in order to develop this model is SOBEK Rural. It schematizes the basin by a one- dimensional and two-dimensional part. The first section is a system of channels containing the river data i and is linked to the 2D grid containing the surface elevation per cell ii . For the purpose of this research, three scenarios are developed, on which two flood years are imposed to evaluate their performances. In this case, the hydrological conditions of the floods of 1998 and 2004 are used as inputs to the model. First, the existing situation as it occurred in 1998 is simulated; the calibration of the model is based upon this scenario. Subsequently, scenarios with the flood control projects as they were designed and without these measures are simulated. The available data of the 1998 flood was most suitable and this year is therefore used as a basis for the model. Subsequently, the 2004 flood is also simulated for scenario B and C. This year is considered as a moderate flood and is therefore a valuable addition to the simulation of 1998. Several factors limit the extent to which the real situation can be met. The main limitations will be discussed here. Firstly, the available data was limited. A coarse grid and missing data on discharges and cross sections complicated the study. A major deficiency of SOBEK is the rainfall input. Although it is possible to introduce precipitation of the grid, this is limited to one station. Consequently, the spatial and temporal variability are not taken into account. For the evaluation of the processes during the monsoon, the temporal variability is however not a determinative factor. Furthermore, the same hydrological input is used for each scenario. For comparison of the different cases, average values therefore satisfy. Another limitation concerns the rainfall-runoff processes. Since the model does not incorporate the groundwater hydrology in its calculations, a large component of groundwater flow is not taken into account. In the middle part of the monsoon, this is, however, acceptable. Since the water levels on the river are high, drainage is impeded anyway. Problems also occurred regarding the schematization. For the purpose of shortening the computation time, the grid was reduced to the area of interest. This caused some complications at the downstream borders, where the water has to leave the grid. This mainly affected the downstream portion of the study area. Therefore, the conclusions are based on the parts of the grid that are not affected by these inaccuracies. Furthermore, the schematizations of river-floodplain connections are merely a rough approximation of reality. Due to a limited amount of information the drainage network is much simplified. The model itself also aggravates the flood by artificially raising the water levels on the Atrai. The cause of this is not exactly clear, but it is very likely that an improvement of the schematization would enhance the model. These limitations have led to the consciousness that caution is bidden for the formulation of the conclusions. i Cross sections, slopes, friction etc. ii A so-called Digital Elevation Model (DEM) was available. 4 The study generated several conclusions on how the water system of the Lower Atrai was affected by the FCD measures. From the results of the simulations it can be concluded that the implementation of the FCD polders did not lead to an improvement of the flood situation in 1998. The floodplains were more or less equally flooded for the three scenarios. In addition the embankments obstruct the flood ways at the end of the flood season, causing the flood to last longer than it would in natural conditions. From the moderate 2004 flood some additional conclusions can be drawn. The implementation of FCD projects led to a considerable rise of the water levels on the Atrai. These high water levels cause an aggravation of the flooding conditions during August and September. The results show that the flood extent highly depends on the rainfall. Therefore the embanking of the rivers has not proven to be a very effective measure. On the contrary, when it causes a water level rise that impedes the drainage of the floodplain it has a negative impact on the flood parameters. These conclusions apply in case of relatively high water levels downstream. In these cases the embankments are thus merely a hindrance for the drainage of the floodplains. The central part of the Lower Atrai does not cultivate during the monsoon. This already indicates that the projects are not functioning as they should, since they were implemented in order to enable cultivation during the monsoon. From the evaluated floods, it can be concluded that it seems unwise to start cultivating this land in the monsoon. The following issues are suggested for further research. It would be interesting to investigate the effect of the implementation of the projects on the water system during low to moderate flood years. It would also be possible to improve the model itself, in order to enhance the accuracy of the results. Further calibration for the water levels at Chanchkair and a better description of the network can improve the results on the downstream part of the study area. Since the downstream water levels are important for the severity of the flooding, a better simulation of these water levels would be a valuable addition to the research. The adjustments to the schematization can also involve an improvement of the grid; by extending it and by using a smaller cell size.