Closure of offtakes in Bangladesh

Closure of offtakes in Bangladesh: Causes and assessment of remedial measures

Vila Santamaria, R.

Uijttewaal, W.S.J. (mentor)
de Jongste, A.L. (mentor)
Mosselman, E. (mentor)
Blom, A. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

Civil engineering

2017-01-25

Bangladesh is characterised by its complex river system, which includes the downstream ends of two of the largest rivers in the world: the Ganges and Brahmaputra. Extremely variable flows and fast morphological changes are the principal characteristics of this system, which presents exceptional challenges for river engineering. Distributary rivers from the major rivers in Bangladesh are a key element in providing fresh water to the South-West and Central regions of the country. With the arrival of the dry season and the drop of water levels in the rivers, some of the distributaries become disconnected during several months from their parent rivers because of aggradation at the offtake during the monsoon season. Acquisition of all the field data required for a detailed morphodynamic study of those offtakes is very often not possible, specially for preliminary designs or during the initial stages of a project. For the rivers of Bangladesh, bed topography data are even outdated in a matter of months because of the extremely dynamic nature of those rivers. In order to overcome this lack of data, the approach of this MSc thesis is to use a physics-based morphodynamic numerical model based only on the most significant characteristics of the offtake system to be analysed. If this model correctly reproduces the most important characteristics and evolution of the river and offtake system, it is then possible to use it for an initial assessment of remedial measures to prevent offtake closure. An analysis of the offtake systems is made before setting up the numerical model. The focus of this analysis is on the relevant physical processes for the evolution of offtakes. Furthermore, the causes of offtake closure occurring at the main distributaries of Bangladesh are identified and possible remedial measures are proposed to prevent them. The setup of the model is based on the findings from these previous analyses. The choice is for a 2D depth average numerical model using the software package Delft3D. A schematised geometry roughly based on the Ganges and Gorai rivers and offtake is used, with a total length of 100 km for the parent river and 20 km for the distributary channel. The simulation is initialised with a flat bed and a constant slope, which then develops into a realistic bed topography shaped by the morphological module of the numerical model. Because this model cannot be properly calibrated, as data available are not sufficient, the order of magnitude of model results is compared with observations of the river system. With this comparison, it is found that the model is able to reproduce the behaviour of the river system with enough accuracy to obtain the correct orders of magnitude for the evolution of the offtake system and the effects of interventions. The offtake behaviour simulated by the model is then analysed and used as reference scenario for the assessment of different engineering measures. The results of this assessment show that dredging the upstream part of the distributary river during the recession period of the monsoon can prevent the discontinuation of flow in the distributary river for the entire dry season, and improves the flow conditions for the following year. However, this measure needs to be applied regularly and does not stabilise the offtake in the long term. Other measures tested in the model are major dredging at the parent river to improve its alignment; construction of submerged erodible weirs; a flow divider and the use of longitudinal training walls to reduce the width of the parent river. However, these measures seem not to be effective in maintaining flow in the distributary river when implemented in the numerical model. In conclusion, the methodology developed in this thesis is able to overcome the lack of data availability and simulate realistic scenarios of the complex process of offtake closures. With this tool, it is possible to perform fast and inexpensive modelling of the behaviour of offtake systems and assessment of engineering measures to prevent offtake closure that otherwise would require much more resources.

offtake
Bangladesh
Gorai
Arial Khan
Old Brahmaputra
Dhaleswari
Delft3D
morphology

http://resolver.tudelft.nl/uuid:691dbc0b-a713-4f29-9354-202c713cc4bc

23.95N, 89.11E

Student theses

master thesis

(c) 2017 Vila Santamaria, R.