Hydrodynamic and morphological analyses of new Dhaleswari river offtake using mathematical model

Abstract

Bangladesh is a riverine delta formed largely by the alluvial flood plains of the three major rivers of Ganges-Brahmaputra-Meghna (GBM). These river systems carry huge sediment loads mostly originated from transboundary river basins. The distributaries of GBM river system have been silting up and the hydraulic conditions are disturbed. As for example, the New Dhaleswari river is one of the distributaries of the Jamuna river. Now-a-day, the mouth of this river becomes dying up due to serious siltation in the vicinity of the offtake. The offtake channel conveys a fraction of flow from it’s discharge of the main river Jamuna. In order to increase the dry season flow of New Dhaleswari, a better offtake management is necessary to feed the downstream part of the river. This study deals with the hydrodynamic and morphological analysis of river offtake of New Dhaleswari using mathematical model. Model has been setup for the study area and simulated for various options. The study area covers 40 km reach of Jamuna river starting from the 22 km upstream of the mouth of New Dhaleswari offtake to 18 km downstream. In model setup, it also covers the 30 km reach of New Dhaleswari River. Morphological analysis i.e. planform analysis, cross section analysis have also been carried out using historical images and cross-sectional data collected from Bangladesh Water Development Board (BWDB). Two-dimensional mathematical model MIKE21C has also been applied for the study. The model has been successfully calibrated and validated against the data for the year of 2015 and 2012 respectively. Simulation runs have been made with three flow conditions such as at high (1998), medium (2005) and low (2001) flood event of Jamuna and New Dhaleswari. For each flow conditions nine different options have been considered for model run. The options are mainly include river training works (Guided banks), inclusion of silt trap, weir and dredging work. Descriptions and outcome of various options have been discussed and presented in this thesis. Thus, a total of 42 model runs with various options have been carried out. Model runs with option-9 produces the amount of flow diversion 3.08% in the monsoon for medium flood event. Here the maximum discharge is 1731 m3/s for medium flood event and net bed degradation is 6.20 Mm3 for 5 km reach of New Dhaleswari River. The model results reveal that, among the various options, the Option-9 which describes the placement of a silt trap, river training works at offtake, a weir near offtake and 50m bed dredging is found to be more favorable way for this offtake management.