Morphological study of Old Brahmaputra offtake using two-dimensional mathematical model

Abstract

Most of the distributaries of major river system of Bangladesh have been silted up. The Old Brahmaputra River is one of the distributaries of the Jamuna River. Nowadays, the mouth of Old Brahmaputra River becomes dying due to serious siltation of sediments in the vicinity of the offtake. The offtake channel conveys a fraction of flow from its original discharge of the main river Jamuna. The maximum diversion ratio also shows a decreasing trend over the years from 1964 to 2011. During lean period, this flow diversion is drastically reduced to below 1% in the last few decades. In order to augment the dry season flow of Old Brahmaputra, a better offtake management is required. In this thesis, a mathematical morphological model has been setup for the study area and simulated for various options. The study area covers 50 km reach of Jamuna River starting from the 25km upstream of the mouth of Old Brahmaputra offtake to 25km downstream of the offtake. It also covers the 20 km reach of Old Brahmaputra River. To setup these morphological model, MIKE21C, an advanced two-dimensional mathematical modelling software developed by DHI, has been applied. The model has been successfully calibrated and validated against the year of 2011 and 2012, respectively. Finally, assessment of morpho-dynamics at high, medium and low flood event of Jamuna for existing and design river bed condition of offtake has been conducted. Four different options have been chosen considering the dredging and river training works for the simulation. The validated model has been run with different types of options. Results of model outputs have been interpreted to understand the outcomes of the options used. The findings from the result suggest that dredging along the left anabranch of Jamuna River (Option 1) does not improve the flow condition of Old Brahmaputra rather than a huge siltation is taken place in front of offtake which impede the flow through the river. Placing of two guide bunds in the mouth of offtake (Option 2) or only dredging in the vicinity of offtake (Option 3) improves the flow condition relatively. About 200 m3/s discharge is available during the month of November for Option 3 whereas it was almost null for base condition. Hence, the percentage of flow for Option 3 varies from 2% to 2.4% during November, while it is less than 2% for Option 2. Again, the connection between two rivers is closed at the end of November for Option 2 whereas it sustains another one month for Option 3. The last option which is the combination of guide bund and dredging (Option 4) provide good results in terms of flow condition. Dredging make the river deeper and confined and guide bunds help to generate less siltation near the mouth of offtake. It is seen that xviii approximately 250 m3/s discharge is available at the end of November if Option 4 is implemented for average flood event condition. In addition, about 32m3/s of flow can be available in the Old Brahmaputra during the lean period for this option. Therefore, it can be suggest that combination of guide bund at the mouth of offtake and the dredging (Option 4) is the most favorable and sustainable engineering solution of the offtake management.