Mathematical modelling study on river improvement of old brahmaputra system

Abstract

This study deals with the Old Brahmaputra system in terms of flow augmentation using two-dimensional morphological model, studying the present condition along with the options like dredging, river training works, etc.The study area covers 45 km reach of Jamuna River starting from 20 km upstream of the mouth of Old Brahmaputra to 25 km downstream of it, 225 km of Old Brahmaputra River (divided into 8 different reaches) starting from its offtake to 13 km downstream near Toke, 10 km of Jhenai, 9 km of Banar (Upper), 12 km of Sutia and 20 km of Lakhya River starting from their respective offtake through downstream direction. To setup these hydro-morphological models, MIKE21C, an advanced two-dimensional mathematical modelling software developed by DHI, has been applied. The models have been successfully calibrated and validated against the year of 2011 and 2012, respectively. Finally, assessment of morpho-dynamics at high, medium and low flood events of Jamuna for existing and design river bed condition of Old Brahmaputra system has been conducted. To augment the flow through Old Brahmaputra from the Jamuna, two different options have been tested. One option contains dredging in the Jamuna and Old Brahmaputra River along with placing two guide bunds, and another option contains dredging in the Old Brahmaputra along with placing two regulators in a new channel connecting the Jamuna and Old Brahmaputra. It has found that in Option 1 maximum amount of flow passes through the Old Brahmaputra during monsoon and dry season flow. Maximum of about 2965 m3/s flow has been found for high flood event. For medium flood event maximum discharge increases from 960 m3/s to 1785 m3/s and in the dry period about 20 m3/s flow passes for low flow condition. Rate of Sedimentation has been found lower in Option 1 compared to Option 2. To improve the flow condition through Jhenai River four different scenarios have been considered. Increasing the upstream flow of Old Brahmaputra River cannot improve the dry season flow of Jhenai. Dredging in the Jhenai and Old Brahmaputra near mouth of Jhenai also shows little improvement of Jhenai river flow. On the other hand, when the left anabranch of the Old Brahmaputra has been closed, it gives sufficient flow (peak flow increases from 290 m3/s to 625 m3/s during monsoon and dry season flow increases from zero discharge to 25 m3/s) through the Jhenai River till dry period. Planform analysis also shows that in Scenario 4, the right channel of Old Brahmaputra River is stable. Total volume of erosion and deposition along with rate of sediment transport is less in this scenario compared with other scenarios. Sufficient water depth has been found in the Old Brahmaputra and Jhenai River with this scenario condition. For Banar (upper) River three different scenarios have been tested for keeping this river connected with the Old Brahmaputra and active. Analyzing the planform, flow diversion, sediment transport rate and water depth, it is found that dredging in the Banar (upper) River and incorporating a loopcut in the Old Brahmaputra at immediate upstream of Banar (upper) shows the most suitable condition. About 320 m3/s flow is found in the monsoon for high flood condition and in the dry period 7 m3/s flow diverts for low flow condition. To improve the flow condition through Sutia, three different scenarios have been considered. Among thoseScenario 3 has been chosen considering new mouth of Sutia at about 3.5km upstream connecting with the Old Brahmaputra River. This scenario shows that the magnitude of flow through Sutia has increased significantly (160 m3/s during high flood event) and till November this river carries flow. For Old Brahmaputra (lower) dredging has been done considering three different lengths along with placing two guide bunds. Analyzing the discharge through Old Brahmaputra (lower) it has been found that for 15km dredging about 40% flow of Old Brahmaputra River passes through this channel during monsoon for high and medium flooding conditions. For low flow condition about 25% flow passes through this channel during monsoon and the flow continues upto January. Analyzing the water depths at different time intervals it has been observed that with the Option/scenario conditions water depth has increased in the Old Brahmaputra River from its offtake to bifurcation of Lakhya and Old Brahmaputra (lower). On average atleast 2.5m, 1.5m and 1m water depth has been found throughout the river at the end of September, November and February, respectively.