Study on siltation at the intake reach of the old Brahmaputra River

Abstract

A study has been conducted to investigate the siltation at the Intake reach of the Old Brahmaputra River. For this purpose, satellite images covering the period from 1973 to 2007, cross-sectional data around the mouth, water level, and discharge data of Old Brahmaputra River covering the period from 1973 to 2005 were collected. In addition, sediment discharge data of Old Brahmaputra River and Brahmaputra River were collected from 1988 to 1993. The length of the Intake Reach was considered 26 km which covers five BWDB transects. For calculation of width the Intake reach was divided into sixteen sections and for calculation of meandering parameter at Intake reach thirteen bends were selected. In order to obtain, superimposition of satellite images of Old Brahmaputra River from 1973 to 2007, digitization of Intake Reach was performed with the help of Arc View software. As a result, superimposition of satellite images shows that the change of plan form of this river is significant. The maximum migration rate the mouth of the Old Brahmaputra River found during 1973-1984. The average migration rate is 157.71 m/yr. The overall trend line of Northing is increasing and Easting is decreasing, which indicates that the mouth of the Old Brahmaputra River is shifting towards north-west ward. It is also visualized that the river has become comparatively narrower. From 1973 to 2007 both narrowing and widening of the channel have been observed in all sections but ultimately channel narrowing process appears to be a dominant process in the area. The average widening of the Intake reach for the last 34 years is 36.49 m/yr and narrowing is 44.16 m/yr. Increase in sinuosity was associated with larger amount of land erosion in the bends. Both the banks of the Old Brahmaputra river are dynamic but the frequency of bank line movements were different at different sections due to erosion and accretion along the respective reaches The maximum migration from left bank was 2149.29 m at section-15 during 1973-1999 and minimum migration was 33.14 m at section-9 during 1973-2002. The maximum migration from right bank was 2297.12 m at section-4 during 1973-1995 and minimum was 30.55m at section-11 during 1973-1995. Considering variation in cross-sectional area it can be found that for Intake reach sedimentation is portrayed in the period 1996 to 2005 (1284.04 m2) and for the remaining cross sections of the river, sedimentation is portrayed in the period 1996 to xiv 2005 (8137.57 m2). Sedimentation dominates over a period of nine years (1996-2005) by raising of mean bed level of about 2.10 m. The movement of thalweg in the vertical direction is very dynamic over the study years. From 1988 the water level of Old Brahmaputra River at Kholabarichar station stops falling at a certain water level of Jamuna River at Bahadurabad station, which may change from year to year. This is the indication of closure of Old Brahmaputra mouth. In 1973, percentage of Jamuan River flow to the Old Brahmaputra River was 6.69% and in 2007 this percentage of flow is reduced to 0.70%. This reduction of flow reveals the siltation problem at the Intake reach of the Old Brahmaputra River.