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1 Department of Geography, University of Nottingham, Nottingham NG7 2RD, UK
2 Sir William Halcrow & Partners, Burderop Park, Swindon SN4 0QD, UK
3 Department of Civil Engineering, Bangladesh University of Engineering & Technology, Dhaka, Bangladesh
The Brahmaputra is one of the world’s greatest rivers, ranking fifth in terms of discharge and eleventh in terms of drainage area. It also has a very high sediment discharge, ranking third in the world. The river is braided with meta-stable islands and nodal reaches, mobile sand bars, shifting anabranches and severe bank erosion. The dominant discharge is about 38 000 cumecs, which is a high in-bank flow. Islands have top elevations that are adjusted to bankfull discharge, with a spacing scaled on the width of the primary channel. Bar top elevations are adjusted to the dominant discharge and bars are scaled on the width of the major anabranches. The dominant bedforms are large dunes scaled on the third order sub-channels of the major anabranches. Sustained right bank erosion has occurred during the last 35 years, with average rates of about 90 m per year. The left bank shows both erosion and accretion that together produce relatively low rates of net movement. Erosion is faster in island reaches and slower in nodal reaches. Recognition of the geomorphic controls of bank erosion allows its spatial distribution to be related to the development of the braided pattern of the river and a tentative hypothesis is proposed to explain recent trends in both bankline movement and the evolution of the channel planform pattern.
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