Prospects of Modeling and Morpho-dynamic Study for Brahmaputra River

Abstract

The Brahmaputra River in the Northeastern State of Assam in India is characterized by its exceedingly high discharge with enormous sediment load, spatio-temporal variation in channel morphology, substantially high bed aggradations, and severe bank line erosion. The river maintains almost continuous channel braiding in most of its segments in the alluvial floodplains of Assam. One-dimensional (1-D) flow models are insufficient to tackle problems of braided streams due to lack of information with regard to transverse flow field. Hence, for better and more realistic flow field assessment, two-dimensional (2-D) or three-dimensional (3-D) numerical models are to be used. 3-D models are numerically too expensive for macro scale river reaches. Hence, 2-D enhanced model with secondary flow corrections in governing equations may be used. After successful implementation of 1-D mathematical model for a braided stretch of Brahmaputra River, 2-D enhanced numerical model with boundary-fitted coordinate system for the same stretch has been developed and verified. A new planform index is proposed namely braid power. It increases with decrease in incoming discharge into the reach in a particular instance of time. The measured braiding indicator, namely, Plan Form Index (PFI) using high-resolution satellite data application, is found to be quite useful for the monitoring and analysis of persistent and complex braiding behavior of a large river like the Brahmaputra. The erosion study based on satellite data based analysis conclusively identifies three to four major geological channel nodal points present along the Brahmaputra River. The variability of stream power with bank erosion and braiding process is investigated. A distinct behavioral pattern between these is observed. For example, with low stream power, braiding appears to intensify which in result may reflect a higher possibility of bank erosion.