NEUROMORPHIC STUDY OF. LONGITUDINAL PROFILE OF THE RIVER 'SRAHMAPUTRA

Abstract

The river Brahmaputra is well known to the world of river hydrologist, geologist and the researchers for its third rank in sediment transportation, fifth in fluvial discharge and eleventh in the size of the drainage basin area. The morphological features of the river Brahmaputra is characterised by highly braiding river channels and higher slopes and ever changing trend due to the high energy environment of the fluvial processes. Unlike in the single channel river in fine alluvium bed for the complexity involved in highly braided channels and higher slopes it has been• a formidable task `'in establishing of interdependencies among the resistance, velocities and other hydraulic parameters due to highly non linearity observed in the phenomena. The river morphological indices: Plan form index, Braiding index, Flow geometry index, sinuosity, -concavity index and slopes constitute the major parameters to describe the morphological aspects. In the present work, basic emphasis has been on the study of the longitudinal profile and other associated morphological parameters. The study area stretch over the length of 622.37km of the river Brahmaputra in the Assam valley of India. The study is based on the hydrographic data (river cross sections) collected over the span of 40 years from 1957 to 1997 comprising seven discrete data years. The foremost strategy of the study has been an application of a soft computing technique in Neuromorphic environment well known as Artificial Neural Network (ANN). The ANN models so developed were applied to generate all the intermediate average bed levels. for entire study period from 1957 to 2005. The observed variations of the average bed levels with the ANN predicted data base have quantified the aggradations and degradations in every single reach between the cross sections and has shown that aggradation phenomena is the most pronounced trend in the later periods, 1985. to 2005. In the three segments of the total length, the middle reach is observed to be more susceptible to degradations due to joining of the tributaries to main stream of the river Brahmaputra in this reach. The slope of the longitudinal profile has been varying from flatter in lower reaches to steeper in upper reach. Segment wise, the upper and the lower reaches have been observed to be decreasing trend in the variation of their slopes but the middle reach shows increasing trend in this respect. The concavity of the river valley profile of the study length has been observed to best fit a second degree polynomial and an exponential curve with Rz value above 0.98. The unsteady perceived in the concavity index value and the total longitudinal slope have been reflected to the plummeting stream power from 1985 to 2005 which has best described the aggradations in the majority of river length of the study reach. The results of the study however, suggest that the morphological changes in the present study length of the river Brahmaputra could be understood as responding to a cyclic hydrological phenomena and maintains its fluvial capacity. IV