Hydraulic modeling for guide bund of rail cum road bridge across river Brahmaputra (Assam)

The paper describes in detail about the physical model studies conducted at Irrigation Research Institute, Roorkee for siting of Rail cum Road Bridge at Bogibil near Dibrugarh (Assam) across river Brahmaputra. The bridge being the longest bridge of its kind in India, connects Dhemaji district to Dibrugarh. Initially it was designed for circular cum straight upstream guide bund and straight downstream one at a flood discharge of 73000 m³/s. Experiments were performed first time in 1998 in the physical model constructed on a Horizontal scale of 1:400 and a vertical scale of 1:50. The performance of circular cum straight guide bunds was not satisfactory in terms of separation of flow near the apex. In due course, model experiments have been continued till present on almost yearly basis to study the effect of post monsoon situation & the responsive behavior of manmade structures i.e. Guide bund, spurs etc. under construction. Based on the outcomes of experiments modifications in original design have been suggested with a suitable bridge length of 4250 m having 34 spans of 125m each with elliptical Guide bund in u/s and straight one in d/s with circular mole head. In addition, in the light of significant changes observed in the topography of river Brahmaputra in post monsoon 2008, the north bund has been proposed to be shifted by 625m i.e 5 no spans towards the north bank with a total bridge length of 4875 m (39 spans). Thus, complex river behavior which keeps on changing with a disturbance in river regime needs to be investigated over the years during construction phase by physical model studies.