Mathematical Modelling and Braid Indicators

Abstract

Characteristics of braided rivers and braid indicators are vital for the design of levees and flood control works. These also provide useful information on the formation of islands. This chapter presents a review of braid indicators and develops a hydrodynamic model for the Brahmaputra River, which can be employed to derive a variety of river morphological features. 11.1 Introduction The plan form of a reach of an alluvial river reflects the hydrodynamics of flow within the channel and the associated processes of sediment transport and energy dissipation. Braided streams occur in high energy environments with large and variable discharges carrying heavy sediment loads on steeper gradients. These streams are characterized by a random pattern of multi-thread channel networks owing to the appearance of braid bars within the overall waterway of the river. The presence of braid bars increases the total flow resistance and the energy losses along the boundary, thereby promoting the development of networks of hydraulically inefficient channels. Simulation of flow propagation in a braided stream is fraught with difficulties on account of its complex channel plan form. These difficulties pertain primarily to the representation of the braided waterway of the stream for solution of the governing equations of a full dynamic wave model. The conceptualization of the hydraulic and topological features of a braided channel poses difficulties because of the highly non-uniform and random pattern of variation of the channel conveyance of the sub-channels across the cross section. The most ideal answer to modeling such a physical situation would be to consider the actual braided geometry for solving the three basic equations of water and sediment continuity and the momentum equations. In that case unresolved problems pertaining to correct estimation of flow division in multiple braid channels and their sub-channels with sharply varying conveyances will be encountered. Whilst a considerable amount of work has been done on modelling of alluvial stream in recent decades, the difficulties of modelling braided alluvial channels briefly indicated above await comprehensive solutions from the modellers. Much work has not been done so far specifically for modelling of braided alluvial streams. The formulation of 229 v.P. Singh et al. (eds.), The Brahmaputra Basin Water Resources, 229-260.