Flow distribution in meandering compound channels

Abstract

Magnitude of flood prediction is the fundamental for flood warning, determining the development for the present flood- risk areas and the long-term management of rivers. Discharge estimation methods currently employed in river modeling software are based on historic hand calculation formulae such as Chezy’s, Darcy-Weisbatch or Manning’s equation. More recent work has provided significant improvements in understanding and calculation of channel discharge. This ranges from the gaining knowledge to interpretation of the complex flow mechanisms to the advent of computing tools that enable more sophisticated solution techniques. When the flows in natural or man made channel sections exceed the main channel depth, the adjoining floodplains become inundated and carry part of the river discharge. Due to different hydraulic conditions prevailing in the river and floodplain, the mean velocity in the main channel and in the floodplain are different. Just above the bank-full stage, the velocity in main channel is much higher than the floodplain. Therefore the flow in the main channel exerts a pulling or accelerating force on the flow over floodplains, which naturally generates a dragging or retarding force on the flow through the main channel. This leads to the transfer of momentum between the main channel water and that of the floodplain. The interaction effect is very strong at just above bank full stage and decreases with increase in depth of flow over floodplain. The relative “pull” and “drag” of the flow between faster and slower moving sections of a compound section complicates the momentum transfer between them. Failure to understand this process leads to either overestimate or underestimate the discharge leading to the faulty design of channel section. This causes frequent flooding at its lower reaches. Due to transfer of momentum between the subsections of the meandering compound channel, the shear distribution is largely affected. For such compound channels, the apparent shear force at the assumed interface plane gives an insight into the magnitude of flow interaction. The results of some experiments concerning the velocity distribution and the flow distribution in a smooth and rough compound meandering channel of rectangular cross section are presented. The influence of the geometry on velocity and flow distribution and different functional relationships are obtained. Dimensionless parameters are used to form equations representing the velocity distribution and flow distribution between main channel and flood plain subsections. Once these equations get formed one can judge the exact flow in main channel and flood channel sections which could possibly guide in flood prediction. The experiments concerning the flow in simple meander channels and meander channel - floodplain geometry have been conducted at the Fluid Mechanics and Water Resources Engineering Laboratory of the Department Civil Engineering, National Institute of Technology, Rourkela, India. Channels of different shapes and sizes have been fabricated in the laboratory with different equipments installed in them. Water is allowed to flow through these channels and the flow is maintained smooth. The Acoustics Doppler Velocitimeter (ADV) installed in the lab is worth mentioning. Taking the aid of a laptop terminal, this equipment helps in determining the three- dimensional velocities (Vx, Vy, Vz) at any point in the water channel. All the velocity readings obtained are recorded and finally velocity contours (i.e. isovels) are plotted with a software 3D-Field. Depending on the flow pattern and shape of the channel, contours are obtained. All the contours are converted to bitmap image and finally inputted in MATLAB software. Now with this software discharge through a channel cross-section is generated which when compared to the actual flow discharge gives a very less percentage of error. Finally equations related to the flow distribution are formed based on the given datas. These formed equations are validated with datas collected from IIT Kharagpur (Bhattacharya, A. K. (1995) and those from Knight and Demetriou( Knight, D.W., and Demetriou, J.D., (1983) which satisfies them as well.