Flow Modelling of Straight and Meandering Compound Channels

Abstract

This Ph.D. research analyses the flow in simple meandering and meandering compound channels of various sinuosities between 1.0 to 4.11. The main channel aspect ratio is of more than 5.0, which is very significant in the field of river hydraulics. Experiments are conducted in the Fluid Mechanics and Hydraulics laboratory of National Institute of Technology, Rourkela, India by casting smooth and rigid, straight and meandering compound channels inside a large tilting flume. The research investigates the distribution of longitudinal velocity; depth averaged velocity; boundary shear stress from inbank to overbank flow cases of series of meandering compound channels.
From the study of isovels of simple meandering and meandering compound channels and straight compound channel with vegetation, velocity distributions are measured and mathematical models are suggested for Roughness coefficient and velocity distribution. Validation of the developed models have been done with present data sets and data sets from previous researchers. New models for subsection discharge are also suggested for meandering compound channels. The mathematical models of stage–discharge relations for simple meandering and meandering compound channels having different width ratios are developed. These developed models are validated with the experimental data as well as data sets of other investigators; modelling and validations has been done by both small-scale data sets and with many real field data sets of river during flood.
The research also analyses the flow in a vegetated compound channel using series of trees, in the Fluid Mechanics and Hydraulics laboratory of Loughborough University, UK. Here, nine model trees are made of fabric using a 3D printer and are placed at an equal distance at the edge and centre of floodplain. Projected areas of the trees are obtained using image analysis. A force-measuring device is designed to measure the drag force exerted on the trees. Flow velocities in front of a tree are measured using an Acoustic Doppler Velocimeter (ADV). The drag force and its coefficient are analysed for trees in series of trees keeping both at the edge of the floodplain and at the centre of the floodplain of a compound channel. An approach velocity to the object is measured using Acoustic Doppler Velocimetry (ADV). The recovery velocity and turbulent intensity behind the object are also measured to understand turbulence in wake. A new estimation method of project area of the object using a photogrammetry technique is introduced. The measured drag force and the estimated project area and measured velocity distributions over water depth are used to calculate the drag coefficient. The influence of velocity distribution on the drag coefficient is demonstrated. The development of shear layer along the trees is observed and the behaviour of the drag force is also revealed. The drag coefficient is then estimated with the detailed measured velocity and small project areas within the tree. Three different tree distances are examined with two different places, as a result, the drag coefficient are found to varies with the tree distance so a drag correction factor are newly introduced which also varies with the tree spacing. The drag coefficient, Cd, of a tree amongst the trees are estimated by adopting different cases of approach velocity. The ratio of Cd obtained using mean velocity to the Cd from local velocity is found to be less than 1 for a tree in the shear layer due to wakes. It is greater than 1.0 for no tree in upstream condition. The Cd value of a tree in the developing shear layer are found to be increasing with the number of trees upstream till the 5th tree and became a constant. The drag coefficients are modelled and compared well with the exiting empirical formulae in the literature.