Ponnambalam Rameshwaran

Quasi two-dimensional model for straight overbank flows through emergent vegetation on floodplains

The paper presents a quasi two-dimensional (2D) model calculating depth-averaged velocity and bed shear stress in a straight compound channel with a vegetated floodplain. The model numerically solves the depth-averaged Navier–Stokes equation for the streamwise motion of flow (quasi 2D). Reduction in volume of water due to vegetation is modelled as porosity. Drag force due to vegetation is modelled as an additional momentum sink term in the Navier–Stokes equation. Estimation methods for model parameters—drag coefficient, shading factor, porosity, friction factor, eddy viscosity and advection term—are discussed. The predictive capability of the model is assessed against experimental data with regard to distributions of depth-averaged velocity, bed shear stress, transverse shear stress and stage-discharge. The results show that the quasi 2D model reproduces a reasonable simulation of the flow field.

The effect of floodplain roughness on flow structures, bedforms and sediment transport rates in meandering channels with overbank flows: Part I

Studies were carried out to understand the effect of floodplain roughness on flow structures, sediment transport rates and bedforms in a mobile meandering channel with overbank flows. Three floodplain roughnesses were examined in this study. Flow structures and bedforms were measured using a three component laser Doppler anemometer (LDA) system and digital photogrammetry, respectively. Comparisons of flow structures and bedforms between different floodplain rougheness are made. Considerable changes in the flow structure and bed form were observed. In particular, multiple secondary flow cells along the meandering channel occur at deeper water depths as the floodplain roughness increases. These cells also cause a series of wavy bedforms along the meandering channel. This paper is separated into two parts. Part I concentrates, in detail, on the flow structure with bedforms in the meandering channel for overbank flow as to change of floodplain roughness. The sister paper, Part II, concentrates on bed form formation during flood, sediment transport rates and flow resistance in the main channel and floodplain.

Flow characteristics in meandering channels with non-mobile

Experiments were conducted in meandering channels with non-mobile and mobile beds to measure flow rates, velocities, turbulent kinetic energies, bedforms and sediment transport rates for overbank flows. The behaviour of bedform in meandering channels with overbank flows was observed using digital photogrammetry, with velocity measurements taken with a Laser Doppler Anemometer. The bedform structure and velocity distributions along the meandering channel were obtained for bank full flow and three overbank flow depths. Important interactions between the flow structure and bedform were observed along the meandering channel. The sediment transport rates collected during the experiment showed three phases; an increase in the sediment transport rate up to the bankfull level, a small decrease as the flow goes overbank up to a relative depth ratio of 0.3 and then an increase again for higher flow depths. The regions of higher turbulent kinetic energy were identified. The total energy losses due to friction, secondary flow and interfacial turbulence in the lower layer flow of the main channel were compared in both the non-mobile and mobile bed cases.

Modelling vegetation effects in irregular meandering river

A numerical investigation of vegetation effects on the hydrodynamics of the Blackwater River, Hampshire, UK, is presented. The computational code Telemac-2D was applied to simulate a flood event in an irregular meandering reach. Velocities were measured at three cross-sections using Acoustic Doppler Current Profiler and Acoustic Doppler Velocimeter. With these, the friction factors for the main channel and the floodplain were calibrated. A uniform Manning coefficient method and an emergent vegetal drag force method to specify flow resistance of vegetation on the floodplain were used to compare flow simulations. An attempt was made to assess the accuracy of the constant viscosity model, the Elder model and the k-ϵ model. Simulated hydraulic parameters were used to assess the boundary shear stress. The vegetation greatly influenced flow behaviour, reducing both velocity and boundary shear stress as the drag force increases. These results can guide practising engineers in estimating flood conveyance and improve the understanding of bed shear behaviour in rivers with vegetated floodplains.

PIV and LDA Measurements of Secondary Flow in a Meandering Channel for Overbank Flow

Secondary flow in a compound meandering channel with straight floodplain banks for overbank was investigated by a visualization method and velocity measurement using three-component laser Doppler anemometor (LDA). The secondary flow in a cross section was visualized by the neutral buoyant tracer method with a submergible video camera. Secondary flow vectors in a cross section were obtained by using PIV software with captured frames from video source through PC and also by LDA measurements. From the comparison of the PIV and LDA results, it is found that PIV data show good agreement in quality with LDA measurements when the secondary flow is strong and stable as shown in this paper.