Straight Compound Research Articles

Resistance studies of overbank flow in rivers with sediment using the flood channel

Some large scale experiments concerning equilibrium sand channels with overbank flow are described. The experiments were undertaken in the UK Flood Channel Facility (FCF) configured into a straight compound channel with fixed banks, a mobile main channel composed of uniform sand with a d50 of around 0.8mm, and two symmetric floodplains. The main channel alluvial resistance changed with flow depth and discharge in a complex way arising from the effects of both bedforms and the floodplain/main channel interaction processes. Overall, zonal and local resistance coefficients for this type of channel are reviewed and the data discussed in relation to practical modelling of alluvial channels with overbank flow.

The influence of planform on flow resistance in mobile bed compound channels

A large-scale test facility, the United Kingdom Flood Channel Facility, has been used to compare observations of a range of overbank flows in straight compound channels and meandering compound channels with similar geometries, floodplain roughnesses and mobile main channel bed material. For straight and meandering channels with smooth floodplains, the single channel Manning's n values were similar, but single channel Manning's n values for the meandering channel with roughened floodplains are approximately 50% greater than those for the straight channel with similarly roughened floodplains. At higher relative depths, Yr > 0·25, the floodplain Manning's n values become similar for both the straight and meandering channels indicating that most of the overbank flow for the meandering channel is in the valley direction at these depths. The meandering main channel with roughened floodplains was found to have the largest mobile bed features.

The influence of planform on flow resistance in mobile bed compound channels

A large-scale test facility, the United Kingdom Flood Channel Facility, has been used to compare observations of a range of overbank flows in straight compound channels and meandering compound channels with similar geometries, floodplain roughnesses and mobile main channel bed material. For straight and meandering channels with smooth floodplains, the single channel Manning's n values were similar, but single channel Manning's n values for the meandering channel with roughened floodplains are approximately 50% greater than those for the straight channel with similarly roughened floodplains. At higher relative depths, Yr > 0·25, the floodplain Manning's n values become similar for both the straight and meandering channels indicating that most of the overbank flow for the meandering channel is in the valley direction at these depths. The meandering main channel with roughened floodplains was found to have the largest mobile bed features.

GEOMETRICAL AND ROUGHNESS EFFECTS ON COMPOUND CHANNEL RESISTANCE.

This paper presents an analysis of resistance coefficients derived from measurements taken in phases A, B and C of the research programme of the UK Flood Channel Facility (FCF). The FCF is a large-scale compound channel facility and the programme has included straight and meandering planforms, smooth and mobile main channel roughnesses and smooth and rod-roughened floodplain boundaries. Floodplain roughness consisted of 25 mm surface-penetrating dowel rods. Data were also obtained for straight trapezoidal channels for comparison by confining flow to the main channels. Stage–discharge relationships were derived which enabled calculation of Manning's roughness coefficient values for a range of selected relative depths for each geometry, planform and roughness case. Graphs are presented of the variation with depth of Manning's coefficient for each geometry and roughness expressed as a ratio of smooth straight compound, or straight trapezoidal channel data. These presentations quantify the increase or decrease of roughness coefficient resulting form cross-sectional geometry, planform and boundary roughness. It is suggested that this represents a possible valid approach to the estimation of Manning's roughness coefficient in compound channels and rivers.

Geometrical and roughness effects on compound channel resistance

This paper presents an analysis of resistance coefficients derived from measurements taken in phases A, B and C of the research programme of the UK Flood Channel Facility (FCF). The FCF is a large-scale compound channel facility and the programme has included straight and meandering planforms, smooth and mobile main channel roughnesses and smooth and rod-roughened floodplain boundaries. Floodplain roughness consisted of 25 mm surface-penetrating dowel rods. Data were also obtained for straight trapezoidal channels for comparison by confining flow to the main channels. Stage–discharge relationships were derived which enabled calculation of Manning's roughness coefficient values for a range of selected relative depths for each geometry, planform and roughness case. Graphs are presented of the variation with depth of Manning's coefficient for each geometry and roughness expressed as a ratio of smooth straight compound, or straight trapezoidal channel data. These presentations quantify the increase or decrease of roughness coefficient resulting form cross-sectional geometry, planform and boundary roughness. It is suggested that this represents a possible valid approach to the estimation of Manning's roughness coefficient in compound channels and rivers.

Generation Mechanism of Secondary Flow in a Straight Compound Open Channel Based on Momentum Equations

Large eddy simulation of fully developed turbulent flow in a compound open channel was carried out using the Smagorinsky model. The Reynolds number based on hydraulic radius and cross sectional averaged velocity is 5300. The mechanism of secondary flow is discussed by evaluating terms in the momentum equations for mean secondary flow. To clarify the important role of pressure gradients in driving the secondary currents, pressure is decomposed into several components that account for the contributions of convection, external force, the Reynolds stresses, and the viscosity. It is found that not only the Reynolds stresses but also the pressure gradients induced by the normal and shear Reynolds stresses play a major role in generating secondary flow.

HYDRAULIC CHARACTERISTICS OF MEANDERING MOBILE BED COMPOUND CHANNELS.

The experimental arrangement for examining the hydraulic characteristics of overbank flows in a meandering mobile bed compound channel at the UK Flood Channel Facility (FCF) is described. The FCF was designed to bridge the gap between small and prototype scales and has been used in a series of experiments on straight compound, skewed and fixed-bed meandering channels. For the present study the sinusoidal meandering main channel had a mobile sand bed. Several overbank flows were measured with smooth or roughened floodplains. Floodplain roughness was provided by surface-penetrating rod elements. It was found that, for floodplain flows with a relative depth >0.2, the average flow velocities over the floodplain, in the valley direction, were greater than in the meandering main channel. The principal main channel bed-forms were repeating long dunes. There was also evidence of plunging flow over the main channel at the apex of bends.

ESTIMATING THE DISCHARGE CAPACITY IN STRAIGHT COMPOUND CHANNELS.

Keywords ADELAIDE UNIVERSITY, ULSTER UNIVERSITY CHANNELS, DISCHARGES, CAPACITY, ESTIMATING, STRAIGHT, COMPOUND, PREDICTION, FLOODS, FLOOD PLAINS, ESTIMATES, VELOCITY, HYDRAULICS, HYDRODYNAMICS, RIVERS, FLOW, DISTRIBUTION, WEIGHTED, DIVIDED, METHODS, ROUGH, ASYMMETRIC, SLOPES, BEDS, WATER... Show All

Discharge prediction in straight compound channels using the mixing length concept

A method for predicting the depth-discharge relationship in a compound channel is developed and applied to two different sets of experimental results. The method uses a mixing length formulation to account for the turbulent interaction between the main channel and the floodplain and the resulting momentum exchange. This momentum transfer tends to reduce the discharge in the main channel and increase the discharge on the floodplain. The net effect is a reduction in the overall discharge capacity of the compound channel. As a result, practical methods which can allow for the interaction effect are needed. In this formulation a variation of Prandtl's mixing length hypothesis is applied to calculate the apparent shear stresses, indicative of the turbulent interaction, on the sides of small vertical elements which comprise the compound channel cross-section. The approach suggested is to use the mixing length approximation to calculate the correction for the momentum interaction effects that are neglected when ...