Resistance to flow and the influence of boundary shear stress on sediment transport in smooth rigid boundary channels
PhD thesis by Mohammadi, M., 1998

Mainly experimental work in V-shaped and specialist drainage channels on bridge decks, aimed at obtaining velocity and boundary shear stress data for such channels. Sediment transport rates were examined for a range of slopes and discharges, seeking optimum ways of satisfying maximum conveyance capacity with also an adequate sediment transport rate. Two flumes were used, a dedicated rig for spatially varied flow, simulating runoff from roads into kerbside drainage units, and a 15m long sediment transport flume set at various slopes and with different drainage units added.


Fully developed turbulent uniform flow characteristics in smooth open channels of V-shaped bottom have been examined experimentally. In order to explore the effect of cross sectional shape on the flow behaviour, and the influence of flow resistance and boundary shear stress on sediment transport in rigid boundary channels, three types of practical bridge drainage channels known as "CIS Bridgeflow II" 300mm and 450mm wide units, and "Glynwed" units, and a specially designed "V-shaped bottom" channel were tested in 15m and 9m long tilting flumes.

The stage-discharge curves were produced for all channels at five channel bed slopes of 0.1%, 0.2%, 0.4%, 0.9% and 1.6% for a number of flow discharges, from which the overall resistance coefficients, n,f, and ks, were analysed. It was found that parabolic type equations of order two and three were the best to construct the stage-discharge relationships, especially for the CIS 450mm and V-shaped bottom channels. It was also found that the channels gave a greater resistance to flow at steep slopes, and less resistance at mild slopes. The rib roughness analysis of Glynwed channel illustrates that the Pavlovskii composite roughness formula is the best among the many composite roughness equations.

In order to show the lateral distribution of depth-averaged velocity within the CIS 300mm and V-shaped bottom channels, the local primary velocities were measured by means of miniature propellers for each relevant flow depth. The measured velocities were then averaged over the depth and the depth-averaged velocities obtained. The velocity distributions together with isovel plots show that the maximum velocity does not necessarily occur at the centreline of the channel. The universal log-law is also less applicable to the present channels, because of the influence of cross sectional shape and secondary currents.

The boundary shear stress was measured using a Preston tube of outer diameter 4.075 mm. The Preston tube readings were converted to the shear stresses using the Patel (1965) calibration equations. In order to check the accuracy, the boundary shear stress data were numerically integrated along the wetted perimeter and compared to the mean energy shear stress ie to = pgRSo. The experimental results were then adjusted to this shear stress and plotted against wetted perimeter. The lateral distributions show that for the mild slope channels they are fairly flat, but that for the steep slope channels this is not the case. The distributions for the latter case point to the role of secondary currents and the influence of cross sectional shape. The percentage shear force carried by the walls, %SFW, was also analysed. Relevant previous experimental data for smooth rectangular, trapezoidal and part-full pipe channels were reassessed and compared with the results obtained from the above set of experiments. The result shows that the %SFW is favourably correlated with the geometry parameter, Pb/Pw. This supports the previous study results. However it was found that the present channels give low %SFW as the aspect ratio, B/h, increases. This again ratifies the cross sectional shape effects. Particular attention is also focused on the effect of Froude number, Fr, and Reynolds number, Re, on the %SFW. The analysis of eddy viscosity shows that the dimensionless eddy viscosity, X, decreases as the flow discharge increases. Using the velocity and; boundary shear stress data, the depth-averaged apparent shear stress and force and local bed friction factors are also analysed.

Sediment transport in loose boundary channels has been widely investigated. However, there is a scarcity of literature regarding the transport of sediment in rigid boundary channels, except possibly for smooth pipe beds. The present study is concerned with an investigation on the initiation of motion of two types of sediment particles in V-shaped bottom channels. The experimental results show that the shape of channel cross section is more effective on the threshold condition of particles than rectangular shaped channels. The results were also compared with the Shields (1936) and Ackers and White (1973) threshold formulae. This comparison shows that the Ackers and White equation gives better results for both sediment sizes than the Shields one.


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