Effect Of Bed Roughness Research Articles

Statistical properties of sediment bed profiles in alluvial channels

The objective of this study is to investigate in detail the statistical properties of series of bed elevations measured on gravel-bed and sand-bed alluvial channels, in order to identify means of quantifying bed roughness effects on streamflow. The semivariogram is used as the basic statistical method for investigating roughness properties of bed profiles obtained from field work and laboratory experiments. For sand bedforms, the semivariograms include exponential and periodic components from which can be obtained reliable measures of bedform spacing and height, as well as information on the degree of regularity of bedform arrangement. Because of the irregular nature of gravel-bed profiles, the approach in this case uses the semivariogram to investigate fractal properties of series of bed elevations to determine scales of bed roughness associated with grain sizes and small-scale bedforms and to estimate the Hausdorff dimension corresponding to each scale. These superimposed scales of roughness may be responsible for the greater flow resistance generally observed in gravel-bed rivers rather than predicted from the theoretical friction equation.

MEASUREMENTS OF MASS TRANSPORT OVER A ROUGH BED

Measurements have been made with a laser doppler anemometer of the time-mean velocity of the fluid close to the bed in a wave flume. Both a rough bed, consisting of gravel of median diameter 11 mm, and a smooth bed were investigated. With the rough bed the time-mean velocity at a given height was found to be strongly dependent on position relative to prominent roughness elements. At one point the time-mean drift at a given height might be in the direction of wave propagation while, at another, in the opposite direction. Significant variation in time-mean drift with horizontal position was observed at all values of Reynolds number tested. The effect of bed roughness on the average value of the time-mean velocity at a given height was found to be most marked at low Reynolds numbers: the maximum near bed value with this gravel bed was about 3 times that for a smooth bed at the lowest Reynolds numbers tested. At the highest Reynolds numbers there was no clear difference between the rough and smooth bed values even though the boundary layer over the rough bed was fully turbulent whereas that over the smooth bed was laminar. However, at these high Reynolds numbers both the rough and the smooth beds showed a reduction in drift velocity below that predicted by Longuet-Higgins (9) because of the increased importance of higher harmonics in the flow.

NEW FRAMEWORK FOR PREDICTION OF LONGSHORE CURRENTS

The accuracy of prediction of longshore sediment transport depends largely on the accuracy with which the wave-driven longshore currents within the breaker zone can be predicted. Longuet-Higgins (1970) developed a formulation for longshore transport which is widely used today. In the present paper the basic theory of Longuet-Higgins is reexamined. The effect of bed roughness on the magnitude of the longshore current is quantified with the aid of over 350 individual data sets and the theory is theoretically extended to include the effect of random waves, in a similar way to Battjes (1974), and higher-order waves. For this latter purpose the Vocoidal water wave theory of Swart (1978) is used. It is shown that the use of Vocoidal theory leads to a velocity distribution which is in closer correspondence to measured data than that predicted by using linear wave theory.

Interaction between Main Channel and Flood-Plain Flows

The experimental results presented in this paper have demonstrated the transport of longitudinal momentum from the main channel to the flood plain. This transport increases the bed shear stress in the flood plain and produces the opposite result on the main channel. The velocity profiles in the region affected by this interaction have been found to be similar if viewed with respect to the undisturbed flow in the flood plain. The velocity and length scales have been correlated with mainly the ratio of the depth of flow in the main channel to that in the flood plain. In the present experiments, the main channel was narrow and thus the interaction extended inwards to the center line of the main channel. For the concepts introduced in this paper to be used in solving practical problems, further experiments should be conducted with wider main channels including the effects of bed roughness.

Derivation of the velocity profile from a statistical model of turbulence

A statistical model of turbulence for two‐dimensional uniform flow in open channels is developed, and this model is used to derive the vertical velocity profile. This profile is defined by a three‐parameter hyperbolic function, with two parameters reflecting the effect of bed roughness and fluid viscosity on the shape of the profile. The third parameter is the mean velocity in the vertical. The hyperbolic function is fitted to velocity data for water in natural channels, a laboratory flume, and air in a wind tunnel. A brief comparison of the hyperbolic function with the logarithmic function is given.