[1] The drag coefficient (C D ) is a fundamental parameter in the determination of shear stress or the drag force between a fixed object and fluid moving over it. In natural settings, conventional methods of defining it are largely impractical and so either smooth-bed C D or constant values are used irrespective of bed roughness or flow strength. This paper deals with the determination of C D over naturally roughened beds. The work was carried out in two annular flumes of known, constant water mass. In an otherwise balanced system, flow deceleration is a manifestation of the total drag force exerted at the rigid boundaries (Newton's second law). The inversion of this relationship is used to yield the bed drag coefficients. The advantages of this method include its accurate use over rough and irregular beds, as shown by experiments over patchy and homogeneous gravel beds and over a wide range of Reynolds numbers. The value of C D was found to converge to the constant value of 3 Â 10 À3 determined by Sternberg [1968] at intermediate velocities, and a reduction in the drag coefficient occurred at high velocities. Results showed that patch spacing did not influence the shear stress value in the case of one-grainthick gravel patches. A modification of the equipment for field use may give advantages where traditional methods fail due to difficulties in obtaining accurate velocity profile measurements.
The effect of suspended sediment concentrations (SSC) on fluid turbulence in an annular flume was investigated. Flow speed was held constant at 0.57 m s À1 , and the resulting turbulent conditions were recorded using a 3-D Acoustic Doppler Velocimeter (ADV) at height (z) of 8.5 cm above the bed. The suspended material was composed of a natural glacial clay made up of particles smaller than 6 lm. The SSC in the flume were increased from clear water to 4800 mg l À1 in nine discrete increments; temporal variations of SSC were monitored using three optical backscatter sensors (OBS) mounted in the flume wall at heights of 0.03, 0.10 and 0.20 m above the flume base. The results showed that turbulent intensity (q 2 =U 2 ) and energy dissipation rate (e) did not change significantly between clear water and 200 mg l À1 , but decreased by nearly 30% in the SSC range between 200 and 2400 mg l À1 . Above 2400 mg l À1 , no further decrease was observed. Analyses of the velocity variances over narrow frequency bands (0.2 Hz wide) from 0 to 12.5 Hz showed that most of the flow turbulent energy ($70-80%) was contained within the lower frequencies i.e. larger eddies, and that these eddies experienced the greatest decrease in energy due to turbidity. It is proposed that these patterns are the consequence of the increase in suspended sediment concentrations and of the vertical stratification of sediments for SSC >200 mg l À1 .
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