John F. A. Sleath

Turbulent oscillatory flow over rough beds

Velocity measurements are presented for turbulent oscillatory flow over rough beds. Two components of velocity were measured with a laser-Doppler anemometer and the rough beds consisted of a single layer of sand, gravel or pebbles on a flat surface. Turbulence intensities showed significant variation during the course of the cycle. Maximum turbulence intensity propagated out from the bed at a more or less constant velocity for all beds. Variation of time-mean turbulence intensity with height was qualitatively similar to that observed in steady flows. Reynolds stress showed several interesting features. Near the bed, maximum Reynolds stress was in phase with one of the two peaks of turbulence intensity but further out it was in phase with the other, i.e. the phase of maximum Reynolds stress showed a 180° phase shift at a certain height above the bed. A related effect was seen in the time-mean eddy viscosity which was negative near the bed but positive further out. It is suggested that these effects are caused by the jets of fluid associated with vortex formation and ejection in oscillatory flow over rough beds. Maximum Reynolds stress was also significantly less than the horizontal force per unit area of bed obtained from the momentum integral. Eddy viscosity and mixing length were found to vary significantly during the course of the cycle. Variation with height of time-mean values of these variables showed similar trends, except in the near-bed region, to those observed in steady flow but derived values of the Karman constant were significantly lower. Non-dimensional defect velocity appeared to show dependence on a / k s as well as on y /δ in the outer layer away from the bed, even at high Reynolds numbers.

Velocities and shear stresses in wave‐current flows

A model is presented for the velocity distributions in combined wave-current flows over rough beds. The model is based on a new analysis of turbulence measurements in oscillatory flow. The resulting formulas for the velocity distribution are remarkably simple and show good agreement with experiment. The formula derived for friction factor also shows good agreement with experiment.

Mobile layer in oscillatory sheet flow

Measurements are reported of mobile layer thickness and the velocity and concentration distributions within the mobile layer in oscillatory sheet flow. Tests were carried out with sand, PVC, and acrylic granules in an oscillatory water tunnel. Velocities were measured with a laser Doppler anemometer, and concentrations were measured with conductivity probes. The measurements of mobile layer thickness appear to confirm the importance of the parameter S = (ρU o ω/(ρ s -ρ)g). At low values of this parameter the measurements tend toward the unique curve which would be expected if pressure gradient and inertia effects were negligible. On the other hand, the velocity profile measurements show some influence of pressure gradient and inertia even for values of S as low as 0.12. At higher values of S a type of plug flow is observed in which the sediment comes to rest as the flow reverses and then begins to move again as a solid block.

Velocity measurements close to rippled beds in oscillatory flow

The results are reported of velocity measurements in oscillatory flow over rippled beds. Velocities were measured with a laser-doppler anemometer in both an oscillating tray rig and an oscillatory flow water channel. Both self-formed and artificial ripples were examined. In addition, some measurements were made with an apparently plane bed with intense sediment motion. The experimental results were compared with the predictions of Kalkanis (1964), Kajiura (1968), Bakker (1973, 1974) and Sleath (1974). The closest agreement between theory and experiment was obtained with the method of Sleath. Measurements of the Eulerian drift velocities showed drift towards the adjacent ripple crest in the immediate vicinity of the bed and away from the crest further out.

Conditions for plug formation in oscillatory flow

Abstract The conditions under which plug formation may occur in oscillatory flow over beds of sediment are examined. The term plug formation is used to describe the situation where sediment starts to move as a solid block when the flow reverses. It is shown analytically that there is a lower bound on the value of the parameter S = ρU 0 ω /( ρ s − ρ ) g for plug formation. For values of S above this limit plug formation is determined by the degree of compaction of the sediment. It is suggested that the degree of compaction depends on the parameter W 0 / ωδ s where W 0 is the fall velocity of the sediment and δ s is the maximum thickness of the mobile layer. Re-examination of the measurements of Dick and Sleath (1991) and Zala Flores and Sleath (1998) showed that for their acrylic sediment plugs formed when W 0 / ωδ s was less than about 0.12. All of the tests which showed plug formation were in Bagnold’s (1954) ‘viscous’ regime. For these conditions there is good agreement between the measured velocity distribution in the shear layer below the plug and the velocity distribution calculated from Bagnold’s constitutive equations provided an appropriate value of effective viscosity is selected. Comparison of tests in which plug formation occurred with those in which it did not suggests that plug formation leads to much larger values of the effective viscosity in the shear layer close to the bed. For plug flows the measured values of effective viscosity were more than 1000 times those suggested by Bagnold (1956) .

Velocity measurements close to the bed in a wave tank

Measurements of the velocity distribution close to the bed have been made under laminar flow conditions in a wave tank. The classical solution for the velocity distribution was found to be valid when the bed was smooth, but considerable deviations between theory and experiment were observed with beds of sand. It is suggested that these deviations were caused by vortex formation around the grains of sand. The similarity between the velocity profiles obtained in these tests and those reported by other writers under supposedly turbulent conditions suggests that even at high Reynolds numbers vortex formation may continue to be the dominant effect in oscillatory boundary layers of this sort.

Friction coefficients of rippled beds in oscillatory flow

Abstract Dissipation and friction factors have been computed for rippled beds in oscillatory flow using a finite-difference solution of the two-dimensional equation of vorticity. The computations show good agreement with available experimental results and also, at very low Reynolds numbers, with the results of a small-perturbation solution of the vorticity equation.

Sediment transport by waves and currents

A bed-load model, based on a generalization of Bagnolds (1954) constitutive equations together with the assumption of similarity profiles for velocity and concentration, was proposed for sediment transport by waves by Sleath (1994). This model is now extended to wave/current flows and applied to three different situations: (a) oscillatory flows with superimposed currents, (b) net transport produced by progressive waves over a flat horizontal bed, and (c) longshore transport induced by waves approaching a beach. In all three cases the model gives reasonable agreement with experiment. In the case of longshore transport the model produces almost the same formula as that of the Coastal Engineering Research Center (1977), apart from additional terms for density and grain size.

Measurements of lift in oscillatory flow

Mesures de la portance sur une sphere dans un ecoulement oscillatoire a proximite de parois planes et rugueuses

DEPTH OF EROSION UNDER STORM CONDITIONS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.