With a view to explaining the phenomena of sediment transport in the open sea, outside the wave breaking area, the author carried out a laboratory investigation of wave action on a horizontal bed. He puts forward a number of new results regarding :
1 - The state of turbulence near the bed and the stability of the oscillatory laminar boundary layer.
2 - The setting in notion of materials under the influence of wave alone.
3 - The entrapment current caused by wave action close to the bed.
4 - The transport of material under wave action only.
5 - The indirect action of wave on the bed.
The main conclusions reached are as follows :
1/ - The results given by Kuon Li regarding the onset of turbulence within the oscillatory boundary layer overestimate the range of laminar conditions. Vo (maximum orbital velocity) and e (roughness) are the principle factors governing the transition. Test waves are either generally laminar, or are only slightly turbulent within the body of liquid, but they are, however, more often turbulent in the immediate neighbourhood of the bed.
2/ - The Investigation of conditions for the onset of grain movement of the bed material shows that the action of wave can be appreciable, even at depths of several tens of metres. A wave of 6 metres amplitude, with a total length of 120 metres, would be capable of putting a 0.3 mm sand grain into motion at a depth of 60 metres.
3/ - The experimental investigation, as well as the viscous fluid theory, shows the existence, close to the bed, of an entrainment current of liquid particles which always works In the direction of wave propagation.
4/ - In test flumes, this entrainment current forms part of a mass transport within the liquid, the vertical distribution of which varies with the characteristics of the fluid motion. On a horizontal bed, It generally gives rise to an effective sediment transport, in the direction of wave propagation, as the preponderant part of the liquid velocity component, near the bed. is in this direction.
5/ - Owing to the existence of the pass transport current and the onset of suspension of material above the bed, some sediment transport can exist out to sea. These results give an explanation of why, under the action of long and regular wave . material tends to be carried in the direction of the waves and build up on the beach whereas, under storm conditions, a strong resultant turbulence produces suspension and favours erosion of the beach.
6/ - On a sloping bed, transport towards the shore is counterbalanced by the effect of gravity, currents caused by winds from seaward and density currents set up in the wave break area so that finally material eroded from land surfaces are, In part, gradually carried away towards the open sea.
Velocity Structure of Density Currents Propagating over Rough Beds
