Flow velocities and bed shear stresses in a stone cover under an oscillatory flow

I order to get a better understanding of the interaction between the wave-induced, near-bed oscillatory flow, the stome cover and the sea bed, physical model tests were carried out. The tests were conducted in an oscillating water tunnel. The bottom of the tunnes was covered by one, two and three layers of stones.

The flow velocities in the pores of the stones were measured using LDA (Laser Doppler Anemometer). In addition to the velocity measurements, the bed shear stresses were also measured using a hotfilm (Constant Temperature Anemometry). It is found that the boundary layer of the outer flow penetrates quite a substantial amount of thickness into the stone cover, deoending on the flow conditions above.

Below this level the velocity remains constant. The level of turbulence in between the stones is found to be very high: 3 to 4 times higher than turbulence level over sipple covered bed in steady current boundary layer without any externally generated turbulence. The bd shear stress is found to be very low, more than ten times smaller than in the case of a smooth base bottom without stone cover.