Field and Laboratory Measurements of Wave-Bottom Interaction

Abstract

Field observations were carried out at Lake George, New South Wales and laboratory experiments were conducted in a wave flume and water tank at the Australian Defence Force Academy, Canberra. Measurements in the bottom boundary layer were performed by means of acoustic and laser Doppler velocimeters, as well as by means of a specially designed shear plate capable of measuring the bottom stresses directly.

In the laboratory studies, for bed conditions ranging from smooth to rough and for JONSWAP-like wave spectra, it was found that, in terms of the loss of wave energy due to the bottom friction, the spectrum can be represented by a monochromatic wave, the bottom velocity amplitude of which is equal to 1.88urms and with a period equal to the spectral peak period. Also, two sets of laboratory experiments were conducted to study conditions of ripple formation on a silt-covered bottom and to study the effects of direct injection of turbulence into the bottom boundary layer by breaking waves.

The bottom interaction term was estimated for Lake George silt-bottom conditions, and the estimates were independently verified by means of comparisons with other energy sources and sinks, simultaneously measured, which together had to satisfy the energy balance. The bottom dissipation term was found to be an important part of the total energy balance in the finite depth wave field, measuring up to 20% of the total dissipation in extreme wave cases.