Long Wave Problems in Laboratory Models

For the design of harbors, offshore terminals, floating platforms, etc. it is of great importance that the long waves in nature are correctly represented in the physical model, because of the dominant influence of the long waves on mooring forces and slow-drift oscillations. Mostly, the long waves are a function of the grouping of wind waves. The present second order wave generation analysis reveals several long wave terms of practical interest. Various terms dominate in different frequency ranges, and this influences the control of the wave generators. For practical purposes a simple procedure for estimating the group-induced long waves and the necessary amplitude of the generator piston is proposed. These quantities are obviously important for the choice of model scale, since the practical limits of piston stroke, resolution, water depth, etc. must be observed. The long wave errors in traditional model tests with only first order wave generation are investigated. In shallow water model tests the long wave effect tends to be underestimated, while it is exaggerated in deep water tests.