Wave impact tests were conducted at two different scales within the Sloshel Joint Industry Project. In 2009 unidirectional breaking waves were generated in a flume at scale 1:6. The waves impacted on an instrumented wall. These tests were repeated at scale 1:1 in 2010. The piston motions were scaled by maintaining the Froude number. The compressibility of the liquid and the gas, and other properties such as the surface tension of the liquid were therefore equal at both scales. The measured impact pressures at scale 1:6 and 1:1 are compared deterministically in this paper to define by how much the Froudescaling of the impact pressures is biased by the fact that the properties of the liquid and the gas are not scaled. This deterministic comparison requires that the global flow at scale 1:1 starting from the wave piston to the instant just before the first contact with the wall is Froude-similar to the flow at scale 1:6. Only then the differences between the measured pressures at the two scales can entirely be attributed to the fact that the properties of the liquid and the gas were not scaled. Froude-similar global flows were obtained for these tests by carefully controlling the piston motion and the water depth at both scales and by minimising the effect of the wind at full scale. The comparison of the impact pressures for the Froude-similar global flows shows that the loading processes ‘building jets along the wall from the impact area’ and ‘compression of entrapped air’ are not Froude-similar when the compressibility of the gas is not scaled. The one-dimensional model of Bagnold (1939) is used to correct the loading process ‘compression of entrapped air’ measured at scale 1:6, resulting in a similar load to the one measured at scale 1:1.

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