Wave ‘set-down’ and set-Up

`Set-down' and set-up, the negative and positive changes in mean water level due to the presence of a train of surface waves, was measured in a wave channel. Well outside the break point the experimental results are in good agreement with the theoretical relationship determined by Longuet-Higgins and Stewart. Near the break point, where the wave height is greater than predicted by first-order wave theory, the measured `set-down' was consistently less than theory would predict from the deep water wave height. Inside the break point the bore height was found to be a linear function of the mean water depth. In this region, the gradient of the set-up, , was related to the beach slope tan β and the mean ratio of wave height to water depth by the equation .