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A new technique has been used to make field measurements of wave-induced set-up. The technique is reliable and accurate, and the results presented show the variation of mean set-up at the shoreline with incident wave height. The amount of set-up is well predicted by a linear function of incident wave height and is found to be in good agreement with theoretical predictions based on the observed decay of wave height across the surf zone. The maximum set-up observed was about 40 cm. mean water level. The slope in mean water level (dq/dx) is proportional to the bottom slope, with the constant of proportionality depending on the ratio of wave height to depth inside the surf zone. Bowen et al. [1968] proposed that H is limited to a constant proportion •, of the total mean water depth h+q (rather than simply the still-water depth h), so that H h+• -Y (1) The ratio of set-up slope to bottom slope is then given by d• K'-'• K ' 3 •'=-(h + •) = •.•, 2 (2) or d-•---K ---K= 1 + dx -dx The results of Longuet-Higgins and Stewart [1962] outside the surf zone and Bowen et al. [1968] inside the surf zone were combined by Battjes [1974, p. 58], who deduced that 1Now at Proudman Oceanographic Laboratory, Bidston Observatory, Bidston, Birkenhead, Merseyside L43 7RA, UK. Here, tim denotes the maximum set-up, i.e., the set-up at the shoreline. It is thus possible to predict the mean set-up at the shoreline from simple theory if the wave height immediately outside the surf zone and the ratio of height to depth inside the surf zone are both known. Laboratory experiments using monochromatic incident 22,289 22,290 KING ET AL.: WAVE-INDUCED SET-UP ON A NATURAL BEACH 22,294 KING Er AL.: WAVE-INDUCED SEt-UP ON A NATURAL BEACH 0.3

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