Wave run-up along the north shore of Oahu has an annual cycle with a maximum centered on boreal winter. An understanding of the variability of high wave wash is important for coastal planning, transportation, safety, and property protection. Wave run-up increases with increasing surf size and tidal height. This study analyzed hourly historic wave data and predicted tides from 1981 to 2007 to better understand wave run-up potential based on the frequency and duration (in hours) that high surf and tides coincided as categorized by thresholds of surf and tidal height.

The Waimea buoy, located just 5 km offshore, was the primary source for deep-water wave height and period. However, this series only began in 2001. The National Data Buoy Center platform 51001 extends back to 1981. Its location is sufficiently remote from Oahu to warrant a correction in significant wave height. The correction was made based on a regression analysis between daily mean wave heights from these two buoys. From the final, deep-water, nearshore Oahu, hourly series of swell height and period, surf heights were calculated using an empirical transformation scheme.

Thresholds were defined for four surf heights and three tidal elevations. Recurrence was calculated for each of the 12 categories. Duration was computed by summing consecutive hours for each event above each threshold. Historical evidence of sand wash onto select portions of the coastal highway was used to qualitatively rank the 12 categories as marginal, significant, or extreme. One important application of these results would be improvement to surf-related coastal flood forecasts by the National Weather Service in pursuit of protection of life and property. For future design considerations, a joint probability model was constructed to better understand the annual average number of hours exceeding any given paired surf and tidal height.