Why Do Surf Heights in Hawaii Vary So Greatly From Place to Place and Moment to Moment?

This is an educational outreach product intended for a general audience. Feedback is welcome.

Tagging numbers to surf heights for a given day along a shoreline facing a similar direction is a daunting task because the heights vary so much from place to place and in time. This web page explains why surf heights vary spatially and temporally. Using this information, explicit definitions are given to surf heights as used in forecasts.

Chuns, Jockos, and Holtons Reefs, N.Shore, Oahu (courtesy of Univ. of Hawaii Coastal Geology Group)

Waves 101 Background Information

One must first understand the basic definitions of wave characteristics and the different words used to describe the life cycle of wind-generated water waves.

Spatial Variability

Surf heights vary from location to location, especially in Hawaii where coasts face at various orientations, the sea floor shape is complex, and upstream islands cause shadowing, dependent upon incident swell direction and period. Transformation of wave height from deep to shallow water waves, then on to breakers, is caused by various physical phenomena. The most important are:

There are other factors that affect surf height such as localized currents and winds, and wave-wave interaction (e.g. backwash, which happens when a wave reflected off the shore heads seaward and meets an incoming breaker, or double-ups, which means crests of two waves of differing wave period or direction arrive simultaneously at the same location). These are not considered in this review.

Scientists have models, such as the Simulating Waves Nearshore (SWAN), that can help visualize the primary physical actions of shoaling and refraction. As the surf grows, the location of the breakers occurs in deeper water, shifting to the outer reefs for the north shore of Oahu. Waimea Bay, which has some of the largest waves closest to shore, is roughly 30% lower than in zones of maximum refraction on outer reefs, such as Outer Log Cabins.

Surf height can also vary along the front of any single wave. In zones of high refraction, the difference between the peak face and wave shoulder is more exaggerated.

Temporal Variability

At a fixed surf zone location, wave heights vary with time. For an individual wave, the height is greatest at the moment of maximum cresting just before the top portion of the wave falls forward, excluding wave heights associated with backwash or double-ups. Over a period of time, the heights of waves follow a Rayleigh distribution. Thus the most numerous wave heights are less than the average wave height. In oceanography, statistics are used to define frequency-of-occurrence parameters related to waves:

Waimea Bay (photo: Kimbal Milikan, Dept. of Oceanography, University of Hawaii)

Given any single one of these parameters, one can calculate the others since they differ by a multiplicative constant as defined by the Rayleigh Distribution.

An application of the Rayleigh Distribution can be seen in the way visual surf observations are made. Reports emphasize the smaller percent of larger waves and are given in a range which estimates the heights of commonly arriving sets with the arrival frequency decreasing as the value within the range increases. Nominally, these observations represent the H1/3 to H1/10, occasionally H1/100 heights.

The amount of wave energy arriving along a shore also has longer scale variations on the order of 30 minutes to a few hours. These groups of groups are referred to as wave envelopes. The number of waves per set and the spacing between wave envelopes is related to the width and proximity of the wave generating zone, with wider and closer sources making more frequent arrivals.

NOAA NWS/NESDIS Collaborative Surf Forecast Table

The Honolulu Forecast Office of the NOAA National Weather Service (NWS), in collaboration with the NOAA National Environmental Satellite and Information Service (NESDIS) Hawaii/Pacific Islands Liaison Office produces a 5-day nearshore swell, surf, and wind forecast table and discussion.

Background

Extensive research was undertaken to seek the most accurate surf forecasts, based on historical north shore Oahu visual surf observations and buoy data. The daily surf reports are made by experienced observers of the H1/3 to H1/10 breakers during the most active period of the daylight hours in the zones of highest refraction, such as Sunset Beach. For days when outer reefs have the zones of highest refraction, Waimea Bay is used as the observing location since breakers are closer to shore and surfers serve as height benchmarks. The H1/10 records were assembled by Mr. Larry Goddard (1968-1987) and Mr. Patrick Caldwell (1987-present). Since historic reports have been made in the colloquial Hawaii scale, the first task was to learn if these observations were valid. The study showed the observations were consistent in time. The next tasks were to translate the observations from Hawaii scale to trough-to-crest heights (peak face) and to develop an empirical formula that estimates surf heights based on deep water swell. It was designed by fitting north shore Oahu H1/10 visually observed surf heights to the H1/3 and dominant wave period as measured by a buoy 3 miles offshore of Waimea Bay. In Hawaii, all coastal regions have relatively narrow shelves, steep sea bed slopes, and zones of high refraction; thus, the empirical scheme is applicable to other coasts. Additional research has been completed to understand north shore Oahu wave-induced coastal flooding potential based on historic records of coinciding high surf and tides.

Collaborative Table Surf Height Definition

Surf height refers to peak face. The output of the empirical formula was designed to match the observing scheme, so visual reports can be used as validation.

This scheme emphasizes the smaller percent of larger waves, which are the ones that pose the greatest risk to safety. Even on the largest days of surf, breakers can be low during lulls and in the divergent wave energy regions between zones of high refraction. This elusive nature of surf height variability gives false security to novices, and makes surf the number one weather-related killer in Hawaii.

Questions or comments: Mr. Patrick Caldwell, patrick.caldwell@noaa.gov

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