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Swell Rating System (SRS)
Quantifying the Effects of Swell Height, Period and Quality on Wave Size

Thesis
Introduction
History
Measurement Considerations
Observations
Swell Category Table
           Limitations
Category Characteristics Table
           Limitations
Swell Quality
Swell Quality Factor Table
Swell Rating System (SRS)
Swell Profile
Sample Standard Swell Profile
Swell Rating Computation
Swell Comparison Table
Scaling Factors
Conclusion

Thesis

This paper documents a process for identifying the signature of swell events. The process is known as the Swell Rating System (SRS). The SRS process is based on the following tenants:

• Swell amplitude, period and quality are the most significant factors in determining the size of a breaking wave from a surf riding perspective.
• Swell amplitude (height) and period measurements can be correlated to predict the face height of breaking set waves.
• Swells of different amplitudes and periods can be grouped together and categorized based on resulting breaking wave face heights.
• Swell Quality can be measured.

It is with these understandings that the Swell Rating System is proposed and submitted for use by the surf riding and forecasting community.

Introduction

This paper identifies a system for categorizing breaking waves based on the amplitude, period and quality of the swells from which the waves originate. A system is provided to methodically compare and contrast waves based on size and quality from one day to the next. This is of particular importance when trying to compare rare, large-swell events which occur only several times a decade.

Currently surf from seasons past are memorialized through anecdotal stories and the experiences of those who had the opportunity to observe the conditions. As is common with stories, they are only as good as the memory of the person telling the story, and like all stories, they change and grow with time. This paper presents a scientific system to accurately and concisely define a swell's signature at any point in a swell event.

The Swell Rating System (SRS) was created to eliminate the ambiguity and uncertainties that currently exist when measuring breaking waves. The SRS was created from data obtained through a systematic process of recording and analyzing swell data obtained from buoys and the resultant breaking wave height observations. A by product of this analysis is the ability to accurately predict the face height of breaking waves based on corresponding offshore buoy readings, even when the buoy is located over distances approaching 1000 miles from shore. Given this relationship, it becomes possible to categorize breaking waves based on the relative size and strength of the underlying swell.

History

The purpose of offshore buoy networks around the world is aimed at warning boaters and beackgoers of impending dangerous marine and surf conditions. But the focus of these programs, and most published data, is non-specific to the interests of those who actually utilize breaking waves for pursuit of recreational and professional purposes, like surfers, body boarders, and the like. To fill this need, various commercial services have evolved. They utilize observational buoy data in conjunction with forecast models and satellite imagery to predict and report surf conditions worldwide. But the products of these services tend to be homogenized to attract and maintain a large client base, and typically lack sufficient detail and depth required by those who pursue the forward most fringes of the sport or who are performing research. This paper is aimed at fulfilling this need.

In the past, the only easily available real-time tools for provided ocean surface conditions to the public were buoy observations provided via the National Weather Service (NWS) on local weather radios. More recently, similar data became available via the Internet. Both these services reported ‘significant sea heights’ observed at various buoys on either an hourly or 3-hour basis. Once again, the purpose of such observations were mainly to warn mariners and beachgoers of dangerous conditions to reduce loss of life and property. Typically an appropriate marine warning would be issued if ‘dangerous’ conditions were present or immanent., including high surf advisories. Using significant seas height data, the study began correlating near-shore hourly buoy observations to real-time surf observations at Cocoa Beach Florida, (and more recently) Half Moon Bay California., and to a much lesser extent, the northern shore of Oahu. By documenting the results, it became apparent, as was expected, that a correlation existed.

Clearly, progressively larger waves would be observed as the size and period of the seas increased. But the objective was to determine the face height (in feet) of waves breaking on unobstructed beaches given a particular buoy reading and similar swell characteristics. A baseline table was completed in 1989, in Florida. It should be noted that buoy observations were not publicly reported until one year prior to this time. Once accomplished, the table was found to be accurate roughly 50% of the time. In the instances where the table was not correct, the observed breaking wave heights were normally less than the height predicted by the table. Subsequent observations drove updates to the table as understanding of the baseline and buoys evolved, but the confidence factor could not be improved beyond the 65% factor.

All measurements were taken at a break known as RC’s, in Satellite Beach, Florida and using Buoy 41009, located 20 nmiles east of Cape Canaveral. By definition, RC’s only breaks on swells originating from between 45 - 90 degrees and within approximately 1500 nautical miles (nmiles). It was discovered that a straight-line percentage factor could be applied to ‘scale’ the prediction for other breaks within the region with a high degree of confidence. (Such methods are discussed in the section titled ‘Scaling Factors’.)

The original study included wave periods ranging from 8 seconds to 17 seconds. Though not included in the current table, wave periods less than 13 secs were included early-on because Florida rarely receives legitimate swells (those which equal or exceed 13 secs), and they are part of the east coast swell riding experience. It was determined the predictive process defined herein is probably applicable to windwaves as well, but since such conditions represent the least treasured end of the spectrum, research was discontinued.

In 1995, research began at Half Moon Bay, California using buoy 46012 and a variety of unobstructed breaks in the region. Once again, a baseline table was established and additional observations were used to verify the baseline. Because the quantity of swells is much higher in California, the pace of research rapidly increased. This also coincided with the proliferation of data via the Internet/World Wide Web. Because Northern California receives many swells in the 14-20 second interval range, and occasionally greater, the upper end of the table was expanded. A correlation between the Florida based table and the newer California based table was observed for the data elements they shared.

But a new more dramatic component was also present. A local break known as Mavericks, was becoming publicly known for it’s ability to be board rideable at sizes up to 30 ft. This made Mavericks a contender for the largest rideable wave in the world, in a class with Waimea Bay (North Oahu) and Todos Santos (Mexico), and arguably the best of the three. Research at Mavericks permitted the study to take a broader scope, and focus on waves covering the highest realm of the rideable spectrum.

Measurement Considerations

Directly measuring the face height of a breaking wave is an imprecise science. Often the observation is prejudiced by one’s viewing angle, lack of an object to use as a reference point, or personal tendencies to exaggerate or underestimate. Therefore, the accuracy of measuring a breaking wave’s height is only as good as the objectivity of the observer. For this study, all measurements are made by the author, based on the distance from trough to crest of the wave as it is initially breaking (commonly known as the peak). The author has over 25 years of experience in wave measurement. Whenever possible, the measurements were collaborated with other observers of equal or greater experience at the time of occurrence, or from measuring photographs of riders and waves taken during measurement ‘events’. Whenever a measurement is in question, it is discarded or the lowest reading within the probable range was utilized.

Buoy measurements are not an exact science either. Energy frequency data is obtained through a variety of accelerometers contained within the buoy. Data is transmitted to a satellite network and down-loaded to a central computer for processing. Readings are taken once every hour for a ten minute interval. The frequency data is decoded and processed, resulting in an average of the highest one-third of waves recorded during the sample period. Initially the National Weather Service only reported ‘significant seas’. Significant seas are defined as the average of the highest one-third of combined swells and wind waves. In the open ocean, chop and swells generated from a variety of sources (storms, gales, high pressure systems, and high/low pressure gradients) interact. The buoys record these conditions, and filtering is applied to eliminate erroneous and transitional spikes. The resultant readings are broadcast.

Lately, additional algorithms are being used which group wave energies by frequency, permitting wind waves and swell (ground swell) to be separately identified. Observation has determined that the most desired feature required for producing rideable waves is swell. Locally generated wind waves have an additive effect on overall height, but significantly degrade quality as the ratio of windwave to swell increases. It was this interaction of wind wave and ground swell that caused the relatively low 65% accuracy rate identified previously. With the publication of ground swell readings from the buoys (i.e. equal to or greater than a period of 13 secs), the accuracy rate has significantly improved.

Additionally, the University of California (San Diego) has implemented another buoy network which identifies predominant direction (in feet, period and degrees) of both north and south bound swells. This data has been used to validate NWS provided swell data.

Based on descriptions of how measurement data is obtained, it is apparent that both the wave face height and buoy measurements have a probability for variability, error or inaccuracy. Namely, breaking wave face heights are measured by inference rather than direct scaled means, and swells (as measured by the buoys) are transcribed through a inferred method. This is not to say an exact correlation between swell heights and breaking wave height is not possible, but that an appropriate deviation range has been considered when producing the final results.

Observations

Swell Category Table

This section contains the most recent generation table of buoy and breaking wave observations. The table groups swells of various periods and sizes into categories. There are 2 features worthy of note:

• The table establishes Swell Categories, based on their resultant expected breaking wave face height. Much like the Saffir-Simpson and Fujisaki scales for indicating hurricane and tornado destructive potential, this table is broken into categories based on traditional size barriers recognized within the wave riding community.
• The table consolidates translation of swell readings of different periods into one standard set of measures. That is, waves of different periods, though they contain different levels of total energy, can be grouped together based on their expected breaking wave face heights.

The horizontal axis identifies standard swell periods. The vertical axis identifies swell categories. The interior of the table groups derived swell measurements based on size/amplitude (in feet) and period (in secs) from the buoys and assigns them to a category. Swells that result in similar sized breaking wave face heights are grouped together.

Swell Category Table

Limitations

The list below identifies parameters within which the table is applicable. If any of the parameters are not met, scaling factors should be applied to produce a valid measurement. (Note: Application of scaling factors may degrade resultant accuracy.)

• Beach must be unprotected . Unprotected is defined as:
  • No obstacle must be present that creates drag or refraction of the swell or prevents a swell from arriving at the beach unimpeded. Examples of impediments include a point (which causes swell redirection/wrapping), isthmus, island, shoal, peninsula, or submerged obstacle (including waters less than 90 meters deep within 20 nmiles from shore).
  • Readings must be from a near-shore buoy. Near-shore is defined as the buoy being located within 20 nmiles of shore and in approximately 90 meters of ocean depth. (Note: Buoy 51001 off Kauai , typically used on the North shore of Oahu, HI does not meet this criteria because it is anchored in a depth of 3000 meters . Therefore, scaling factors should be applied. Buoys 46006 and 46059 off the northern California coast also are in a similar situation.)

Category Characteristics Table

The Category Characteristics Table correlates swell categories to expected breaking wave face height measurements. Several measurement types are provided to address the variety of scales typically used by various sub-factions of the wave riding community.

The horizontal axis identifies a category and a breaking wave face height size measurement using each of the different scales. Each scale is described below:

• Apparent Size: The size of the wave face measured by comparing it to the size of standing rider (surfer) on that wave. (Assuming a ‘average’ rider is about 5’9" tall and assuming a normal surfing position, total height would be about 5’0".
• Measurable Size: The actual breaking wave face height if a photograph of the wave was taken and a standard of measure (in feet) was applied to it.
• Hawaiian Size Equivalent: A regional scaled applied mainly at breaks in Hawaii. This scale measures the back of a breaking wave rather than its face.
• Mavericks (Apparent) Size: Same as apparent size, only occurring at Mavericks. Most unobstructed sandbar and reef breaks become unrideable once the swell size is sufficient to start becoming rideable at Mavericks. They become closed-out. Conversely, when the swell is small enough to be rideable at most sand-bar and reef breaks, no waves break at Mavericks because the water depth at the reef is too deep to allow waves to break.
• Mavericks Measurable Size: Same as Measurable Size. Because of the reef at Mavericks, the size is actually magnified (positive scaling) compared to other breaks experiencing the same swell conditions.

The vertical axis identifies each of the 7 Swell Categories.

Swell Category Characteristics

** Mavericks data available but intentionally not included here to protect the break and those that have earned the right to surf it without a crowd.

Limitations

It should be understood that the breaking wave face heights defined above are presented as general guidelines for swell classification purposes, and can vary depending upon the characteristics of the ocean floor where the waves break. Some ocean floor configurations may amplify the swell’s breaking wave size potential (positive scaling), others reduce it (negative scaling), some become unrideable above a certain size while other only break when the size becomes extreme. Some waves break on sandbars, others on reefs, rock shelves, or points. Regardless of the breaks configuration, the table above has been proven to hold generally accurate if used according to the limitations established herein.

Swell Quality

The previous discussion has been based on the observance of clean ground swells. If viewed graphically, the swell would ideally be focused within a narrow amplitude and frequency band range.

The presence of secondary or even tertiary ground and/or wind swells can have an additive effect of overall breaking wave face height. It is not the intent of this paper to research and discuss findings related to such an effect, but merely to state that the presence of wind waves and other ground swells of less significant size and/or period could cause the size predicted by the tables to be understated.

It is the interaction of ground and wind swells that caused a low confidence in early versions of the swell table. Namely, the use of ‘significant sea’ heights instead of ‘swell’ introduces a variable component which can’t be accurately measured at this time.

Also, there are some instances where the buoys report significant seas with a period of 13 sec or greater, yet also report an identical wind wave reading and no swell reading.

This phenomenon is still being investigated, but it is suspected it is caused by a swell that has a marginal 13 sec period, and is actually spread between 11 to 13 sec., making it qualify more as wind wave than ground swell.

A similar condition exists when a ground swell and wind swell of nearly equal amplitude converge on a buoy simultaneously. Often the readings over time will be displayed as follows:

The resulting breaking wave face height will not correlate well with the significant sea readings (being something less), but will be greater than the sporadic swell reading. It appears, based on observation, to be a combination of the two, primarily influenced by the swell, and secondarily amplified by the wind wave.

Such conditions can be difficult to interpret and make it difficult to construct an accurate breaking wave face height forecast. And the resulting breaking waves are less than ideal to ride, commonly called ‘junky’ or ‘choppy’, even though local winds could be calm. This persistent condition has resulted in the creation of swell quality criteria. When multiple swells or waves with periods less than 13 seconds converge at a near-shore buoy, a confidence or Quality factor has been devised to be broadcast with the Swell Category to indicate the presence of multiple frequencies occurring simultaneous with the ground swell. A simple 5 point scale is presented below:

Swell Quality Factor Table

It is often difficult to determine exactly which of the above Quality Factors to apply to a swell without the tools provided by the National Data Buoy Center via their web site. This site provides graphics of the various energy frequencies present at each buoy on an hourly basis. The lower the frequency, the higher the associated energy level, and period. It appears that energy levels of .5 hertz equate to a period of about 20 seconds. A simple review of the hourly graphic clearly identifies the presence of one or multiple frequency levels. The presence of multiple lower frequency swells occurring simultaneous with a large primary ground swell often makes the primarily swell unrideable, or at least, less desirable.

Reviewing data at the prime buoy URL (http:www.nws.fsu.edu/buoy/) for the same time period indicates the presence and amplitude of both predominant ground swell and lesser energy waves (wind waves).

Swell Rating System (SRS)

Swell, wind, secondary swells, wind waves and tide change hourly and interact on a minute by minute basis to produce the dynamic medium on which wave riding sports are practiced, and large waves of quality are at best exceedingly rare and fleeting in nature. This is well understood by the wave riding population, and large surf of quality is something to be coveted, remembered and memorialized. It is with this historical background that a system to ensure accurate comparison is proposed.

Swell Profile

Though a swell rating can be computed at any point during a swell event, the swell signature should be computed at the peak of the swell event. This is easily determined by reviewing hourly buoy reports over the duration of the swell event.

The resulting hourly measurements can be plotted to create a ‘swell profile’ which maps the evolution of the swell over it’s lifespan (See table below). The peak of the swell often occurs at the point where the swell period transitions from one of it’s early high-energy frequencies (typically accompanied by lower amplitudes) to the next lowest frequency band (usually accompanied by a subsequent increase in amplitude). The combination of Swell Amplitude and Period that result in the largest Swell Rating define when the Peak is reached.

Sample Standard Swell Profile

Swell Rating Computation

Once one computes the peak Swell Category, the corresponding Swell Quality Factor should be determined. Once that’s complete, the Swell Rating can easily be assigned.

The Swell Rating is determined by concatenating the Swell Category and Swell Quality components. The resulting number defines the value, or signature for the swell and resultant breaking waves at the time the observations are taken. This is achieved by first identifying the Swell Category rating, then the Swell Quality rating. They should be separated with a decimal point (‘.’). This method is similar to one created for rating difficulty of rock climbing paths.

Format: XX.YY

Where ‘XX’ equals the Swell Category and ‘YY’ equals the Swell Quality.

For example:

If Swell Category rating = 7

And Swell Quality rating = 5

Then the Swell Rating = 7.5,

This would be the best of all possible conditions.

Like wise

If Swell Category rating = 0

And Swell Quality rating = 1

Then the Swell Rating = 0.1

This would be the worst of all possible conditions.

This method provides a means of quantifying swell size, period, and quality in one concise value.

From the examples above, it’s now possible to construct a matrix with 40 degrees of fidelity to describe all possible surf conditions. The X axis identifies swells in increasing size from left to right, while the Y axis identifies swells in increasing quality from top to bottom. The worst of all swells appears in the upper left hand corner of the table, while the best of all swells appears in the lower right hand corner of the table.

Swell Comparison Table

                     INCREASING SIZE ------>

The SRS does not consider swell direction, local winds, tides, or provide a means to rate how the wave breaks relative to other breaks, ie hollow, mushy, peeling versus 'sectiony', point break versus beach break. It is well understood that swell size and quality are the two most important factors for determining the potential for rideable surf. If those two elements are present, it then becomes an academic and personal preference issue of finding the best break (location), at the correct tide, with acceptable wind conditions to cause the swell to reach it’s maximum potential from a surf riding perspective. But without sufficient swell, all other influential elements are insignificant.

This system does not attempt to rank surf riding breaks (locations) because it’s purely a matter of personal preference. Each break has it’s own set of rules regarding optimal swell direction, tide, local winds etc, and each person has their own criteria for what is optimal surf based on their wave riding abilities. But again, such a debate is inconsequential, because there must first be swell.

SCALING FACTORS

Many NOAA and CDIP buoys are positioned sufficiently clear of obstacles which impede the progress and strength of swells. Points, isthmus’s, under sea ledges, continental shelves, islands and the like, can block a swell or cause it to refract, drag, or otherwise loose energy. This results in a decrease in either swell period or amplitude and can lead to errors when calculating expected breaking wave face heights. The actual face height may be less than that predicted from the Swell Category Table, depending on the magnitude of the obstruction.

The buoys used in production of this study off California and Florida were 20 nautical miles from the coast and generally unobscured. At certain beach locations within the area serviced by these buoys, obstacles were present, and the Swell Category Table overstated expected breaking wave face heights. Depending on the nature of the obstacle, a variety of scaling factors were applied to develop an accurate swell rating for the location where the waves broke. For example, buoy 46026 (20 nmiles off San Francisco) lies in unprotected waters. But 10 miles to it’s north are the Farralon Islands. When a swell arrives from 282-290 degrees, it hits the buoy unobstructed, but the island partially blocks the swell on it’s journey to the beaches of Half Moon Bay. The resulting breaking wave face heights are diminished. Experience has proven that generally, swells arriving from this direction loose 15% of their face height potential. The swell rating for such an occurrence would be calibrated based on the known Scaling Factor (in this case 15%) and computed accordingly.

This paper only identifies the need for calibration at obstructed beaches. Skill and databases must be developed independently for such locations.

CONCLUSION

Swell size, period and quality are the most significant factors in determining the potential for surf and are the central elements used in the Swell Rating System (SRS). With the SRS, it is now possible to record the occurrence of any ‘swell event’ in a manner that accurately measures it’s size and intensity. Correlating buoy derived swell height measurements with breaking wave face heights eliminates much of the error, exaggeration and hype associated with the current method of surf measurement. The SRS is several magnitudes of improvement better than the current system, but there is still potential for error when affected surf breaks require the application of Scaling Factors. Databases still need to be developed on a break-by-break basis.

It is with this understanding that this proposal is submitted for use by the surf riding and forecasting community.

Copyright © STORMSURF 1999, 2000, 2001, 2002

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