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The wave height (Hs) represents a 30-minute average of the 1/3 highest waves at a sensor. The height is the distance between the trough and the crest of the wave. Statistically, the highest wave during the measurement period is likely to be approximately twice the reported wave height (1.8*Hs).
The period of waves is the time it takes two consecutive crests to pass a single spot, and the direction is the compass angle (0-360 degrees clockwise from true North) that the waves are coming from. In the ocean, however, no two waves are perfectly identical - they are constantly coming from different directions at different frequencies.
Since there is never just a single direction or period for ocean waves, we can only measure the peak period (Tp) and peak direction (Dp). The peak period is the most common period between consecutive waves, while the peak direction is the most common direction. To come up with these values, all of the wave energy for a station over a specified period of time - approximately 30 minutes, in most cases - is grouped into different bands. For instance, in CDIP's 9-band products, all waves with periods from 6 to 8 seconds go into the 7-second band, those from 8 to 10 secs go into the 9-second band, etc. After all of the wave energy has been divided into these bands, the band with the most energy is selected as the peak band.
The cutoffs between bands, however, are arbitrary, and changing the cutoffs and/or number of bands affects the resulting Tp and Dp values. At CDIP we use two formats for analysis, 9-band and parameter. In the the 9-band products, all of the wave energy is divided into 9 broad bands. In the parameter products, the energy is split into 64 (or 128) narrow bands. As a result, the 9-band Tp and Dp give more general, broad values, while the parameter Tp and Dp identify finer, subtler peaks.
So which Tp/Dp values are better? It depends what you're looking for. For instance, the broad bands of the 9-band values are better for addressing general questions about the sea state (e.g. which is currently predominant - local seas or ground swell?). To pick up more subtle features - like the arrival of long-period swell from a distant storm - the parameters values may be more helpful.
Over any given period, the waves at a point in the ocean are actually coming from a number of different sources. There may be short-period waves from local winds, long-period waves from one or more distant storms, and a range of other wave fields originating from different weather systems. The peak period and peak direction describe the strongest of all the sources of wave energy. When one source becomes stronger than another, the Tp and Dp values can suddenly change dramatically. And when two sources of wave energy have near-equal strength, the Tp and Dp values may bounce back and forth between them.
For instance, in summer on the US west coast we often have long-period swell from large storms in the southern hemisphere and short-period seas from local winds off the coast. Whenever the south swell is the stronger the peak period could be 18 seconds and the peak direction 180 degrees. Whenever the seas are a bit stronger, the Tp could be 5 seconds and the Dp 290 degrees. And if these two sources have near equal energy, we may find that from hour to hour the Tp and Dp values jump up and down repeatedly, showing whichever of the two sources is briefly the more energetic.
There are often big differences between inshore and offshore temperatures due to various phenomena. In the surfzone, for instance, mixing may result in temperatures much colder than in calm surface waters offshore. Our buoys are all located offshore and measure sea surface temperature using a sensor located about 18 inches directly below the buoy.
Very often waves from different parts of a distant storm arrive here at the same time. Their period (or lengths) can be nearly the same size. For a while the crests of the two wave trains begin to coincide. The two trains adding together result in waves that grow bigger. Later, the troughs of one begins to coincide with the crests of the other. The combination wave grows smaller. We see this as a "set" of large waves, followed by an interval of smaller waves. See figure below.
Local winds generate very short-period, high frequency waves, and not all of CDIP's instruments can measure these waves effectively. This is especially true of pressure sensors postioned deep underwater. At Kings Bay, for example, the sensors are mounted at a depth of 55 feet. Due to attenuation, these sensors do not feel high frequency waves, and so on windy days the high frequency energy will not be reflected in the reported Hs.
To determine the wave conditions, you can't simply look at the ocean for a few seconds. Instead, you need to sample data over a long period. For most of CDIP's wave calculations, a data sample of approximately 30 minutes is used. And unlike the NDBC and other data providers who use sample end times on their spectra, CDIP uses sample start times.
This means that when we update our site based on a data sample that ended just a few minutes ago, the time assigned to the data - the start time - will already be more than 30 minutes old. For Datawell buoys, at the end of a 30-minute sampling period, the sensor calculates a wave spectrum and starts to transmit it; it's repeatedly sent in 4-minute blocks over the next half-hour.
This means that the when a CDIP station updates, the new spectrum will have a time that is at least 35 or 40 minutes old (30 min sample + 4 min transmission + a few minutes to process). But sometimes newly-updated data may have a start time that is up to 1 hour and 10 minutes old; it simply depends on which point in the half-hour cycle the buoy is contacted. (I.e. CDIP only grabs the data from stations once each half-hour, and it may happen that we're grabbing the spectrum near the end of its transmission cycle.)
When looking at buoy data, it's important to distinguish between deep-water measurements and shallow-water measurements. In deep water, swell direction is primarily determined by the location of the fetch that produced the swell. So deep-water buoys on the West Coast register swells from the Gulf of Alaska as NW swells, and swells from the South Pacific as S or SW swells.
In shallow water, on the other hand, the swell direction is determined primarily by the local bathymetery; swell is refracted such that wave crests approach the coast parallel to shore. I.e. whether it's a NW swell or S swell, at your local beach the waves always come in and line up at nearly the same angle, with only slight shifts to the N or S.
For example, the San Francisco Bar buoy is a shallow-water buoy at a location where the shore normal points to the WSW. Thus all long-period swell, regardless of source, has been refracted to the WSW at that spot. So the readings you see are correct given the water depth (15m) at the buoy's location.
The relative amounts of swell and wind waves (or seas) reported for a buoy depend upon the cutoff period used to distinguish swell from seas. NDBC buoys include anemometers and use wind measurements to determine the sea/swell cutoff period based on current conditions. CDIP buoys, on the other hand, do not measure winds, and instead of a dynamic sea/swell cutoff they use a fixed 10-second cutoff when reporting swell and wind wave measurements to NDBC.
Sometimes there are significant amounts of wave energy with periods in the 8 to 10 second range. Depending on wind conditions, NDBC may report this energy as swell, whereas CDIP buoys will always report it as wind waves. Hence the NDBC reports will show more swell, and the CDIP reports will show more wind waves. By looking at the swell and wind wave periods, however, it should be clear when this sort of discrepancy occurs; the NDBC buoys will be reporting a low swell period, one that falls below CDIP's 10-second cutoff.
For a more in depth discussion of wave measurements and standards, we recommend the Coastal Engineering Manual (CEM), published by the United States Army Corps of Engineers' Coastal and Hydraulics Laboratory.
|Annual maximum wave heights from waverider buoy data||Seymour, R.J., Castel D.||Shore and Beach||2017-01-01 00:00:00|
|The California coastal wave monitoring and prediction system||O'Reilly, W.C., Olfe, C.B., Thomas, Julianna, Seymour, R.J., Guza, R.T.||Coastal Engineering||2016-10-15 00:00:00|
|CDIP - Public Data Management Plans created with the DMPTool||Jennifer McWhorter, Darren Wright, Julie Thomas||Research Ideas and Outcomes||2016-04-15 00:00:00|
|La Jolla Shores March 8, 2016||Jen McWhorter||2016-03-08 00:00:00|
|Validated coastal flood modeling at Imperial Beach, California: Comparing total water level, empirical and numerical overtopping methodologies||T.W. Gallien||Coastal Engineering, 111||2016-01-14 00:00:00|
|CDIP wave observations during a strong El Nino year||Seymour, R.J.,Thomas J.O., Castel D.||Shore & Beach||2016-01-01 00:00:00|
|How Are High Resolution Wave Observations and HF Radar Derived Surface Currents Critical to Decision-Making for Maritime Operations?||Thomas, J.,Hazard, L., Jensen, R., Otero, M., Terrill, E., Keen, C., Harlan, J., Fake, T.||Coastal Ocean Observing Systems, 1st Edition Edited by Liu, Y., Kerkering, H., Weisberg, R.||2015-06-23 00:00:00|
|Biofouling effects on the response of a wave measurement buoy in deep water||Thomson, J., et. al.||J. of Atmospheric and Oceanic Technology||2015-06-01 00:00:00|
|CBS New 8 Interview||J. Thomas||2015-03-19 00:00:00|
|Research Highlight: Waves Bring the Heat||Mario C. Aguilera||Scripps Institution of Oceanography||2014-11-03 00:00:00|
|Support letters, FY 2014||J. Thomas||2013-05-04 00:00:00|
|Support letters, FY 2013||J. Thomas||2012-05-04 00:00:00|
|Equilibrium Shoreline Response of a High Wave Energy Beach||Yates, M.L, R.T. Guza, W.C. O'Reilly, P. Barnad and J Hansen||J. Geophysical Research, 116, C04014, doi:10.1029/2010JC006681||2011-09-25 00:00:00|
|Coastal cliff ground motions from local ocean swell and ifragravity waves in southern California||Adam P. Young, P.N. Adams, W.C. O'Reilly, R.E. Flick, R.T. Guza||J. Geophysical Research, 116, C09007, doi:10.1029/2011JC007175||2011-09-01 00:00:00|
|Support letters, FY 2012||J. Thomas||2011-05-04 00:00:00|
|The Effect of Temporal Wave Averaging on the Performance of an Empirical Shoreline Evolution Model||M.A. Davidson, I.L. Turner and R.T. Guza||Coastal Engineering, 58||2011-02-11 00:00:00|
|Short-term retreat statistics of a slowly eroding coastal cliff - Point Loma, California, USA||Young, A.P., Guza, R.T., O'Reilly, W.C., Flick, R.E. and Gutierrez, R.||Natural Hazards and Earth System Sciences, 11||2011-01-21 00:00:00|
|California's Ocean||Ari Daniel||2010-09-03 00:00:00|
|Support letters, FY 2011||J. Thomas||2010-05-04 00:00:00|
|Coarse Sediment Yields from Seacliff Erosion in the Oceanside Littoral Cell||Adam P. Young, J.H. Raymond, J. Sorenson, E.A. Johnstone, N.W. Driscoll, R.E Flick, and R.T. Guza||Journal of Coastal Research||2010-05-01 00:00:00|
|Comparison of Airborne and Terrestrial LIDAR Estimates of Seacliff Erosion in Southern California||Adam P. Young, M.J. Olsen, N. Driscoll, R.E. Flick, R. Gutierrez, R.T. Guza, E. Johnstone, and F. Kuester||Photogrammetric Engineering & Remote Sensing||2010-04-01 00:00:00|
|A Portable Airborne Scanning Lidar System for Ocean and Coastal Applications||Benjamin D. Reineman, Luc Lenain, David Castel, and W. Kendall Melville||J. Atmospheric and Oceanic Technology||2009-12-01 00:00:00|
|Huntington Beach Surfzone Currents and Beach Water Quality Case Study||S. Grant, W.C. O'Reilly, F.Feddersen and R.T. Guza||2009-10-13 00:00:00|
|Local Scientists Use Dye In Ocean Pollution Study||10 News||2009-10-08 00:00:00|
|An Ocean Study to Dye For||Jeff Zevely||CBS 8||2009-10-08 00:00:00|
|Little Buoy Takes On Big Job||Deeda Schroeder||The Daily Astorian||2009-10-05 00:00:00|
|Equilibrium shoreline response: Observations and modeling||Yates, M.L, R.T. Guza, and W.C. O’Reilly||Journal of Geophysical Research||2009-09-18 00:00:00|
|Rain, Waves, & Short-Term Evolution of Composite Seacliffs in Southern California||Adam P. Young, R.T. Guza, R.E. Flick, W.C. O'Reilly, and R. Gutierrez||Marine Geology||2009-09-01 00:00:00|
|A Portable Airborne Scanning LiDAR System for Ocean and Coastal Applications||Benjamin D. Reineman, Luc Lenain, David Castel, and W. Kendall Melville||AMS Journals Online||2009-07-13 00:00:00|
|Comparison of short-term seacliff retreat measurement methods in Del Mar, California||Adam P. Young, R.E. Flick, R. Gutierrez, and R.T. Guza||Geomorphology||2009-06-18 00:00:00|
|Support letters, FY 2010||J. Thomas||2009-05-04 00:00:00|
|Local fishermen recover valuable weather buoy||Greg Thomas||Half Moon Bay Review||2009-04-29 00:00:00|
|San Francisco PORTS||J. Thomas||2009-01-15 00:00:00|
|Persistence of a Small Southern California Beach Fill||M.L. Yates, R.T. Guza, W.C. O'Reilly, R.J. Seymour||J. Coastal Engineering||2009-01-01 00:00:00|
|Overview of seasonal sand level changes on southern California beaches||Yates, M.L., R.T. Guza, W.C. O'Reilly, and R.J. Seymour||Shore & Beach||2009-01-01 00:00:00|
|Malibu's Vanishing Broad Beach a sign of rising sea levels, experts say||Kenneth R. Weiss||Los Angeles Times.com||2008-12-31 00:00:00|
|Central California Wave Buoys||2008-12-01 00:00:00|
|Pacific Northwest Wave Buoys||2008-12-01 00:00:00|
|California Ocean Protection Council - LA/Long Beach||J. Thomas||2008-11-21 00:00:00|
|San Diego's Changing Climate: A Regional Wake-Up Call||2008-11-17 00:00:00|
|Santa Barbara & Ventura County Wave Info||2008-11-01 00:00:00|
|The San Diego Foundation Regional Focus 2050 Study||2008-11-01 00:00:00|
|The Power of Ocean Waves||R. Seymour||UCSD TV||2008-10-08 00:00:00|
|Southern California Beach Processes Study||Thomas, Julie||CDIP||2008-09-01 00:00:00|
|A Technique for Eliminating Water Returns from Lidar Beach Elevation Surveys||Yates, M.L., R.T. Guza, R. Gutierrez, and R.J. Seymour||J. Atmospheric and Oceanic Technology||2008-09-01 00:00:00|
|Southern California Wave Buoys||2008-08-01 00:00:00|
|Implementation of Google by CDIP||J. Thomas||2008-07-24 00:00:00|
|Hydrographic Survey Research Panel||J. Thomas||2008-07-24 00:00:00|
|Support letters, FY 2009||J. Thomas||2008-05-04 00:00:00|
|Data Quality Control||J. Thomas||2007-10-17 00:00:00|
|Scenarios for Coastal Flooding in San Diego County Caused by Sea Level Rise||R. Guza, R. Seymour, W. O'Reilly, J. Thomas||2007-08-24 00:00:00|
|Sea Level Rise||R. Guza, R. Seymour, W. O'Reilly, R. Bucciarelli, J. Thomas||2007-08-24 00:00:00|
|Waves & Beaches||2007-08-09 00:00:00|
|CSBPA - CDIP Wave Measurements and Model Predictions||J. Thomas||2007-04-26 00:00:00|
|CSBPA - Airborne LIDAR Mapping of Beach Changes||R. Seymour||2007-04-26 00:00:00|
|Sea Range partners with UCSD/Scripps||Patti Sauers||Air Time||2007-03-01 00:00:00|
|Evolution of Surface Gravity Waves Over a Submarine Canyon||R. Magne, K.A. Belibassakis, T.H.C. Herbers, Fabrice Ardhuin, W.C. O'Reilly, and V. Rey||J. Geophysical Research, 112, C01002||2007-01-09 00:00:00|
|Performance Evaluation of Seacliff Erosion Control Methods||Adam P. Young and Scott A. Ashford||Shore and Beach, 74,(4)||2006-09-12 00:00:00|
|Application of Airborne LIDAR for Seacliff Volumetric Change and Beach-Sediment Budget Contributions||Adam P. Young and Scott A. Ashford||J. Coastal Research, 22,(2)||2006-03-01 00:00:00|
|Changes in Sand Level and Wave Energy on Southern California Beaches||Marissa L. Yates, R. Guza, R. Seymour, W. O'Reilly, R. Gutierrez||2006-02-17 00:00:00|
|Retaining Sandy Beaches in North County||O'Reilly, Guza, Seymour, Flick, Yates, Thomas, et al||2006-01-15 00:00:00|
|City of Oceanside Harbor & Beaches Advisory Committee||J. Thomas, R. Seymour, R. Guza||2005-12-08 00:00:00|
|Headlands to Oceans Conference||J. Thomas, R. Guza, R. Seymour, W. O'Reilly, R. Bucciarelli||2005-10-26 00:00:00|
|SCBPS overview||2005-03-01 00:00:00|
|Rapid Erosion of a Southern California Beach Fill||Seymour, R.J., R.T. Guza, W. O'Reilly and Steve Elgar||J. Coastal Engineering, 52,(2)||2004-05-15 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 07||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2002-11-30 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 06||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2002-08-31 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 05||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2002-05-31 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 04||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2002-02-28 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 03||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2001-11-30 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 02||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2001-08-31 00:00:00|
|Torrey Pines Beach Nourishment Study Quarterly Report - Quarter 01||R.T. Guza, W.C. O'Reilly, R.J. Seymour, S.L. Elgar||2001-05-31 00:00:00|
|Coastal Data Information Program||Oceanside Magazine||2001-03-01 00:00:00|
|Systematic Underestimation of Maximum Crest Height in Deep Water Using Surface-Following Buoys||Proceedings, Offshore Mechanics and Arctic Engineering Conference, Lisbon, Portugal, July 5-9||1998-07-05 00:00:00|
|Assimilating Coastal Wave Observations in Regional Swell Predictions. Part 1: Inverse Methods||W. C. O'Reilly and R. T. Guza||J. Physical Oceanography, 28,(4)||1998-04-01 00:00:00|
|Undersea pumped storage for load leveling||Proceedings, California and the World's Oceans, 1997, San Diego, CA.||1997-06-15 00:00:00|
|Observations of seiche forcing and amlification in three small harbors||Okihiro, M., Guza, R.T.||J. of Waterway Port, Coastal, and Ocean Engineering||1996-10-01 00:00:00|
|Wave climate variability in Southern California||Seymour, R.J.||J. of Waterway, Port, Coastal, and Ocean Engineering, ASCE||1996-07-01 00:00:00|
|A Comparison of Directional Buoy and Fixed Platform Measurements of Pacific Swell||W. C. O'Reilly, T. H. C. Herbers, R. J. Seymour and R. T. Guza||J. Atmospheric and Oceanic Technology, Vol. 13,(1)||1996-02-01 00:00:00|
|Field Wave Gaging Program, Wave Data Analysis Standard||Marshal D. Earle, David McGehee, and Michael Tubman||USACE Instruction Report CERC-95-1||1995-03-01 00:00:00|
|Effects of Southern California Kelp Beds on Waves||M. Hany S. Elwany, William C. O'Reilly, Members, ASCE, Robert T. Guza, and Reinhard E. Flick||J. Waterway, Port, Coastal and Ocean Eng., 121,(2)||1995-03-01 00:00:00|
|New Technology in Coastal Wave Monitoring||Richard Seymour, David Castel, David McGehee, Julianna Thomas, and William O'Reilly||Ocean Wave Measurement and Analysis, Proc. 2nd Int. Symp.||1993-07-25 00:00:00|
|Wave Monitoring in the Southern California Bight||William C. O'Reilly, R. J. Seymour, R. T. Guza, D. Castel||Ocean Wave Measurement and Analysis, Proc. 2nd Int. Symp.||1993-07-25 00:00:00|
|A Comparison of Two Spectral Wave Models in the Southern California Bight||William C. O'Reilly and R. T. Guza||Coastal Engineering||1993-05-01 00:00:00|
|A Comparison of Spectral Refraction and Refraction-Diffraction Wave Propagation Models||William C. O'Reilly and R. T. Guza||J. Waterway Port, Coastal and Ocean Eng.||1991-05-15 00:00:00|
|Wave observations in the storm of 17-18 January 1988||Seymour, R.J.||Shore & Beach||1989-10-01 00:00:00|
|Automated remote recording and analysis of coastal data||J. Waterway, Port, Coastal and Ocean Engineering, Proc. ASCE||1985-03-02 00:00:00|
|Influence of El Ninos on California's wave Climate||B.L. Edge, ed.||Proc. 19th Int. Conf. on Coastal Engineering, ASCE, Houston, Texas, 3-7 September||1984-09-03 00:00:00|
|Continuous estimation of longshore sand transport||Coastal Zone '78, Proc. Symp. on Technical, Environmental, socioecomonic and Regulatory Aspects of Coastal Zone Management, ASCE, San Francisco, CA, 14-16 March||1978-03-14 00:00:00|
|Measuring the nearshore wave climate: California experience||M.D. Earle and A Malahoff, eds.||Ocean Wave Climate, Plenum Press, New York, Marine Science||1977-07-12 00:00:00|
|Regional network for coastal engineering data||Proc. 15th Coastal Engineering Conf. American Society Civil Engineers (ASCE), Honolulu, HI, July||1976-07-01 00:00:00|
|Effects of El Nino on the West Coast Wave Climate||Seymour, R.J.||Shore & Beach||0000-00-00 00:00:00|
|Unusual marine erosion in San Diego County from a single storm||Dayton, P.K., R.J. Seymour, P.E. Parnell, and M.J. Tegner||Estuarine, Coastal and Shelf Science||0000-00-00 00:00:00|
|The Relationship Between Incident Wave Energy and Seacliff Erosion Rates: San Diego County, California||Bunumof, B.T., Storlazzi, C.D., Seymour, R.J., Griggs, G.B.||California. Journ. Coastal Research||0000-00-00 00:00:00|
|[Editorial] The great storm of January 1988||Seymour, R.J.||Shore & Beach||0000-00-00 00:00:00|
|Evidence for Changes to the Northeast Pacific Wave Climate||Seymour, R.J.||Journal of Coastal Research||0000-00-00 00:00:00|
|Storm wave induced mortality of giant kelp, Macrocystis pyrifera, in Southern California||Seymour, R.J., M.J. Tegner, P.K. Dayton, and P.E. Parnell||Estuarine, Coastal and Shelf Science||0000-00-00 00:00:00|
|COASTAL FORUM: Unusual damage from a California storm||Seymour, R.J.||Shore & Beach||0000-00-00 00:00:00|