NRL Measures Record Wave During Hurricane Ivan

The possibility of a super wave is often suggested by anecdotalevidence such as damage caused by Hurricane Ivan in September of 2004to an offshore rig in the Gulf of Mexico that was nearly 80 feet abovethe ocean surface. Hence, some of the destruction done by Ivan has beenattributed to a rogue wave.

According to industry and national weather sources, the damage doneby waves during Ivan has been on the extreme high end for a category 4hurricane. Ivan has been the most expensive hurricane ever for the oiland gas industry in the Gulf. The Minerals Management Service (MMS)reported that Ivan amazingly forced evacuation of 75% of the mannedplatforms in the Gulf (574 platforms) and 59% of the drilling rigs (69rigs), set adrift 5 rigs and sunk 7 rigs entirely. However, the damageby Hurricane Ivan in the oil fields in the Gulf cannot be measured byhow many platforms or rigs were destroyed. The most costly damage isbelieved to have been made to the underwater pipelines. Aside fromobvious leaks, some pipelines were reported to have moved 3000 ft whileothers were buried under 30 feet of mud and cannot be found. The mostextensive damage to the pipelines is attributed to undersea mudslides(equivalent to a snow avalanche) and to extreme waves. The completefindings of this study are published in the August 5, 2005 issue ofScience.

During NRL's Slope to Shelf Energetics and Exchange Dynamics (SEED)field experiment, six current profiler moorings that also containedwave/tide gauges (Sea-Bird Electronics SBE 26) were deployed on thecontinental shelf at water depths ranging between 60 and 90 meters justwest of the DeSoto Canyon, about 100 miles south of Mobile Bay,Alabama. An additional eight deep moorings were deployed down the shelfslope but did not contain wave/tide gauges. Fortuitously, between 8:00pm CDST and midnight on September 15, the eye of Ivan passed throughthe center of the array, and almost directly over moorings 2, 5, 8, and11. Historically, instruments in the ocean do not even survive nearmisses of such powerful storms, much less direct hits. Fortunately, allof the SEED moorings survived this powerful storm, and provided thebest ocean measurements of currents and waves ever obtained directlyunder a major hurricane.

During the approach of Ivan, a moored buoy (ID 42040),deployed by the National Data Buoy Center (NDBC) near the west side ofthe SEED array, registered a significant wave height of 16.0 meters (53ft). Unfortunately, the NDBC buoy broke loose and was set adrift onSeptember 15 at 5:00 pm CDST, just before the arrival of the main forceof the hurricane. According to a spokesman at NDBC, this wave heightappears to be the largest ever reported by NDBC from a hurricane andcomes within a few tenths of a meter of NDBC's all-time record reportedin the North Pacific.

Note that the wave heights reported by the NDBC buoys are derivedfrom wave spectra. Buoy measurements do not report time series ofsurface wave elevations, and hence, maximum individual wave heights canonly be statistically postulated from spectrum-derived significant waveheights. The SEED wave/tide gauges, however, provided direct timeseries measurements of surface wave elevations. The maximum individualcrest-to-trough wave heights can be reliably obtained. At mooring 3,located under the most intense winds, the maximum measured wave heightwas 27.7 meters (91 feet) which was part of a group of large waves withperiods of approximately 10 seconds where several waves reached heightsof about 20 meters (66 feet).

These waves recorded by the NRL SEED gauges are by far the largestwaves ever directly measured. Even larger waves could have been missedentirely on the shelf since the surface wave data from the SEED gaugeswere not closely sampled in time, but were instead sampled at 1 hertzover a 512 second data burst only every 8 hours. Analysis of the wavedata with the winds suggests that the wave heights likely exceeded 130feet near the eye wall of the hurricane. Orbital wave velocitiesgenerated by such large waves during Hurricane Ivan (not rogue waves)exceeded 2 meters/second at the ocean bottom (in addition tolower-frequency measured currents that exceeded 1 meter/second) andcould certainly have caused much damage to underwater structures andpipelines.

The measurement of "super waves" cannot be planned and are indeedvery rare. These in-situ measurements made by NRL directly under acategory 4 hurricane are very valuable since they can be used toprovide an assessment of potential impacts to offshore structures andoperations by energetic storm waves.