Leslie C. Bender

A Comparison of Methods for Determining Significant Wave Heights—Applied to a 3-m Discus Buoy during Hurricane Katrina

Abstract In August 2005, the eye of Hurricane Katrina passed 90 km to the west of a 3-m discus buoy deployed in the Mississippi Sound and operated by the Central Gulf of Mexico Ocean Observing System (CenGOOS). The buoy motions were measured with a strapped-down, 6 degrees of freedom accelerometer, a three-axis magnetometer, and from the displacement of a GPS antenna measured by postprocessed-kinematic GPS. Recognizing that an accelerometer experiences a large offset due to gravity, the authors investigated four different means of computing wave heights. In the most widely used method for a buoy with a strapped-down, 1D accelerometer, wave heights are overestimated by 26% on average and up to 56% during the peak of the hurricane. In the second method, the component of gravity is removed from the deck relative z-axis accelerations, requiring pitch and roll information. This is most similar to the motion of the GPS antenna and reduces the overestimation to only 5% on average. In the third method, the orient...

Observing and forecasting coastal currents: Texas Automated Buoy System (TABS)

The Texas Automated Buoy System operates buoys at seven sites off the Texas coast from Brownsville to Sabine in water depths ranging from ten to one hundred meters. The system is supported by the Texas General Land Office as part of its mission to mitigate the effect of catastrophic oil spills on the Texas Coast. Buoys communicate surface currents and water temperature measured at 2m depth in near real time via cell phone and commercial satellite digital data links. One buoy located at East Flower Garden Banks has a 300 kHz ADCP, a meteorology package with an ultrasonic acoustic wind velocity sensor, and a conductivity sensor. Data are posted regularly to a Web page http://www.gerg.tamu.edu/tglo and are available to the public and governments within a few hours after data collection. On the TABS Web page, a graphical map presentation of TABS current vectors has links to data tables and historical databases. Links are also provided to other data resources for oceanographic data in the Gulf of Mexico. Also on the Web page are links to an automated continental shelf forecast system that predicts currents over the Texas-Louisiana shelf on an operational basis. There are four major components in this system: (1) forecast wind field retrieving and preparation, (2) shelf circulation model module, (3) simulation plotting module and (4) Web display and file transfer module. The wind field used is a 3-hour interval ETA-22 forecast gridded wind from NOAA NCEP based on 00, 06, 12, and 18UTC model runs. The shelf circulation model is a 3-D version of Princeton Ocean Model (POM) on a domain extending from the coast to a curved line extending from 25/spl deg/N on the Mexican coast to 85/spl deg/W at the coastline of Florida. The operational POM model used at this time is a simplified barotropic version that permits us to reduce computational time to allow prediction of surface currents twenty-four hours into the future. The data vs. model comparison from April through December, 1999 of nine nearshore TABS buoys indicates modest skill of the model in predicting the wind driven circulation. A fully baroclinic version of TABS-POM model is undergoing tests and will be implemented on an operational basis when sufficient computational resources become available. We are also developing data-assimilating models of the whole Gulf of Mexico and beyond to couple to our shelf model to supply outer boundary conditions.

Wave Heights during Hurricane Katrina: An Evaluation of PPP and PPK Measurements of the Vertical Displacement of the GPS Antenna

Abstract In August 2005 the eye of Hurricane Katrina passed 49 n mi to the west of a 3-m discus buoy operated by the Central Gulf of Mexico Ocean Observing System (CenGOOS). Buoy motions were measured with a strapped-down 6 degrees of freedom accelerometer, a three-axis magnetometer, and a survey-grade GPS receiver. The significant wave heights were computed from the buoy’s accelerometer record and from the dual-frequency GPS measurements that were processed in two different ways. The first method was postprocessed kinematic (PPK) GPS, which requires another GPS receiver at a fixed known location, and the other was precise point positioning (PPP) GPS, which is another postprocessed positioning technique that yields absolute rather than differential positions. Unlike inertial measurement units, either GPS technique can be used to obtain both waves and water levels. The purpose of this note is to demonstrate the excellent reliability and accuracy of both methods for determining wave heights and periods from...

The effects of marine fouling on the performance of a single-point acoustic Doppler current sensor mounted on a TABS-II spar buoy

Following the successful results of an experiment conducted during June and July, 2000 to determine how a single-point, acoustic Doppler current sensor would perform on a TABS-II spar buoy, a second experiment was conducted to determine the effects of marine fouling on the sensor. The TABS-II spar buoy is one of two types of spar buoys used by the Geochemical and Environmental Research Group (GERG) to monitor and report near real-time surface currents, in support of oil spill response and trajectory modeling for the Texas General Land Office in Austin, Texas. An Aanderaa DCS3500R Doppler current sensor was mounted on a TABS-II spar buoy and moored at a location approximately 22 nmi southwest of Galveston, Texas in 19 in of water. The mooring remained in place from June 1 to December 15, 2000 during which time it reported hourly near-surface current components and temperature via satellite telephone to GERGs office in College Station. On November 15, 2000 a second TABS-II spar buoy, equipped with a clean, calibrated, Marsh-McBirney, Inc., (MMI) Model 585 electromagnetic sensor, was deployed 200 in downcoast of the DCS3500R-equipped buoy. A 600-kHz RD Instruments directional-wave ADCP was mounted on the bottom between them in an upward-looking configuration. On November 15, 2000, prior to deployment of the MMI-equipped buoy and the ADCP wave meter, the DCS3500R-equipped buoy was recovered to remove marine growth from the hull and mooring. Barnacle growth on the DCS3500R was extensive, covering the entire sensor and cable.

Wave heights from a 3m discus buoy during Hurricane Katrina

In August 2005 the eye of Hurricane Katrina passed 49 nm to the west of a 3-m discus buoy (USM3m01), in 20 m water depth, operated by the Central Gulf of Mexico Ocean Observing System (CenGOOS). Buoy wave heights were measured with an onboard 6-axis accelerometer and from the displacement of a GPS antenna as measured by Real-Time-Kinematic (RTK) GPS. The computed wave heights are compared to the nearby NDBC 42007 buoy and show reasonable agreement for wave heights less than 2-3 m. At higher wave heights there is a potential bias in the wave heights if the buoys heel is not accounted for. This is believed to be the result of swell in shallow water, a heel in the buoy caused by wind and currents, and the failure to tilt-correct the accelerometer data.