At the peak of the 2008 hurricane season and with several tropical disturbances on the radar, the National Oceanic and Atmospheric Administration (NOAA) dedicated the first of four new Sentinels on a clear day with blue skies in Bay St. Louis, Miss.
The Sentinels are 25-ft. (7.6 m) tall single-pile steel structures with water level observing stations. They provide valuable real-time data, including water level and meteorological information during storms and they are designed to withstand a Category 4 hurricane. Elevated atop substantial single pile platforms and painted International Safety Yellow, they are hard to miss. Their color is a designated Coast Guard shade.
NOAA has constructed four Sentinels along the Louisiana-Mississippi Gulf Coast. There are three Sentinels in Louisiana located at Amerada Pass, Calcasieu Pass and Lake Borgne, which is off of Shell Beach in St. Bernard Parish. The fourth Sentinel, where the Aug. 22 dedication occurred, is near Waveland, Miss., in Bay St. Louis.
The Center for Operational Oceanographic Products and Services (CO-OPS), part of NOAA’s National Ocean Service, is responsible for operating the National Water Level Observation Network (NWLON). CO-OPS turns all of the operational oceanographic data into meaningful information for the United States.
John Stepnowski, a CO-OPS engineering technician and PORTS Site Rep, explained, “Once the data is accepted then we disseminate the data to the general public.”
Another beneficial aspect of NWLON is archived data.
“Archived data are used in many coastal protection engineering projects like levee construction and evacuation route planning,” Stepnowski said.
According to Stepnowski, the Shell Beach Sentinel is “the only one that is operating but not operational.” Basically, that Sentinel will be fully online, like the other three, once it goes through various levels of quality assurance, including a 45-day initial data gathering process.
Woods Hole Group Inc., East Falmouth, Mass., was the prime contractor responsible for installing the electronics equipment, the platforms, and the sensors on the Sentinels. Woods Hole Group specializes in observational program design and engineering services that blend custom and standard instrumentation for real-time data acquisition.
Woods hired Berry Bros. General Contractors Inc., Berwick, La., as the subcontractor to install and paint the Sentinels. The design of the pile was created by TranSystems, headquartered in Kansas City, Mo., with an office in Norfolk, Va.
The design of the Sentinels, single-pile structures, presents a minimal profile to a storm coming from any direction. Research, based on wind and waves generated by a Category 4 hurricane (131 to 155 mph winds), determined that the platforms should stand at least 25 ft. (7.6 m) above the water’s surface, and they should be situated on a 4-ft. (1.2 m) diameter single pile.
Ryan White, Berry Bros. engineer, describes the equipment used to transport the piles and to install the sentinels. The piles are brought to the Sentinel locations via barge.
“The barge was custom built by Berry Bros. to more efficiently complete the types of work that we deal with on a day to day basis,” said White. “The pile driving barge is a 150- by 40- by 9-foot barge with a Link Belt 338 crane.”
Berry Bros. used rental hammers made by Pileco to drive the piles 60 to 80 ft. (18 to 24 m) into the seafloor. A Pileco D46-32 diesel hammer with offshore bell leads and a Pileco vibratory hammer with a 550 power pack and remote control operations were used to drive the piles. The piles for the Sentinel installed at Calcasieu Pass withstood 15 blows of a hammer per foot.
“The strategy,” said Stepnowski, “was to use a vibratory hammer to drive [a pile] within 15 feet of final elevation, then we would switch to the diesel hammer to get within a tenth of the final elevation.”
The piles are mostly driven underwater and are driven deep to ensure stability. The bottom of the piles, mostly below the water’s surface, are 4 ft. (120 cm) in diameter then the pile tapers or narrows as it reaches to the sky. At the top it is 36 in. (91 cm) in diameter.
Three of the four Sentinels are made of 1-in. (2.5 cm) thick steel. The Sentinel at Calcasieu Pass was 1.25-in (3.2 cm) thick steel. Calcasieu Pass also was unique because, since it was 120 ft. (36.6 m) long, the piles had to be shipped in two sections. The barge carries piles up to 95 ft. (29 m) long. The 48-in. (120 cm) diameter section of the Sentinel was 94 ft. (28 m) long, and the 36-in. (91 cm) diameter section was 26 ft. (7.9 m) long.
The Sentinel at Amerada Pass was originally planned to be 85 ft. (26 m) long. However, according to Stepnowski, “We had to use an emergency length of pipe because the soil was mud and went down further than thought.” As a result, the Sentinel ended up being 130 ft. (40 m).
“More sections had to be welded below the taper,” Stepnowski said.
There are several factors that determine where the Sentinels will be built. The foremost objective is to situate them in coastal areas that are most vulnerable to severe storms. Another requirement that determined location was to replace or re-establish the NWLON stations that were either destroyed or heavily damaged by recent hurricanes. New station locations also were determined, said Kate Bosley, outreach coordinator, CO-OPS, based on areas that were identified as “gaps in our network.”
To build and install the four Sentinels, NOAA obtained the funds, approximately $2 million, from post-Hurricane Katrina federal recovery money.
“Future installations,” Bosley explained, “depend on Congress to approve funding.”
The dedication ceremony was attended by U.S. Sen. Thad Cochran of Mississippi; Michael Szabados, director, NOAA’s CO-OPS; David Mooneyhan, director Gulf Coast Geospatial Center; and La Donn Swann, director, Mississippi-Alabama Sea Grant Consortium. Bosley was in attendance for the ceremony as well.
For more information, visit www.tidesandcurrents.noaa.gov. CEG
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