Infrared image of Hurricane Opal on October 4, 1995, approaching land along Santa Rosa Island, Florida.
On October 4, 1995, over 2000 kilometers of coastline stretching from southwest Florida to Louisiana was struck by storm-generated waves as Hurricane Opal moved northward across the Gulf of Mexico toward landfall east of Pensacola Beach, Florida (Figure 1). Approximately 12 hours before landfall on October 4, Opal neared category 5 strength (measured on the Saffir/Simpson scale) with sustained wind speeds of over 65 meters per second. Storm surge levels of about 5 meters were estimated across the Northwest Florida shelf by the National Hurricane Center (NHC), resulting in the overwash of most of Santa Rosa Island, the most affected section of coast in the Gulf.
Opal ranked the strongest of the eighteen hurricanes that have struck the Pensacola area since 1900, and it was probably the most devastating. Beach profile stations surveyed along Santa Rosa Island 1 month before hurricane landfall were reoccupied 2 days after Opal crossed the coast, providing a unique opportunity to quantify the morphological impacts of the hurricane along the coast.
Although Hurricane Opal was very strong and removed dunes higher than 5 m above sea, the island lost very little sand throughout the entire landfall event. This has important implications for the morphodynamics of coastal systems during storm events, and specifically for the redistribution of sediment.
After drifting over the Yucatan Peninsula and the southwestern Gulf of Mexico for more than 5 days, Hurricane Opal turned northeast toward the Florida peninsula. National Hurricane Center advisories and satellite imagery from the Louisiana State University Earth Scan Laboratory indicated that the system significantly intensified as it passed over the central Gulf, attaining category 4 status on the Saffir/Simpson scale by 6:00 a.m. Eastern Standard Time on October 4.
Twelve hours later, Opal was just east of Pensacola. Only minutes before, a peak gust of 64 meter per second wind was recorded at Hurlburt Air Force Base, approximately 50 kilometers east of Pensacola, while calm winds were reported between these two locations. These conditions indicated that Opal's center had indeed made landfall.
Hourly observations of significant wave height and dominant wave period were obtained from the National Data Buoy Center's buoys 42001 and 42036, which were approximately 50 km west and 325 km east of the storm track, respectively (Figure 2). The sea surface began responding to Hurricane Opal approximately 18 hours before the system passed over buoy 42001. Wave heights peaked at 8 meters as the leading portion of the hurricane moved over the buoy, dropping to approximately 7 meters as the trailing portion of the system moved through. Dominant wave period was relatively consistent at 13 seconds. Although buoy 42036 was further from the storm track, maximum wave heights and periods were similar to those recorded at 42001 because the system strengthened to a strong category 4 some 12 hours before it hit land.
Eighteen hurricanes, including Opal, have touched down in the Pensacola area since 1900. Prior to Opal, the most devastating of these was Frederic, a category 3 hurricane that hit Alabama in 1979. An airplane survey of the Florida panhandle 2 days after Opal touched down revealed that it caused more damage to infrastructure and greater morphological response of the coast than Frederic had. Dunes along the coast were vegetated and stable and 5 meters high before the storm, but they were entirely removed along Santa Rosa Island and Destin during the storm.
To quantify the morphological response of the area to the hurricane, beach profiles were surveyed west of Pensacola Beach. On Santa Rosa, dunes approaching more than 5 meters were reduced to approximately 1.5 meters high as the island experienced severe overwashing. Remarkably, between 95<196>99% of the sediment eroded from the beach-dune system landed in the barrier interior and along the bay shoreline. Large fanlike features (overwash deposits) were present along the entire island and in several locations and relocated the bay shoreline by 100 meters into Santa Rosa Sound. This is a common response to severe storms and one that actually permits barrier islands to survive for thousands of years
In addition, the gulf-facing shoreline did not move toward land during the storm as often happens during such events. Instead, virtually all of the change occurred as an erosional-depositional couplet, a zone in which the amount of sediment eroded from one site and deposited along another is balanced.
This particular finding has significant implications for reassessing the widely held assumption that substantial volumes of sediment are transported offshore to the continental shelf during hurricanes. In addition to the marked morphological change to the barrier, a significant coarsening of the sandy component of the beach occurred, and no noticeable increase in the carbonate (shell) fraction was observed<197>an unusual situation for beach response to strong hurricanes.
Scientists are still studying the effect of the hurricane on Santa Rosa Island. They hope to quantify recovery rates across the entire barrier system, interrelationships between beach volume increase and dune growth, identify periods of maximum sand accumulation, and measure the sediment size and composition against conditions before the storm. Source: Eos, May 7, 1996, p. 181.
I first became interested in science when I worked in industry for 3 years after graduating from high school as a chemist's assistant. I loved the job, and got a chance to understand more about environmental chemistry--water analysis for pollutants, air analysis for pollutants, etc. I also got the opportunity to participate in a release program that the company encouraged in which I would be released from work for a day each week to attend a local college and study chemistry, physics, and mathematics. This is when I knew I wanted to go to college and study the Earth sciences.
I grew up in Londonderry, Northern Ireland, and completed my bachelors degree at the University of Ulster in a town called Coleraine in Northern Ireland. I won a scholarship from the British government to study for a master's degree in the United States at the University of Florida and then completed my Ph.D. in coastal geomorphology at the University of Maryland. I currently study the impacts and recovery of beaches and coastal systems during and after hurricanes and numerically model wave behavior along coasts. I offer the following advice to aspiring scientists: Become very aware of your scholastic strengths in the sciences. Make sure you have a thirst for knowledge in this field you have identified before devoting your life to it at graduate school. Identify the top scientists in your field, the institutions they are teaching/conducting research at, and make contact. Visit them, their facilities, campus, and secure ample funding before accepting a place in their graduate program. Love your research and you will excel.--Gregory W. Stone
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