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Shelf Width, Shoreline Curvature, and Hurricanes: Continued Development of a New Hurricane Impact Scale

Shelf Width, Shoreline Curvature, and Hurricanes: Continued Development of a New Hurricane Impact Scale David M. Bush 1, Robert S. Young 2 , and Chester W. Jackson 1 1 State University of West Georgia 2 Western Carolina University

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Shelf Width, Shoreline Curvature, and Hurricanes: Continued Development of a New Hurricane Impact Scale

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  1. Shelf Width, Shoreline Curvature, and Hurricanes: Continued Development of a New Hurricane Impact Scale David M. Bush1, Robert S. Young2, and Chester W. Jackson1 1State University of West Georgia 2Western Carolina University

  2. Recent NC HurricanesSource: NOAA Tropical Prediction Center Reports Several similarly sized hurricanes have hit the North Carolina coast in recent years with very different impacts.

  3. Some of the difference is due to the shoreline setting. Here is the retreating shoreline and overwash apron of Oak Island.

  4. Bogue Banks, meanwhile, is a higher sand volume island, retreating bluff setting.

  5. The Saffir-Simpson Scale The classical way to rank hurricanes is the Saffir-Simpson Scale, fine for hurricanes over open water but not so good with predicting erosion and overwash impacts at landfall.

  6. Hurricane Impact Scale The Hurricane Impact Scale would consider storm surge height and storm surge spread (length of shoreline affected), as well as the Saffir-Simpson Scale, to rank hurricanes.

  7. The important controls on storm surge at landfall are: (1) storm physical characteristics, (2) movement of the storm, and (3) shape of the shoreline relative to storm track.

  8. Shelf width and shoreline shape are two important controls on storm surge: Wide shelf = higher storm surge Narrow shelf = lower storm surge Concave shoreline = higher storm surge Straight or convex shoreline = lower storm surge

  9. miles along coast Hurricane Opal hit a curved stretch of the Florida panhandle in 1995 and clearly shows a classic storm surge profile on a shoreline with a curvature at a scale matching the size of the hurricane.

  10. Factors Andrew Hugo Frederic Opal SSS (wind) 4 4 3 3 Surge Height (above SHT/rank) 13.6/4 10.4/3 15.4/4 18.4/5 Surge Spread 1 5 3 4 HIS Rating 9 12 10 12 Normalized HIS 3.0 4.0 3.3 4.0 HIS Applied Applying the HIS for recent major storms shows that the impacts of the storms at landfall were at scales somewhat different than their Saffir-Simpson Scale category rankings.

  11. Factors Fran Floyd SSS (wind) 3 2 Surge Height (ft above SHT/rank) 6.5/2 5.8/2 Surge Spread 1 1 HIS Rating 6 5 Normalized HIS 2.0 1.7 Fran vs. Floyd The same is true for Hurricanes Fran and Floyd. Most of Floyd’s damage was from rainfall which is not considered by the Hurricane Impact Scale.

  12. Shoreline Curvature Index C Land Sea P Curvature = C/P Future work calls for applying a shoreline curvature parameter. Shoreline curvature may be quantified by dividing the chord length (C) by the perpendicular length (P) for various coastal segments.

  13. The ratio of shelf width (light blue line) to shoreline curvature (C/P) yields a width/curvature index (white numbers). This index has yet to be incorporated into the HIS.

  14. Predictive (pre-storm) Shelf width Shoreline curvature (at several scales) Predicted storm track Predicted storm strength Predicted storm forward speed Historical storms Range of HIS scales Comparative (post-storm) Field measurements Surge height Surge spread Wind speeds NHC/NWS Single HIS rank assigned HIS Modes

  15. With shoreline settings quantified, local coastal managers will be able to predict the range of possible landfall impacts of a hurricane for their particular stretch of shoreline, given the National Hurricane Center’s predicted strength of the hurricane.

  16. In Terms of Earthquakes • Saffir-Simpson Scale = Richter Scale • Measure of energy released • Absolute scale • Hurricane Impact Scale = Mercalli Scale • Measure of impacts • How the storm was actually felt • Both EQ scales are post-event measurements • SSS and HIS both predictive and comparative • HIS attempting to refine predictive capability

  17. Another Way to Look at it: • Every hurricane is different, has its own personality • SSS doesn’t take that into account, too rigid • SLOSH maps good for their application (evacuation and sheltering) but too inclusive • HIS will consider and predict “hurricanes as individuals” impacts

  18. Conclusions I • SSS good for indicating hurricane strength • Storm surge height and extent is a good indicator of energy flux • Many controls on storm surge not handled by SSS • New scale needed to work in conjunction with SSS

  19. Conclusions II • Same storm in different setting means different impacts • Hurricane Impact Scale will allow better communication of potential storm impacts as storm approaches • Pre-storm, a range of HIS values given • Post-storm, one final HIS rank determined

  20. Acknowledgements • Federal Emergency Management Agency • Public Entity Risk Institute • Natural Hazards Center • Orrin Pilkey, Duke University • Wil Shaffer, NOAA • Steve Lyles, NOAA/NOS

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