1. Methods 1. Dune area delineated with GIS before and after storms 2. Calculate total loss and mean loss in dune area for foredunes (N = 26) and secondary dunes (N = 53) 3. Examine changes in dune structure for both types of dunes from hurricanes 4. Evaluate importance of structural and spatial features as predictors of erosion with classification trees Structural variables measured: Dune height (m) Dune area (ha) Dune width (perpendicular to shoreline) (m) Dune length (parallel to shoreline) (m) Spatial variables measured: Distance from eye of hurricane at landfall (km) Island width at center of dune (km) Size of gap between nearest dunes (m) Presence / absence of foredunes for secondary dunes Spatial and structural features influencing dune erosion from hurricanes and implications for barrier island restoration Alexander J. Pries †, Deborah L. Miller ††, and Lyn C. Branch † † Department of Wildlife Ecology and Conservation, The University of Florida, Gainesville, FL USA †† Department of Wildlife Ecology and Conservation, West Florida Research and Education Center, Milton, FL USA Introduction With increases in tropical storm activity and frequency predicted (Emanuel 2005), protection and restoration of coastal dunes is becoming increasingly important for coastal managers. While laboratory research and mathematical models have examined dune erosion extensively, few studies have been able to quantify the relationship of dune structure to storm-related erosion in the field. Additionally, little research has examined erosion in secondary dunes on barrier islands, which are found furthest from the shoreline. Future efforts to protect and restore coastal dunes also would benefit from information on additional physical and spatial factors that influence erosion potential. We assessed coastal dune erosion along Santa Rosa Island, a barrier island ecosystem in the Gulf of Mexico (USA), after Hurricanes Ivan (2004) and Dennis (2005). In the last decade, storm impacts along the northern portion of the Gulf of Mexico have dramatically altered coastal dune structure on this barrier island. This has lessened the dunes’ ability to protect infrastructure and to provide habitat for wildlife. Our objectives were: (a) To document losses and changes in dune structure to established foredunes and secondary dunes on Santa Rosa Island from hurricanes. (b) To evaluate the role of landscape context and structural features of dunes as predictors of dune vulnerability from hurricanes. Study Area Foredunes near shoreline were once continuous (avg. height prior to Hurricane Opal (1995) = 3.8 m). Secondary dunes on landward side. Study on ~20 km portion managed by Eglin Air Force Base. Location of Santa Rosa Island, FL. Hurricanes Source: NOAA Dennis (July 10, 2005) Source: NOAA Ivan (September 16, 2004) Landfall as category 3 storm (121 mph winds) Passed ~ 100 km west of study area Storm surge = 3 – 4.5 m Most destructive hurricane in 100 years Landfall as category 3 storm (121 mph winds) Passed ~ 10 km west of study area Storm surge = 1.8 – 2.2 m Higher storm surge than predicted Remains of established foredunes after Hurricane Ivan’s impact on Santa Rosa Island, FL. Established foredunes prior to Hurricane Ivan. Foredunes are built by wind forces and vegetation that traps sand. Vegetation also provides ecosystem function, further stabilizes the dune, and provides food and cover for wildlife. Conclusions Hurricane impacts severely reduce total area and alter structure of foredunes; loss of foredunes leads to further erosion of secondary dunes during later storms. Structural features explaining patterns of dune erosion different for foredunes and secondary dunes suggesting different processes act on dunes farther from shoreline. Results confirm laboratory and mathematical models of dune erosion; dune height and dune shape (i.e., width) predicted to be important based upon processes of sediment slumping and continual wave action from storm surge during storm events. Spatial context also important; increased secondary dune erosion further from eye of hurricane likely caused by funneling of storm surge through a narrowing Santa Rosa Sound behind barrier island. References Emanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688. Wassilieff. M 2006. “Coastal Shoreline”,Te Ara - The Encyclopedia of New Zealand, URL: http://www.TeAra.govt.nz/EarthSeaAndSky/MarineEnvironments/CoastalShoreline/en Generalized model of dune erosion Source: Wassilieff 2006 Established secondary dunes prior to Hurricane Ivan. These dunes are located on the landward side of the island and are no longer controlled by wind forces. Vegetation on these dunes is dominated by woody species. Classification tree explaining patterns in dune erosion for (a) foredunes and (b) secondary dunes from Hurricane Ivan. Percentages are mean percentage of dune loss and N is number of dunes for the grouping. Results Dune Loss & Change in Dune Structure Dune area Dune heightGap between dunes Mean % loss in dune area Wider dunes (regardless of type) on Santa Rosa Island experienced less erosion than narrower dunes. Erosion of foredunes reduced with increasing distance from eye of hurricane. Taller foredunes experienced less dune erosion. Presence of foredune reduced dune erosion in secondary dunes. Erosion in secondary dunes increased with increasing distance from eye of hurricane. Patterns in dune erosion less clear after Dennis, likely because of more variability and reduced number of dunes. Restoration Implications Use sand fencing techniques and multiple fences to “layer” dunes of greater height and width. AND Plant native vegetation that traps sand early in dune development to ensure dunes are more stable during storm events. Make considerations for spatial features (i.e., offshore sand bars, or narrow sounds) that may predispose areas to more or less erosion during storms. After storm events, plant coastal species that trap sand and use sand fences to rebuild dunes. a b c * ** a a a * * * * a b c * *