Numerical Model Study of Breakwaters at Grand Isle, Louisiana
Julie Rosati1994
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Abstract
Chapter 1 Introduction configured breakwater project was recently constructed in Cameron Parish in the western part of Louisiana and provides a model for designing a breakwater project at Grand Isle. This task reviewed the observed morphological response at the Cameron Parish project and made comparisons to the proposed project at Grand Isle. The most comprehensive study of Grand Isle is the 1979 U.S. Army Corps of Engineers report on Grand Isle. This report contains extensive information on coastal erosion, coastal processes, and sand resources, and designs for the Corps of Engineers beach erosion and hurricane protection project which was built in 1984. Combe and Soileau (1987) reported on construction details and the successful performance of this project during and after the unusual 1985 hurricane season in which Hurricanes Danny, Elena, and Juan impacted the island. A report related to this investigation of proposed breakwaters at Grand Isle was prepared by engineering firm HNTB under contract for the Louisiana Department of Natural Resources, Coastal Restoration Division (1993). This report reviews Louisiana State Senate Concurrent Resolution Number 83, which urges the Wetlands Conservation and Restoration Authority, the Department of Transportation and Development, and the U.S. Army Corps of Engineers to examine the feasibility and advisability of placing breakwater protection along the coast of Grand Isle, Louisiana. The report provides a good summary of coastal projects constructed at Grand Isle between 1951 and 1991, reviews the performance of the breakwater project at Holly Beach, Louisiana, and proposes a conceptual segmented detached breakwater project design and layout for Grand Isle. This chapter describes procedures and results of the wave data preparation and nearshore wave refraction task of the study. This task consisted of three steps and resulted in the preparation of wave information that was subsequently employed in the numerical shoreline evolution simulations presented in Chapter 3. The first step involved the transformation of hindcast estimates of the offshore wave climate to the Grand Isle project area. In the second step, the time history of wave information at the Grand Isle project area was statistically analyzed and 22 categories of potential wave approach and wave period were identified. The results of the statistical analysis were also used to select a 4-year-long representative time history of wave conditions from the 20-year-long hindcast database. The third step involved the use of a numerical wave refraction model to obtain representative nearshore wave conditions in shallow water at fixed points alongshore. No long-term wave measurements are available for the vicinity of the Grand Isle project. Therefore, the required wave information was generated by means of the Wave Information Study (WIS) hindcast technique. The WIS provides a 20-year hindcast for the U.S. Gulf of Mexico coast for the years 1956 through 1975. The Gulf of Mexico hindcast wave information ) includes a 20-year time history of wave height, wave direction, and wave period at 3-hr intervals for both sea and swell at 50 stations along the Gulf of Mexico coasts of Florida, Alabama, Mississippi, Louisiana, and Texas. Hindcast Station 21 located offshore of Grand Isle at 28.5 deg north latitude and 90.0 deg west longitude was selected for use in this study. Phase 111 transformation. Wave information at WIS Station 21 is representative of wave conditions in 300 ft of water at this geographic location, however, since the location is gulfward of the Mississippi River delta which blocks wave energy approaching from the east, these wave conditions Chapter 2 Wave Refraction Analysis Chapter 3 Long-Term Shoreline Change Chapter 3 Long-Term Shoreline Change zone). If an energy window is open (no diffraction source) on one side, the diffraction coefficient for that side is set equal to 1.0.
