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Impacts of Evolving Nearshore Topography on Flood Risk

This case study explores how changing topography affects base flood elevations, with implications for stakeholders in Pacific County, Washington. The study uses data from NDBC and NOAA to analyze wave heights, erosion, and flood risk implications.

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Impacts of Evolving Nearshore Topography on Flood Risk

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  1. Case Study on Impacts ofEvolving Nearshore Bathymetryand Topography on Base Flood Elevations andImplications for Stakeholders D. Michael Parrish, PhD, PE, CFM ASFPM May 2011 How does topography affect base flood elevations? Discussion / Practical Implications Next Slides: Methods / Results

  2. Site Description • Setting • Plane Beach backed by Dune • Open Coast • Location • Pacific Northwest • Pacific County, Washington • Long Beach Source: Ruggiero et al.

  3. Data Sources • NDBC buoy 46029 • Significant Wave Heights • Peak Period • Mean Wave Direction • NOAA gage 9440910(Toke Point) • Still Water Level • Profile 28, USGS Report DS260 (Ruggiero et al.) • Surf Zone and Backshore Topography / Bathymetry • Grain Size • GG NDBC: National Data Buoy Center USGS: United States Geological Survey NOAA: National Oceanographic and Atmospheric Administration

  4. Methods • Event Selection • MaximumWave Heights • Beach Slope • Measured Manually from Data • Wave Runup • Direct Integration Method (DIM), FEMA Guidelines • Storm Durations • Full Width at Half Maximum (FWHM), Parrish • Erosion • Modified Komar & Allan Geometric Model (MK&A), FEMA

  5. Event Selection

  6. Event Selection 1995 2000 2005 2010

  7. Surf Zone and Backshore Topography—Ground Profiles

  8. Surf Zone and Backshore Topography—Ground Profiles

  9. Surf Zone and Backshore Topography—Ground Profiles

  10. Surf Zone and Backshore Topography—Ground Profiles

  11. Surf Zone and Backshore Topography—Ground Profiles

  12. Surf Zone and Backshore Topography—Ground Profiles

  13. Surf Zone and Backshore Topography—Ground Profiles

  14. Surf Zone and Backshore Topography—Ground Profiles

  15. Surf Zone Topography—Slopes Year 1998 1999 2000 2001 2002 2003 2004 2005 1/m = cot α 34 65 50 52 52 56 54 56

  16. Base Flood Elevations (BFE’s)—Wave Runup 13 12

  17. -100 -50 0 Horizontal Extent

  18. Summary • BFE’s stable • SFHA movingseaward SFHA: Special Flood Hazard Area

  19. Implications for Stakeholders • Property Owner or Resident • Threat moving seaward. • Potential to overpay for insurance. • Builders • Topography changes annually, but structure intended to last decades. • A 1998 flood study might lead to stricter building codes. • Floodplain Managers • Evolving topography complicates management decisions. • Flood insurance rate map (FIRM) might constitute misinformation

  20. Implications for Stakeholders • Emergency Services • As coastal threat declines, other threats become more important. • Funding agency • Value of ground survey? • Value of recent survey in comparison to two older surveys? • National Flood Insurance Program • Potentially overcharging policy holder • Inability to represent extent of inundation on a static map

  21. Discussion D. Michael Parrish mparrish@g-and-o.com

  22. References • Ruggerio P et al. (2007) Beach Morphology Monitoring in the Columbia River Littoral Cell: 1997–2005. USGS Data Series 260. http://pubs.usgs.gov/ds/2007/260/ • NOAA Tides and Currents http://tidesandcurrents.noaa. gov/geo.shtml?location=9440910 • NDBC Staion 46029 http://www.ndbc.noaa.gov/station_history.php?station=46029

  23. Surf Zone and Backshore Topography—Ground Profiles

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