Lumber River Basin
Floodplain Remapping Project

Coastal flood hazard analysis for Holden Beach and Ocean Isle Beach, North Carolina.

Following Hurricane Floyd’s widespread devastation, and in light of a high probability for future hurricane flooding, there was an urgent need to update floodplain maps throughout North Carolina. As a result, the State partnered with FEMA and 11 other federal agencies to launch the North Carolina Floodplain Mapping Program (NCFFMP), with the goal of creating up-to-date digital flood insurance rate maps for major waterways within its reaches. The significant amount of engineering and mapping work and the fast-track timetable required that advanced technologies be employed for floodplain mapping.

PBS&J modeled the subbasins within the Lumber River basin, studying more than 500 stream-miles of models including redelineations, limited detail, and detailed models.  PBS&J used geographic information system (GIS)-based hydrology and hydraulics packages for all of the modeling including one that automatically delineated basin boundaries on digital elevation models using sophisticated light detection and ranging (LIDAR) mapping to collect digital topographic data. Based on these delineations, United States Geological Survey (USGS) regression equations were used to determine flow quantities at specific locations within the drainage basins.  PBS&J completed the hydraulic modeling using HEC-RAS in tandem with HEC-GeoRAS.

PBS&J also revised a coastal flood hazard analysis for 14 miles of Atlantic shoreline along Holden Beach and Ocean Isle Beach, in Brunswick County, North Carolina. LIDAR topographic data was again used to develop a digital elevation model from which more than 25 shore-normal transects were derived. Coastal transects were processed through the standard FEMA erosion procedure, and wave height and wave run-up was modeled in CHAMP v. 1.0. Modeling results were mapped and interpolated on an aerial photographic base in ArcView (v. 3.2) GIS and, once approved by FEMA and the State of North Carolina, were linked with adjacent riverine flood hazards.