Coastal Hydrodynamic and Morphodynamic Response to Tropical Cyclones I
Presiding: G W Stone, Louisiana State University; B A Murray, Duke University
OS33A-01 13:30h
Dune Retreat and Shoreline Change on the Outer Banks, North Carolina, 1997-1998
LIDAR data were collected in late September of 1997 and 1998 along a 175 km stretch of the Atlantic coast of the Outer Banks, North Carolina between Cape Lookout and Oregon Inlet. These data, available from NOAA, provide the basis for quantitative determination of the changes in beach morphology during this one-year interval. There had been no recent large storms preceding the 1997 data collection. The 1998 data were collected 4 days and 10 days after Hurricane Bonnie. Beaches east and north of Ocracoke Inlet have been stabilized and maintained since the 1930s while beaches to the south and west have been left in their natural state. We examine shoreline perpendicular profiles, taken every 20 m along the shoreline, that extend from an onshore baseline to the shoreline. Using these profiles, we determine horizontal shoreline position and, for the most seaward dune, dune height and the position and elevation of the dune base. These parameters change between 1997 and 1998 due to changes in beach morphology. For locations where the 1997 beach width was wider than approximately 20 m, there is a nonlinear relationship between the 1997 beach width and the associated maximum dune retreat. This relationship varies from south to north. Greater maximum dune retreats are found for comparable beach widths north of Ocracoke Inlet, the region where beaches have been actively maintained.
OS33A-02 13:45h
Morphologic Response of the North Carolina Inner Shelf to Hurricanes Isabel (September 2003) and Alex (August 2004)
The inner continental shelf between Cape Hatteras and Cape Lookout, North Carolina was mapped four times (Jun 2003, Sep 2003, Jul 2004, Aug 2004) before and after landfalling hurricanes Isabel (18 Sep 2003) and Alex (03 Aug 2004) using sidescan sonar, multibeam and interferometric swath bathymetry, and high-resolution CHIRP seismic systems. These data provide continuous coverage of the seafloor, extending from approximately the 7 m isobath near the shoreline to 10 km offshore (28-30 m water depth). Pre-Isabel imagery shows that the seafloor in this area is characterized by a variety of shore-perpendicular to shore-oblique morphologic features recently termed "sorted bedforms" in reference to the sorting of grain size that has been observed in such bedforms at many inner shelf locations. The bedforms have typical wavelengths of 100-300 m and heights up to 1 m, and extend seaward across the entire study area. Post-storm surveys used sidescan sonar and multibeam bathymetry systems to resurvey selected areas within a week following both hurricanes' landfall. On the inner shelf off Ocracoke Inlet, a 63 km2 field of well-organized, low-amplitude (0.5 m), 250 m wavelength sorted bedforms was rearranged by Isabel such that coarse and fine sediments were largely dispersed across the inner shelf, in an apparent reorganization towards a larger-scale pattern adjusted to the storm flow regime. The seafloor morphology returned to its pre-Isabel state by Jul 2004. Alex was a smaller storm than Isabel; only minimal seafloor changes were noted following its passage over the study area. This suggests there is a storm-energy threshold for significant bed reworking that was exceeded by Isabel but not by Alex. These storm responses are consistent with a numerical model of seafloor bedform evolution that treats the transport of coarse and fine sediment fractions as functions of the local bed composition. Large-scale sorted patterns exhibiting the main characteristics of the natural features are a robust prediction of the model and provide insight into physical processes affecting the inner shelf during the storms.
OS33A-03 14:00h
Development of Flaser and Lenticular Bedding in Response to Tropical Cyclones
Flaser and lenticular deposits (sensu Reineck and Wunderlich) are common in the sedimentary record and generally found associated with rippled seafloors in tidally influenced coastal waters. These features are thought to be created by deposition of suspended fine-grained mud onto a rippled sandy seafloor during quiescent periods, followed by migration of sand ripples during periods of strong tidal currents. The size of these mud deposits range from small lenses the size of ripple wavelengths (flasers) to large interconnected mud beds containing sand deposits (lenticular deposits). The development of flaser and lenticular bedding was observed on a mostly sandy shelf (5-20 m water depths) in the northeastern Gulf of Mexico after the passage of Hurricane Ivan and subsequent tropical storms. The mud was probably derived from backwash from lagoonal areas behind barrier islands after the passage of Hurricane Ivan (16 September 2004) and the mobilization of sand ripples was a result of storms. Suspended mud was deposited over the rippled seafloor during a period of relative calm. Subsequent storms Hurricane Jeanne (26 September) and, later, Tropical Storm Matthew (8-10 October) mobilized sediments creating 50-100 cm wavelength sand ripples that migrated over the freshly deposited mud layers creating buried features ranging from small, isolated flaser deposits to lenticular deposits within mud to mud deposits covering 100's of square meters. Mud deposit thicknesses ranged from 2 cm to as much as 15-20 cm. The temporal persistence of these features is unknown but similar flaser and lenticular bedding has been observed in chirp sonar records and sediment vibra-core samples from sediment depths below that conceivably mobilized by Hurricanes.
OS33A-04 14:15h
An evaluation of the causes of breaching of Hatteras Island by Hurricane Isabel
The processes responsible for incising channels across Hatteras Island, North Carolina, during Hurricane Isabel are investigated using a suite of numerical models. The hindcast wind, wave, and current fields are used to force a three-dimensional numerical sedimentation model. The erosion potential of the coastal sediment transport system is predicted for a grid with a horizontal resolution of approximately 50 m. This erosion potential is compared to the available sand pool comprising the beach-dune system. Penetration of the dunes into the barrier islands is predicted when the erosion potential exceeds the estimated dune-beach volume, whereas breaching is dependent on prior dune erosion and the difference in water levels between the ocean and lagoon sides of the island. The model results compared favorably with aerial photographs taken after landfall.
OS33A-05 14:30h
On the Role of Tropical Cyclones and Winter Storms in the Short-Term Evolution of the Northeastern Gulf of Mexico.
Recent data suggest that the North-central Gulf of Mexico coast has undergone an increase in the number of tropical cyclone landfalls over the past decade. Louisiana State University and the USGS has monitored the Florida panhandle and Alabama coasts since the mid 1990's using airborne LIDAR and field surveys. The resultant data sets provide a unique time series capturing morphological change and post-storm adjustment due to two powerful events (Opal, 1995 and Ivan, 2004), weaker hurricanes and numerous tropical storms. In this paper we document a unique response of this coast to storm surge/wave inundation and present the concept of "barrier mass conservation". We also demonstrate the significance of locally generated, high frequency waves in estuaries/bays during winter storms and discuss their role in the short-term evolution of Holocene barriers along this region of coast. In an otherwise tectonically stable region of the northeastern Gulf of Mexico, the coast appears to be storm dominated and undergoing a reduction in sediment volume in the sub-aerial barrier unit, a phenomenon that appears attributable to storm dominance.
OS33A-06 14:45h
Morphologic Response and Sediment Redistribution of the Beach and Nearshore Sand Bars due to Extratropical and Tropical Storm Forcing: a Spatial and Temporal Analysis
Shore-oblique bars and associated exposures of an underlying geologic stratum in the nearshore have been documented along the US East Coast and have been linked to shoreline erosional hotspots. While earlier studies acknowledged that the bedforms responded to extratropical and tropical storms, neither quantified the extent of sediment redistribution after the events. An approach that encompasses actual volume measurements across the nearshore-beach down to a non-sandy stratum and quantifies the response of the beach and the nearshore to the same hydrodynamic forcing will enable a better understanding of the exchange of sediment between the two regions. Total nearshore sediment volume has been shown to be a first-order contributor to the behavior of the shoreline. This volumetric approach is employed in the analysis of morphological changes and the redistribution of sediment in the nearshore and beach following storms. A regional survey from 2002 provides the initial, fair-weather morphologic state of the nearshore (1.5-15m water depth) spanning 40 km of the North Carolina Outer Banks. Four small-scale surveys were conducted in subsequent years, focusing on four 1-km2 regions within the initial 2002 survey area. The smaller regions were selected on the basis of the morphological state observed during the 2002 survey and historical shoreline behavior. Data were collected in March 2003 following a Northeaster; in May 2003 following an extended period of fair weather conditions; in November 2003 following Hurricane Isabel; and finally, in June 2004 after another period of fair weather. A swath bathymetry system was used to collect bathymetry and side scan sonar (acoustic backscatter) and a high-resolution chirp sub-bottom profiler imaged the shallow sub-surface geology of the nearshore. In addition, RTK-GPS was used to map the sub-aerial beach at each 1-km2 site from the toe of the dune to the water line for the May 2003, November 2003, and June 2004 sampling periods. This sampling regime and data set offers a unique opportunity 1) to compare the simultaneous response of the beach and the nearshore to storms and 2) to investigate how the response of the beach and the nearshore varies depending on sandbar morphology and differences in the geologic framework. Three-dimensional maps of the beach and nearshore, from the toe of the dune to the 15m isobath, are used to demonstrate changes in total shoreface volume after storms and during recovery. Understanding the redistribution of sediment in the context of volumetric change enables the quantification of the exchange of sediment between the beach and the nearshore during large-scale forcing events.