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Scientists evaluated and improved the accuracy of pre-landfall forecasts of storm-induced coastal erosion hazards for Northeast beaches using data from post-Sandy lidar sruveys, beach morphology, and storm hydrodamics.
USGS forecasts of coastal change from hurricanes provide critical information used to identify areas vulnerable to extreme, and potentially catastrophic, erosion during landfall. Using post-Sandy lidar elevation data, this project produced updated assessments of storm-induced coastal erosion hazards for Northeast beaches. Proposed and alternative rebuilding efforts (such as dune restoration) can now be included in vulnerability assessments to evaluate efficacy. Additionally, National Climate Assessment scenarios for future sea level rise were incorporated to examine corresponding changes in vulnerability. Updated assessments and GIS data layer are available through various online systems, such as the USGS Coastal Change Hazards Portal, NOAA's Digital Coast, and data.gov.
Following Hurricane Sandy (landfall on October 29, 2012), the USGS obtained lidar data from multiple missions (dated from November 5-29, 2012) and examined coastal change and erosion. The data have been published as a USGS data series.
Sandy beaches provide a natural barrier between the ocean and inland communities, ecosystems, and resources. However, these dynamic environments move and change in response to winds, waves, and currents. During a hurricane, these changes can be large and sometimes catastrophic. High waves and storm surge act together to erode beaches and inundate low-lying lands, putting inland communities at risk. A decade of USGS research on storm-driven coastal change hazards has provided the data and modeling capabilities to identify areas of the coastline that are likely to experience extreme and potentially hazardous erosion during a hurricane.
The analysis is based on a storm-impact scaling model that uses observations of beach morphology collected after Hurricane Sandy combined with sophisticated hydrodynamic models to predict how the mid-Atlantic coast will respond to the direct landfall of category 1-4 hurricanes. Hurricane-induced water levels, due to both surge and waves, are compared to beach and dune elevations to determine the probabilities of three types of coastal change.
Following the passage of Hurricane Sandy, 74% of dune-backed beaches along the mid-Atlantic coast were very likely to experience collision for a category 1 hurricane landfall, compared to 89% pre-storm. Hurricane Sandy eroded the dune face in many places, resulting in a narrower dune with a higher dune toe elevation and a decreased likelihood of dune erosion due to collision. In other locations, the mean beach slope decreased, resulting in lower values for wave-induced water levels. For a category 4 hurricane landfall 95% of the post-Sandy mid-Atlantic beaches are very likely to experience overwash, an increase of 3% over pre-storm. The fraction of coastline very likely to experience inundation also increased from 66% to 68% following Hurricane Sandy.
See USGS Open-File Report National assessment of hurricane-induced coastal erosion hazards—Northeast Atlantic Coast for more results.
Using the latest wave and surge information available as the hurricane was making landfall, USGS predicted the likelihood of a range of coastal change impacts within the region affected by the storm. Collision was very likely for the majority of the sandy beaches along the Virginia, Maryland, Delaware and New Jersey coasts with widespread overwash also very likely in many areas. Along the south shore of Long Island, New York, the models predicted extensive beach and dune erosion as well as intermittent overwash.
The USGS is working to develop quantitative methods to evaluate the accuracy of these pre-landfall predictions of coastal change. Using lidar-based surveys of beach topography, coastal change impact metrics such as dune-elevation change, shoreline change and beach volume change, along with pre- and post-storm photo comparisons can be used to verify pre-landfall predictions.
Extreme dune elevation changes of 4 m or more were observed along much of the Outer Banks of North Carolina. From Cape Hatteras north to Oregon Inlet (distance from landfall (DFL) = -420 km, negative numbers indicate locations to the south of landfall) nearly 14% of the dunes experienced 2 m or more in dune height erosion. In Rodanthe, storm surge and waves eroded the dunes and the beach, depositing sand inland and shifting the shoreline landward. Prior to the storm several houses sat on top of, or just seaward of, the dunes or highest elevations on this part of the island. Dune erosion and landward migration of the shoreline has left many of these houses more vulnerable to future storms. Storm surge and waves produced overwash that deposited sand on the coastal roads. Further north, waves eroded the face of dunes in Corolla, N.C.
Along the undeveloped Virginia coastline south of Virginia Beach, relatively little volume change occurred and high dunes averaging 6.6 m in elevation protected against overwash and inundation. However, waves and elevated water level contributed to dune erosion and scarping from the North Carolina/ Virginia border northward. For the approximately 5 km length of the coast centered on Virginia Beach, the beach is backed by a boardwalk, which represents the peak dune height here. The boardwalk remained intact through Hurricane Sandy therefore dune height is unchanged and dune erosion here is zero. However, volume loss and narrowing of the beach occurred in front of the boardwalk. Virginia's barrier island system north of the mouth of Chesapeake Bay experienced extensive landward deposition of sand as low dunes were overtopped by waves and surge. North of Wallops Island, the storm response along Assateague Island varied spatially with dune morphology. Much of the southern end of Assateague Island experienced erosion and volume loss as sand was transported landward to form large overwash deposits. Breaching occurred through the narrow and low-lying beach just north of Little Toms Cove (distance from landfall (DFL) = -190 km, negative numbers indicate locations to the south of landfall). Dunes are larger for the middle section of Assateague Island, averaging 4.5 m in height. Elevated water levels and storm-driven waves eroded the dune face causing scarping along the entire extent of the Assateague coastline but dune height remained intact. Overwash occurred only in limited areas where previous storms cut through the line of dunes.
Maryland and Delaware
The majority of the northern end of Assateague Island was fronted by a 2 m high berm prior to Hurricane Sandy. Storm waves and elevated water levels destroyed the majority of this low dune structure, carrying sand inland and creating overwash deposits in the bay behind the island in narrow, low-lying places. In the few places along this stretch where a dune existed that exceeded the 2 m high berm, the dune crest remained intact.
While the dunes along the developed coastline of Maryland from Ocean City Inlet northward remained largely intact, waves and surge generated by Sandy eroded and narrowed the beaches. The greatest amount of dune erosion in Maryland north of Assateague Island occurred just north of the Ocean City inlet where a dune elevation reduction of 2 m was observed and the Ocean City Fishing Pier was damaged.
In October 2011 through February 2012, a renourishment project was completed that spanned the beaches from Fenwick Island, Del. (DFL = -121 km) north to Lewes Beach just south of Cape Henlopen. Sand was pumped onto face of the beach, increasing beach width, and dunes were constructed. Despite this addition of sand, Delaware beaches from Bethany Beach to just south of Dewey Beach (DFL ~ -100 km) suffered significant dune erosion, volume loss and shoreline retreat. Areas where the created dunes and widened beach system remained intact after the passage of Sandy appear as positive shoreline and volume change.
Destructive waves and storm surge associated with Sandy severely eroded the beach and dune systems that represented, in many places, the first line of defense for the New Jersey coastline. The majority of the New Jersey coast from Brigantine, where Sandy made landfall, northward experienced severe dune erosion of 2 to 6 m vertical loss. Along the undeveloped barrier coast from Brigantine to Beach Haven (DFL ~ 16 km) dunes that averaged 3.2 m in peak elevation pre-storm were eroded in height by an average of 1.7 m. The dunes along New Jersey's Island Beach State Park (DFL = 47-60 km) were also severely impacted with an average elevation loss of 1.4 m and a mean loss of sand volume from the beach of 69.0 m3/ m along the barrier island dune system. Waves atop elevated water levels eroded the face of the dunes, resulting in dune scarping and breaching of the dune line in places allowing waves to carry sand inland and deposit sand on roads and in parking lots.
The greatest impacts from Sandy were observed along the shoreline from the northern boundary of Island Beach State Park north through Mantoloking, N.J. (DFL = 76 km). Dunes here experienced an average of 2.7 m elevation loss. In Seaside Heights, for example, maximum dune elevation erosion of 7 m was observed and many oceanfront houses were severely damaged. Storm waves and surge destroyed the Seaside Height Pier and the boardwalk in places, eroded the beach, overwashed dunes and washed sand inland, damaging or destroying homes. In Mantoloking, waves and surge breached the narrow island in several locations, cutting through roads and homes and washing houses into Barnegat Bay.
Some of the regions of New York hardest hit by Sandy storm surge include the highly developed communities of Coney Island, the Rockaway Peninsula and Long Beach barrier island (DFL = 131-145 km). Waves and surge attacked dunes averaging 4.3 m in elevation in this region, overtopping the dunes and transporting sand into coastal communities and causing an average of 1.4 m of vertical dune erosion. Along Fire Island, a mean dune-height erosion of 2 m was observed with dune erosion as high as 5 m in many places. Nearly half of the island's dunes experienced overwash resulting in deposition of sand landward and an overall flattening of beach topography. For example, overwash of the dunes at Ocean Bay Park, just east of Ocean Beach, N.Y., resulted in the transport of large volumes of sand inland from the beach system, severe decrease of dune height and significant property damage. Breaching occurred in several locations along Fire Island, including at Old Inlet (the location of breaching during previous storms, DFL = 192 km) where volume loss and shoreline retreat are apparent. Positive shoreline change observed along much of Fire Island is evidence of recovery of the seaward-edge of the beach system immediately following the passage of the storm. A more detailed analysis of coastal changes occurring on Fire Island can be found on the Fire Island Coastal Change project webpage.
From Moriches Inlet east to Montauk, higher dunes (on average 6.6 m in height) protected much of the coastline from overwash and inundation, and the storm surge threat lessened along the eastern portion of Long Island (for example, a peak of approximately 1.8 m above normal tide was reported at Montauk, Blake and others, 2013) resulting in mean dune erosion of 1.0 m. However, several narrow and low-lying places were breached during the storm including Cupsogue Beach just north of Moriches Inlet and W. Scott Cameron Beach south of Mecox Bay (DFL = 240 km).
Automated Coastal Change Probability Forecasts and Updates
USGS forecasts of coastal change from hurricanes provide critical information to identify areas vulnerable to extreme, and potentially catastrophic, erosion during landfall. Both real-time and scenario-based forecasts are produced. Scenario-based predictions have been published for the Gulf of Mexico, Southeast Atlantic, and mid-Atlantic coastlines for generalized Category 1-5 hurricane storm surge and wave conditions. As the USGS expands regional predictions into the Northeast Atlantic and Pacific coastlines, other storm scenarios must be explored including nor'easters, and winter storms. Tropical storms conditions will also be modeled for the Gulf of Mexico and Atlantic coasts to produce scenario-based forecasts of coastal change.
View the forecasts in the Coastal Change Hazards Portal.
Below are publications associated with this project.
Below are news stories associated with this project.
When the next hurricane heads toward a coastal community in the United States, residents and emergency managers busily readying for the storm will...