U.S. Geological Survey Scientific Investigations Report 2006–5309

Effect of Storms on Barrier Island Dynamics, Core Banks, Cape Lookout National Seashore, North Carolina, 1960–2001

By Stanley R. Riggs and Dorothea V. Ames

U.S. Geological Survey Scientific Investigations Report 2006–5309


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Abstract

The effect of storms on long-term dynamics of barrier islands was evaluated on Core Banks, a series of barrier islands that extend from Cape Lookout to Okracoke Inlet in the Cape Lookout National Seashore, North Carolina. Shoreline and elevation changes were determined by comparing 77 profiles and associated reference markers established by the U.S. Army Corps of Engineers (USACE) on Core Banks from June 1960 to July 1962 to a follow-up survey by Godfrey and Godfrey (G&G) in 1971 and a survey by the Department of Geology at East Carolina University (ECU) in 2001, in which 57 of the original 77 profiles were located.

Evaluation of the baseline data associated with the USACE study supplies an important record of barrier island response to two specific storm events—Hurricane Donna in September 1960 and the Ash Wednesday extra-tropical cyclone in March 1962. The 1962 USACE survey was followed by 9 years characterized by no major storms; this low-energy period was captured by the G&G survey in 1971. The G&G survey was followed by 22 years characterized by occasional small to moderate storms. Starting in 1993, however, and continuing through 1999, the North Carolina coast experienced a major increase in storm activity, with seven major hurricanes impacting Core Banks.

Both the USACE 1960–1962 and G&G 1962–1971 surveys produced short-term data sets that reflected very different sets of weather conditions. The ECU 2001 survey data were then compared with the USACE 1960 survey data to develop a long-term (41 years) data set for shoreline erosion on Core Banks. Those resulting long-term data were compared with the long-term (52 years) data sets by the North Carolina Division of Coastal Management (NCDCM) from 1940–1992 and 1946–1998; a strong positive correlation and very similar rates of average annual erosion resulted. However, the ECU and NCDCM long-term data sets did not correlate with either of the USACE and G&G short-term survey data and had very different average annual erosion rates.

The average annual long-term rate of shoreline erosion for all of Core Banks and for both the ECU 1960–2001 and the NCDCM 1946–1998 surveys was -5 feet per year (ft/yr). These long-term rates of shoreline recession are in strong contrast with the short-term, storm-dominated rates of shoreline erosion for all of Core Banks developed by the USACE 1960–1961 and USACE 1961–1962 surveys, which have average annual erosion rates of -40 ft/yr and -26 ft/yr, respectively, and range from -226 feet (ft) to +153 ft. The combined short-term, storm-dominated shoreline erosion rate for the USACE surveys (1960–1962) was -36 ft/yr. In contrast, the average annual short-term, non-stormy period G&G 1962–1971 survey demonstrated shoreline accretion for all of Core Banks with an average annual rate of +12 ft/yr. In general, North Core Banks has higher erosion and accretion rates than South Core Banks.

In the 1961 survey, the USACE installed 231 reference markers (RM-0 is closest to the ocean and RM-2 is farthest from the ocean) along the 77 profiles, as well as 33 reference markers labeled RM-4, RM-6, and RM-8 in the wider portions of the islands. The G&G survey recovered a total of 141 reference markers (61 percent), and the ECU survey recovered a total of 83 reference markers (36 percent) of the RM-0, RM-1, and RM-2 markers. The average ground elevation measured by the USACE in 1961 was RM-0 = +5.8 ft, RM-1 = +5.2 ft, and RM-2 = +4.8 ft. The G&G 1970 survey measured average ground elevations of RM-0 = +6.7 ft, RM-1 = +6.4 ft, and RM-2 = +6.1 ft, and the average ground elevation measured by ECU in 2001 was RM-0 = +10.1 ft, RM-1 = +9.1 ft, and RM-2 = +8.5 ft. The latter numbers represent approximately an overall 72-percent increase in island elevation from 1961 to 2001. Based on aerial photographic time-slice analyses, it is hypothesized that this increase in island elevation occurred during the post-1962 period with storm overwash systematically raising the island elevation through time, which in turn led to decreased numbers of overwash events. The latter processes and responses in turn led to a substantial increase in vegetative growth on the barrier island, as well as submerged aquatic vegetation on the back-barrier sand shoals.

Integration of the USACE, G&G, ECU, and NCDCM shoreline erosion data for Core Banks shows several important points about shoreline recession. (1) The ECU and NCDCM data sets demonstrate that there is an ongoing net, long-term, but small-scale shoreline recession associated with Core Banks; (2) the USACE short-term data sets demonstrate that processes associated with individual storm events or sets of events produce extremely large-scale changes that include both erosion and accretion; (3) the short-term, non-stormy period data set of G&G demonstrates that if given enough time between storm events, barriers can rebuild to their pre-storm period conditions; and (4) the post-storm response generally tends to approach the pre-storm location, but rarely reaches it before the next storm or stormy period sets in. The result is the net long-term change documented by both the ECU 1960–2001 and NCDCM 1946–1998 Core Banks data sets that resulted in erosion rates ranging from 0 to -30 ft/yr with net annual average recession rates of -5 ft/yr.

Analysis and comparison of these data sets supply important information for understanding the dynamics and responses of barrier island systems through time. In addition, the results of the present study on Core Banks supply essential process-response information that can be used to design and implement management plans for the Cape Lookout and Cape Hatteras National Seashores and for other seashores in the U.S. National Park Service system.

Contents

Introduction

Purpose and Scope

Previous Core Banks Surveys

U.S. Army Corps of Engineers Surveys

Godfrey and Godfrey Survey

East Carolina University Geology Department Survey

North Carolina Division of Coastal Management Data Sets

Description of Core Banks

The Barrier Islands

Inlet History

Ocracoke Inlet

Barden Inlet

The Drum Inlets

Ephemeral Inlets

North Carolina Storm History

Active Storm Period: 1940–1962

Low Storm Activity Period: 1963–1970

Moderate Storm Activity Period: 1971–1990

Most Active Storm Period in Recorded N.C. History: 1991–2005

Methods of Data Collection and Analysis

Effect of Storms on Barrier Island Dynamics

Shoreline and Elevation Changes on Core Banks

U.S. Army Corps of Engineers Data Sets

Godfrey and Godfrey Data Sets

East Carolina University Data Sets

North Carolina Division of Coastal Management Data Sets

Patterns of Shoreline Change Along Core Banks

Elevation Changes on Core Banks Through Time

Summary and Conclusions

Acknowledgments

References Cited

Appendixes

Figures

1. A regional satellite image showing the location of the Core Banks study area in the North Carolina coastal system

2. A four-part, georeferenced aerial photograph time series (A, 1940; B, 1962; C, 1998; and D, 2003) showing the evolution of Old Drum Inlet, New Drum Inlet, and New-Old Drum Inlet, Cape Lookout National Seashore, North Carolina

3. A three-part aerial photograph time series (A, 1940; B, 1962; and C, 1983) showing the Portsmouth Village Overwash Plain with Whalebone and Swash Inlets on the southwestern part of the photographs, Core Banks, Cape Lookout National Seashore, North Carolina

4. A five-part aerial photograph time series (A, 1866; B, 1933–1937; C, 1940; D, 1962; and E, 1998) showing Cape Lookout, Cape Lookout Bight, and Barden Inlet, North Carolina

5. Graph showing total number of hurricane strikes in North Carolina by decade for the period from 1900 to 1999

6. Diagrams showing the 77 profiles and associated topographic and bathymetric surveys done from 1960 to 1962 by the U.S. Army Corps of Engineers (USACE) on North Core Banks (A), and South Core Banks (B), Cape Lookout National Seashore, North Carolina

7. Diagrams showing the layout of reference markers (RMs) developed for the (A) 1960 to 1962 surveys and (B) 2001 survey of North and South Core Banks, Cape Lookout National Seashore, North Carolina and the methods used in 2001 to determine shoreline and elevation changes

8. Photographs showing relocated U.S. Army Corps of Engineers 1960–1962 reference markers (RM) on Core Banks, Cape Lookout National Seashore, North Carolina

9. Diagram showing the 57 profiles located by the 2001 East Carolina University survey of the original 77 profiles established by the U.S. Army Corps of Engineers on North and South Core Banks, Cape Lookout National Seashore, North Carolina

10. Schematic diagrams showing the method used by the U.S. Army Corps of Engineers, Godfrey and Godfrey, and East Carolina University to determine shoreline changes in their 1960 (A) 1961, 1962 (B) 1971, and 2001 (C), shoreline surveys, Core Banks, Cape Lookout National Seashore, North Carolina

11. Net shoreline change data for 77 profiles by the U.S. Army Corps of Engineers between the 1960 and 1961 surveys (blue) and the 1961 and 1962 surveys (red), Core Banks, Cape Lookout National Seashore, North Carolina

12. Average annual rate of shoreline change for the 77 profiles between the U.S. Army Corps of Engineers 1960 and 1961 surveys (blue) and the 1961 and 1962 surveys (red), Core Banks, Cape Lookout National Seashore, North Carolina

13. Horizontal distance from the shoreline to the 6-, 12-, 18-, 24-, and 30-foot depth contours for alternate profiles from the U.S. Army Corps of Engineers 1961survey, Core Banks, Cape Lookout National Seashore, North Carolina

14. Horizontal distance from the shoreline seaward to the 6-, 12-, 18-, 24-, and 30-foot depth contours for alternate profiles from the U.S. Army Corps of Engineers 1961 survey, plotted opposite the long-term NCDCM 1946–1998 average annual shoreline erosion rates in feet per year for Core Banks, Cape Lookout National Seashore, North Carolina

15. Average rate of shoreline change data from the USACE 1962 survey to the Godfrey and Godfrey 1971 survey for Core Banks, Cape Lookout National Seashore, North Carolina

16. Average annual rate of shoreline erosion and accretion from the U.S. Army Corps of Engineers 1960–1962 survey data (red) and the Godfrey and Godfrey 1962–1971 survey data (blue), Core Banks, Cape Lookout National Seashore, North Carolina

17. Cross-sectional profile of South Core Banks at Codds Creek showing the shallow stratigraphic interpretation of the history of overwash fan deposition developed from a series of trenches dug across the island

18. Net shoreline change (red) and average annual rate of change (blue) from the East Carolina University 1960–2001 survey for each profile where the U.S. Army Corps of Engineers reference markers were recovered, Core Banks, Cape Lookout National Seashore, North Carolina

19. Average annual rate of shoreline change from the U.S. Army Corps of Engineers 1960–1962 surveys for the 77 USACE profiles and the East Carolina University 1960–2001 survey for the 57 profiles where one or more of the U.S. Army Corps reference markers were recovered, Core Banks, Cape Lookout National Seashore, North Carolina

20. Average annual rate of shoreline change from the Godfrey and Godfrey 1962–1971 survey and the East Carolina University 1960–2001 survey, Core Banks, Cape Lookout National Seashore, North Carolina

21. Net increase in ground elevation at U.S. Army Corps of Engineers reference markers based on changes between the U.S. Army Corps of Engineers 1961 and East Carolina University 2001 surveys for each profile where U.S. Army Corps of Engineers reference markers were recovered, Core Banks, Cape Lookout National Seashore, North Carolina

22. Shoreline change data from the North Carolina Division of Coastal Management 1940–1992 and 1946–1998 data sets, Core Banks, Cape Lookout National Seashore, North Carolina

23. Average annual rate of shoreline change from the long-term East Carolina University 1960–2001 data set and the long-term North Carolina Division of Coastal Management 1946–1998 data set, Core Banks, Cape Lookout National Seashore, North Carolina

24. Aerial photographs showing specific features that are relevant to the dynamics of shoreline change, Core Banks, Cape Lookout National Seashore, North Carolina

25. Histogram showing elevation changes measured by the Godfrey and Godfrey 1970 and East Carolina University 2001 surveys as compared to the U.S. Army Corps of Engineers survey when the reference markers were installed on 77 profiles, Core Banks, Cape Lookout National Seashore, North Carolina

26. A five-part, aerial photograph, time-slice sequence showing the Swash Inlet to Whalebone Inlet segment of North Core Banks, Cape Lookout National Seashore, North Carolina

27. Photographs showing changes with time on Core Banks, Cape Lookout National Seashore, North Carolina

Tables

1. Maximum and average annual rates of shoreline erosion and accretion for each of the shoreline survey data sets for North and South Core Banks combined, Cape Lookout National Seashore, North Carolina

2. Summary of the elevation change data for 231 reference markers on 77 profiles from the 1961 U.S. Army Corps of Engineers survey for Core Banks, Cape Lookout National Seashore, North Carolina

3. Summary of the elevation change data for 141 reference markers on 69 profiles from the 1970 Godfrey and Godfrey survey for Core Banks, Cape Lookout National Seashore, North Carolina

4. Summary of the elevation change data for 83 reference markers on 57 profiles from the East Carolina University 2001 survey for Core Banks, Cape Lookout National Seashore, North Carolina

5. Maximum and average annual rates of shoreline erosion and accretion for each of the shoreline surveys or data sets for North and South Core Banks, Cape Lookout National Seashore, North Carolina

6. Summary of net ground elevation and percent change data for all reference markers from surveys by the U.S. Army Corps of Engineers, Godfrey and Godfrey, and East Carolina University for Core Banks, Cape Lookout National Seashore, North Carolina

Appendixes

1. Summary of the U.S. Army Corps of Engineers survey data developed in the 1960, 1961, and 1962 surveys on Core Banks, Cape Lookout National Seashore, North Carolina

2. Summary of the U.S. Army Corps of Engineers 1961 survey data of the shoreface bathymetry for Core Banks, Cape Lookout National Seashore, North Carolina

3. Summary of the Godfrey and Godfrey 1970 survey data of elevation change along 69 of the 77 U.S. Army Corps of Engineers profiles on Core Banks, Cape Lookout National Seashore, North Carolina

4. Summary of the Godfrey and Godfrey 1971 survey data of shoreline change along 39 of the 77 U.S. Army Corps of Engineers profiles on Core Banks, Cape Lookout National Seashore, North Carolina

5. Latitude and longitude locations for the recovered U.S. Army Corps of Engineers reference markers that were located and surveyed in 2001 by East Carolina University for the present study on Core Banks, Cape Lookout National Seashore, North Carolina

6. Summary of the East Carolina University survey data for 57 of the 77 U.S. Army Corps of Engineers profiles located in the 2001 survey on Core Banks, Cape Look out National Seashore, North Carolina.

7. Summary of the North Carolina Division of Coastal Management shoreline recession data (1940–1992 and 1946–1998) for the average annual shoreline erosion rates on Core Banks, Cape Lookout National Seashore, North Carolina


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