Effects of Hurricane Ginger on the Barrier Islands of North Carolina
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ROBERT DOLAN Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia 22903 PAUL GODFREY Department of Botany, University of Massachusetts, Amherst, Massachusetts 01002 Effects of Hurricane Ginger on the Barrier Islands of North Carolina ABSTRACT National Park Service and the state of North Carolina for both preservation and manage- The two barrier-island systems of North ment reasons. Carolina responded to the storm waves and surges of Hurricane Ginger in a strikingly dif- HURRICANE GINGER ferent manner. Within the northern sector, which has been stabilized by man, erosion and Hurricane Ginger was originally detected on dune recession were extensive. In the southern 10 September, 1971, as a tropical depression sector, as yet relatively unmodified, overwash south of Bermuda, and the storm dissipated and associated deposition were the dominant offshore from Cape Hatteras on 5 October. processes. This difference offers important Although Hurricane Ginger was far from the geologic, ecologic, and land management im- most destructive storm of record, it had the plications. distinction of being the longest-lived tropical cyclone to track in the North Atlantic. The INTRODUCTION hurricane crossed Core Banks on 30 September, Hurricane Ginger moved inland near Cape 1971; however, the storm's presence off Cape Lookout, North Carolina, on 30 September, Lookout several days earlier resulted in strong 1971, providing an excellent opportunity to windfields which generated large waves. By 30 compare the impact of a severe storm upon September, winds in excess of 100 kmph were two adjacent barrier-island systems, one recorded at Cape Lookout and in excess of 75 stabilized, the other natural (Fig. 1). Simul- kmph at Cape Hatteras; waves of up to 4 m in taneous field investigations were underway at height were reaching the beaches (U.S. De- Cape Hatteras and Cape Lookout, and NASA partment of Commerce, 1971). The storm's (Wallops Island) provided color infrared peak surge or set up of 2.5 m above mean sea aerial photographs of the barrier islands, before level, which is slightly less than the sand level and after Hurricane Ginger. of the beach backshore, resulted in increasing water depths inshore so that very large waves OUTER BANKS reached the beach face. This pattern can be The Outer Banks extend from the Virginia- seen in Figure 2, which also illustrates the North Carolina border south to Cape Lookout, before-and-after condition of both Core Banks a distance of 240 km. These islands were and Cape Hatteras. As the eye crossed the originally one continuous biophysical system, islands the winds shifted 180°, reaching a but the upper section has experienced three maximum of 75 kmph from the south on 2 decades of dune and beach stabilization coupled October. with extensive public and private development Since the storm moved inland south of Hat- (Dolan, 1972). In 1957 the stabilized islands teras, the winds there remained north-north- became the Cape Hatteras National Seashore. east, reaching a speed of 100 kmph on 1 The southern segment, which includes Core October. Maximum inshore wave height re- Banks, remains essentially in a natural state ported near Cape Hatteras was 4 to 5 m. There and is designated to become part of the new are no wave gauges near Cape Lookout, but Cape Lookout National Seashore. The re- wave hindcasting coupled with general ob- sponses of these two different systems to the servations suggest that deep-water heights of 6 inshore processes are of special interest to the m and over were generated on 1 October. Geological Society of America Bulletin, v. 84, p. 1329-1334, 4 figs., April 1973 1329 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1329/3418249/i0016-7606-84-4-1329.pdf by guest on 30 September 2021 1330 DOLAN AND GODFREY The storm surge drove sea water completely across the islands from Cape Lookout north to Portsmouth Island. Drift lines in the vicinity of the Cape Lookout Lighthouse were up to 0.7 m above the level of the sand flats, and about 0.3 m above the level 43 km to the north- east on Portsmouth Island. The surge was slightly lower near Cape Hatteras. SHORELINE IMPACT Profiles surveyed at 2-m intervals across Core Banks and Hatteras Island during the summer of 1971 were resurveyed shortlv after the hurricane. Elevation changes of the berm surface relative to bench marks on the Core Banks profile were also measured. Figure 2 allows comparison of before-and-after profiles along two transects, illustrating a distinct in- crease of land elevation on Core Banks and erosion of the beach face on Hatteras Island. In the case of Core Banks, the storm flattened the beach face by moving sand offshore, as well as over the berm, while the beach crest receded about 40 m. However, the mean-sea-levcl in- tersect retreated very little. At the time of resurvey, nine days after the storm, a new berm was forming. A substantial portion of the berm material moved onto the backshore by Hurricane Ginger was transported up to 100 m inland into the prestorm open dune lines (Fig. 2). Material buildup on the berm surface reached a maximum of 0.50 m at the new berm crest, 0.30 m at bench mark P-3 (Fig. 4a and 4b), and 0.10 m occurred in the interdune troughs, tapering down to no change near the backside of the dune zone. Winds from the south, fol- lowing the storm's passage across Core Banks, deposited new sand to the dune line, raising the seaward face and crest by about 0.2 m. A re- survey in July 1972 shows that the Core Banks profile had almost completely recovered from the effects of Hurricane Ginger; whereas sands lost along the dune face of Hatteras have not yet been recovered (Fig. 2). The patterns along most of the Core Banks, as determined from the Wallops Island aerial photography and site visits, were similar: flattening of beach face, retreat of berm crest, rier islands of North Carolina. burial and deposition on the backshore, en- past (Savage and Wcodhouse, 1968). Some of trapment of sand by grasslands, and general these dunes near the beach were severely under- flooding. Damage or erosion, in the classical cut. Within the stabilized segment of the sense, was observed where experimental cunes Outer Banks, the massive and unbroken man- had been created by grassing projects of years made dunes act as an impenetrable obstruction Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1329/3418249/i0016-7606-84-4-1329.pdf by guest on 30 September 2021 EFFECTS OF HURRICANE GINGER ON BARRIER ISLANDS 1331 Figure 2. Before-and-after Hurricane Ginger profile Island. See photographs of site P-3 in Figure 4. comparison for transects on Core Banks and Hatteras Figure 3. Core Banks and Hatteras Island, three the broad beaches on Core Banks versus the man-made days after Ginger. Note the striking difference between barrier dune on Hatteras Island. to the storm surge, so the energy dissipation barrier dunes near Cape Hatteras, as indicated and sediment transfer processes differ signifi- from several measured traverses, averaged be- cantly. tween 3 and 5 m. Photographs taken three days after the seas Based upon these measurements, natural of Hurricane Ginger had subsided (Fig. 3) and barrier islands appear to be better adapted to before-and-after profiles (Figs. 2 and 4) all extreme events than are artificially stabilized illustrate the important implications of these islands. Since there is little resistance to the differences. The man-made dunes absorb the storm surge, wave energy is dissipated across full impact of the surge and storm waves and the beach face, across a wide berm, through erode rapidly. The run-up profile is restricted low dunes, and finally into the grasslands and to the narrow beach face, and the sands are adjacent marshes. The post-Ginger photo- not transferred inland, but rather offshore or graphs of Hatteras Island and Core Banks alongshore. Some of this material is, of course, shown in Figure 3 support this. The inland returned to the beach face, but much of it is areas actually gain material from the beach as lost permanently. In the case of Hurricane the surge moves across the islands, and such Ginger, recession of the seaward face of the deposits serve as sources of supply for new dune Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/4/1329/3418249/i0016-7606-84-4-1329.pdf by guest on 30 September 2021 1332 DOLAN AND GODFREY il AUGUST 12,' 1971 • Figure 4. Close-up of site of P-3 (Fig. 2), bench August, 1971), and the same site on 9 October, 1971. mark on Core Banks before Hurricane Ginger (12 growth (Godfrey, 1970). Dunes are an integral mental to their origin, and that continue to be part of the natural system, since they do pro- important if they are to be preserved in a vide some resistance to surge, but they do not natural state. stop overvvash. ACKNOWLEDGMENTS IMPLICATIONS This investigation v/as supported by the Overwash is the major means by which the Office of Natural Science, National Park low barrier islands of the mid-Atlantic ret :eat Service, Department of the Interior. Aerial before the rising sea (Hicks, 1971), and it is the photographs were provided by the Chesapeake only manner in which great quantities of coarse Bay Ecological Program, National Aeronautics sand and shell can be transported inland from and Space Administration, Wallops Island, the beach. Coring shows that the barrier-island Virginia. systems are, in fact, constructed mostly of overwash-type sediments gained from the REFERENCES CITED littoral zone (Hoyt and Henry, 1967; Hoyt, 1971; Kraft, 1971).