CAPE LOOKOUT Barrier Island Ecology of Cape Lookout National Seashore and Vicinity, North Carolina NPS Scientific Monograph No. 9

Transects across the Codd's Creek salt marsh that formed where overwash deposits went into the lagoon are shown in Figs. 26 and 27. This new, expanding marsh developed in less than a decade and is much more productive than the older, eroding marshes that adjoin it. Tidal water moves freely into the new Spartina alterniflora (salt marsh cordgrass) marsh that is growing here at a lower elevation than the older, and hence higher, marsh nearby. The presence of beach shells under this luxuriant marsh grass is clear evidence that the substrate originated from overwash.

Fig. 26. Differences between old salt marsh surfaces and recent overwash deposit. (A) View from the end of Codds Creek Transect 1, showing eroded peat around the marsh and short Spartina alterniflora. The tall grass at location 2 on the right is Spartina growing in what was part of the creek before overwash filled it during the late 1950s. (B) Transect 2 was made through this remarkably tall and productive stand of Spartina alterniflora. Here the grass is nearly 2 m tall and is typical of new marsh growth on recent overwash deposits.

Fig. 27. Diagrammatic cross-section which contrasts the new salt marsh on Transect 2 with the older one on Transect 1. In Transect 2, the overwash created a gradually sloping surface into the creek, which was ideal for colonization by S. alterniflora. The marsh is readily flooded by the tides and is of great value to the estuary. Waves are easily dissipated in the grass and no erosion occurs. The marsh can also advance further into the creek on the sediments. It is tall, thick, and highly productive, with nearly three times the standing crop of the nearby old marsh. In Transect 1, the marsh substrate is peat and the plants are of the short form. Waves erode the edge of the marsh peat and the grass cannot invade the creek bottom. Several overwashes may have been involved in creating the substrate for the new marsh. The original creek bottom was very fine sand mixed with silt, and is distinct from the overlying oceanic sediments. The fine gray sand and shells may have come from an earlier, less severe overwash that preceded the major deposition on which the marsh now grows. Gray sand indicated reducing conditions and iron sulfides. Refer to Fig. 21E for the location of these transects. (click on image for an enlargement in a new window)

The photographs (Fig. 21) also demonstrate that the island retreats as a unit; it apparently maintains the minimum width which will allow storm waves to dissipate the brunt of their energy before getting to the dune line. Waves churn across this berm, demolishing any dunes too close to the beach, and then flow more quietly between the dunes into the grasslands behind them. Energy is further dissipated in the grass lands, where the flow slows down quickly. Thus, most of the sand load settles out in the dunes and grassland.

Since we began our observations, several storms have struck the islands, and in each case sand was deposited as expected. Hurricane Ginger, in the fall of 1971, was not a really powerful hurricane, but it did considerable damage to some developed beaches and put the Cape Lookout islands under water (Dolan and Godfrey in press). On these islands, instead of damage, there was a definite build-up around elevation markers at Codd's Creek (Fig. 28). On a rather narrow section of Core Banks opposite Atlantic, N.C., as much as 50 cm of sand were deposited on grasslands about 150 m back from the beach (Figs. 29 and 35).

Fig. 28. Effects of Hurricane Ginger, 30 September 1971, on Core Banks at Codds Creek. The hurricane crossed Core Banks between Drum inlet and Portsmouth Island. Storm surge was 2.5 m above sea level, with winds of 100 kph at Cape Lookout. The banks were completely inundated, with about 0.8 m of water over the backshore at the dune line. (A1) Codds Creek transect, shown by dotted line, before Hurricane Ginger. (A2) Codds Creek transect after Ginger. (9 October 1971.) Very little evidence of the storm passage can be seen from the air, illustrating the ability of these islands to withstand storm surges. (B1) View toward the ocean end of the transect, with plastic pipe benchmark CC3 visible in the foreground, (August 1971.) (B2) After Ginger, fresh sand was pushed up over the beach, burying CC3 and covering the sea oat seedlings in the old driftline. The beach face was flattened and the berm crest moved back about 40 m, although the location of m.s.l, did not retreat, (C1) Close-up of benchmark CC3 before Ginger, August 1971. The shell pavement on the island surface is readily visible in this view, (C2) Benchmark CC3 after Ginger. The sand level was raised 29 cm around this pipe. The deposits contain fine sand and shells, and the surface no longer has a shell pavement. (D1) View along the transect toward Codds Creek before Ginger, showing the line of benchmark pipes 30 m apart at which the cores shown in Fig. 27 were taken, and the general features of the overwash pass, dune system, and grasslands. CC4 is in the foreground. (D2) Transect 1 after Ginger. The drift line in dunes at the left shows that this area was totally submerged. The fine texture of the new deposit contrasts with the old surface. The sand surface at CC4 was raised 14 cm. At CC5 near the edge of the grassland it rose 19 cm, while at CC6, the third core back, which was in a hollow in the overwash pass, the level dropped 9 cm. At CC7,just visible in the distance, there was no change. The vegetation was unharmed by the storm surge, and only before and after measurements really show what occurred.

Fig. 29. Effects of Hurricane Ginger on Core Banks, where the storm crossed the Outer Banks causing heavy overwash just south of the Old Atlantic Coast Guard Station. (A1) View looking south of the berm and dune line in June 1971 (photo taken with a wide-angle lens). (A2) The same general area following Hurricane Ginger. The berm crest moved back and edges of the dunes along overwash passes were eroded. Even when the photo was taken in October, a new berm was building on the beach (photographed with a normal lens). (B1) An old overwash pass before Ginger—view is across the island from the view in Fig. 29 (photo taken with a wide-angle lens). (B2) The same area following Ginger. The dunes near the telephone pole were eroded, while the low dunes on the berm, seen as dark patches on the berm in Fig. 29, B1 were completely buried; just the culms of the grasses are visible (photographed with a normal lens). (C1) View across Core Banks toward the beach from the opposite side of the area in B1 before the storm. (C2) Ginger left a major overwash deposit on what was once a grassland in a former overwash pass. (D1) The overwashed area in C as seen from a dune in June 1971. The grassland was relatively open, although patches of dense vegetation were present. (D2) After Ginger, the new sand at the end of the overwash deposit was 50 cm (20 in) deep in places. Not all the grass was flattened; some Spartina patens is protruding from the sand. In a short time, the buried grass will recolonize the new surface as shown in Fig. 22. The view here is slightly to the left of that in D1; the telephone pole on the right in D1 corresponds to the nearest pole on the left in D2.

When storm waves strike directly against a stabilized dune line, scarping such as that shown in Fig. 30 is the result. Such erosion may occur when a beach retreats up to a natural or artificial continuous dune line, or when dune lines are built too close to the beach. In either case, wave energy is expended directly against the dune base, undermining the dune and causing the front of the structure to slump. No amount of grass on the dune can prevent this.

Fig. 30. A tropical depression in the fall of 1970 crossed a stabilized section of Bogue Banks, with the results we have learned to expect on a stabilized barrier island. The beach was cut back considerably and the dune face badly eroded. Waves from the storm dissipated their energy directly against the dunes and what berm existed was cut away. High tide now reaches the dune line. Contrast this scene with those in Fig. 29.

Occasionally, stabilized dune lines are broken by storm waves and an overwash fan is formed as sand from the dune is carried back. Figure 31 shows a once-continuous man-made dune on Core Banks which has been broken by storm water. Here, however, the damage was only to the artificial dune itself, since the sand was caught and colonized by grasses behind the dune. It is when there are developments behind such a continuous dune that difficulties result. Figure 32 shows the results of overwash in an area where cottages were built right behind the barrier and are now subject to flooding. Such breaching and flooding can be expected; they are part of the natural pattern of retreat.

Fig. 31. The same storm drove water over the beach on Core Banks, with no damage except to experimental dune lines built with beachgrass on northern Core Banks; this straight dune line was continuous and storm surges piled up against it until a weak area broke through. Sand from the dune line was spread out in back as an overwash fan.

Fig. 32. Dune breaks and overwash caused considerable damage to cottages built too close to the beach. A storm on 28 March 1971 broke over the low dunes at Avon, a town within the boundaries of Cape Hatteras National Seashore, causing what is generally termed damage, although it was a predictable and natural event. (Photo by Cape Hatteras National Seashore Staff.)

Contrasting with the erosion and breaching of stabilized dunes is the pattern seen on islands with natural, discontinuous dune fields. Here, overwashes carry sand through the dune fields and create fans in low areas behind, as shown in Fig. 33 on Shackleford Banks. This fan is typical of natural beaches; when Hurricane Ginger swept across the Outer Banks, many such fans were produced on Core Banks (Fig. 34). A summary of the overwash process is shown in Fig. 36.

Fig. 33. Tropical storm Doria, in August 1971, drove storm surges across Shackleford Banks. The dune line, while substantial, has breaks and passes. All along this beach, classic overwash fans were laid out between the dunes. The edge of this deposit was about 17 cm thick. (Telephoto view from a high rear dune appears to compress the distance between fan and ocean.)

Fig. 34. The same part of Core Banks as shown in Fig. 29, after Hurricane Ginger. The photographs in Fig. 29 were taken opposite the number "34." Contrast this view with Fig. 32. On the undeveloped beach. overwash builds up the inner part of the island. Past overwash deposits are clearly visible in this view. The area is just south of the view of the Coast Guard Station in Fig. 23. In December 1971, the Corps of Engineers cut a new inlet through the narrow neck near the top of the photograph. (See Fig. 10D.)

Fig. 35. The effects of Hurricane Ginger in Cape Lookout (Core Banks) and Cape Hatteras National Seashores are contrasted in this diagram from Dolan and Godfrey (in press). On Core Banks, the berm crest was knocked back, and the beach face flattened; overwash raised the backshore. Following the storm, the berm crest moved seaward and 10 months later all that was lost in the storm was eventually regained; see profile in Fig. 25 which shows the same transect in more detail. Dunes were actually raised by wind blowing the new sand into the dune line. On the stabilized section, the whole beach system was cut back, and the berm and dune line severely eroded, with the inevitable alarm and call for erosion control. The contrast between profiles of stabilized and natural beaches is in itself revealing. (click on image for an enlargement in a new window)

Fig. 36. Diagrammatic interpretation of the overwash process and recovery of a low barrier island such as Core Banks, (From Godfrey 1970.) (1) Cross-section of a low barrier island at any starting time. (2) A severe storm will push high tides over the beach berm; as the water flows back across the island it carries sand into the grasslands. (3) A second, less severe storm carries a smaller quantity of sand onto the earlier deposit. Abnormally high tides have pushed a little sand over the edge of the berm. Grass has pushed up through the earlier layer. The front of the island has retreated slightly. Following the storm, the berm will build seaward, (4) A severe storm inundates the island and adds a great deal of sand to the earlier layers. Old dunes have been knocked down and the sand spread out, The beach front has retreated substantially. In some places, the berm front may be lowered as sand is removed. (5) After a few years of relative storm quiet, the island surface has completely recovered, although the vegetation zones have been displaced slightly. Low salt marsh has grown up on the new base in the sound, the former low marsh has become high marsh, and the sand flat has extended into the old high marsh border.