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Practical solution to a beach erosion problem
Coastal Engineering, 1 (1977) 3--16 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
3
PRACTICAL SOLUTION TO A BEACH EROSION PROBLEM
PER BRUUN Technical Unwersity of Norway, Trondheim (Norway) (Received August 27, 1976; accepted February 2, 1977)
ABSTRACT Bruun, P., 1977. Practical solution to a beach erosion problem. Coastal Eng., 1 : 3--16.
This paper deals with a severe erosion problem, its initial solution and maintenance. The problem area is part of the Hilton Head Island, South Carolina, a member of the large Carolina, Georgia and Florida Island-chain. The only practical solution, under the actual circumstances, was artificial nourishment of the beach which then had also to be maintained by nourishment. The latter problem has not been finally solved at this time, but a "status quo" accepting slow erosion is upheld until pumping of sand to the beach from the ocean bottom can be initiated. In the introductory sections of the paper, some data of historic and g£~omorphological nature are given. INTRODUCTION T h e H i l t o n Head Island, S o u t h Carolina is l o c a t e d a b o u t 60 k m n o r t h e a s t o f t h e large s e a p o r t Savannah, Georgia, w h i c h is o n the historic Savannah River. T h e island, which was a U n i o n s t r o n g h o l d d u r i n g t h e Civil War, n e x t b e c a m e an a l m o s t f o r g o t t e n l u m b e r island, b u t e m e r g e d again in t h e fifties as a r e s o r t area w h i c h is n o w k n o w n all over t h e world. G e o g r a p h i c a l l y it is a b o u t 20 k m long and varies in w i d t h f r o m 1 k m (at t h e c r e e k or s o u n d w h i c h a l m o s t separates t h e island in t w o parts) t o a b o u t 8 km (Fig.lA). A l t h o u g h possibly sighted b y earlier explorers, D e Q u e x o s o f Spain und o u b t e d l y was t h e first t o see t h e island's high b l u f f t o w a r d s n o r t h e a s t . This h a p p e n e d o n August 18, 1521 ( H o l m g r e n , 1 9 5 9 ) . On the same d a y a n o t h e r island, which h e called Santa Elena, n o r t h w e s t o f H i l t o n Head was sighted ( n o w Pan'is Island, Marine Corps base; see F i g . l B ) . T h e land was claimed f o r Spain. T h e Spanish, h o w e v e r , did n o t s h o w a n y particular interest in t h e i r n e w possession. In 1 5 6 2 a F r e n c h vessel arrived. Its c a p t a i n was Jean Ribaut. T h e c h i e f s p o n s o r b e h i n d t h e e x p e d i t i o n was t h e n o b l e m a n Gaspard, c o m t e de Coligny, seigneur de Chatillon-sur-Loing, an admiral o f France, and it was r u m o r e d t h a t t h e Q u e e n M o t h e r herself, t h e ever-scheming Catherine d e Medici, had also c o n t r i b u t e d t o it. T h e colonists a b o a r d t h e vessel were F r e n c h Huguenots seeking refuge f r o m a n t i - P r o t e s t a n t p e r s e c u t i o n in Catholic France. T h e r e
4
Fig.l.A. Hilton Head Island, South Carolina. B. The harbour of Port Royal.
is some evidence to prove that the sponsors, at least -- if n o t the colonists themselves -- hoped to make their fortune by robbing Spanish treasure ships homeward bound with their b o o t y of Inca and Aztec gold. Ribaut did not set up camp on the island where any passing Spanish ship might have sighted him, but went on a good three leagues within the channel
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(or at least most historians think so) to the island later called P a r r i s - f o r the Englishman Alexander Parris who later bought it. Ribaut had already set up at least one stone pillar engraved with the royal arms of France somewhere along the Florida coast and n o w he erected another near the new town site. He named the town "Charlesfort". It did n o t take the Spanish a long time to hear of the French colony at Charlesfort, and they decided to wipe it out. Hernando Manri_que de Rojas set o u t in the spring of 1564. Little did he guess that all b u t one of the French Protestants had already left the settlement. The Spanish t o o k over again, b u t vanished gradually, massacred by the Indians. The final clean-out was n o t due to Indian opposition, however, b u t to the coming of the English (Holmgren, 1959). Charles the First of England was easily persuaded that the early voyages of the Cabots in 1497 gave him full claim to the whole North American continent. True, the Catholic Pope had decreed these lands to be Spanish, b u t Protestant England owed no allegiance to His Holiness. Consequently, the English were free to settle where they pleased, and they had been pleased to settle in Virginia and New England and several places in between. Now they began to look southward to the tempting green lands of Spanish Florida and Santa Elena. The land would need a new name, of course, to go with the new owner, a name that honored King Charles of England. From the Latin "Carolus" they formed the name "Carolana", soon changed to "Carolina". It was a good choice, regardless of whom it honored, for a Spanish king named Charles had sponsored the land's first discovery and a French King Charles had been the namesake of its first Protestant colony. The English with their new Charles were the last to come and they would be the last to leave. Carolina, North and South, the land would remain, and the sea islands of the Carolina shore would bear witness to their triple heritage. In 1663 Captain William Hilton t o o k off from Barbados for the islands. Referring to Fig.lB, Capt. Hilton had already explored the harbor of St. Ellen's (Elena) or Port Royal from the inland waterway, and there he decided to investigate its seaward approach. Now, apparently for the first time, he saw the headland that bears his name, standing o u t bluff and bold at the southwest of the entrance, the perfect landmark for taking safe bearings to enter in deep water and avoid the shoals (Holmgren, 1959). "The said headland is bluft," he wrote, "and seems so steep as though trees hung over the water." Then withl carefull mariner's directions he went on to explain h o w any captain keeping the bluff headland as landmark would find all bold steering and safe entry. From that m o m e n t on it was named "Hilton H e a d " and so it has been called ever since, written plainly on many a sailor's chart and in many a sea journal. At the beginning of the English period, the island developed slowly, b u t became a prosperous c o t t o n field in the 18th--19th century until the Civil War in the 1860's when, after battles with the Confederates, it became a Union stronghold. Many of the southern c o t t o n farmers left. Two forts were now built, but no attacks followed. The most recent history of the island has
already been mentioned above. From a deserted lumber land, it emerged as an attractive recreational resort area in the 1950s. COASTAL GEOMORPHOLOGY
The enormous flatlands on the southeastern seabord of the United States were built up b y sediments which originated from erosion of mountains in the area and northwest of the area. During the Pleistocene the position of the shorelines followed the glacial variances, being submerging in interglacial and emerging during glacial periods. Sea-level changes in the Quaternary evoked some ingenious theories. It is n o w established that during the half-million years of the Quaternary, the sealevel has oscillated in a manner as rapidly and extreme as was ever before observed in geological history. During the warm, interglacial phases, the shorelines advanced inland leaving erosion cliffs, terraces and platforms behind. During the cool, glacial phases the shorelines advanced seaward and new land was built up leaving ridges and plateaus emerged for later submergence (Bruun; 1962). The ice-age oscillations involved withdrawals of huge water masses from the sea on the polar land masses. The present total volume of sea water is approximately 1 3 7 0 - 106 km 3, b u t there is still a considerable quantity of water locked in the present-day continental ice caps and glaciers. For the Northern Hemisphere the glacier area is estimated at somewhat over 2- 106 km2; for the Southern Hemisphere 13 - 106 km 2, thus a total of 15 • 106 km 2 or approximately 37.5 • 106 km 3 of ice averaging 2.5 km thick. Because the area of ocean surface is 3 6 0 . 1 0 6 km 2, a total melting of 3 7 . 5 . 1 0 6 k m 3_of ice would cause a sealevel rise of 95 m (315 ft.). Due to oceanic crustal lowering, marginal to rising continental areas, and to the fact that the rising sea would spill over enormous lowlands greatly expanding the present ocean area, the final level of the ocean might be perhaps only approximately 50 m above the present datum. (Present as used herein is in a geological rather than literal sense.) The area of the last glaciation m a x i m u m has been determined at approximately 4 0 - 106 km 2 or to approximately 40- 106 km 2 ice. This corresponds to the maximum measured fall of sea level, 100 m (330 ft.), or to " t h e Wisconsin" Glacial Period in the United States. However, owing to the progressive build-up of Antarctica over four or five glacial cycles of the Pleistocene, the total removal of water m a y approach 200 m (660 ft.) The eustatic oscillations during the post-Glacial transgression demonstrate an almost constant sea-level during the last 5,000 years with smaller fluctuations (Bruun, 1962). The so-called "Florida Emergence" that occurred approximately 2,100--1,600 years ago had sea-level elevations of approximately 2 m below present, probably adding 1/8 mile to 1 mile to the general Florida shores, depending on offshore b o t t o m slope. It coincides with a slight advance of northern glaciers. From a historic point o f view, it is interesting to note that this period covers the R o m a n Era and that data from Britain, Italy, and the Mediterranean suggest a low sea-level at this time. Apart from notoriously
unstable and volcanic areas, there is widespread evidence of the "drowning" of Roman coastal structures. The deep foundations of some ancient harbor works may not have been so difficult to construct as they seem today. Climatically, the Florida Emergence, which also effected the Carolinas, coincides with a universal cool phase (Bruun, 1962). Fig.2 gives an impression of eustatic oscillations during the post, Glacial transgression including the latest 2,000 years (Bruun, 1962). i0
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Fig. 2. Eustatic oscillations during the post-Glacial transgression. In examining the sea-table fluctuations of the past 100 years, note that the average world rise for the 1 9 0 0 - - 1 9 5 0 period was 1.2 mm, annually, corresponding to the average rise since the Roman Era of approximately 2,000 mm. Meanwhile deglaciation can result in sea-levels rising at 25 m m or approximately 1 inch per year. In the 1 9 4 6 - - 1 9 5 6 period, the rise was 5.5 m m per year. H.A. Marmar (Bruun, 1962) lists approximately 8 mm, annually, from 1930 to 1948, for the entire eastern seaboard of North America. Yet, an appreciable c o m p o n e n t in this sea-level rise, only approximately 20% is "eustatic" (overall sea-level) and is possibly due to a secular decelaration of the Gulf Stream. This decreases pressure differences between its right and left-hand side caused by the Coriolis force. Such pressure fluctuation is noted in the annual changes in the difference of sea-level between stations on opposite sides of the Gulf Stream; for example, between Miami and the West Bahamas. This difference is normally of the order of approximately 0.6 m (2 ft.). The seasonal variation in water temperature (density of water) is clearly indicated in records along the southeast coast of Florida where the maximum sea-level occurs in September and
October, apparently caused b y the summer heating of the water. The same happens, b u t to a minor extent, in the Carolinas. The littoral drift along the Hilton Head Island is northeastward during the spring and summer months and southwestward during the rest of the year, the predominant drift being southwestward. The shelf in front of the island is very wide and shallow with 3--5 m depth. It has u n d o u b t e d l y been emerged above sea-level although not facing the ocean with a continuous shoreline, b u t penetrated by tidal and river entrances. During periods of emergence the shorelines progradated with large beach ridge systems as evidenced by the numerous ridges which m a y be seen in Fig. 5. At the entrances shoals developed and the beach ridges bent oceanward in large, cuspate forelands. At the Hilton Head Island the existence of such forelands in the past is evidenced b y the beach ridge geometry turning seaward in the middle and on the northeast part of the island and b y the bump in the shureline alignment at the middle of the island (Fig.lA). It is natural that erosion is maximum at this b u m p where the curvature of the shoreline is maximum. This happened to be the place where the so-called "Palmetto Dunes Development" was undertaken (Bruun, 1976). Main reasons for erosion should be sought in the slow rise of the sea-level (Bruun, 1962) and -- n o t least -- in the existence of a rather small and innocentAooking tidal creek called "The F o l l y " with an ocean entrance as seen in Fig.lA. Regardless of its small size -- max. discharge is a b o u t 5--10 m3/sec -it has caused the development of ocean shoals n o w storing some hundred thousand cubic meters of sand (Fig.lA) causing severe erosion of the immediate adjoining beach towards the southwest called the Singleton-Collier Beach which has a length o f a b o u t 500 m. As the Port Royal Sound (Fig.lB) functions as a littoral drift barrier for the overall predominant southward littoral drift, the Folly creek in this way acts as a local barrier for the southward drift on Hilton Head Island. The total length of heavily eroding beaches is a b o u t 5 km. Before 1970-1971 there was no beach at all at normal high tide. Mean tidal range is a b o u t 2 m (7 ft.), with spring range at 2.5--3 m (9--10 ft.) and neap range is 1.5--2 m (5--6 ft.) Due to a shoreline recession of 1--1.5 m (4--5 ft.) per year, high tides washed up towards the low beach dunes and w o o d e d areas developing vertical erosion scarps which contributed further to erosion b y reflection of waves. The erosion situation was demonstrated by a great number o f tree stumps and peats exposed on the beach. At low tides the beach was a b o u t 75 m (250 ft.) wide b u t very wet, n o t only because of the receding tides, b u t also due to ground seepage of water from troughs between the beach ridges (Fig.5) through the narrow dune zone. This in turn contributed further to erosion b y run-down, as well as b y lift forces on the sand at low tides. During storms and high tides uprush penetrated in the w o o d e d area, and the result was that palmettos, oak trees and shrubbery were lying on the upper beach in one big mess. To stabilize the beach there was apparently no other solution than replacement of the eroded material, raising the beach a b o u t 1.2--1.5 m (4--5 ft.) and widening it b y 40--50 m ( 1 2 0 - 1 5 0 ft.). Furthermore, in order to protect the dunes and the area behind the dunes against overwashing and
10 flooding, it was necessary to build an artificial dune of 20 m (60 ft.) crown width at elevation 3.3 m (11 ft.) above mean sea-level (MSL), slope 1 in 7 (220 m 3 per meter) or 1.6 million cubic yards (100 cu. yds. per ft. of shore front), a total of about 1.2 million cubic meters. Grain size of beach sand was on average 0.18--0.20 mm. Fill was as large or larger, averaging 0.18--0.22 mm (Fig.3). 3 50'
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Fig.3. Beach a n d d u n e g e o m e t r y a f t e r t h e artificial n o u r i s h m e n t .
Fill for beach nourishment was obtained from dredging of canals and lagoons in the development, thereby creating a great number of canal and lagoonfront lots for homesites. For reasons of e c o n o m y it was decided to limit pumping distances to a m a x i m u m of approximately 1 km (3,000 ft.) from the dredge to the discharge point. In addition some interior dredging and filling was done (Bruun, 1976). The working comprising dredging and dumping of 220 m 3 per meter was undertaken by Norfolk Dredging Company, Norfolk, Virginia, which used its 14-inch dredge, "Jekylll Island". The 100 tons heavy dredge was brought in on the bay side of the island and pulled by a D6C Caterpillar Dozer across approximately 1/4 mile of mainland, then across a highway and finally across a golf course and launched at an interior lake. The air-filled rubber bags, or mattresses, were 30 inches in diameter by 30 ft. long (0.75 m by 9 m long) filled to a capacity of 7 PSI (0.5 kg/cm: ) when carrying the dredge across the highway. Departure after completion of work was accomplished on a relatively calm day by rolling the dredge out of the lake over the beach dunes and into the ocean. It has been determined through numerous moves made with these particular bags and this particular machine that one must have no steeper than a 7 horizontal to 1 vertical slope for ascension and decent. The entire moves on this large project were made with ease and minimum cost compared to assembly and reassembly. Fig.4 shows the 14 inch (0.35 m) discharge pipe spoiling on the beach, building it up and widening it. The existing partly swamped beach may be seen in the background. Bulldozers used to give the artificial dune the right geometry (Fig.3) are also seen. The unit cost per cubic meter was about $0.60. The ultimate result is shown in Figs. 5 and 6, a wide and beautiful beach. The inner part of the crown and the landward slope of the dune was vegetated by American Beach grass (Ammophila brevigulata). At either end of the beach curved rock groins 50 and 80 m long were built
11
Fig.4. Spoiling on beach.
Fig. 5. The artidicial dune and beach.
12
Fig.6. Dune and beach after nourishment.
out to the MSL shoreline to "encase" the fill to some extent w i t h o u t damaging the adjoining shorelines (Fig.lA). South of the southern boundary line, a 1,200 ft. (360 m) rip-rap protection supported by palmetto piles was built in 1960, located about 40 ft. (12 m) inside the MHWL to protect the homes located right on the eroding dune exposing 1--2 m scarps. Due to their curvature and extension to the MSL line only, the two curved groins have worked satisfactorily with only minor adverse effects -- if any -- on their downdrift sides. This artificial nourishment was completed in 1970 and during the first two years the erosion apparently was only minor, at least n o t visible on the dry beach. The fact u n d o u b t e d l y was that overflow material from the nourishment stabilized the nearshore b o t t o m for some time. In the latter part of 1972 erosion started again and developed high scarps during a severe storm which peaked exactly during a spring high tide. The result may be seen in Fig.7. In order to bring the profile back to normal shape, another type of "artificial nourishm e n t " of the dune and of the upper part of the beach was undertaken. It was observed repeatedly that while the dune slope and the MHWL line moved shoreward during a severe storm the MLWLline moved oceanward due to the deposition of eroded sand on either side of the MLWL line. For t h a t reason a scraper and dozer operation was undertaken attempting to bring the profile back to the original shape and location. In addition an application was filed for federal as well as state authorities to close The Folly (the creek) which as mentioned above constituted a partly littoral drift barrier. It was anticipated that such a closure would decrease the anticipated annual maintenance of about 40,000 m 3 to about half of this quantity. Misunderstood and exaggerated en-
13
Fig.7. Erosion scarps in December 1971.
vironmentalism and lack of proper understavcding of the problem and of the environment as well, combined with the lack of practical state laws, has, however, up to this time upheld action on this project, which is very easy to undertake and is supported by all engineering professional agencies including the federal U.S. Army Corps of Engineers and generally supported by all local governments and authorities who k n o w the problem. Storms in 1973 and 1974 made it necessary to repeat this " n o u r i s h m e n t " by which the dune and upper beach were stabilized at the cost o f the lower shore and nearshore area. Admitting that this does n o t contribute to the nourishment of the entire eroding areas and b o t t o m profile, such a procedure u n d o u b t e d l y has some advantages extending b e y o n d the assistance in keeping the erosion " p e r p e t u u m mobile" running. It is well k n o w n from practical experience as well as research in Holland (Bakker, 1969) that the higher an erosion scarp is, the more rdaterial is eroded. It is therefore important to keep the inner beach high (above MHWL + most uprushes) and have as gentle a slope of the dune as possible. This slows d o w n the rate of erosion of the upper beach and stabilizes the dune against erosion and -- most important -- offers protection against overflows and flooding. Certain precautions should be considered, however. Scraping should, if possible, be undertaken during the late summer and the early-fall months. If so, the summer swells have built up a beach ridge between the MSL and the MLW line, and if material from this ridge is scraped or dozed up in the dune and upper beach, the ridge most likely will re-build by material from the nearshore b o t t o m due to gentle swell action. In other words, the swells are helpful in extending the "artificial nourishment" further offshore! If, on the other hand, scraping is undertaken during the winter months, one may risk that storm waves penetrate more easily to the upper levels of the beach and reach the dunes, thereby increasing erosion. Summer-scraping therefore has double advantages.
14 Tracer experiments using fluorescent tracers undertaken following a scraper operation proved that material on either side of the MSL line, regardless of the scraper operation which goes max. 0.3 m down in the beach, did n o t limit or prohibit migration of material in either direction from the scraped area, most rapidly in the direction of the predominant drift, that means southwestward. This would also be rather unlikely, as neither the wave energy nor the refraction pattern has been changed! -- (although some tend to believe t h a t it has!) It is, however, considered as being of importance that scraping is n o t undertaken close to the boundary groins, but stays 300--500 m away from these areas to avoid any trap-function of the scraped areas. To save on maintenance and due to the continued but slow, general shoreline recession, tests have been performed by placing new dune fill under a steeper slope angle, 1 in 4, and at the same time letting the upper (seaward) edge of the dune recede about 5--10 m. Tests are now planned to use a "Sea Carpet" of Dutch import, placed on a slope, 1 in 4, compacted to 75% max. density, and held in place in its upper and lower edge by casting it in a concrete box. Tests carried out earlier (1961--1965) at the Port Royal Inn, located at the northeast c o m e r of the Hilton Head Island, placing a plastic carpet with sandfilled 0.3 m tubes for every 1.5 m on an about 1 in 10 beach slope were very successful. The carpet buried itself in the sand very rapidly! During storms the fill washed out -- but it always covered itself up after the storm (Fig.S).
Ouno
Beach after placement of carpet
stitched to form 0.3m tubes which were sandfilled.
Fig.8. Placement of plastic sheet on eroding beach.
Another measure of a temporary maintenance nature has been undertaken frequently. In fall and winter, sometimes also in spring, storms tend to develop the so-called "sloughs" in the beach profile (Figs. 9 and 10). Such a slough is in fact a " t r o u g h " inside a " b a r " and, at the same time as it makes the beach inside the slough (too) steep, the slough carries a longshore current which as a rip current transports material eroded in offshore direction (Fig.8). Scrapers and bulldozers are used to bar or fill such sloughs entirely to eliminate their adverse effect on erosion of the beach. As it will be understood from the above-mentioned, these maintenance measures must be considered of partly temporary nature. The final solution to the maintenance will u n d o u b t e d l y become the closing of the Folly and regular (once every 3 to 5 years) nourishment from offshore sources. In Florida a dragline has been used (Bruun, 1967). New methods include a jet pump ( F i g . l l ) (De Graca, 1975; McNair, 1976)
15
Fig.10. Slough and its run-off channel.
16
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Dune
Fig.9. Cross-section in beach with slough.
operating 600--1,000 ft. (200--300 m) from shore, or a "Norwegian t y p e " split-hull (SELMER) barge of 6--8 ft. (2--2.5 m) draft fully loaded. It may e.g. be filled by a sidecaster (Bruun, 1976). Tests have now been undertaken by the U.S. Army Corps o~ Engineers, Wilmington District, on the use of such a barge. So - the battle even though not yet won -- is being won. Discharge
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Fig.11. A r t i f i c i a l n o u r i s h m e n t b y j e t - p u m p .
REFERENCES Bakker, W.T, 1969. The influence of Dune Height on Dune Erosion. Rijkswaterstaat Rep., 69-1, The Netherlands. Bruun, P., 1962. Sea-level rise as a cause of shore erosion. Proc. ASCE, J. Waterw., Harbors Coastal Eng., Div., 88 (WW1). Bruun, P., 1967. Bypassing and backpassing with reference to Florida. Proc. ASCE, J. Waterw., Harbors Coastal Eng. Div., 93 (WW2). Bruun, P., 1976. Dredging and reclaiming and exclusive development. Terra Aqua, 10. Bruun, P., 1976. Port Engineering. The Gulf Publishing Co., Houston, Texas, 600 pp. De Graca, H., 1975. Jet-pump - - A possible answer to Santa Cruz port shoaling". World Dredging, October, 1975. Holmgren, V.C., 1959. Hilton Head, A Sea Island Chronicle. Hilton Head Island Publishing Co. McNair, E.C., 1976. Abstract on "Sand bypassing system using a jet pump". Int. Conf. Coastal Eng., Honolulu, 15th.
3
PRACTICAL SOLUTION TO A BEACH EROSION PROBLEM
PER BRUUN Technical Unwersity of Norway, Trondheim (Norway) (Received August 27, 1976; accepted February 2, 1977)
ABSTRACT Bruun, P., 1977. Practical solution to a beach erosion problem. Coastal Eng., 1 : 3--16.
This paper deals with a severe erosion problem, its initial solution and maintenance. The problem area is part of the Hilton Head Island, South Carolina, a member of the large Carolina, Georgia and Florida Island-chain. The only practical solution, under the actual circumstances, was artificial nourishment of the beach which then had also to be maintained by nourishment. The latter problem has not been finally solved at this time, but a "status quo" accepting slow erosion is upheld until pumping of sand to the beach from the ocean bottom can be initiated. In the introductory sections of the paper, some data of historic and g£~omorphological nature are given. INTRODUCTION T h e H i l t o n Head Island, S o u t h Carolina is l o c a t e d a b o u t 60 k m n o r t h e a s t o f t h e large s e a p o r t Savannah, Georgia, w h i c h is o n the historic Savannah River. T h e island, which was a U n i o n s t r o n g h o l d d u r i n g t h e Civil War, n e x t b e c a m e an a l m o s t f o r g o t t e n l u m b e r island, b u t e m e r g e d again in t h e fifties as a r e s o r t area w h i c h is n o w k n o w n all over t h e world. G e o g r a p h i c a l l y it is a b o u t 20 k m long and varies in w i d t h f r o m 1 k m (at t h e c r e e k or s o u n d w h i c h a l m o s t separates t h e island in t w o parts) t o a b o u t 8 km (Fig.lA). A l t h o u g h possibly sighted b y earlier explorers, D e Q u e x o s o f Spain und o u b t e d l y was t h e first t o see t h e island's high b l u f f t o w a r d s n o r t h e a s t . This h a p p e n e d o n August 18, 1521 ( H o l m g r e n , 1 9 5 9 ) . On the same d a y a n o t h e r island, which h e called Santa Elena, n o r t h w e s t o f H i l t o n Head was sighted ( n o w Pan'is Island, Marine Corps base; see F i g . l B ) . T h e land was claimed f o r Spain. T h e Spanish, h o w e v e r , did n o t s h o w a n y particular interest in t h e i r n e w possession. In 1 5 6 2 a F r e n c h vessel arrived. Its c a p t a i n was Jean Ribaut. T h e c h i e f s p o n s o r b e h i n d t h e e x p e d i t i o n was t h e n o b l e m a n Gaspard, c o m t e de Coligny, seigneur de Chatillon-sur-Loing, an admiral o f France, and it was r u m o r e d t h a t t h e Q u e e n M o t h e r herself, t h e ever-scheming Catherine d e Medici, had also c o n t r i b u t e d t o it. T h e colonists a b o a r d t h e vessel were F r e n c h Huguenots seeking refuge f r o m a n t i - P r o t e s t a n t p e r s e c u t i o n in Catholic France. T h e r e
4
Fig.l.A. Hilton Head Island, South Carolina. B. The harbour of Port Royal.
is some evidence to prove that the sponsors, at least -- if n o t the colonists themselves -- hoped to make their fortune by robbing Spanish treasure ships homeward bound with their b o o t y of Inca and Aztec gold. Ribaut did not set up camp on the island where any passing Spanish ship might have sighted him, but went on a good three leagues within the channel
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(or at least most historians think so) to the island later called P a r r i s - f o r the Englishman Alexander Parris who later bought it. Ribaut had already set up at least one stone pillar engraved with the royal arms of France somewhere along the Florida coast and n o w he erected another near the new town site. He named the town "Charlesfort". It did n o t take the Spanish a long time to hear of the French colony at Charlesfort, and they decided to wipe it out. Hernando Manri_que de Rojas set o u t in the spring of 1564. Little did he guess that all b u t one of the French Protestants had already left the settlement. The Spanish t o o k over again, b u t vanished gradually, massacred by the Indians. The final clean-out was n o t due to Indian opposition, however, b u t to the coming of the English (Holmgren, 1959). Charles the First of England was easily persuaded that the early voyages of the Cabots in 1497 gave him full claim to the whole North American continent. True, the Catholic Pope had decreed these lands to be Spanish, b u t Protestant England owed no allegiance to His Holiness. Consequently, the English were free to settle where they pleased, and they had been pleased to settle in Virginia and New England and several places in between. Now they began to look southward to the tempting green lands of Spanish Florida and Santa Elena. The land would need a new name, of course, to go with the new owner, a name that honored King Charles of England. From the Latin "Carolus" they formed the name "Carolana", soon changed to "Carolina". It was a good choice, regardless of whom it honored, for a Spanish king named Charles had sponsored the land's first discovery and a French King Charles had been the namesake of its first Protestant colony. The English with their new Charles were the last to come and they would be the last to leave. Carolina, North and South, the land would remain, and the sea islands of the Carolina shore would bear witness to their triple heritage. In 1663 Captain William Hilton t o o k off from Barbados for the islands. Referring to Fig.lB, Capt. Hilton had already explored the harbor of St. Ellen's (Elena) or Port Royal from the inland waterway, and there he decided to investigate its seaward approach. Now, apparently for the first time, he saw the headland that bears his name, standing o u t bluff and bold at the southwest of the entrance, the perfect landmark for taking safe bearings to enter in deep water and avoid the shoals (Holmgren, 1959). "The said headland is bluft," he wrote, "and seems so steep as though trees hung over the water." Then withl carefull mariner's directions he went on to explain h o w any captain keeping the bluff headland as landmark would find all bold steering and safe entry. From that m o m e n t on it was named "Hilton H e a d " and so it has been called ever since, written plainly on many a sailor's chart and in many a sea journal. At the beginning of the English period, the island developed slowly, b u t became a prosperous c o t t o n field in the 18th--19th century until the Civil War in the 1860's when, after battles with the Confederates, it became a Union stronghold. Many of the southern c o t t o n farmers left. Two forts were now built, but no attacks followed. The most recent history of the island has
already been mentioned above. From a deserted lumber land, it emerged as an attractive recreational resort area in the 1950s. COASTAL GEOMORPHOLOGY
The enormous flatlands on the southeastern seabord of the United States were built up b y sediments which originated from erosion of mountains in the area and northwest of the area. During the Pleistocene the position of the shorelines followed the glacial variances, being submerging in interglacial and emerging during glacial periods. Sea-level changes in the Quaternary evoked some ingenious theories. It is n o w established that during the half-million years of the Quaternary, the sealevel has oscillated in a manner as rapidly and extreme as was ever before observed in geological history. During the warm, interglacial phases, the shorelines advanced inland leaving erosion cliffs, terraces and platforms behind. During the cool, glacial phases the shorelines advanced seaward and new land was built up leaving ridges and plateaus emerged for later submergence (Bruun; 1962). The ice-age oscillations involved withdrawals of huge water masses from the sea on the polar land masses. The present total volume of sea water is approximately 1 3 7 0 - 106 km 3, b u t there is still a considerable quantity of water locked in the present-day continental ice caps and glaciers. For the Northern Hemisphere the glacier area is estimated at somewhat over 2- 106 km2; for the Southern Hemisphere 13 - 106 km 2, thus a total of 15 • 106 km 2 or approximately 37.5 • 106 km 3 of ice averaging 2.5 km thick. Because the area of ocean surface is 3 6 0 . 1 0 6 km 2, a total melting of 3 7 . 5 . 1 0 6 k m 3_of ice would cause a sealevel rise of 95 m (315 ft.). Due to oceanic crustal lowering, marginal to rising continental areas, and to the fact that the rising sea would spill over enormous lowlands greatly expanding the present ocean area, the final level of the ocean might be perhaps only approximately 50 m above the present datum. (Present as used herein is in a geological rather than literal sense.) The area of the last glaciation m a x i m u m has been determined at approximately 4 0 - 106 km 2 or to approximately 40- 106 km 2 ice. This corresponds to the maximum measured fall of sea level, 100 m (330 ft.), or to " t h e Wisconsin" Glacial Period in the United States. However, owing to the progressive build-up of Antarctica over four or five glacial cycles of the Pleistocene, the total removal of water m a y approach 200 m (660 ft.) The eustatic oscillations during the post-Glacial transgression demonstrate an almost constant sea-level during the last 5,000 years with smaller fluctuations (Bruun, 1962). The so-called "Florida Emergence" that occurred approximately 2,100--1,600 years ago had sea-level elevations of approximately 2 m below present, probably adding 1/8 mile to 1 mile to the general Florida shores, depending on offshore b o t t o m slope. It coincides with a slight advance of northern glaciers. From a historic point o f view, it is interesting to note that this period covers the R o m a n Era and that data from Britain, Italy, and the Mediterranean suggest a low sea-level at this time. Apart from notoriously
unstable and volcanic areas, there is widespread evidence of the "drowning" of Roman coastal structures. The deep foundations of some ancient harbor works may not have been so difficult to construct as they seem today. Climatically, the Florida Emergence, which also effected the Carolinas, coincides with a universal cool phase (Bruun, 1962). Fig.2 gives an impression of eustatic oscillations during the post, Glacial transgression including the latest 2,000 years (Bruun, 1962). i0
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Fig. 2. Eustatic oscillations during the post-Glacial transgression. In examining the sea-table fluctuations of the past 100 years, note that the average world rise for the 1 9 0 0 - - 1 9 5 0 period was 1.2 mm, annually, corresponding to the average rise since the Roman Era of approximately 2,000 mm. Meanwhile deglaciation can result in sea-levels rising at 25 m m or approximately 1 inch per year. In the 1 9 4 6 - - 1 9 5 6 period, the rise was 5.5 m m per year. H.A. Marmar (Bruun, 1962) lists approximately 8 mm, annually, from 1930 to 1948, for the entire eastern seaboard of North America. Yet, an appreciable c o m p o n e n t in this sea-level rise, only approximately 20% is "eustatic" (overall sea-level) and is possibly due to a secular decelaration of the Gulf Stream. This decreases pressure differences between its right and left-hand side caused by the Coriolis force. Such pressure fluctuation is noted in the annual changes in the difference of sea-level between stations on opposite sides of the Gulf Stream; for example, between Miami and the West Bahamas. This difference is normally of the order of approximately 0.6 m (2 ft.). The seasonal variation in water temperature (density of water) is clearly indicated in records along the southeast coast of Florida where the maximum sea-level occurs in September and
October, apparently caused b y the summer heating of the water. The same happens, b u t to a minor extent, in the Carolinas. The littoral drift along the Hilton Head Island is northeastward during the spring and summer months and southwestward during the rest of the year, the predominant drift being southwestward. The shelf in front of the island is very wide and shallow with 3--5 m depth. It has u n d o u b t e d l y been emerged above sea-level although not facing the ocean with a continuous shoreline, b u t penetrated by tidal and river entrances. During periods of emergence the shorelines progradated with large beach ridge systems as evidenced by the numerous ridges which m a y be seen in Fig. 5. At the entrances shoals developed and the beach ridges bent oceanward in large, cuspate forelands. At the Hilton Head Island the existence of such forelands in the past is evidenced b y the beach ridge geometry turning seaward in the middle and on the northeast part of the island and b y the bump in the shureline alignment at the middle of the island (Fig.lA). It is natural that erosion is maximum at this b u m p where the curvature of the shoreline is maximum. This happened to be the place where the so-called "Palmetto Dunes Development" was undertaken (Bruun, 1976). Main reasons for erosion should be sought in the slow rise of the sea-level (Bruun, 1962) and -- n o t least -- in the existence of a rather small and innocentAooking tidal creek called "The F o l l y " with an ocean entrance as seen in Fig.lA. Regardless of its small size -- max. discharge is a b o u t 5--10 m3/sec -it has caused the development of ocean shoals n o w storing some hundred thousand cubic meters of sand (Fig.lA) causing severe erosion of the immediate adjoining beach towards the southwest called the Singleton-Collier Beach which has a length o f a b o u t 500 m. As the Port Royal Sound (Fig.lB) functions as a littoral drift barrier for the overall predominant southward littoral drift, the Folly creek in this way acts as a local barrier for the southward drift on Hilton Head Island. The total length of heavily eroding beaches is a b o u t 5 km. Before 1970-1971 there was no beach at all at normal high tide. Mean tidal range is a b o u t 2 m (7 ft.), with spring range at 2.5--3 m (9--10 ft.) and neap range is 1.5--2 m (5--6 ft.) Due to a shoreline recession of 1--1.5 m (4--5 ft.) per year, high tides washed up towards the low beach dunes and w o o d e d areas developing vertical erosion scarps which contributed further to erosion b y reflection of waves. The erosion situation was demonstrated by a great number o f tree stumps and peats exposed on the beach. At low tides the beach was a b o u t 75 m (250 ft.) wide b u t very wet, n o t only because of the receding tides, b u t also due to ground seepage of water from troughs between the beach ridges (Fig.5) through the narrow dune zone. This in turn contributed further to erosion b y run-down, as well as b y lift forces on the sand at low tides. During storms and high tides uprush penetrated in the w o o d e d area, and the result was that palmettos, oak trees and shrubbery were lying on the upper beach in one big mess. To stabilize the beach there was apparently no other solution than replacement of the eroded material, raising the beach a b o u t 1.2--1.5 m (4--5 ft.) and widening it b y 40--50 m ( 1 2 0 - 1 5 0 ft.). Furthermore, in order to protect the dunes and the area behind the dunes against overwashing and
10 flooding, it was necessary to build an artificial dune of 20 m (60 ft.) crown width at elevation 3.3 m (11 ft.) above mean sea-level (MSL), slope 1 in 7 (220 m 3 per meter) or 1.6 million cubic yards (100 cu. yds. per ft. of shore front), a total of about 1.2 million cubic meters. Grain size of beach sand was on average 0.18--0.20 mm. Fill was as large or larger, averaging 0.18--0.22 mm (Fig.3). 3 50'
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Fig.3. Beach a n d d u n e g e o m e t r y a f t e r t h e artificial n o u r i s h m e n t .
Fill for beach nourishment was obtained from dredging of canals and lagoons in the development, thereby creating a great number of canal and lagoonfront lots for homesites. For reasons of e c o n o m y it was decided to limit pumping distances to a m a x i m u m of approximately 1 km (3,000 ft.) from the dredge to the discharge point. In addition some interior dredging and filling was done (Bruun, 1976). The working comprising dredging and dumping of 220 m 3 per meter was undertaken by Norfolk Dredging Company, Norfolk, Virginia, which used its 14-inch dredge, "Jekylll Island". The 100 tons heavy dredge was brought in on the bay side of the island and pulled by a D6C Caterpillar Dozer across approximately 1/4 mile of mainland, then across a highway and finally across a golf course and launched at an interior lake. The air-filled rubber bags, or mattresses, were 30 inches in diameter by 30 ft. long (0.75 m by 9 m long) filled to a capacity of 7 PSI (0.5 kg/cm: ) when carrying the dredge across the highway. Departure after completion of work was accomplished on a relatively calm day by rolling the dredge out of the lake over the beach dunes and into the ocean. It has been determined through numerous moves made with these particular bags and this particular machine that one must have no steeper than a 7 horizontal to 1 vertical slope for ascension and decent. The entire moves on this large project were made with ease and minimum cost compared to assembly and reassembly. Fig.4 shows the 14 inch (0.35 m) discharge pipe spoiling on the beach, building it up and widening it. The existing partly swamped beach may be seen in the background. Bulldozers used to give the artificial dune the right geometry (Fig.3) are also seen. The unit cost per cubic meter was about $0.60. The ultimate result is shown in Figs. 5 and 6, a wide and beautiful beach. The inner part of the crown and the landward slope of the dune was vegetated by American Beach grass (Ammophila brevigulata). At either end of the beach curved rock groins 50 and 80 m long were built
11
Fig.4. Spoiling on beach.
Fig. 5. The artidicial dune and beach.
12
Fig.6. Dune and beach after nourishment.
out to the MSL shoreline to "encase" the fill to some extent w i t h o u t damaging the adjoining shorelines (Fig.lA). South of the southern boundary line, a 1,200 ft. (360 m) rip-rap protection supported by palmetto piles was built in 1960, located about 40 ft. (12 m) inside the MHWL to protect the homes located right on the eroding dune exposing 1--2 m scarps. Due to their curvature and extension to the MSL line only, the two curved groins have worked satisfactorily with only minor adverse effects -- if any -- on their downdrift sides. This artificial nourishment was completed in 1970 and during the first two years the erosion apparently was only minor, at least n o t visible on the dry beach. The fact u n d o u b t e d l y was that overflow material from the nourishment stabilized the nearshore b o t t o m for some time. In the latter part of 1972 erosion started again and developed high scarps during a severe storm which peaked exactly during a spring high tide. The result may be seen in Fig.7. In order to bring the profile back to normal shape, another type of "artificial nourishm e n t " of the dune and of the upper part of the beach was undertaken. It was observed repeatedly that while the dune slope and the MHWL line moved shoreward during a severe storm the MLWLline moved oceanward due to the deposition of eroded sand on either side of the MLWL line. For t h a t reason a scraper and dozer operation was undertaken attempting to bring the profile back to the original shape and location. In addition an application was filed for federal as well as state authorities to close The Folly (the creek) which as mentioned above constituted a partly littoral drift barrier. It was anticipated that such a closure would decrease the anticipated annual maintenance of about 40,000 m 3 to about half of this quantity. Misunderstood and exaggerated en-
13
Fig.7. Erosion scarps in December 1971.
vironmentalism and lack of proper understavcding of the problem and of the environment as well, combined with the lack of practical state laws, has, however, up to this time upheld action on this project, which is very easy to undertake and is supported by all engineering professional agencies including the federal U.S. Army Corps of Engineers and generally supported by all local governments and authorities who k n o w the problem. Storms in 1973 and 1974 made it necessary to repeat this " n o u r i s h m e n t " by which the dune and upper beach were stabilized at the cost o f the lower shore and nearshore area. Admitting that this does n o t contribute to the nourishment of the entire eroding areas and b o t t o m profile, such a procedure u n d o u b t e d l y has some advantages extending b e y o n d the assistance in keeping the erosion " p e r p e t u u m mobile" running. It is well k n o w n from practical experience as well as research in Holland (Bakker, 1969) that the higher an erosion scarp is, the more rdaterial is eroded. It is therefore important to keep the inner beach high (above MHWL + most uprushes) and have as gentle a slope of the dune as possible. This slows d o w n the rate of erosion of the upper beach and stabilizes the dune against erosion and -- most important -- offers protection against overflows and flooding. Certain precautions should be considered, however. Scraping should, if possible, be undertaken during the late summer and the early-fall months. If so, the summer swells have built up a beach ridge between the MSL and the MLW line, and if material from this ridge is scraped or dozed up in the dune and upper beach, the ridge most likely will re-build by material from the nearshore b o t t o m due to gentle swell action. In other words, the swells are helpful in extending the "artificial nourishment" further offshore! If, on the other hand, scraping is undertaken during the winter months, one may risk that storm waves penetrate more easily to the upper levels of the beach and reach the dunes, thereby increasing erosion. Summer-scraping therefore has double advantages.
14 Tracer experiments using fluorescent tracers undertaken following a scraper operation proved that material on either side of the MSL line, regardless of the scraper operation which goes max. 0.3 m down in the beach, did n o t limit or prohibit migration of material in either direction from the scraped area, most rapidly in the direction of the predominant drift, that means southwestward. This would also be rather unlikely, as neither the wave energy nor the refraction pattern has been changed! -- (although some tend to believe t h a t it has!) It is, however, considered as being of importance that scraping is n o t undertaken close to the boundary groins, but stays 300--500 m away from these areas to avoid any trap-function of the scraped areas. To save on maintenance and due to the continued but slow, general shoreline recession, tests have been performed by placing new dune fill under a steeper slope angle, 1 in 4, and at the same time letting the upper (seaward) edge of the dune recede about 5--10 m. Tests are now planned to use a "Sea Carpet" of Dutch import, placed on a slope, 1 in 4, compacted to 75% max. density, and held in place in its upper and lower edge by casting it in a concrete box. Tests carried out earlier (1961--1965) at the Port Royal Inn, located at the northeast c o m e r of the Hilton Head Island, placing a plastic carpet with sandfilled 0.3 m tubes for every 1.5 m on an about 1 in 10 beach slope were very successful. The carpet buried itself in the sand very rapidly! During storms the fill washed out -- but it always covered itself up after the storm (Fig.S).
Ouno
Beach after placement of carpet
stitched to form 0.3m tubes which were sandfilled.
Fig.8. Placement of plastic sheet on eroding beach.
Another measure of a temporary maintenance nature has been undertaken frequently. In fall and winter, sometimes also in spring, storms tend to develop the so-called "sloughs" in the beach profile (Figs. 9 and 10). Such a slough is in fact a " t r o u g h " inside a " b a r " and, at the same time as it makes the beach inside the slough (too) steep, the slough carries a longshore current which as a rip current transports material eroded in offshore direction (Fig.8). Scrapers and bulldozers are used to bar or fill such sloughs entirely to eliminate their adverse effect on erosion of the beach. As it will be understood from the above-mentioned, these maintenance measures must be considered of partly temporary nature. The final solution to the maintenance will u n d o u b t e d l y become the closing of the Folly and regular (once every 3 to 5 years) nourishment from offshore sources. In Florida a dragline has been used (Bruun, 1967). New methods include a jet pump ( F i g . l l ) (De Graca, 1975; McNair, 1976)
15
Fig.10. Slough and its run-off channel.
16
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Fig.9. Cross-section in beach with slough.
operating 600--1,000 ft. (200--300 m) from shore, or a "Norwegian t y p e " split-hull (SELMER) barge of 6--8 ft. (2--2.5 m) draft fully loaded. It may e.g. be filled by a sidecaster (Bruun, 1976). Tests have now been undertaken by the U.S. Army Corps o~ Engineers, Wilmington District, on the use of such a barge. So - the battle even though not yet won -- is being won. Discharge
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REFERENCES Bakker, W.T, 1969. The influence of Dune Height on Dune Erosion. Rijkswaterstaat Rep., 69-1, The Netherlands. Bruun, P., 1962. Sea-level rise as a cause of shore erosion. Proc. ASCE, J. Waterw., Harbors Coastal Eng., Div., 88 (WW1). Bruun, P., 1967. Bypassing and backpassing with reference to Florida. Proc. ASCE, J. Waterw., Harbors Coastal Eng. Div., 93 (WW2). Bruun, P., 1976. Dredging and reclaiming and exclusive development. Terra Aqua, 10. Bruun, P., 1976. Port Engineering. The Gulf Publishing Co., Houston, Texas, 600 pp. De Graca, H., 1975. Jet-pump - - A possible answer to Santa Cruz port shoaling". World Dredging, October, 1975. Holmgren, V.C., 1959. Hilton Head, A Sea Island Chronicle. Hilton Head Island Publishing Co. McNair, E.C., 1976. Abstract on "Sand bypassing system using a jet pump". Int. Conf. Coastal Eng., Honolulu, 15th.