Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 72
6 Alternative Responses As previously outlined, there has not been a Tong history of coping with sea level rise in the United States. Because of shore erosion, a portion of which is due to sea level rise, buildings have been lost and significant engineering projects have been under- taken during the past few centuries. Other countries have coped with relative sea level rise for thousands of years. Alternative re- sponses to sea level rise, derived from worldwide experiences are described in this chapter. COASTAL STRUCTURES AND PROTECTIVE TECHNIQUES The performance and effectiveness of different types of coastal structures and protective techniques will be affected to varying degrees by a relative rise in sea level. Common to each type of erosion control structure under the action of sea level rise is the diminished efficiency due to submergence and overtopping. Structural failure becomes more likely as well, because wave forces can be greater due to the greater wave heights possible in deeper water and the higher-moment arm for the forces, providing greater fluid power. 72
OCR for page 73
ALTERNATIVE RESPONSES 73 Groins These shor~perpendicular structures serve to reduce the local littoral drift rate, fostering sand impoundment on their upUrift sides until they are filled to capacity, after which the longshore drift is allowed to bypass. If groins are allowed to fill from a natural sediment supply rather than from an alternative source as part of their construction, erosion of the adjacent shoreline will always occur. Groins are most effective along coastlines where a significant littoral drift occurs. They are often used to protect a long segment of coastline by the emplacement of a groin field. The variety of groins in use, with differing lengths, widths, heights, permeabilities to sand, orientation, and spacing (between groins), has resulted in varying degrees of success in reducing ero- sion problems along the protected beaches. Examples of success- fuT groin fields can be seen in such places as Rehoboth Beach, Delaware; Westhampton Beach, Long Island, New York; and Madeira Beach, Florida. However, Ocean City, Maryland has shifted away from the use of groins until a more complete under- standing of all effects are known. The beach downdrift of a groin field Is often a location of accelerated erosion, and special treatment is necessary to protect this region. Often groin fields terminate at inlets, requiring no special measures; however, some groin fields terminate abruptly, requiring the use of beach nourishment, discussed below, or revet- ments of some kind. The erosion downdrift of the Westhampton Beach, New York groins shows the consequence of neglecting to provide for this effect. The landward end of a groin typically extends into the dune line. As the sea level rises, the retreat of the dune line may leave the groin susceptible to flanking during high or storm tides, thus permitting sand to bypass the groin, reducing its electiveness. The more readily the structure is flanked during normal weather conditions, the less the groin's sand-trapping and stabilizing ca- pacity. Submergence of the groin by sea level rise brings on the same flanking effect, as well as overtopping of the groin by the Ton gsh ore current and waves that transport the sand, again result- ing in a Toss of efficacy. Groins constructed of durable material, such as stone, and appropriately designed can have a useful life exceeding 50 years. Using the three scenarios of sea level rise, the design of a rubble -
OCR for page 74
74 RESPONDING TO CHANGES IN SEA LEVEL structure should include the capability to raise the crest elevation in keeping with the relative sea level rise. Groins constructed with wood, gabions, steel-sheet piling, and other less durable materials probably will have a useful life of less than 50 years. Therefore, no unusual measures accounting for sea level rise are needed at the present time in the design of these groins, using the adopted scenarios. Bulkheads and Sea WaDs These structures are often used on shorelines above the mean high-water line to provide protection for the upland. Another use is to reduce flooding due to storm surges (e.g., the Galveston sea wall). These structures are often constructed as a vertical wall, facing the sea, thus occupying the least amount of land. A successful sea wall or bulkhead must be able to withstand not only the forces of incoming waves during a storm, but also the effects of overtopping, which permits a significant amount of water to add to the passive earth load exerted on the wall and can further result in a scouring or eroding of the backfill. A common result of sea wall and bulkhead placement along the open coastline is the loss of the beach fronting the structure. This phenomenon, however, is not well understood. It appears that during a storm the volume of sand eroded at the base of a sea wall is nearly equivalent to the volume of upland erosion prevented by the sea wall. Thus, the offshore profile has a certain ~demand" for sand and this is "satisfiers by erosion of the upland on a natural beach or as close as possible to the natural area of erosion on an armored shoreline. The practice of placing rubble at the toe of a wall to dissipate wave energy reduces or distributes this erosive effect. As the mean shoreline retreats toward a bulkhead or sea wall as a result of rising relative sea level, the erosion in front of the wall is enhanced and overtopping increases. Dean and Maurmeyer (1983) provide a means, based on the concept of an equilibrium beach profile, to predict the amount of change in the beach profile due to changes in mean water level. Sea level rise can be incorporated into the design of a sea wall in two ways. The first is to build the wall initially to account for the anticipated sea level rise during the life of the structure. Provided the freeboard is sufficient for the design life of the structure and
OCR for page 75
ALTERNATIVE RESPONSES 75 it is engineered correctly for the forces it will experience, the sea wall should be immune to sea level rise effects. The other method is to design the wall with lower initial elevations (with less cost), increasing the elevation in the future as dictated by the relative sea level rise actually experienced and/or projected over relatively short (about a decade) time frames. :Revetments A revetment consists of either loose or interlocking units laid on a slope, from the upland to some point on the profile, often below the depth of anticipated scour or fixed by a toe wall to prevent undermining by scouring. This structure serves the same objective as a bulkhead or sea wall, protecting the upland. While a revetment occupies a larger land area, the existence of a slope and the roughness provided by the structural elements may reduce the amount of erosion immediately seaward of the structure. Sea level rise and appropriate methods of accommodation are the same for revetments as for sea walls and bulkheads. Beach Nourishment Replenishing an eroding beach with sand is an effective means to restore a beach temporarily. Depending on the type and volume of nourishment sand, the temporary restoration may last for years. The massive (10.5-m~le) effort along Miami Beach has lasted since 1980 without substantial volumetric erosion. An attractive advantage of beach nourishment is that it is a soft solution to the erosion problem, i.e., no rigid structures are required. The drawback of beach nourishment, however, is that the processes that created the original erosion problem remain and continue to remove the nourishment sand. The length of time beach nourishment can be expected to last will depend on wave conditions. Other factors that can influence the duration of a fill are the characteristics of the fill sand and the methods of placement. Sand that is finer than the original beach sad (particularly if it contains a significant silt fraction) will be eroded faster than the original sand. Fill not unifor}nly placed over the beach profile creates an out-of-equilibrium profile, which usually fosters offshore sediment transport, with attendant beach recession. Although this process of "profile equilibrations is accompanied by a shoreline recession
OCR for page 76
76 RESPONDING TO CHANGES IN SEA LEVEL and may be interpreted as an indication of poor performance of the project, In reality it should be viewed as an adjustment toward the natural profile with the recognition that the relocation sand is not lost, but remains in the nearshore system. Using present technology, beach fill on stabilized shorelines will become more costly as sea level rises. As the offshore region deepens, the beach profile must steepen due to the fixed shoreline position. Using fill sand of the same grain sizes (or smaller) as the original beach sand will require far larger volumes of sand as the water level rises and the beach will become increasingly unstable. An alternative is to utilize coarser sand in future beach fills. Coarse sand permits a steeper beach profile and less transport offshore (Bascom, 1951~. A very approximate measure of the increased rate of Tosses can be developed by considering that the transport of sand away from the nourishment site is proportional to the wave height to the 2.5 power (Dean, 1976~. The resulting percentage increase in beach nourishment volumes due to a sea level rise is ttt~ F') Is1] x 100% = 7% (Case A) and + ~ 1] x 100% = 200% (Case B)' accounting for the effects of increased wave heights In the two examples presented in Chapter 4 (pp. 38-393. It is of interest to examine the approx~nate costs of nourish- ment required to mamta~n the existing shoreline. This requires accurate projections of the rate of sea level rise. The calculations presented below will be based on two different formulations. First, Bruun's rule will be used for various sea level rise rates, which requires quantification of W (the active profile width) and h* (the associated vertical dimension of this profile, including the berm elevation). Secondly, based on present (S.) and projected (S2) sea level rise rates, the ratio R2 52 R1 51 can be formed, where Rat and R2 are the present and anticipated recession rates, respectively.
OCR for page 77
ALTERNATIVE RESPONSES Method ~ 77 For illustrative purposes, consider the case of Florida's east coast. The long-term estimates of past relative sea level rise are 30 cm/century, and it is estimated that the limiting depth of motion hB is on the order of 7 m and the berm height is 2 m, resulting in a h* value of 9 m. The associated width could be determined from profiles or from the equilibrium beach profile h Ax2/3 in which a representative value of A for this area has been deter- mined from analysis of numerous profiles to be 0.1 m,/3. Thus, W (hB) / = ( 9 ) = 854 m. Therefore, the recession rate multiplier for sea level rise, defined by the ratio of retreat R to sea level rise as determined from Bruun's rule, is h = (584/9) = 9s. * The present relative sea level rise rate is 30 cm/century, which appears to include a eustatic component of 12 cm and a neotectonic (subsidence) component of 18 cm. Assuming that the neotectonic component IS unchanged over the next century, the relative rates of rise adopted in this report are as presented ~ the third column of Table ~1. The volume per unit length of beach V to maintain the shore- line position can be determined by considering a general form of the equation for the total retreat rate Ret, composed of retreat due to sea level rise Ret and advancement A due to additions of sand to the profile. The resulting equation is 1?~ = Rat + A, where the volume V required to result in an advancement A = Rs such that the shoreline is stable, is ~ = SW.
OCR for page 78
78 RESPONDING TO CHANGES IN SEA LEVEL TABLE 6-1 Projected Shoreline Retreats and Costs of Maintaining the Shoreline by Nourishment over the Next Century on Florida's East Coast (Method I) Average Average Annual Volumetric Annual Costsa/ Eustatic Relative Shoreline Requirements/ Unit Length Rise Rise Retreat Unfit Length of Shoreline Scenario (m) (m) (m) (m /m) ($/m) I 0.5 0.7 45 4.1 33 II 1.0 1.2 78 7.0 56 III 1.5 1.7 111 10.0 80 aCosts are based on $8/m3 and are 1987 approximate costs. The annual volumetric requirements for the various scenar- ios are presented in column 5 of Table ~1. These volumes are converted to annual costs using a 1987 cost of sand of $8/m3. The ranges of annual maintenance nourishment costs associated with the three scenarios range from $33/m to $80/m of beach front. For comparison, the approximate range of values of beach- front property along the east coast of Florida is from $6,000/m to $60,000/m. The annual maintenance nourishment cost, expressed as a percentage of the value of the property, ranges from 0.06 to 1.3 percent. Thus, one could consider shoreline stabilization through nourishment as a "taxi or cost of living on a shoreline subject to natural erosive forces. Method II This method is much more direct. The annual average shorn line retreat rate R due to natural causes (relative sea level rise) along the east coast of Florida is approximately 0.5 m/yr. With the present sea level rise rate So ~ 30 cm/yr) and projected rates S2, the projected retreat rates R2 are R2 = R1, sl and the associated required annual volumetric maintenance nour- ishment rates for shoreline stabilization V are V2 = R2h*-
OCR for page 79
ALTERNATIVE RESPONSES TABLE 6-2 Projected Shoreline Retreats and Costs of Maintaining the Shoreline by Nourishment offer the Next Century on Florida's East Coast (Method II) 79 Average Average a Annual Volumetric Annual Costs / Eustatic Relative Shoreline Requirements/ Unit Length Rise Rise Retreat Unfit Length of Shoreline (m) (m) (m) (m /m) ($/m) I 0.S 0.7 117 10.5 84 II 1.0 1.2 200 18.0 144 III 1.5 1.7 283 25.5 204 aCosts are based on S8/m3 and are approximate 1987 costs. Adopting as before a value of h* = 9 m, the results, including annual maintenance costs per unit length, are presented in Ta- ble 6-2. Summarizing briefly, the annual costs (in 1985 dollars) of stabilizing the shoreline range from $84/m to $204/m. For the same range of band values from $6,000/m to $60,000/m, this repre- sents annual maintenance costs expressed as a percentage of value ranging from 0.1 to 3.4 percent. The shoreline stabilization costs through beach nourishment as predicted by the two methods diner by a factor of approx~nately 2.5 and reflect the inexact nature of this methodology. Beach Nourishment with Groins The use of groins with beach fill increases the time that the beach nourishment remains on the beach and reduces the down- drift erosion since the filled groins will begin to bypass sand im- mediately after construction. The response to sea level rise is the same as groins and beach fill, mentioned earlier. Perched Beach An interesting concept for rebuilding bathing beaches, the perched beach is an attempt to raise the local profile with fill and an onshore submerged sill that is oriented parallel to the shoreline. The sill is intended to ret awn the fill, simply acting as a "dame or impediment to limit offshore sediment transport. The advantage of this technique is that beach fill Is only required in
OCR for page 80
80 RESPONDING TO CHANGES IN SEA LEVEL the region shoreward of the sill, rather than along a large portion of the beach profile. The perched beach should be enclosed by shore-perpendicular structures, especially at the ends, to reduce the longshore loss of fill material. A test case for the perched beach has been carried out at Slaughter Beach, Delaware; however, no conclusions were drawn from the installation (U.S. Army Corps of Engineers, 1981~. The perched beach concept requires more testing because it consists of some design considerations that are poorly understood, such as the appropriate depth of water for the offshore sill. Also, it is not clear whether the offshore sins may act as a diode, permitting the Toss of material in the offshore direction, but acting as a barrier to beach building by onshore transport of sand during favorable wave conditions. Sea level rise will affect a perched beach in the same manner as beach nourishment, with the exception that the sill structure will become less efficient as the sea level rises, resulting in reduced sand retention. The sill should be anchored by shore-perpendicular return walls situated well inland in order to prevent flanking. Offshore Breakwaters The use of above-water, shore-parallel breakwaters to reduce wave heights at the shoreline and the potential for littoral drift is a very popular and effective international erosion control measure. In the United States, Winthrop Beach, Massachusetts; Lorraine, Ohio; and Presque Isle, Pennsylvania contain working examples of these structures, whose effectiveness is based on limiting the penetration of wave energy behind the breakwater. In Japan, more than 2,000 of these structures are in place (Toyoshima, 1982~. Often a series of such structures is used; the spacing between breakwaters Is an important parameter, as distance affects the amount of wave energy that passes to the protected beach. Without shoreline stabilization provided by beach nourish- ment, rising water levels will effectively move the shoreline farther away from the breakwater, increasing the ability of the waves to diffract behind the structure and reducing the sheltering and effi- cacy of the device. Overtopping will obviously diminish the ability of onshore breakwaters to reduce the wave energy in the sheltered region. To be effective, designs must anticipate sea level rise, be- cause the design lives of these structures are likely to be long. For example, they could be designed with higher initial top elevations
OCR for page 81
ALTERNATIVE RESPONSES 81 or with features that make it possible to increase elevations in the future. With increases in sea level, waves that attack the structures may increase in height, thus posing a greater threat. For example, the weight of stone, Wa, in a jetty or breakwater is chosen based on a design wave height. Using Hudson's formula in the Shore Protection Manual (U.S. Army Corps of Engineers, 1984), the design stone weight is proportional to the cube of the wave height. If the wave parameters in Chapter 4 are used, the increase in design stone weight due to relative sea level rise is or wa ~1 + F,'3 = 1.08 (Case A) 3 = ( ) = 1.24 (Case B). H Thus, the increase in stone weight for these two examples would be 8-24 percent. The unplication is that the margin of safety built into existing structures Is reduced. Stolen Surge Barriers Several barriers have been built in the United States to protect coastal cities from inundation during storm surges. Examples are the barriers at New Bedford, Massachusetts; Providence, Rhode Island; and Texas City, Texas. Others have been designed but not built (PerdikLs, 1967~. Internationally, probably the best known barriers are the Thames barrier, designed to protect the city of London, and the Delta Project to protect low-lying lands in the Netherlands. These barriers were designed with heights to exceed the surge elevations of certain design storms. As relative sea level rises, the factors of safety of these structures will be reduced. Other Devices There are numerous other devices used for beach erosion con- trol. Several of them are available commercially but do not have the proven capability to eliminate or reduce beach erosion. Some of these devices are bottom mounted and would become more ineffective as sea level rises.
OCR for page 82
82 RESPONDING TO CHANGES IN SEA LEVEL Polders are used in many countries for the reclamation of land from the sea. A polder, by definition, is land surrounded by dikes kept dry by the use of pumping. The Dutch have historically been the most active users of polders. In low-lying U.S. lands, as sea level rises and the need for land increases more use of polders may be made. Effective management of estuar~ne sediments and sedimen- tation offers some potential for building up coastal wetlands. Dredged materials can be used to reinforce marshlands. Another alternative, especially in the Mississippi River delta, is to periodi- cally divert sediment-laden river waters (usually contained behind levees) into marshes to allow natural deposition to take place. Engineering Case Studies The practicality of effective engineering response to increased future sea level rise can be addressed, in part, through the ex- am~nation of case studies. Some facilities have been in place Tong enough to have experienced significant sea level rises. The case studies presented here include the Galveston sea wall and landfill} at .ton, Texas; the Delta Project (dikes and surge barrier) in the Netherlands; the Harrison County, Mississippi beach nourishment project; Miami Beach, Florida beach nourishment; and the Tybee Island, Georgia sea walls, groins, and jetties. Galveston, Texas The city of Galveston is located on Galveston Island, a long barrier bounded on the east by the Bolivar Roads Inlet to Gal- veston Bay. In the late 1800s, Galveston was a summer resort community with extensive development. Existing sand dunes were removed for fill and beach access (Davis, 1961~. The elevation of much of the island was extremely low; the average elevation in 1900 was 5.8 ft above mean low water (Engineering News, 1902~. On September 8, 1900 Galveston was demolished by a major hurricane. More than 6,000 people were killed and most of the buildings were flattened. To protect the city, a concrete sea wall, 16 It high (with a crest elevation at 17 It above mean low water), was constructed between October 1902 and July 1904. The wall characteristics included a curved face towards the sea and rubble toe protection to help dissipate wave energy and reduce wave scour. The sea wall was constructed on the beach along the +3 ft
OCR for page 85
ALTERNATIVE RESPONSES 85 was the reclamation of the Zuider Zee, which had been expanding constantly from its origin as a small freshwater lake (Lake FIevo) into a saltwater estuary. Although this project was controversial from the beginning due to its extremely high cost, the spirit of early engineers such as Andries Vierlingh, alike master to William the Silent, prevailed. VierI~ngh wrote in 1570 in his treatise, Tracteet van Dickagie, as quoted by Wagret (1968), "The more one retreats, the more the sea prepares to expel one completely. The economic problems were difficult to overcome because the cost of the project exceeded the value of the recIa~rned land. However, the benefits for future generations outweighed the merits of taking no action. In 1932, the Zuider Zee dike was completed and 550,000 acres of farm land were added to the Dutch nation (a 9 percent increase). Because of the high costs and environmental concerns associates] with polders, not all of the sea bottom was reclaimed. On February 1, 1953 the St. Ignatius flood, caused by a large winter storm moving across the North Sea, occurred with the loss of 1,850 lives and the flooding of many thousands of acres of crop land (almost 8 percent of the country) due to hundreds of breaches in the dikes, particularly south of Rotterdam. This massive storm created the pressure for the Delta Project, the worId's largest coastal engineering work, which has resulted in the closing off of three major estuaries In the Rhine-Meuse delta region. This project will no longer permit intensive storm surge flooding. The Delta Project consists of several phases. The first was the closure of the Har~ngviiet estuary, with the use of sluices to permit the efflux of Rhine River flows at low tide into the North Sea. The Grevelingenmeer was closed at both ends, creating a saline lake, with no apparent loss of water quality, to the surprise of most involvecI. Environmental concern about enclosed lakes led to the use of storm surge barrier gates for the largest of the estuaries, the Osterschelde. A total of 64 massive gates, which will be shut during major storms, peanut tidal flows into the estuary to maintain existing water quality. The cost of the surge barriers (or stoTmvioe~keTing) is approximately $2 billion. A recent article in the National Geographic (October 1986) describes the construction of the barriers. The design life of the Osterschelde barriers is 200 years, based on a design storm flood with a frequency of 1 in 4,000 years. This is a far longer design life and greater design storm than those used for any other coastal structure ever constructed. The people of the Netherlands, with their limited land mass
OCR for page 86
86 RESPONDING TO CHANGES IN SEA LEVEL and expanding population, have demonstrated that it is possible to defend against an encroaching sea, with its ever higher storm surges, using dikes and pumps. This has not been accompanied by a sacrifice of the beaches. Coastal resorts, located on diked islands, remain popular and are complete with bathing beaches. Exam- ples are the beaches at Voorne, Goeree, Schouwen, and Walcheren islands in the Rhine estuary area. Additionally, the resort com- munity of Scheveningen, nearly a part of The Hague now, has very wide beaches held in part by groins. In the north, the Frisian Islands beaches along the adjacent West German coast have been maintained in the face of relative sea level rise by migrating, as documented by Nurnmedal and Peniand (1981~. Miarn~ Beach, Florida Between 1976 and 1980, a large beach nourishment and flood protection project was constructed] by the U.S. Army Corps of Engineers at Miarru Beach and cost $64 million. Over 14 million y33 of sand were placed on 10.5 miles of beach, from Bakers Harbor at the north end to Government Cut Entrance at the south. The resulting nourished beach averaged 300 ft wider than before. In addition to perforrn~ng as a recreational beach, the project provides a flood and storm buffer for expensive property and rejuvenates the beach, the premier attraction of the city. The fill material, dredged from offshore, had a large portion of fine and carbonate sands, leading to concerns about the stability of the fill. Measurements based on aerial photographs show that the shoreline at the north end of the fill retreated 100 It within the first 5 years and remained stable over the next 4 years (up to 1985~. It is likely that the initial shoreline retreat was a readjustment of the fill profile to an equilibrium profile. The nourishment project has withstood some moderate hurri- cane activity (e.g., Hurricane David, 1979), and it has clearly met the needs of the coastal cities located behind the fill (Bal Harbour, Surfside, and Miami Beach). Harrison County, Mississippi The longest ~d one of the earliest beach restoration projects constructed was in Harrison County, Mississippi. This cooperative project, conducted by Harrison County with federal aid, encom- passed some 26 miles of Mississippi Sound shoreline between Biloxi
OCR for page 87
ALTERNATIVE RESPONSES 87 and Henderson Point. This area is shelterer] by barrier islands from the direct attack of waves from the Gulf of Mexico. The original project was constructed during 1951-1952 and included the placement of nearly 6 Anion y]3 of fill from a borrow trench dredged to a depth of 15 It and located about 1,500 It offshore. The cost of the material placed was $0.22 /y]3, and the project resulted in some 700 acres of new beach with a width in excess of 300 ft and a berm height of 5 ft. A sea wall some 25 ankles Tong had been constructed during the years 1925-1928 to protect property and highway U.S. 90, irnrnediately upland of the sea wall. The longshore transport along this beach is from east to west. A terminal structure Is located at Henderson Point at the entrance to Bay St. Louis, the western (downdrift) end of the project. Numerous concrete drainage trenches were constructed across the beach and function as groins, thereby helping to stabilize the placed beach. This project is generally considered to have performed well. Annual losses were estimated to be on the order of 100,000 yd3/yr, with a considerable portion of this amount due to sand being blown inland. In 1969, Hurricane Camille, one of the two most intense storms on record ~ the Gulf of Mexico, made landfall near the western end of the project, causing record storm tides in excess of 22 ft and, understandably, causing some sand losses. During 1972-1973 the project was renourished with 1.9 million y33 of sand. The project was inspected in the summer of 1985, prior to Hurricane Elena, and appeared to be performing well. Undoubtedly, this is an example of a project that has provided both protection to the upland against severe storms and a valuable recreational facility. Based on the data of Hicks et al. (1983) the est~rnated relative sea level rise over the period encompassing the beach restoration project (1952-1985) Is approximately 8 cm, too small to be indicative of the stability of a nourishment project in an era of sea level rise and in the presence of substantial Tongshore sediment transport. In comparing the relative longevity of this project with others, one must consider the sheltering provided by the offshore islands that form the gulfward boundary of Mississippi Sound. Tybee Island, Georgia Tybee Island, Georgia is a barrier island some 6 km Tong lo- cated just south (downdrift) of the entrance to the Savannah River.
OCR for page 88
88 RESPONDING TO CHANGES IN SEA LEVEL Navigational improvements to Savannah River include jetties and a deepened channel that have effectively eliminated any sediment supply from the north. This lack of sediment supply is reflected, in part, by the landward migration of offshore contours and the erosive stress on Tybee Island. Tybee Island represents an interesting case study due to the long history of erosion studies and variety of erosion control mea- sures employed. The earliest studies date back to 1855. Erosion control measures have included shore parallel structures (revet- ments and sea walIs), groins, and beach nourishment. Shoreline positions documented by these studies are presented in Figure ~1. In 1882, three rock groins were constructed at the north end of the island, although it Is not evident whether these were for erosion control or river training. Between 1912 and 1930, several additional groins and portions of a sea was were constructed. ~ 1931, additional erosion control efforts were initiated, in- cluding a 2,650 It long bulkhead and 5 groins extending from the bulkhead. Numerous structures were tried, and in the late 1930s and early 1940s a concrete sea wall was constructed extending along the entire length of Tybee Island. Hurricane Dora In 1964 caused failure of a portion of the sea wall. This failed section, am prox~nately 1.5 km In length, was protected by a rock revetment. A Corps of Engineers study culminated in 1971 with the rec- omunendation for three substantial groins and a beach nourishment project. The sand was to be placed at the north end of the island, with one groin to be located at the northerly limit of sand place- ment and the other two near the north end and center of the project. This project was constructed in the period 1974-1976. However, only the northerly structure was built; it extended 800 ft from the sea wall. Total sand placed was 2.26 million y33. The borrow area for the project was a shoal extending southeast from the island. The project performance was monitored and initial results indicated more rapid losses than anticipated. These early Tosses from the project areas occurred (1) over and through the permeable north groin resulting in 10-12 acres accumulation of dry sandy area on the north end of the island, and (2) at the south end of the island where material appeared to be "drawn" to the substantial depression resulting from the borrow operation. The center of the island Secreted. In summary, the erosion stresses at Tybee Island are abnor- mally high due to the navigational works at the entrance to the Savannah River. With more than 100 years of erosion control ITS ~ . ~ . ~ I
OCR for page 89
ALTERNATIVE RESPONSES /. ~ ~ ~ ,~' .. .' 89 it\ ~ /_~ - . .;~~' TYBEE ISLAND . /} ski 11 Lo_.` : ( // . : ~ ~ /} ii ' ','3~fe ~ ~ ~ / LEGEND / N 1 867 1875 1900 1918 1931 FIGURE 6-1 Tybee Island mean-high-water shoreline positions for various years. Source: Oertel et al. (1985~.
OCR for page 90
go RESPONDING TO CHANGES IN SEA LEVEL efforts, during which relative sea level has risen over 40 cm, the shoreline has not eroded as much as might be expected and the erosion control efforts have been moderately successful to date. The historic shorelines (Figure ~1) have experienced substan- tial fluctuations, but the dominant changes between 1867 and 1931 were (1) the loss of a projection near the northeast end of the is- land, and (2) the deposition near the north of the island. The areal changes In these two features appear to balance approxi- mately. The sea wall construction program completed in the early 1940s "fixed" the shoreline position against severe storms. Beach nourishment during 1975-1976 contributed to the formation of recreational beach areas, still present after 10 years. Near the central portion of the island, sand has accumulated, resulting in a fairly substantial dune field up to 70 m wide. Terminal Island, California The extreme rise In relative mean sea level experienced at Terminal Island and a portion of Long Beach, California some years ago was dominantly due to subsidence (Allen and Mayrega, 1970~. The first evidence of the phenomenon occurred in the late 1930s and early 1940s when surveyors began to have difficulty in reproducing leveling measurements. The discrepancies became so prevalent that the U.S. Coast and Geodetic Survey was called upon to run a new first order survey from the mountains on either side of the Los Angeles basin across the waterfront. The results showed that in the few years since the last set of levels, an area about 3 miles wide and 4 miles long had subsided about ~ ft. in a dish-shaped depression. The center of the depression was near the eastern end of Terminal Island, where the largest steam electric-generat~g plant in southern California was located. A series of studies was comrn~ssioned that conclusively iden- tified the major cause of subsidence as the withdrawal of of] and gas from the Wilmington Oil Field, the limits of which closely matched the subsidence contours. The recommended remedy was to maintain pressure in the various strata comprising the field. This was accomplished by organizing the oil field so that some of the existing wells Could be used for production and others for wa- ter injection to maintain pressure, and such that the water would "sweeps the oil to the production wells. The hydrocarbons pro- duced were shared by all companies with a financial interest in the field.
OCR for page 91
ALTERNATIVE RESPONSES 91 By the 1970s subsidence had been arrested and a small re- bound had even occurred. The overall subsidence ranged up to 20 ft with considerable damage to harbor facilities, pipelines, cul- verts, buried cables, and other structures. This damage required substantial remedial efforts, including dying In areas of extreme subsidence, reconstruction of damaged facilities, bridge repair, and redrilling of of] wells that had experienced casing damage. The experiences in the Long Beach/Term~nal Island area re- sulted in measures to counter an extreme relative mean sea level rise. The changes occurred much more rapidly than those expected with rising relative sea levels elsewhere, and they includes] hori- zontal movements of points on land that would not be expected with a general rise In sea level. This experience illustrates the na- ture and effectiveness of some of the measures that may be needed along the sea coasts. RETREAT Holding back the sea as water levels rise will almost always be technically feasible; however, in some cases it may not be economically or environmentally sound. In areas where the long- range cost or environmental damage due to shoreline stabilization is unacceptable, it will be advisable for development to retreat or move back from the shore. Although stabilization measures can be deferred until an accelerated rise makes moves necessary, a planned decision to retreat would require a lead time of years. A retreat can occur as either a gradual process or as a catas- trophic abandonment. Examples of the former would include re- moving buildings as they are threatened or as they interfere with use of the beach, ~d avoiding major renovations of buildings or new construction that would soon be threatened by higher sea levels. The latter might involve prohibiting the reconstruction of buildings destroyed or damaged by storms. This approach is being taken on Galveston Island by the state of Texas in the wake of Hurricane Alicia ~ 1983. A recent conference of coastal scientists, engineers, and policy analysts (Howard et al., 1985) concluded that it may be prefer- able for some communities to move back from the shoreline in a planned and orderly fashion. Otherwise, as sea level rises there is a significant likelihood that a number of communities will retreat involuntarily as a result of unpredictable disasters.
OCR for page 92
92 RESPONDING TO CHANGES IN SEA LEVEL Melanism of Retreat There are three basic ways to retreat from an eroding shore- Tme: (1) buildings can be moved as the shoreline approaches, (2) buildings can be written oh and the remnants removed after be- ing destroyed in storms, or (3) the construction of buildings near beaches can be avoided altogether. An example of the third approach is the anticipatory land-use planning for erosion in North Carolina. A movable house must be set back from the shore the distance of the erosion expected in the next 30 years; immovable buildings, such as high rises, must be set back a distance equal to 60 years of expected erosion (North Carolina Office of Coastal Management, 19843. In Maine, new buildings must be set back far enough to permit 100 years of erosion. Both states assume that current erosion trends will not accelerate as a result of projected sea level rise. North Carolina and Mame have essentially chosen a policy of gradual retreat from the shore. Both states have enacted regu- lations prohibiting placement of hard structures of any kind on eroding open-ocean shorelines. In 1984, 27 erosion-threatened buildings were moved back from the North Carolina shore; the regulations will be put to a more severe test in the future, when multistory condominiums are threatened by erosion. Putting a policy of retreat in place can be accomplished in various ways by different communities. Areas with low-density coastal development can rely on building codes, setbacks, zoning, and land-use plans. More developed communities will have to address the issues of existing buildings and shoreline stabilization structures. The problems are so diverse that their solutions will require many different actions by different levels of government as well ~ the private sector. The diversity of retreat mechanisms will be governed by the widely varying characteristics of natural shoreline systems. Some of the methods government might use to prepare for retreat are included in Howard et al. (1985~. Of those recom- mendations for implementing retreat, the ones most related to engineering issues follow: 1. Halt stabilization of the shoreline. No more funds should be used to hold the shoreline in place under the retreat alternative. 2. Establish construction setback lines in states that do not have them. Seaward of these setback lines, no construction can be permitted. Setback lines exist in Florida, Maine, North Carolina,
OCR for page 93
ALTERNATIVE RESPONSES 93 Alabama, and Delaware, to name a few states with the necessary enabling state legislation. Furthermore, for rapidly eroding shore- Imes, a t~rne-dependent setback line may be established to allow for further retreat as shorelines recede. 3. Remove coastal stabilization devices that become threats to public safety, as well as structures, including buildings, that become undermined by the sea. 4. Encourage further work in coastal processes research to provide greater scientific backing for the design of setback lines, as well as to develop innovative technologies for sand bypassing at inlets and development of cost-effective coastal protection schemes. Implicit in the philosophy of retreat is the belief that cost- effective coastal protection is not viable for the given locale. Since the state of the art of coastal erosion mitigation is evolving rapidly, any retreat decision should be reviewed periodically. If the benefits of shoreline stabilization exceed its costs, then the retreat decision should be reevaluated. Engineering, Geologic, and Economic Considerations A decision to retreat or not and the choice of retreat mecha- nisms should be based on a sound understanding of coastal pro- cesses. Perhaps the single most important such consideration is the impact the actions of one community can have on neighboring communities whose beaches are connected to the same sand supply system. To reduce the potential for sand loss and damage to recre- ational beaches, communities that do not choose to retreat should ideally be at the terminus of the sand supply line for a given coastal reach. For example, stabilization of eroding blues or headlands should be discouraged if it can be demonstrated that beaches in adjacent communities wall suffer as a result of the loss of eroding material. In general, sources of sand should not be stabilized; areas near Sarah sinks are much more suitable for stabilization by devices such as sea walls and revetments. Recognizing that a retreating shoreline provides a sand source to downdrift shorelines, in situa- tions in which shoreline stabilization is deemed justified, the state of Florida requires annual rrutigation through sand placement in the beach system to offset the material prevented from entering the system through natural erosion processes. Clearly, if some segments of the shoreline remain In place and others are allowed to move back in response to a rising sea
OCR for page 94
OCR for page 95
ALTERNATIVE RESPONSES 95 estation and a gradual shift from fossil fuels to solar and nuclear energy, which do not Ernst CO2. Even a shift from coal to natural gas would decrease CO2 emissions significantly. Nevertheless, the time that it would take to replace completely our fossil fuel infras- tructure suggests that it wiD be very difficult to limit the global warming expected in the next several decades.
Representative terms from entire chapter: