Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Introcluction HISTORICAL BACKGROUND Cape Hatteras Lighthouse, the tallest and best-known brick lighthouse in the U.S. (Figure 1*), faces destruction due to coastal erosion. It is 200 feet (61 meters) tall and weighs approximately 2,800 tons (2~540 metric tons). The present lighthouse was constructed in 1870 to replace a masonry tower built near the present site in 1803 (Holland, 1968~. The principal purpose of the lighthouse and its predecessor was to protect shipping from the dangerous Diamond Shoals that extend 13 nautical miles (24 kilometers) seaward--the "Graveyard of the Atlantic"--where at least 600 ships have been lost. In 1870, the lighthouse was approxi- mately 1,500 feet (460 meters) from the water's edge. By 1935, this distance had diminished to approximately 100 feet (30 meters) due to shoreline erosion. Today, partly due to temporary shoreline protection measures, the lighthouse is approximately 160 feet (49 meters) from the water's edge. * This height was estimated from the engineering drawings obtained from the National Archives (e.g., Figure 1 ) and information from the Coast Guard's Light List ( 1971~. The lighthouse is approximately 200 feet (61 meters) from its base at grade level to the platinum tip of the rod above the roof of the lantern. The focal plane of the light is 1 90.S feet (58.2 meters) above mean low water, approximately 181 feet (55.2 meters) above the ground. 9
10 Saving Cape Hatteras Lighthouse FIGURE 1 Elevation and vertical section of Cape Hatteras Lighthouse. Adapted from engineering drawing, late 1 860s. Photograph courtesy United States National Archives.
Introduction 11 Several remedial and emergency measures were taken dur- ing the past 50 years to protect the lighthouse and other structures in its vicinity (Appendix A). These measures included an artificial dune constructed along Hatteras Island by the Civilian Conservation Corps in the 1930s; a field of three groins abreast and north of the tower constructed in 1969-70, repaired in 1975, and now deteriorating; nourishment of the beach north of the lighthouse with 200,000 cubic yards 153,000 cubic meters) of sand in 1971 and with 1,250,000 cubic yards (955,000 cubic meters) of sand in 1973; a semi- circle of nylon sandbags seaward of the lighthouse installed in the late 1960s and again in 1980; a 150-foot (46-meter) landward sheetpile extension of the southern groin con- structed in 1980 to prevent flanking; and artificial seagrass installed in 1981, 1982, and again in 1984. Some of these measures, notably the groinfield, have reduced the rate of retreat of the shoreline and may temporarily have promoted accretion of the beach (Figure 2; U.S. Army Corps of Engineers, 1985~. However, they do not provide any long- term ~ 100 years) protection to the lighthouse, and prolonging their short-term (20 years) beneficial effects would require costly repairs and new construction. Since 1870, the shoreline has receded approximately 1,600 feet (490 meters; U.S. Army Corps of Engineers, 1985), except for a small promontory at the lighthouse. The apparent reduced rate of shoreline retreat near the lighthouse since the 1930s may provide an unfounded sense of security. Research by coastal geomorphologists during the past two decades has clarified the migratory nature of coastal barriers, including the influence of gradual sea-level rise. The beach gradient in front of the Cape Hatteras Lighthouse is steep and narrow, which suggests that the shoreline is poised to return to equilibrium through sudden recession in the event of a major storm or series of storms (Leatherman, 1985, 1987; Everts, 1987~. At present, even without further shoreline erosion, the lighthouse is vulnerable to damage from major storms. A storm surge or abnormal increase in sea level due to a " 100- year storm" (having a 1 % probability of occurring in any year) is estimated to be 8.S feet (2.7 meters) (MTMA Asso- ciates, 1980~. This temporarily would raise sea level to the base of the lighthouse, which stands ~ feet (2.5 meters)
12 Saving Cape Hatteras Lighthouse FIGURE 2 Aerial view of lighthouse and beach showing effect of groinfield on beach erosion. Photograph by D. Policansky, August 1987.
Introduction 13 above mean sea level (MSL). At high tide, such a storm surge would engulf the base of the lighthouse. To make matters worse, the Average height of the one-third highest waves" breaking above the storm surge elevation would be 15.5 feet (5 meters) (MTMA Associates, 1980, Table 5~. Thus, storm surge and breaking waves would directly attack the lighthouse, undermine its shallow footings, and probably demolish its accessory buildings as well. RECENT PROPOSALS Since 1980, the National Park Service (NPS) has consi- dered diverse measures to protect the lighthouse and associated buildings--two keepers' dwellings and an oilhouse. One proposal, approved by NPS in 1985, involved construction of an octagonal revetment and seawall that would encircle the tower and reach a height of 15 feet (4.6 meters) above grade level. Alternative proposals considered by NPS involved relocation of the lighthouse--either in one piece or in segments--to a new site approximately 2,800 feet (850 meters) southwest of the present location. This would place it about 2,400 feet (730 meters) from the nearest shoreline. Other proposed or attempted options include rehabilitating and expanding the groinfield (with or without a partial revet- ment), submerging objects offshore to create an artificial reef, constructing offshore breakwaters with rehabilitation of the groinfield, installing artificial seagrass, continuing beach Several studies and reports prepared since 1980 addressed the preservation of the lighthouse; examples are cited throughout this report and are described in Appendix A. nourishment. and talcln£ no action THE PRESENT STUDY The Committee on Options for Preserving Cape Hatteras Lighthouse was formed by the National Research Council in July 1987 at the request of NPS. The committee's charge was to
14 Saving Cape Hatteras Lighthouse . . . evaluate the scientific, engineering, environmental impact, and policy aspects of alternative options for preserving the lighthouse from the encroachment of the sea. The feasibility, likelihood of success, long-term dependability and monetary cost of each option will be considered. The study will emphasize the broader con- text of national policy concerning preservation versus conservation and coastal issues. The committee also was charged with providing an interim report summarizing its initial findings within 3 months or as soon as possible thereafter. The interim report was submit- ted to NPS on October 15, 1987. The committee contained within its membership a broad array of experience and disciplinary specialties pertinent to the Cape Hatteras problem. The committee studied many relevant documents, met at Cape Hatteras with additional technical experts and local community spokespersons, visited the site, inspected the lighthouse, and flew over the Cape Hatteras area. Science cannot adjudicate the legislative mandates and public policies under which NPS manages national seashores. The committee recognizes that the final decision concerning options for preserving Cape Hatteras Lighthouse will involve important factors outside its purview--public sentiment and politics in particular. Political feasibility of the various options or the nature and extent of public sentiment associ- ated with them were not within the scope of the charge, and the committee did not critically assess them. The committee was not charged with evaluating the wis- dom of a national policy that would preserve this particular lighthouse. Congress has appropriated more than $4 million to preserve the lighthouse (U.S. Senate, 1986~. Many issues must influence any decision concerning Cape Hatteras Lighthouse. Part II discusses the options considered by the committee and the associated costs. engineering tech- nolo~v. and reliability of protection. Factors discussed in Part I of this report include coastal barrier-island migration sediment transport; rising sea level; historic preservation-- aims, methods, and constraints; and relevant public policies.