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OCR for page 9
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
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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.
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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)
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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.
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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
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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.
Representative terms from entire chapter:
hatteras lighthouse
