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OCR for page 9
Relative Mean Sea Level
A significant portion of the worId's population lives within the
coastal zone, with many buildings and facilities built at elevations
less than 3 m (10 ft) above mean high-tide level along the shoreline.
Without any secular change In the height clef mean sea level or in the
height of the land, the elevations of structures are not adequate to
ensure the safety of people and works in the event of major storms
(including storm surges), especially when such events coincide with
infrequent but predictable perigean spring tides.
This hazard has grown increasingly apparent and serious along
many of the worId's coastlines as local mean sea level has risen
during the twentieth century. Although in some areas of the world
the local sea level is falling, the predominant change is a rising
sea level with rates ranging from 1 to 5 mm/yr. International
attention has been drawn toward this problem by two possibly in-
terrelated sets of observations: (1) relative mean sea level is rising
and beach erosion is being exacerbated in many parts of the world
(Bird, 1985), including many areas of the United States (Figure 1-
1~; and (2) the atmospheric level of Greenhouse gases is steadily
rising as a result of the combustion of fossil fuel and deforestation.
While it is tempting to correlate the two, a cause-and-effect (eu-
static) response has not been proven to date, although a future
eustatic response is a clear possibility (Barth and Titus, 1984~.
9
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OCR for page 11
RELATWE MEAN SEA LEVEL
11
Carbon dioxide and the other trace gases that comprise "green-
house gases create a greenhouse effect in the troposphere. The
combined effects are still poorly understood but seem likely to es-
tablish a sequence of climatic effects that could result in a general
global warming (National Research Council [NRC] 1983, 1982,
1979~. The result wall be an increased rate of rise in glacioeustatic
and steric-expansion eustatic sea level.
Relative mean sea level change at a particular location is
the difference between the eustatic (global) change and any local
change in land elevation. The long-term causes of relative mean
sea level rise are sixfold, but not aD of the processes are operative
in every locality.
1. Eustatic Tise of world sea level. "Ecstatic means a global
change of the oceanic water level. Its most ~rnportant forms at
the present time are regarded as glacio-eustasy, caused by melting
of land-based glacier ice, and the steric expansion of near-surface
ocean water due to global ocean warming. hysterics refers to the
specific volume of the medium, which expands when heated or
shrinks when cooled.
2. Crustal subsidence or uplift of the land surface due to new
tectonics, that is, contemporary, secular, structural downwarping
of the earth's crust. Tectonic phenomena occur In five distinctive
categories: subsidence of former glacio-~sostatic marginal uplift
belts (e.g., the eastern United States); cooling crustal belts fol-
lowing rifting (e.g., parts of the Gulf of California); subsidence in
regions of long continued sediment loading (e.g., East and Gulf
coasts, especially the Mississippi delta); uplift in regions of active
crustal subduction (e.g., Puget Sound); and subsidence due to
loading by volcanic eruptions (e.g., Hawaii, Aleutians Islands).
3. Seismic subsidence of the land surface due to sudden and
irregular incidence of earthquakes.
4. Auto-subsidence due to compaction or consolidation of soft,
underlying sediments, especially mud or peat.
5. Man-made subsidence due to structural loading, as well
as groundwater, and of! and gas extraction. Of the four subsi-
dence processes only this category,^anthropogenic subsidence, can
be reversed or at least partially Instigated by recharge or other
management actions.
6. variations due to climatic fluctuations are a consequence
of oceanic factors including E! Nin - Southern Oscillation (ENSO)
OCR for page 12
12
RESPONDING TO CHANGES IN SEA LEVEL
effects, and are related to secular changes in the size and mean
latitudes of subtropical high pressure cells. Along mainland coasts
(especially east coasts in the Northern Hemisphere), a decreasing
current flow associated with Waring epochs causes a rise in sea
level due to the Coriolis eject, whereas in rn~doceanic gyre re-
gions there is no mean sea level change. This issue requires study
(Cartwright et al., 1985; Barnett, 1983b). It also appears that tide
gauge records contain substantial long-period fluctuations (~100
years), which indicate that the accurate extrapolation of small
sea level rise values from the data is very difficult. Furthermore
determining changes in rates of rise is even more difficult.
;,
Of these identified causes of sea level rise, only the eustatic rise
is a universal, global eject (by definition). For any one area the
other causes come into play in various proportions. It should be
stressed that no national survey of the local extent of the processes
has ever been undertaken, but it is clear that the variations will
be highly regional.
Various segments of the U.S. coastline experience subsidence
or uplift due to factors (2) through (5~. Superimposed on this
regional subsidence is the global eustatic sea level rise. If the
greenhouse e£ect/glacier melt concept is confirmed, its potential
contribution to mean sea level rise will outstrip other causes of
relative sea level rise along most of the U.S. coastIme by 2025.
PAST CHANGES IN RELATIVE MEAN SEA REVEL
Geologic Record of Sea ferret
It has been establishecI that during the last Ice Age (15,000
years ago), mean sea level was perhaps as much as 10~150 m
lower than it is now. Sea level rose rapidly until about 6,000
years ago, when the rate of change of global sea level became
quite low compared to that earlier period of tune. An assessment
developed by Shepard (1963) is presented in Figure 1-2. During
the past 6,000 years there were perhaps fluctuations one or more
meters over a thousand years (or more) apparent ~ some areas
(Fa~rbridge, 1961~.
In regions of very rapid subsidence (e.g., Mississippi delta,
Rhine delta), eustatic trends tend to be obscured (Van de Plassche,
1986~. In contrast, the formerly glaciated regions of the world
OCR for page 13
RELATIVE MEAN SEA LEER
o
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~—D
ID
-
-
Oh
`~ 200
J
Cal 300
x~ x xx
~2~
X.
~ \O ~
Ax
· Texas shelf \
~ Holland \
x Australia \
· Southwest Louisiana in\
O Eastern Argentina
· West Louisiana shelf
O Western Mexico
a
0 10 2~)
13
O
25
m
50 ~
In
75
100
THOUSANDS OF YEARS BEFORE PRESENT
FIGURE 1-2 Sea petrel elevations versus time as obtained from carbon 14
dates in relatively stable areas. Source: Adapted from Shepard (1963~.
(notably, most of Canada, Scandinavia, and Scotland) are regions
of neotectonic tilting or uplift, where rates of uplift have exceeded
past rates of sea level rise.
Sea [eve! Rise In the Twentieth Century
For nearly a century, relative mean sea level has maintained
a steady rise at many tide-record~ng stations around the world.
At the same time the atmospheric greenhouse gases have shown
a steady rise, and, more recently, other greenhouse gases such as
chiorofluorocarbons have been increasing. To some observers the
increase of greenhouse gases in the atmosphere implies a warming
of the worId's climate, although the evidence to date is still being
debated by the scientific community (NRC, 1983, 1982) and most
experts do not expect the warming to be detected until the l990s.
OCR for page 14
14
RESPONDING TO CHANGES IN SEA LEVEL
A tide gauge sunply records the current sea level at a particu-
lar location. Since sea level is a basic consideration for short-range
coastal construction plans, designers of long-lived coastal struc-
tures shouIc3 consider the change in sea level that may occur during
the structures' useful life. For this purpose, simple projection of
the trend of local tide gauge records Is inadequate; the underly-
~ng causes ot sea level change must be addressed and techniques
employed to forecast the effect of each cause.
Tide gauge data are available for approximately the past cen-
tury. However, these data provide the sea level relative to the
supporting base of the gauge, and that base may be either sinking
or rising. For example, Figure 1-3 presents results from long-term
tide gauge records at Atlantic City, New Jersey; San Erancisco,
California; and Juneau, Alaska. The approximate recorded rela-
tive rates of change are
Atlantic City: + 0.40 m/century,
San Francisco: ~ 0.13 m/century, and
Juneau: - 1.38 m/century,
where a positive rate indicates a relative rise. The relative drop
in sea level at Juneau reflects the rebound (rise) of the land with
unloading of the land following the melting of glacial ice.
A question raised early in the development of this study was
whether sea level change trends based on tide gauges located
inside bays and estuaries are representative of open-coast trends.
This concern was posed as early as 1929 in a report sponsored
by the National Research Council (Johnson, 1929~. Specifically,
the issue is whether the effects of engineering works, primarily
channel deepening and the construction of jetties for navigational
purposes, would affect the sea level rise trend measured by bay
gauges.
A special study commissioned by the Marine Board to ad-
dres~ this concern includes a summary of analytical relationships
and empirical results relevant to this problem (Mehta and Philip,
1986~. Mehta and Philip concluded that gauges located inside and
outside bays are each subject to different influences that tend to
degrade the quality of the data. With locations more distant inside
the mouth of the bay, gauges contain a greater amount of "noise,
which Is not representative of the open-coast sea level. Analytical
OCR for page 15
RELATE MEAN SEA LEVEL
-
— 72
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~ 6.4
-
ct
_.
ID
ID
-
0 9.2
J
~ 8.4
7.6
14.6
hi, 13.8
-
a:
r
Yearly Mean Sea Level
Sta. No.8534720
Atlantic City, NJ
5.8 L
1850 1880 1910
n!
.~
i- - r
At'
~ 1 Century
0.40 m
1940 1970
YEAR
Yearly Mean Sea Level
Sea. No.9414290
San Franclaco, CA
_~
1 Century
1 1 1 1 1 1 1 1 1
1910
1850 1880
t
YEAR
Yearly Mean Sea Level
Sta. No.9452210
Juneau, AK
13.0 LO
1850 1880 1910
1940 1970
-1.38 m
14
1 Century \/\|\
1 1 1 1 1 1 _l !
1940 1970
15
YEAR
FIGURE 1-3 Tide gauge data for (a) Atlantic City, New Jersey; (b) San
Francisco, California; and (c) Juneau, Alaska. Source: Hicks et al. (1983~.
OCR for page 16
16
RESPONDING TO CHANGES IN SEA LEVEL
TABLE 1-1 Difference in the Secular Change of Mean Sea Level for
Selected Gauge Pairs (outside minus inside)
Secular Change
Gauge Pair Difference (mm/yr)
Outside Inside Outside- Inside
Long Branch, N.J. New York, N.Y. 13.1
Atlantic City, N.J. New York, N.Y. 1.4
Duck Pier, N.C. Norfolk, Va. 2.1
Springmaid Pier, S.C. Charleston, S.C. 13.6
Daytona Beach Shores, Fla. Mayport, Fla. 6.4
Key Colony Beach, Fla. Fort Myers, Fla. 2.4
Vaca Key, Fla. Fort Myers, Fla. 2.2
Key West, Ftla. Miami, Fla. -0.3
Naples, Fla. Fort Myers, Fla. 0.7
Clearwater Beach, Flu. St. Petersburg, Fla. -0.6
Cedar Key, Fla. Pensacola, Fla. -0.4
Shell Point, Fla. Pensacola, Fla. 1.8
La Jolla, Calif. San Diego, Calif. -0.1
Santa Monica, Calif. Los Angeles, Calif. 0.8
Ricon Island, Calif. Los Angeles, Calif. 3.2
Monterey, Calif. San Francisco, Calif. -3.9
Arena Core, Calif. San E`rancisco, Calif. -13.1
Trinidad, Calif. San Francisco, Calif. 4.8
Crescent City, Calif. Astoria, Ore. -0.3
NOTE: A positive change denotes a higher outside than inside rate.
SOURCE: From Hicks as reported by Mehta and Renji (1986~.
considerations suggest that any short-term (several decades) did
ferences will be biased toward a lower trend from this inside gauge
compared to outside gauges.
In addition, Hicks' (1984) results of long-term sea level trends
were assessed from pairs of gauges inside bays versus open-coast
gauges. The comparison included 19 gauge pairs, with each outside
gauge selected as that in closest proximity to the inside gauge
(Table 1-1~. The average trend difference of the 19 gauge pairs
was 1.S mm/yr with the outside rate exceeding that inside. When
the three absolute differences exceeding 10 rnm/yr are excluded,
the average difference decreases to 1.1 mm/yr. Thus, relative to
outside gauges, this study indicates that the trend rate from inside
gauges has underestimated somewhat the relative mean sea level
rise rate.
OCR for page 17
RELATIVE MEAN SEA LEVEL
TABLE 1-2 Estimates of Eustatic Sea Level Rise per Century Based on
Tide Gauge Data
Author
Estimated Rise (cm)
Thorarinsson (1940)
Gutenberg (1941)
Kuenen (1950)
Lisitzin (1958)
F`airbridge and Krebs (1962)
Hicks (1978)
Emery (1980)
Gornitz et al. (1982)
Barnett (1983a)
11 ~ 8
12 to 14
11.2 ~ 3.6
12
15 (United States only)
30
12a
15
a Ten centimeters excluding long-term trend.
SOURCE: Adapted from Barnett (1983a) and Hicks (19783.
17
Range of Sea [eve] l:sti~n~tes Available
Using tide gauge results from around the world, various esti-
mates have been obtained for mean rate of change (Table 1-2~. The
large variance is partly the result of gross geographical imbalance
of the gauge sites; most are in the Northern Hemisphere m~lat-
itudes, almost Al are on continental shores, and almost none are
in high latitudes. A map of gauge locations is presented in Pugh
and Faull (1983~.
The lack of insular tide gauges, until recently, has deprived
analysts of any means of testing mean sea level change for the
Coriolis effect. Gauges located toward the Huddle of an oceanic
gyre that is speeded or slowed will fall or rise respectively, whereas
those on continental shores show an opposing trend. The lack of
a sufficient number of high-latitude gauges precludes testing for
planetary spin-rate eEects (although there is astronomical proof
of changes in the earth's spin rate which should be registered by
sea level, especially at high latitudes). It is noted that the spin
rate has decreased slightly and would result ~ relative mean sea
level decreases and increases at low and high latitudes, respec-
tively. This has been proposed, but not confirmed, as an indicator
of antarctic melting. It is not clear whether this signal could
be isolated in the presence of the other components and Noise.
OCR for page 18
18
RESPONDING TO CHANGES IN SEA LEVEL
Furthermore, many gauges are located in obviously subsiding delta
areas, and others ~ tectonically rising areas.
The first attempt to filter out the grossly anomalous data of
world tide gauge records was done by Fa~rbridge and Krebs (19623;
their results indicated a mean rise for the first part of the twen-
tieth century of 1.2 mm/yr. Subsequent analyses, using different
filtering procedures but progressively larger data sources, showed
sirn~lar results; for example, Lisitzin (1974), 1.12 mm/yr, and Gor-
nitz et al. (1982), 1.2 mm/yr. Several analysts have suggested a
change during the last few decades; Barnett (1983a) showed for
193~1980 an average of 2.3 mm/yr, and Emery (1980) gave a
value of 3.0 mm/yr.
The newest global results are by Pirazzoli (1986, 1984~. Of
1,178 records provided by the Permanent Service for Mean Sea
Level, 229 stations were selected as having > Midyear records (plus
a few with 30 years) of consistent trends (Figure I-4~. Only 13
percent showed the "mean ecstatic value (1.~1.5 mrn/yr); 22.5
percent showed 1.0-2.0 mm/yr; 20.5 percent showed a rise of 0.1-
1.0 mm/yr. Pirazzoli indicates that the extreme variance between
the stations emphasizes the unportance of local subsidence. Fur-
thermore, he stresses that there is no unequivocal demonstration
of any eustatic rme at all, at least during the last 4~50 years.
Gornitz (unpublished data) has prepared averages for a number
Of m-Pacific atolls: over 2~30 years, Nawiliwili shows a mean
rise of 0.30 mm/yr; Canton Island, 0.31 mm/yr; Eniwetok, 0.81
mun/yr; and Midway, minus 1.34 rnrn/yr. For regional studies
(e.g., Australia), Aubrey and Emery (1983) were unable to iden-
tify "unambiguously a eustatic signal.
Munk et al. (1985) indicate that phenomena that create large
amplitude fluctuations in sea level, such as the ENSO, make it
very di~cult to obtain statistically reliable estimates of rates of
change in mean sea level and even more difficult to detect whether
the rate of change is increasing or decreasing.
In surrunary, the gauge measurements, in a few cases continu-
ing over 10~300 years after correction for known trends, suggest
mean sea leered fluctuations that are generally consistent with the
geological record of the past 6,000 years (Ters, 1986; Tooley, 1978;
Fairbridge, 1961~. The nature of the related climatic changes, in-
sofar as it ho been possible to document them, is also consistent.
OCR for page 19
RELATIVE MEAN SEA LEVEL
60
30 N
o
30S
60
_ _ ~ +5.5?—-5.7
3 251
-2.2 -0.2 +2.6 0.0 +1.8 +~.5-~S
_ 17 5 32 1 17 85 1
+0.7 +5.9 +3.0 +1.6 +3.3
-= +3.6
~ ~~ 10
rid_ ~
4~0~6 +5~0 +1~8
7 5 2
+2~5 +1.3
_ ~
180W 150 120 90 60 30 0 30 60 90 120 t50 180E
19
LONGITUDE
FIGURE 1-4 Geographical distribution of worldwide change of relative
mean sea level (by latitude/longitude coordinates) using tide gauge records
extending more than 50 years (augmented with a few of 30 years). Large
figures and mathematic signs indicate average relative rise or fail in mm/yr;
small figures the number of stations employed. Note that 97.4 percent of the
data is from the Northern Hemisphere. The shaded box is the only rectangle
(Central Asia) with no marine coastline. No data exist for 70 percent of
the boxes, and 70 percent of the stations are located in only four boxes. It
is evident that no statistically valid basis exists for assuming that eustatic
rise is in progress, at least from the data presented here. Nevertheless, it
is important that five boxes with very low rates of change are midoceanic
stations. Four boxes with values over 5 mm/yr are located in areas of
recognized crustal uplift or subsidence. To identify and quantify a global
ecstatic rate requires not only more and better-selected gauging points,
but also more-sophisticated geological and oceanographic analysis. Source:
Pirazzoli (19843.
Additional examination is required. The record of cInnatic fluctu-
ation indicates quite appreciable variations (Lamb, 1984; Wright,
1983).
METHODS O]? OBSERVING
RELATIVE MEAN SEA LEVEL
With reference to any one locality, relative mean sea level is
OCR for page 20
20
RESPONDING TO CHANGES IN SEA LEVEL
measured by different techniques that fall within the expertise of
different groups of specialists, who for the most part do not meet
professionally. The techniques are as follows:
1. Tide gauge analysis is processed globally by a commission
of the International Association of Physical Oceanography and
nationally In the United States by the National Ocean Service
(NOS). Monthly relative sea level maps of the tropical Pacific are
issued by the University of Hawaii.
2. Satellite altimetry is administered by the National Aero-
nautics and Space Administration (NASA) In the United States
and studied internationally by groups within the International
Union of Geology and Geophysics (lUGG). Satellite imaging can
also be employed for geomorphic analysis.
3. Geodetic leveling is giobaDy reported by the Commission
for Recent Coastal Movements of the International Association of
Geodesy (LAG, which adheres to JUGG) and in the United States
by the National Geodetic Survey.
4. Geomorphological and geological analysis is globally coor-
dinated by the Commission on Shorelines and the Commission
on Neotectonics of the International Union for Quaternary Re-
search (INQUA). There is a U.S. National Committee for INQUA
with individual representation on the commissions but no official
governmental participation.
There Is also the Sea Level Project of the International Geologi-
cal Correlation Programme (No. 61, completed in 1984; followed
by No. 200, led by P. Pirazzoli, CNRS-Intergeo Geographical In-
stitute, Paris), which also has a U.S. committee with individual
membership.
Each of these organizations collect data in tune series that
refer to specific locations. However, the time series cover very
different intervals. The relevant geological studies deal with about
the past 6,000 years; the others concentrate on periods ranging
from two or three centuries to a month or less. Nevertheless, from
Al these data, regional ant} global means are obtained by varied
and sophisticated statistical analysis. While good agreement is
possible for the regional means of relative mean sea level (coastal
sectors of 50~1,000 km), there is so far no unanimity as to the
global values for eustatic sea level rise. Efforts are now being
directed at defining the eustatic components, and to delineating
and understanding the local and regional effects.
OCR for page 21
RELATIVE MEAN SEA Lecture
21
Of the various observational techniques, tide gauge analy-
sis furnishes the most detailed, accurate, and directly measured
record of relative mean sea level, but its hour-by-hour variability
is subject to complex disturbances (e.g., by atmospheric pressure
or local rainfall), and the data therefore require extremely careful
analysis. The results provide valuable monthly, seasonal, annual,
and more-than-decadal trends.
Tide gauge data provide basic input for the design of coastal
structures. For short-lived structures, the annual tide ranges and
the mean tide level are sufficient; however, for longer-lived coastal
structures, the long-term trends become important in the design
process. Satellite altimetry is potentially very valuable, but re-
quires a longer base period (Wyrtki, 1985~. Recent availability of
the Global Positioning System (GPS) provides absolute (not rela-
tive) sea level to centimeter accuracy. Future contributions of this
new technology, supplemented by traditional measurement tech-
niques, should be extremely beneficial to understanding eustatic
sea level trends and local neotectonics.
Geodetic leveling involves the periodic releveling of the first-
order vertical topographic survey stations, about once every two or
three decades, and thus provides evidence of secular deformation of
the earth's crust, as wed as indications of local compactional sum
sidence. The method is extremely time consuming, and although
very successful in Japan, Scandinavia, and Eastern Europe, it has
been poorly funded in the United States and many of the available
data have not been analyzed. The analyses provide decadal to
century-Ion" trends. The Japanese have developed and applied
ingenious methods of sensing local and short-term compaction
rates.
Fmally, geomorphological and geological analysis furnishes
century to millennial trends. This method ~ particularly valuable
in demonstrating the natural, long-term response of any area or
region, a trend or pattern of behavior that can then be compared
with the short-term time series provided by geodetic leveling and
tide gauges, which may contain extraneous or anomalous short-
term data. The short-term data should be evaluated in terrors of
known long-term trends whenever possible.
Biro Examples
How these varied approaches can be constructively integrated
OCR for page 22
22
RESPONDING TO CHANGES IN SEA LEVEL
NORTH
3
in,
1
aS (D
~ a)
- O
=
-
-
L
· 0
o
~ ~ in
o
o
o, ~ o
~ I
UPLIFT ~
a,
o
_ o
0
—
~ o
Y
-
1
SOUTH
m a.
Y z
~ 0
3 ~
0
Z
SUBSIDENCE ~
0 100 200 300
KILOMETERS
400 500
FIGURE 1-5 Geodetic leveling profile from Perth Amboy, New Jersey
through New York City to Rouses Point, New York on the Canadian border.
Mean sea level rise at the Battery, New York City, averaged 2.7 mm/yr. Sub-
tracting 1.2 mm/yr as the eustatic component leaves 1.5 mm/yr representing
subsidence and long-term oceanographic factors. Source: U.S. Coast and
Geodetic Survey data, 1902 to 1955; adapted from l?airbridge and Newman
(1968).
is illustrated by reviewing data from two well-documented sites:
New York City and Long Beach, California.
The tide gauge of New York City is located at the Battery and
set on hard crystalline bedrock that ~ not disturbed by sediment
compaction, frost action, or human action, such ~ grour~dwater
withdrawal. For 90 years the record shows a systematic (though
fluctuating) rise of mean sea level of about 2.7 mm/yr (Hicks et
al., 1983~. Deducting about 1.2 mm/yr as the eustatic component,
1.5 mm/yr remains as a probable crustal subsidence factor. This
has been checked by geodetic and geologic techniques.
The geodetic leveling lines were followed up the Hudson River
to the Canadian border and filtered to remove highly deviant
data points (Figure 1-~. A secular tilting of the crust during the
present century is shown, with the Canadian border area rising at
about 1-2 mm/yr and the New York City area sinking at about the
same rate; a null point is situated near Kingston (Fairbridge and
OCR for page 23
RELATIVE MEAN SEA LEVEL
23
Newman, 1968~. The geologic surveys show that over the last 6,000
years the southern end of the section has been sinking slowly and
the Canadian end has been rising. About 9,0()0 years ago, Lake
Champlain lay at sea level. This is a clear-cut case, demonstrated
by independent data sets, that the New York City area is subsiding
at about 1-2 mm/yr. The lowering of land affects the bedrock of
the whole region and is not affected by human activity.
Id California, despite frequent earthquakes, the tide gauge
records are remarkably stable and coherent from station to station.
San Francisco has the longest series (since 1855), which shows that
if interannual variations are removed, mean sea level shows broad
fluctuation but has generally risen at 1.3 mm/yr over 125 years
(Hicks et al., 1983~. No distinguishable change is evident for the
last several decades.
~ contrast to San Francisco and New York, Long Beach Har-
bor, California commenced a sucIden, substantial, relative sea level
rise trend in the 1950s, submerging appreciable parts of Terminal
Island, which is in the harbor. This subsidence is anomalous when
compared with long-term trends and was diagnosed as a short-
term local phenomenon that is related to the withdrawal of oil,
natural gas, and water during exploitation of the Wilmington Oil-
field. Artificial recharge of the porous strata h" slowed the rate
of continued subsidence, but the cost of dike building and other
land preservation measures exceeded $100 million.
The New York City and Long Beach examples demonstrate
that areas must be considered in three contexts: local, regional,
and global.
Representative terms from entire chapter:
mean sea
