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RESPONDING TO CHANGES IN SEA LEVEL
height (freeboard); seismic activity; and seepage. Flooding of
islands can have several adverse impacts, including temporary
detriments to water quality due to ocean water intrusion, increased
loss of water by evaporation, increased seepage on islands adjacent
to the flooded areas, loss of agricultural land, damage to urban
and recreational developments, and fish and wildlife losses.
A letter of February 13, 1986 from Mr. David N. Kennedy,
director of the California Department of Water Resources, indi-
cates that since 1980, about $100 million of emergency funds from
federal, state, and local sources have been spent shoring up delta
levees and reclaiming flooded islands, and that the state Is now
furnishing about $2 million each year to improve the levees.
It appears that a 1-ft increase In relative mean sea level could
have a major impact on the protective capabilities of the levees.
Increases in levee elevations (and base widths) should be made if
and when needed and justified economically. The added weight of
the levees on the soil would increase the rate of subsidence, and this
would require design and construction consideration. Earthquake
resistance should also be accommodated in seismically active areas.
Many of the levees were constructed years ago when it was rel-
atively simple to obtain and transport material for their construc-
tion. Additional information may be needed to identify problems
that could be encountered In the future and to develop poten-
tial solutions. Issues include sources and transport of material,
environmental conflicts, and requirements for widening the base.
Tm some farming areas surrounded by levees, the soil type Is
such that the ground level becomes lower with time as a result of
farming, wmd erosion, fires, biochemical oxidation, and subsidence
due to compaction (Stephens and Speir, 1970~. These effects,
combined with an increase ~ eustatic mean sea level, wait change
levee stability.
Preliminary information from the Delta Levee Subventions
Program inclicates that a long-term data collection program Is
desirable to measure rates of subsidence (Kennedy, 1986~. Deem
measurement compaction recorders may be used to permit the sew
oration of surface oxidation and compaction of the peat soil from
deeper subsidence resulting from water and/or gas withdrawal.
Use of satellite surveying methods is planned to determine ground
elevations, since concern exists that much of the data obtained
with standard leveling techniques in the past is invalid because of
land instability in the delta.
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ASSESSMENT OF RESPONSE STRATEGIES
101
The California Central Valley is quite flat, and the Sacra-
mento River is tidal as far upstream as Sacramento. As a first
approximation, the levees would have to be increased ~ elevation
by the same amount (slightly more in bay areas where deeper wa-
ter allows higher waves to be generated) as any increase in relative
mean sea level.
Gates are an integral component of some levees in the United
States; examples mclude the Fox Point Barrier in Narragansett
Bay, the New Bedford Hurricane Barrier, the hurricane barrier
system at New OrIeans, Louisiana, and the Texas City Hurricane
Protection System. Whether a 0.~ to lift rise in relative mean
sea level during the next 50 years would require modification of
the gates is unknown. Study of the typhoon gates in Japan might
provide information concerning this issue.
New Levees
The construction of new levees may be a solution to protect
some densely populated areas from a substantial rise in relative
mean sea level. The National Research Council (1983) considered
the case of ~ very large rise (5 m) in relative mean sea level for
Boston, Massachusetts. This rise ~ much greater than any of
the values considered ~ this report, but it is noteworthy that a
practice solution may be possible in some Teas. The report states
(pp. 473~74~:
A rudimentary illustration of the economics can be based on the
Boston area. A full 5 m would cover most of downtown Boston.
Beacon Hill, containing the State House, would be an island sepa-
rated by about 3 km from the nearest mainland. Most of adjacent
Cambridge would be awash. But it would take only 4 km of dikes,
mostly built on land that is currently above sea level, to defend
the entire area. Perhaps even more economical, because it would
avoid the political costs of choosing what to save and what to give
up and of condemning land for right-of-way, would be a dike 8 or
10 km in length to enclose all of Boston Harbor.
If that were done, new deep-water port facilities would have to be
constructed outside the enclosed harbor; locks would permit small
boats in and out. The Charles and Mystic Rivers would have to be
accommodated. Whether in a couple of hundred years there would
be any significant Bow in those rivers would depend on changing
climate and increasing demand for water. Levees, a diversion canal,
or pumping could be compared for costs, and ecological impacts.
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102
RESPONDING TO CHANGES IN SEA LEVEL
The study cited above could be redone to determine if any levee
would be needed or justified for a 0.5- to I.~ft rise during the
next 50 years. The dike length of 8-10 km mentioned in the above
study Is very small compared with existing levees in Tokyo and
Osaka, Japan.
Major problems wall be associated with new levees, including
band condemnation, sources and transportation of construction
material, and environmental conflicts. In addition, navigation
gates may be needed in some regions.
S1DD=ENTATION O1? SEAPORTS AND HARBORS,
NAVIGATION CHANNELS, TURNING BASINS, AND
DOCKING AREAS
~ a Marine Board document (NRC, 1985a), it is stated that
there are 102 ports In the United States serving oceangoing traffic
(defined arbitrarily as 30 It or more in depth). Twenty-s~x of
these are primarily shoreline or coastal, 55 are ~ estuaries, and
21 are basically river ports (not entirely exclusive categories). No
pattern of equivalence was found to exist among the ports and
harbors. Each has its unique set of conditions of topography,
bathymetry, tides, current and wind variations, temperature and
climate, salinity and turbidity, ~d sediment transport regimes,
which, with the man-made developments, result in a different
situation for each case.
Changes in tidal and other currents wall occur with changes
in relative mean sea level, and changes in tide range and phase
may occur. These may, in turn, cause changes in siltation. Hydro-
graphic surveys will be required, and changes made in bathymetric
charts, tide tables, and current tables.
The alteration of seiching conditions could be locally signifi-
cant. Wherever the coastline tends to focus long waves of a certain
period in the manner of a lens, a rise in sea level may produce dim
proportionately increased seiche heights. New resonant locations
may develop, and old ones disappear. The effects of these changing
conditions on moorings and cargo handling need to be studied.
One of the findings of the Marine Board (NRC, 1985a:5) is
It is possible that major improvement in dredging will increase
deposition rates in certain locations within a harbor rather than
reducing the problem. This is because deepening certain channels,
especially in estuaries allows seawater intrusion farther upstream
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ASSESSMENT OF RESPONSE STRATEGIES
than before dredging, which can cause deposition of fine sediments
(which normally would be carried seaward) in the upper reaches.
103
This statement was made for the case of no sea level rise. What
would be the effect of a rise in relative mean sea level? At first
glance it Ought be thought that it would result in deeper naviga-
tion channels, turning basins, and docking areas than at present,
which would result in a decreased need for dredging. This might
be true for the short term, but not necessarily in the long term,
as is implied by the above statement. Major changes would prob-
ably occur in the location of the saltwater wedge, and thus in the
location of shoaling of the river channel. Sediment movement up-
strea~n would affect the costs of dredging and disposing of dredged
material. Because of the complexity of siltation and the high costs
of dredging, greater detail is devoted to this issue in this report.
Consider some of the effects on sedimentation of a rise of
relative mean sea level (recall that there are 55 ports in estuarine
environments ~ the United States). As the mean sea level rose
after the last Ice Age, the sea invaded valleys along coasts created
in the past by Duvial erosion and tectonic processes. Whenever a
river draining a valley carried enough sediment to fill the drowning
valley at the rate of sea level rise, a river delta developed (e.g.,
Mississippi River).
Very different estuary configurations evolved where there was
insufficient sediment to fill the valley as sea level rose. These
occurred where the valley was wide, or where the stream car-
ried little sediment. The invading sea created large shallow bays
that cleepened as sea level rose, as in the San Francisco Bay sys-
tem, Delaware Bay, and the Chesapeake Bay. Large, shallow
bays provide hydraulic conditions that facilitate sedimentation of
riverborne sediments and deposition of sed~rnents brought to the
bays during high-flow events. Sediments accumulate at the river
deposition sites and gradually extend into the bay (Krone, 1979~.
Daytime onshore breezes are typical of estuaries in the sum-
mer, and these breezes generate waves on the bays. The ability of
waves to suspend sediment increases rapidly with decreasing water
depth. Many estuaries are shallow enough that the wave action
generated by onshore breezes can suspend deposited material and
hold it in suspension while tidal currents circulate it throughout
the system. At night, when onshore breezes die, the suspended
sediment settles. If it settles where subsequent wave action or tidal
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104
RESPONDING TO CHANGES IN SEA LEVEL
currents resuspend it, it continues to circulate and may continue
to the sea. Under these conditions the upper bays fill to the level
where, over time, suspension by wave action equals the supply of
riverborne sediment (Krone, 1979~.
The supply of riverborne sediment has typically increased
with land development. For example, sediment supply to the
San Francisco Bay system was vastly increased from the 1850s to
1877 by extensive hydraulic mining in the Sierra Nevada foothills.
About 1.9 billion y33 of material deposited in the upper bay Peas
of the San Francisco Bay system (Gilbert, 1917), which filled to the
level at which wave action maintained the water depth. Sea level
has risen about 0.7 It since the hydraulic mining era. Sediment
supply after this period apparently continues to be greater than
that prior to 1850, and the upper bay areas are shallow, so that
the deposition is limited by wave action. The extent of the deposit
continues to progress toward the estuary mouth (Krone, 1979~.
Another consequence of sea level rise in drowned valleys Is
the development of marshes along the shore (Krone, 1985~. Ex-
amination of historical sea levels shows that the rise has not been
continuous, but has fluctuated through tune. Along the shore of
a bay, tidal Hats that developed during a period of higher sea
level are exposed for a greater portion of the tidal cycle when sea
level temporarily falls. Some types of plants become established
on these mud Bats when they are slightly above mean tide level.
Plants trap suspended sediment when they are inundated by tides
and reduce erosion by waves, so that the rate of sedimentation on
the marsh Is enhanced. The elevation of the marsh surface rises
rapidly, gradu~ly slowing as the increasing elevation reduces the
frequency and duration of flooding by the tide. Depending on the
rate of sea level rise and the supply of suspended sediment, the
marsh surface tends to maintain its level relative to the tide as sea
level rises.
The rising marsh surface caused the marsh to invade land
where the shoreline slope ~ gradual, and over time extensive
marshlands developed. Such marshes were significant traps for
suspended sediment and undoubtedly affected the rates of depo-
sition in the bays. In developed countries many estuary marshes
have been diked for use as salt evaporation ponds or filled for agri-
culture and urban development, and thus no longer play a part in
estuarine hydrology. Enough information to calculate the loss of
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ASSESSMENT OF RESPONSE STRATEGIES
105
sediment to marshes has only recently become available, and such
calculations should be reported in the near future (Krone, 1985~.
Changes in relative mean sea levels will also change the geome-
tries of rivers flowing into the ocean. Laboratory studies (Chang,
1967) led to the conclusion that rising sea level at the downstream
end of a river will greatly enhance the river's meandering ten-
dency. FaDing sea level encourages the river to run straight and
can significantly increase its sediment-carrying capacity, and thus
increase estuarine sedimentation. This phenomenon was reported
by geomorphologists studying water-surface fluctuations of large
lakes.
Whether in an estuarial environment or not, coastal harbors
will experience different effects of a change in relative mean sea
level, depending on shoreline and bay bathymetry configuration.
These changes can be quite complex, but estimates of them can
be made with appropriate physical and/or mathematical models.
BREAKWATERS, SEA WALLS, AND JETTI1 :S
Sea defense systems of the rubble-mound type can be easily
increased in elevation by among armor units, stone or cast con-
crete shapes. Normal maintenance often requires adding material
to compensate for settlement and consolidation of the core mate-
rials and foundations, and adjustments for sea level rise may be
part of that process. The core of a rubble-mound breakwater is
usually relatively impervious. Thus, a breakwater that neecis to
be increased in height will require modification to the core, to the
filter layers, and to the armor. In seismic regions the new design
must be analyzed to assure its earthquake-resistance capacity.
Breakwaters and sea wails of solid construction, such as mono-
lithic concrete, will be overtopped more often. Since wave damage
is a function of wave height, which in turn increases in proportion
to water depth, the damage may increase exponentially with sea
level rise. This may require raising top levels and slopes after a
period of years.
~ some regions the design wave for breakwaters (and levees)
is limited due to the water depth so that an increase in water
depth owing to a relative increase ~ mean sea level will result in
a larger design wave, which will require modification to the size
and/or slope of the breakwater.
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106
RESPONDING TO CHANGES IN SEA LEVEL
NAVIGATION GATES
Navigation gates are covered briefly under the section entitled
"Levees.~ Adequate data are not available to determine where
new navigation gates might be needed, and if existing gates need
to be modified.
PIERS AND WHARVES
If the sea level at a pier or wharf were to rise as much as 1
It ~ 50 years, and the design life is taken to be 50 years, then
the deck elevation may have to be designed to be functional for
both present and future conditions. One solution, for which there
is precedent at the Brooklyn Navy Yard (where crane rails were
built at elevations that allowed for subsidence), is to build at a level
6 in. higher than presently needed, thus splitting the difference. If
wave action is expected to be significant, the underside of the deck
structure may have to be kept above future wave crests, or 1 It
higher than is needed now. Piles or caissons would have to be 1 It
longer. Similarly, cranes and loading/unio~ing equipment would
have to be designed to reach 1 ft higher above ship decks.
Piers are located on open coasts as well as harbors. The
case of piers on the open coast would be more complex than
discussed above. With no change in the bottom, the piers in
most exposed locations would be subject to larger waves as sea
level rose, because the deeper water would permit higher breakers,
and thus greater wave-induced forces on structures. However, as
previously discussed, changes wiD occur in the bottoms, and then
the problem becomes quite complex.
It is possible that the change in climate, with the resulting
change in numbers ant} intensity of storms, will be more impor-
tant than a small change In relative mean sea level. For example,
during the winter of 1982-1983, when an E! Nino-Southern Oscil-
ration condition existed, both the number and intensity of storms
increased (Seymour et al., 1984~. More than 12 ocean piers in Cal-
ifornia were either destroyed or severely damaged. It is not evident
to this committee whether the numbers or intensity of storms will
increase or decrease as a result of the greenhouse effect. How-
ever, by and large the expected magnitude of the consequences to
coastal piers is relatively minor.
raters are ~ocarea on open coasts as well as naroors
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RESPONDING TO CHANGES IN SEA LEVEL
bulk material docks, the vertical travel of the land-based unloader
will clear objects on the ships' decks by less and less. Allowance
for any increase, in ah cases, will be difficult to compensate, and
it may be advisable to incorporate anticipated sea level rise in
the original design, despite additional cost. Fluid loading and
unloading docks that depend on jo~nted-pipe loading arms also
will need to be designed for the rising level of ships' decks and
manifolds, since tankers ride higher in the water when they are
empty. Loading systems that use hose connections may have more
flexibility, or may be more easily modified by adjustments in hose
length or supports.
HIGHWAYS, RAIIROADS, BRIDGES, AND
VEHICULAR TUNNELS
As sea level rises, highways and railroads across lowlands near
tidal water wiD experience more frequent flooding during high
tides and storms. This effect may be especially severe in certain
estuaries where the rise in sea level will be amplified, the more
so because these same estuaries are more vulnerable to storm
surges as water funnels into a gradually narrowing arm of the sea.
The levels of such highways and railroads may have to be raised
by reballasting or adding pavement from time to time. Some
highways and railroads already experience flooding during heavy
rains and high tides, aIld these events would increase, especially at
underpasses. Increased pumping capacity would be needed. Much
can be learned from experiences ~ Japan with the rapid subsidence
of some heavily populated areas that have much infrastructure of
this type.
The clearance above high water will gradually diminish for
bridges across water in the tidal zone. The amount of the reduction
will be greater in the case of bridges upstream in estuaries where
the rise of water level is amplified by funnel ejects, as in Delaware
Bay. Although the rise may be slow and gradual, the consequences
of damage to a bridge may be so catastrophic as to warrant regular
· —
monltormg.
The clearance between the center of a suspension bridge and
the water surface depends upon mean sea level, tide stage' tem-
perature, winds, freshwater flow into an estuary, and static and
dynamic loads on the bridge. An example of how elevation can
be affected by structural changes in the bridge is the Golden Gate
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ASSESSMENT OF RESPONSE STRATEGIES
109
Bridge over San E`rancisco Bay, California. About three decades
ago a lower lateral bracing system and maintenance platform rails
were instated, resulting in a downward deflection of about 3 It
from the original dead-Ioad position at the mid-point of the cen-
ter span. Recently, a lighter-weight deck element was installed.
The maximum additional camber of the center span during the
replacement operations was about 7 ft and occurred when the new
lighter-weight deck had been placed on the Marin backspan and
the mainspan, with the heavier original concrete deck on the San
Francisco backspan (D. E. Mohn, Golden Gate Bridge, Highway,
and Transportation District, San Francisco, California, letter, De-
cember 11, 1986~.
COMMERCL\[ AND INDUSTRIAL BUIIDINGS
Structures near tidal water will suffer increased flooding. Sur-
face water levels will rise, and groundwater levels, which generally
are driven in part by nearby harbor and estuary levels, will fol-
low with a time lag and amplitude that increase and decrease,
respectively, with distance.
Results are likely to be increased seepage into basements,
poorer storm drainage, and ponding in parking areas and am
preaches. The former may be countered with better waterproofing
or by sump pumps and drainage; the latter may eventually require
added paving.
POWER PLANTS
Most power plants on coasts use sea water for cooling, and
power plants on estuaries use estuarine water for cooling. Studies
should be made of the effects of rising relative mean sea level on
these facilities. The "design lifer of a power plant is about 40
years, a period over which mean sea level could vary from a few
inches to almost 1-ft, considering the scenarios used in this report.
Engineering changes required in those aspects of operation most
affected by a 1-ft rise in relative mean sea level, such as cooling
water intake and discharge systems, would be handled as normal
changes. Environmental effects would also need to be considered
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RESPONDING TO CHANGES IN SEA LEVEL
PIPELINES
With rising sea levels, groundwater might rise above some
pipelines in cities and ports, affecting corrosion rates.
Large diameter pipes are used in sewer outfalis in many coastal
cities and, in some cases, for intake and discharge conduits for
power plant cooling water systems. To counter the effects of sea
level rise in these systems, it might be necessary to increase pump
ing capacity, as well as to increase the capability of outfall systems
to dilute their effluent.
Another possible effect is associated with the amount of ero-
sion that knight occur on sand coasts. Pipe sections buried under
the beaches and current surf zone might become exposed, while
discharge ports could become covered with sand.
FLOODING AND STORM DRAINS
Flooding could be a major problem ~ many low-ly~ng por-
tions of coastal cities with a sea level rise. Examples include New
Orleans, portions of which are below present sea level, and areas of
San Francisco. Additional pumping will be required. More flood-
ing would occur than at present in many areas, and levees, tide
gates, and channels would be needed in some regions. Environ-
mental conflicts would have to be exarn~ned and reconciled.
Considerable detail on planning and installation of a flood
control system for the Koto delta region of Tokyo, Japan is given
by Ukena et al. (1970~. Because of subsidence, about 50 percent
of the Koto delta area became lower than the daily low-tide level
by 1953. Natural runoff from this area became nearly nonexistent,
and forced drainage had to be used on a very large scale.
Subsidence in lands adjacent to the south end of San Francisco
Bay between 1934 and 1967 was measured at several hundred
benchmarks. About 100 ran subsided more than 3 ft. with the
magnum subsidence of about 8 ft. As a result, several miles
of levees were raised to prevent flooding by bay waters, and flood
control levees were added near the ends of the streams running into
the bay (Poland, 1970~. Poland states that salt water from the bay
moved upstream, and channel grades crossing the subsidence area
became downwarped. This resulted in sediment deposition near
the stream mouths, with a reduction in channel capacity, which in
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ASSESS OF RESPONSE STRATEGIES
111
turn required an increase ~ levee heights. Even with the higher
levees, flooding occurs at tunes of very high runoff.
Williams (1985) points out that at present there are no stan-
dard assumptions for backwater calculations in the design of flood
control projects; the usual practice is to design for the Goodyear
flood at a title level of mean higher high water. An increase in rel-
ative mean sea level could result in higher flood waters upstream,
with the possibility of levee overtopping. In addition, he states
that the flood control systems of many areas presently rely on
storage of flood waters and gravity releases at low tide. Inade-
quate information is available to see how a rise in relative mean
sea level would affect specific areas.
In some parts of the country (for example, the New Jersey
Meadows tidal marsh), mosquito control commissions have in-
stalled elaborate systems of drainage ditches and tide gates to
Intuit backflood~ng of marshland when the tide rises. The flow is
very sensitive to changes in mean tide level, and will be affected
seriously by a rise in relative mean sea level.
La Roche and Webb (1986) est~rnate the cost of the expected
overhaul of an urban gravity drainage system in Charleston, South
Carolma for (1) present sea level, (2) an 11-~n. rise, and (3) a 1~.
rise. They also estimate the cost of an overhaul if the system is
built for today's sea level and later retrofitted for 11- and lawn.
rises. Presently, designing for an 1l-iIl. rise would require larger
pipes at an additionad cost of $260,000 (a 5 percent increase).
However, a retrofit would cost $2.4 Bullion not including indirect
costs of closing the streets.
Waddell and Blaylock (1987) conducted a similar evaluation
for a watershed in Ft. Wadton Beach, Florida, which is more
lightly developed and employs a variety of independent measures
to reduce flooding. They conclude that there are no savings to
designing the system for a future rise, compared with upgrading
the system if and when a rise occurs.
Titus et al. (1987) evaluate the planning implications of the
preceding studies. They conclude that future sea level rise is rele-
vant to today's design decisions where cities are overhauling urban
gravity drainage systems, but not where drainage improvements
are achieved by new parallel systems. In the former case, designing
for a future rise is similar to insurance, with the economic mer-
its being site specific. The report also discusses forced drainage,
which was not investigated in detail by the studies.
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112
RESPONDING TO CHANGES IN SEA LEVEL
A segment about 30 km Tong by 7 km wide along the coast
of the Sea of Japan in Nilgata and the nearby vicinity subsided
by about 50 to 150 cm between 1959 and 1968 (kimono, 1970;
Takeuchi et al., 1970~. Larger areas subsided by lesser amounts;
most of the Niigata Plain, about 8,300 km2, had some subsidence.
Takeuchi et al. (1970) mention that in the Nugata lowland on the
flood plain of the Shinano River, the subsidence decreased the
ability of pumps to drain the area and also damaged the drainage
canal network. They estimated the cost of reconstruction to be
about 20 billion yen (then about $56 million).
HOTELS AND WILLIS
The effects of a sea level rise on these facilities will be similar
to those for commercial buildings, but greater emphasis wall be
needed to preserve the amenities that attract patronage. Remedies
may be applied sooner as the effects of even a small rise in sea level
become apparent. This may be especially pertinent for facilities
close to the oceans. The threat of damage to a waterfront hotel,
for example, might warrant extensive measures to reinforce sea
wails, add beach materials, or otherwise protect the shore and
hose! foundations from storm damage.
Many mass and hotels in the United States become "aged" in
a few decades and undergo major and extensive renovation. It Is
likely that measures to accommodate small rises ~ relative mean
sea level would be taken as a part of the renovation.
RESID1:NTL\L CENTERS
As more and more people move to shore housing sites, a
rise in sea level will become more evident to greater numbers
of people. The effects wait be most noticeable In beach erosion,
sea wall damage, and flooding of lower levels, drives, and swales.
Since many such areas are also subject to subsidence from earth
compaction, groundwater pumping, or tectonic movement, the
effect of the rise may be accentuated.
For example, in the area around Baytown, Texas, on Galveston
Bay, subsidence has caused frequent high-tide flooding of land that
slopes toward the bay at gradients of 1 ft/mi, on the average.
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ASSESSMENT OF RESPONSE STRATEGIES
WAT1:R SUPPLY SYSTEMS
113
A number of effects on water supply systems (particularly on
water quality) can be realized with changes ~ relative mean sea
level. Some of these are considered below for both groundwater
and surface-water sources.
Perched Mesh Water
The increase in hydrostatic pressure with depth is about 3
percent greater in sea water than In fresh water. If the surface of
the freshwater table is X feet above mean sea level, under static
conditions, the freshwater-seawater interface would be about 40X
It below mean sea level.
As sea level rises, this "bubbles of fresh water should simply
float In the salt water at an elevation that is higher by the amount
of sea level rise. Thus, no significant effects are expected. Addi-
tionally, in many locations on barrier spits and islands the water
supply is presently brought from other regions and thus a rise in
sea level should present no additional problem.
Aquifers
Coastal aquifers normally flow toward adjacent surface waters
such as lakes, rivers, estuaries, or the sea. Excessive pumping
for irrigation and municipal water supplies can reverse the flow so
that water is recharged to the aquifer. If such recharge occurs near
the mouth of a river, a rise In sea level can recharge the aquifer
with sea water. Conditions are aggravated during droughts, when
the saltwater wedge advances upstream and when pumping for
irrigation is augmented. An example of a potential problem is the
Delaware River, which recharges the Potomac-Raritan-Magothy
aquifer (the source of water for many wells in New Jersey) above
river mile 98 (Hull and Titus, 1986; Camp, Dresser and McKee,
Inc., 1982; Hull and Tortoriello, 1979~.
Some possible engineering responses to this problem are: (1)
modifying the elevation of the aquifer's connection to the estuary
to reduce the landward penetration of the salt wedge; (2) reducing
the permeability of the sediment where the aquifer communicates
with the estuary to reduce the rate of seawater recharge; and (3)
increasing recharge during periods of high precipitation.
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RESPONDING TO CHANGES IN SEA LEVEL
Freshwater Intakes prom Upstream Regions of Estuaries
A rise in relative mean sea level could have far-reaching effects
on taking fresh water from upstream regions ~ estuaries. As
an example, consider the Sacramento-San Joaqum delta region
of California from the standpoint of maintaining water quality
standards. The California Water Resources Control Board (1983
and 1984) has stated:
The Delta is a vital link between river systems of the Sacramento
Valley and the water deficient areas to the south and west of
the Delta. Two major systems—the State Water Project (SWP)
operated by the Department of Water Resources (Department) and
the federal Central Valley Project (CVP) operated by the United
States Bureau of Reclamation (Bureau) withdraw supplies from
the Delta for use in areas of need. These projects are the two
largest water diversions from the Delta. They provide municipal
supplies to areas where over 14 million people live and support
an extremely productive agricultural economy in the San Joaquin
Valley.
The underlying principle of these standards is that water quality
in the Delta should be at least as good as those levels which
would have been available had the state and federal projects not
been constructed, as limited by the constitutional mandate of
reasonable use. The standards include adjustments in the levels of
protection to reflect changes ~ hydrologic conditions experienced
under different water year types.
In a recent decision of a state court of appeal, it was ruled that
the California Water Resources Control Board has "compromised
its important water quality role by defining its role too narrowly
in terms of enforceable water rights (Einstein, 1986~.
A possible rise of mean sea level of between 0.5 and 1.5 m
by the year 2100 could have an important imp act on the struc-
tures and methods necessary to maintain water quality standards.
According to Kennedy (1986), there are many alternatives to
transferring fresh water through and around the delta; several
are described by the California Department of Water Resources
(1983a).
In the Ned' York Times (March 21, 1986), a newly developed
scheme was announced to liberate New York City from the periodic
water shortages that result from its dependence on the drought-
prone tributaries of the Delaware River. An existing pump station
on the Hudson River at Chelsea, on the east bank near Poughkee~
sie, would be enlarged and integrated into a plan of new reservoir
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ASSESSMENT OF RESPONSE STRATEGIES
115
construction. During high-discharge stages of the river, the fresh
water would be ~skimmed" and put in the storage reservoirs. At
Tow-discharge times the salt water comes dangerously close to the
pump station. At present, the Hudson River is tidal as far as
Albany. A sea level rise of 0.5 m would bring the saltwater wedge
above the level of Poughkeepsie.
Hull and Titus (1986) estimate an increased salinity of the
Delaware estuary during a repeat of the 1960s drought for 2.4-
ft and 8.2-ft rises in sea level. A 2.4-ft rise would cause the
25~ppm isochIor (the desalt frontal to move 7 miles upstream,
on the average. During 15 percent of the tidal cycles, the river
at Philadelphia's Torresdale drinking water mtake would have
elevated sodium concentrations exceeding 50 ppm (the New Jer-
sey drinking water standard) if no countermeasures were taken.
Lennon et al. in HuD and Titus (1986) conclude that the elevated
salt levels in the river could contaminate parts of the Potomac-
Raritan-Magothy aquifer, which is pumped at a point below sea
level and recharged by the river. The report cites several mitiga-
tion measures, such as new reservoirs, and recommends long-term
planning for consequences of sea level rise as well as possible
changes ~ drought frequency caused by the greenhouse effect.
LANDPII`LS AND WASTE DISPOSAL SITES
A rise in sea level can affect landfills and disposal sites in two
ways: (1) direct overtopping and erosion, or (2) changes in the
level of the aquifer and the groundwater leaching pattern. Dikes,
similar to those currently used in some containment areas, could
be designed and constructed to counter both of these effects.
OFFSHORE PLATFORMS AND ARTIFICL\[ ISLANDS
The productive lives of offshore platforms and artificil] islands
used ~ the production of oil and gas is on the order of 25 years.
They should not be affected very much by eustatic rise in mean
sea level In that time span. The problem of relative rise in mean
sea level owing to subsidence can be of much greater importance.
An article in News of Norway (May 22, 1985) describes how the
Ekofisk platforms have sunk 2 m (6 ft) and continue to subside
at a rate of 1 - cm/mo or 12~8 cm/yr (about 5-9 in.~. The
article mentions a conclusion in a recent report by the Norwegian
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116
RESPONDING TO CHANGES IN SEA LEVEL
classification society, Det Norske Veritas, that several platforms
on the field would be total losses in the event of a major storm.
An article in the Financial Times of London (April 11, 1986)
states that a massive rescue plan has been proposed to raise six
steel of! platforms by 6 m. It claims the platforms have sunk nearly
3 m because of the weight of 3,000 m of rock overlying the field's oil
ant] gas reservoir. Subsidence due to the removal of hydrocarbons
is also a likely contributing factor. A report commissioned by
Phillips Petroleum to study two of the peripheral platforms in
the field concluded that total loss, given a Goodyear wave, could
not be ruled out if the platforms sank 8.5 ft. and even a heavy
storm would cause serious damage. A number of alternatives were
considered. The lowest cost alternative, and one which could be
done with only 18 days production shutdown, would cost $286
million (Anonymous, 1986~. The plan would consist of jacking up
the decks of the platforms and installing extensions. The work is
expected to start in June 1987.
Representative terms from entire chapter:
mean sea
