v>EPA
United States
Environmental Protection
Agency
Delaware
River Basin
Commission
May 1986
EPA 230-05-86-010
Greenhouse Effect, Sea Level
Rise, and Salinity in the Delaware
Estuary
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Library of Congress Cataloging-in-Publlcation Data
Greenhouse effect, sea level rise, and salinity in the Delaware Estuary.
Biblliography:
1. Sea Level—Delaware River Estuary (N.Y.-Del.-Penn. and N.J.)
2. Salinity—Delaware River Estuary (N.Y.-Del.-Penn. and N.J.)
3. Greenhouse effect, Atmospheric—Delaware River Estuary Region
(N.Y.-Del.-Penn. and N.J.) 4. Delaware River Estuary Region
(N.Y.-Del.- Penn. and N.J.)—Climate 5. Water, Underground—
New Jersey—Quality. I. Hull, C.H.J. II. Titus, James G.
GC89.G75 1986 363.7'394 86-11454
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GREENHOUSE EFFECT, SEA LEVEL RISE, AND
SALINITY IN THE DELAWARE ESTUARY
Edited by
C.H.J. Hull
Delaware River Basin Commission
James G. Titus
Environmental Protection Agency
Other Contributors:
Gerard P. Lennon
Lehigh University
M. Llewellyn Thatcher
Cooper Union College
Richard C. Tortoriello
Delaware River Basin Commission
Gary M. Wisniewski
Lehigh University
Gary A. Yoshioka
ICF Incorporated
Protection Agency
5, L.!bra-,',i (E'L-_,/;)
£wO £. Dearborn t:ruet, Boom 1670
Chicago, 11 60504
This document has been reviewed in accordance with the U.S.
Environmental Protection Agency and Delaware River Basin
Commission peer and administrative review policies and
approved for publication. Mention of trade names or
commercial products does not constitute endorsement or
recommendation for use. Please send comments to James G.
Titus (PM-220), Strategic Studies Staff, U.S. Environmental
Protection Agency, Washington, B.C. 20460.
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11
SUMMARY
Increasing atmospheric concentrations of carbon dioxide and other gases
are expected to warm the earth a few degrees (C) in the next century by a
mechanism commonly known as the "greenhouse effect." Such a warming could
alter precipitation patterns and raise sea level. Although it is not yet
possible to predict whether particular areas will receive more or less
rainfall, there is a general agreement that sea level will rise. Unfortu-
nately, estimates for the year 2025 range from 5 to 21 inches above current
sea level, while estimates of the rise by 2100 range from 2 to 11 feet.
Several issues must be resolved for society to rationally address the
possibility of significant changes in climate and sea level. Officials making
decisions about near-term projects with long lifetimes must examine the
potential consequences and determine whether these risks justify a shift to
strategies that are less vulnerable to changes in sea level or the frequency
or severity of droughts. Research officials must assess the opportunities for
improving predictions and decide whether the need for these improvements
justifies accelerating the necessary research. Decision makers must decide
whether to base policies on today's inadequate knowledge or ignore the
implications until they are more certain.
One potential impact of a global warming and rise in sea level would be an
increase in the salinity of estuaries, which might threaten drinking water and
aquatic ecosystems. The Delaware River Basin Commission (DRBC) has long
considered the implications of droughts on management of water resources in
the Delaware estuary; since 1979, it has also considered the implications of
recent sea level trends. However, the DRBC has not previously focused on the
possibility that the "greenhouse warming" could exacerbate salinity problems.
The Environmental Protection Agency has initiated studies on the impacts of
sea level rise and climate change on erosion, flooding, and wetland
protection, but has not previously examined the impacts on salinity.
This joint report by the Environmental Protection Agency and the Delaware
River Basin Commission examines the implications of the greenhouse warming for
salinity control in the Delaware estuary. The study focuses on the implica-
tions of (1) a 21-inch rise in global sea level expected by 2050, which would
imply a rise of 2.4 feet in the Delaware estuary; and (2) a 7-foot global rise
by 2100, which would imply an 8.2-foot rise in the Delaware estuary. The
authors estimate the increase in estuary salinity, estimate the possible
increase in salinity of the Potomac-Raritan-Magothy aquifer system, discuss
the implications, and examine possible responses. Potential changes in
precipitation are not evaluated.
CONCLUSIONS
1. Sea level rise could substantially increase the salinity of the
Delaware estuary in the next century. If no countermeasures are taken, a
repeat of the 1960s' drought with a 2.4-foot rise would send the salt front
upstream to river-mile 100, compared with mile 93 for current sea level.
Moreover, the chloride concentration at mile 98, the DRBC salinity control
point, would increase from 136 parts per million (ppm) to 305 ppm. An
8.2-foot rise would send the salt front upstream to mile 117 and would
increase salinity to 1560 ppm at the salinity control point.
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2. Accelerated sea level rise could cause excessive salinity concentra-
tions at Philadelphia's Torresdale intake if no countermeasures are taken.
For a 2.4-foot rise, sodium concentrations would exceed 50 ppm (the New Jersey
drinking water standard) during 15 percent of the tidal cycles during a
recurrence of the 1960s drought. For an 8.2-foot rise, sodium concentrations
would exceed 50 ppm during 50 percent of the tidal cycles.
3. Accelerated sea level rise could threaten the New Jersey aquifers
recharged by the Delaware River. During the 1960s drought, river water with
chloride concentrations as high as 150 ppm recharged the Potomac-Raritan-
Magothy aquifer in the vicinity of Camden, raising chloride concentrations of
some wells from 20 ppm to 80 ppm. A repeat of the 1960s' drought with a
2.4-foot rise in sea level would result in river water with concentrations as
high as 350 ppm recharging the aquifer in this area. During the worst month
of the drought, over one-half of the river water recharging the aquifer would
have chloride concentrations in excess of 250 ppm. With an 8.2-foot rise, 98
percent of the recharge during the worst month of the drought would have
chloride concentrations greater than 250 ppm, and 75 percent of the recharge
would be greater than 1000 ppm. (The EPA drinking water standard is 250 ppm,
and water with chloride concentrations greater than 78 ppm generally exceeds
the 50-ppm sodium standard.)
4. Planned but unscheduled reservoirs could offset salinity increases
expected in the next forty years. Salinity increases resulting from a one
foot rise in sea level expected in the next forty years would require
increased reservoir capacity of at least 110 thousand acre-feet. However,
reservoirs planned by the DRBC but not yet scheduled would have a combined
capacity of 592 thousand acre feet.
5. Possible shifts in precipitation resulting from the greenhouse warming
could overwhelm salinity increases caused by sea level rise. Excessive
salinity has been a problem only during droughts. Unfortunately, it is not
possible to determine whether the Delaware River Basin will receive more or
less rainfall in the future. A recent study by NASA suggested that a tenfold
increase in drought frequency cannot be ruled out. On the other hand, some
researchers have suggested that most coastal areas will experience a 10
percent increase in precipitation.
6. Uncertainties regarding future climate change do not necessarily imply
that waiting for better predictions is the most prudent strategy. There is
no guarantee that accurate climate projections will be possible when they are
needed. Moreover, some measures may have potential benefits so far in excess
of their costs as to be warranted in spite of current uncertainties. For
example, identifying potential reservoir sites long before they are necessary
and not developing them for other uses can ensure that they are available if
and when they are needed, without imposing substantial costs. Waiting until
they are needed could result in no satisfactory sites being available.
7. A regional study should be initiated that examines the potential
impacts of precipitation changes as well as sea level rise for the Delaware
estuary and adjacent river basins. A thorough understanding of the water
resource challenges faced by the Delaware River Basin is not possible without
considering the needs of New York City and other areas outside the Basin that
depend on the Delaware for water supply.
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TABLE OF CONTENTS
Page
I. INTRODUCTION 1
II. THE BASIS FOR EXPECTING A RISE IN SEA LEVEL
By James G. Titus 3
Past Changes in Climate and Sea Level 3
The Greenhouse Effect 4
III. SALINITY IN THE DELAWARE ESTUARY
By C.H.J. Hull, M. Llewellyn Thatcher, and
Richard C. Tortoriello 15
Saltwater Intrusion 15
The Delaware Estuary 16
Saltwater Intrusion and the DRBC 21
Estimating Impacts of Sea Level Rise on Salinity 28
Implications 36
IV. IMPACT OF INCREASED RIVER SALINITY ON NEW JERSEY AQUIFERS
By Gerard P. Lennon, Gary M. Wisniewski, and Gary A. Yoshioka .. 40
The Relationship Between Sea Level and Aquifer Salinity .... 40
The Potomac-Raritan-Magothy Aquifer System 41
Impact of a Drought on the Aquifers--Current and Future
Sea Level 45
Improved Estimates 53
V. RESPONSES TO SALINITY INCREASES
By C.H.J. Hull and James G. Titus 55
Preventing Salinity Increases 55
Adapting to Increased River Salinity: Surfacewater
Users 58
Adapting to Increased River Salinity: Groundwater
Users 60
VI. NEXT STEPS
By C.H.J. Hull and James G. Titus 64
Considering Climate Change 64
Necessary Research 66
Conclusion 67
APPENDIX A: TECHNICAL DESCRIPTION OF DELAWARE ESTUARY SALINITY MODEL
By M. Llewellyn Thatcher 68
APPENDIX B: MODIFICATION OF SALINITY-MODEL GEOMETRY FOR LARGE
RISES OF SEA LEVEL
By M. Llewellyn Thatcher 77
REFERENCES 82
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vi
LIST OF FIGURES
Figure 1 Global Temperatures and Sea Level Rise in the Last
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Century
Concentrations of Selected Greenhouse Gases Over Time
Estimated Global Warming Due to a Doubling of Greenhouse
Gases
Global Sea Level Rise Scenarios
General Location of the Delaware Basin
The Delaware Estuary
Change in Yearly Mean Sea Level at Philadelphia
Chlorinity Response of Delaware Estuary to Projected
35-Year Rise in Sea Level, 1965-2000
Chloride Concentrations vs . River Mile
Percent of Tidal Cycles in Which Specified Concentration
is Exceeded at Torresdale for Three Sea Level Scenarios . . .
Saltwater Intrusion in a Coastal Aquifer
Modeled Area Showing Outcrop Area and River Reaches of the
Potomac-Raritan-Magothy Aquifer System
Potentiometric Surface of the Potomac-Raritan-Magothy
Aquifer System, November -December 1973
Potentiometric Surface of the Potomac-Raritan-Magothy
Aquifer System at Year 2000
Chloride Concentrations at Wells in the Camden City
Vicinity
Injection-Type Seawater Intrusion Barrier
5
7
10
13
18
19
29
31
35
37
42
44
46
47
48
62
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LIST OF TABLES
Page
Table 1 Scenarios of Future Sea Level Rise 14
Table 2 Existing and Scheduled Reservoirs That Can Control
Salinity in the Delaware Estuary 24
Table 3 Maximum 30-Day Chloride Concentration at Different River
Miles 34
Table 4 Recharge of Delaware River Water into Potomac-Raritan-
Magothy Aquifer System 49
Table 5 Velocities, Contact Times, and Penetration Distances for
Ground Water with Elevated Chloride Concentration 52
Table 6 Unscheduled Reservoirs in DRBC's Plan That Could Be Used
to Offset Increases in the Delaware Estuary 56
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I. INTRODUCTION
Increasing atmospheric concentrations of carbon dioxide, methane,
chlorofluorocarbons, and other gases are expected to raise the earth's average
surface temperature several degrees in the next century by a mechanism
commonly known as the "greenhouse effect." Such a global warming would
probably raise sea level and substantially change precipitation patterns
worldwide, altering water quality and availability and upsetting wetland and
aquatic ecosystems. Scientific understanding is not yet sufficient to
estimate the impacts accurately, but it is sufficient to expect that the
changes will be substantial.
Although it is not yet possible to project future climate change for
specific regions, there is a consensus on the probable increase in average
temperatures. Because sea level depends mostly on the global average
temperature, it is possible to estimate the likely range of its rise. Recent
reports by the National Academy of Sciences and the Environmental Protection
Agency project a worldwide rise in sea level of sixty to one hundred fifty
centimeters (two to five feet) in the next century. Such a rise would be a
substantial acceleration over the rise of thirty centimeters (one foot) that
has taken place along the Atlantic coast in the last century.
One of the impacts of a rise in sea level is an increase in the salinity
of estuaries and aquifers. In 1979, the Delaware River Basin Commission
(DRBC) investigated the impact of recent sea level trends on salinity in the
estuary and determined the measures that would be necessary by the year 2000
to counteract the increased salinity caused by droughts and sea level rise.
Because no projections on the impact of the greenhouse effect were available
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at the time, that study did not consider the implications of an acceleration
of the current rate of sea level rise.
This report examines the potential impacts of accelerated sea level rise
on salinity in the Delaware estuary and adjacent aquifers in New Jersey.
Although the impacts we examine are uncertain and contingent upon particular
rates of sea level rise occurring in the future, this type of analysis is
useful because it may be possible to identify cost-effective opportunities to
prevent or mitigate possible consequences that warrant consideration even
today. We hope that this report stimulates interest in the long-term planning
necessary for management of the Delaware estuary to meet successfully the
challenge of a rise in sea level.
This report first describes the basis for expecting a rise in sea level.
It then explains how droughts and rising sea level increase the salinity of an
estuary, describes the impact of droughts on salinity that would result from a
73- and 250-centimeter (2.4- and 8.2-foot) rise in sea level, and discusses
some of the consequences. Section 4 discusses the impact of increased river
salinity on the adjacent Potomac-Raritan-Magothy aquifer system in New
Jersey. Section 5 provides a qualitative, discussion of possible responses,
including ways of preventing salinity increases in the estuary and the
aquifer, and ways of adjusting to the increases.
The report concludes by outlining the next steps that should be taken to
determine the best responses to the greenhouse effect. Problems with
increased salinity generally occur during droughts, the frequency of which may
be different in the future. Although this effort is limited to sea level
rise, a more in-depth assessment must also consider possible changes in
precipitation.
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II. THE BASIS FOR EXPECTING A RISE IN SEA LEVEL
Past Changes in Climate and Sea Level
Throughout geologic history, sea level has risen and fallen by over three
hundred meters (one thousand feet). Although changes in the size and shape of
the oceans' basins have played a role over very long periods of time (Hays and
Pitman 1973), the most important changes in sea level have been caused by
changes in climate. During the last ice age (18,000 years ago), for example,
the earth was about five degrees celsius colder than today, glaciers covered
most of the northern hemisphere, and sea level was one hundred meters (three
hundred feet) lower than today (Bonn, Farrand, and Ewing 1962).
Although most of the glaciers have melted since the last ice age, polar
glaciers in Greenland and Antarctica still contain enough water to raise sea
level more than seventy meters (over two hundred feet) (Untersteiner 1975). A
complete melting of these glaciers has not occurred in the last two million
years, and would take tens of thousands of years even if the earth warmed
substantially. However, unlike the other glaciers, which rest on land, the
west antarctic ice sheet is marine-based and more vulnerable to temperature
increases. Warmer ocean water would be more effective than warmer air at
melting glaciers, causing West Antarctica to melt. Mercer (1970) suggests
that the West Antarctic Ice Sheet completely disappeared during the last
interglacial period (which was one or two degrees warmer than today and
occurred 100,000 years ago), at which time sea level was five to seven meters
(about twenty feet) above its present level.
Over relatively short periods of time, climate can influence sea level by
heating and thereby expanding (or cooling and contracting) sea water. In the
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last century, tidal gauges have been available to measure relative sea level
in particular locations. Along the Atlantic Coast, sea level has risen about
30 centimeters (one foot) in the last century (Hicks, Debaugh, and Hickman
1983). (Figure 7 shows the rise that has taken place at Philadelphia.)
Studies combining all the measurements have concluded that average worldwide
sea level has risen ten to fifteen centimeters (four to six inches) in the
last one hundred years (Barnett 1983; Gornitz, Lebedeff, and Hansen 1982). At
least part of this rise can be explained by the thermal expansion of the upper
layers of the oceans resulting from the observed warming of 0.4°C in the last
century (Gornitz, Lebedeff, and Hansen 1982). Meltwater from mountain
glaciers has also contributed to sea level rise (Meier 1984). Figure 1 shows
that global temperature and sea level have been rising in the last century.
Nevertheless, questions remain over the magnitude and causes of sea level rise
in the last century.
The Greenhouse Effect
Concern about a possible acceleration in the rate of sea level rise stems
from measurements showing that concentrations of carbon dioxide (CCO,
methane, chlorofluorocarbons, and other gases released by human activities are
increasing. Because these gases absorb infrared radiation (heat), scientists
generally expect the earth to warm substantially. Although some people have
suggested that unknown or unpredictable factors could offset this warming, the
National Academy of Sciences (NAS) has twice reviewed all the evidence and
found that the warming will take place. In 1979, the Academy concluded: "We
have tried but have been unable to find any overlooked physical effect that
could reduce the currently estimated global warming to negligible proportions"
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FIGURE 1
GLOBAL TEMPERATURES AND SEA LEVEL
RISE IN THE LAST CENTURY
0.2 .
Temperature o
-0.2
10
Sea Level
(cm)
-5
1880
i
1920
1960
Year
Sources: Temperature curve from: HANSEN, J.E., D. JOHNSON, A. LACIS, S.
LEBEDEFF, D. RIND, AND G. RUSSELL, 1981. Climate Impact of
Increasing Atmospheric Carbon Dioxide, Science 213:957-966.
Sea
level curve adapted from: GORNITZ, V., S. LEBEDEFF, and J. HANSEN,
1982. Global Sea Level Trend in the Past Century.
215:1611-1614.
Science
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(Charney 1979). In 1982, the NAS confirmed the 1979 assessment (Smagorinsky
1982).
A planet's temperature is determined primarily by the amount of sunlight
it receives, the amount of sunlight it reflects, and the extent to which its
atmosphere retains heat. When sunlight strikes the earth, it warms the
surface, which then reradiates the heat as infrared radiation. However, water
vapor, CCL, and other gases in the atmosphere absorb some of the energy
rather than allowing it to pass undeterred through the atmosphere to space.
Because the atmosphere traps heat and warms the earth in a manner somewhat
analogous to the glass panels of a greenhouse, this phenomenon is generally
known as the "greenhouse effect." Without the greenhouse effect of the gases
that occur in the atmosphere naturally, the earth would be approximately 33°C
(60°F) colder than it is currently (Hansen et al. 1984). Thus, the greenhouse
effect per se is not just something that will happen in the future; it is an
existing natural characteristic of the atmosphere.
In recent decades, the concentrations of these "greenhouse gases" have
been increasing. Since the industrial revolution, the combustion of fossil
fuels, deforestation, and cement manufacture have released enough C02 into
the atmosphere to raise the atmospheric concentration of carbon dioxide by 20
percent. As Figure 2 shows, the concentration has increased 8 percent since
1958 (Keeling, Bacastow, and Whorf 1982). Recently, the concentrations of
methane, nitrous oxide, chlorofluorocarbons, and some other trace gases that
also absorb infrared radiation have also been increasing (Lacis et al. 1981).
Ramanthan et al. (1985) estimate that these gases will warm the earth as much
as CCL alone.
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FIGURE 2
CONCENTRATION OF SELECTED GREENHOUSE GASES OVER TIME
§ 340
5 «°
e
e
« 32S
O"
« »»
1
Carbon Dioxide
Concentrations
l»70 l»'5 l»« IMS
308
O. 3°2
Q.
i 300
298
2. Nitrous Oxide
Concentrations «
300
29«
299
1877 1878
1878
1880
200 -
I 1 1 1 i
3.CFC11 Concentrations
360
S.32Q
Q.
$280
»-
z
240
200
- 4. CFC12 Concentrations
rfJl'lH
1976 1977 1978 1979 I960 1981
• . ' i ' i ' i ' i ' i ' i ' i ' i ' r
5. Methane Concentrations
3000 2000
1000 700 500 300 20O
Timt (yrj 8 P )
1. C.D. Keeling, R.B. Bacastow, and T.P. Whorf, 1982. "Measurements of the Concentration of
Carbon Dioxide at Mauna Loa, Hawaii." In Carbon Dioxide Review 1982, edited by W. Clark.
New York: Oxford University Press, 377-382. Unpublished data from NOAA after 1981.
2. R.F. Weiss, 1981. "The Temporal and Spatial Distribution of Tropospheric Nitrous Oxide."
Journal of Geophysical Research, 86(C8):7185-7195.
3. D.M. Cunnold et al., 1983. "The Atmospheric Lifetime Experiment. 3. Lifetime Methodology
and Application to Three years of CFCL3 Data." Journal of Geophysical Research, 88(C13):8379-8400.
4. D.M. Cunnold et al., 1983. "The Atmospheric Lifetime Experiment. 4. Results for CF2CL2 Based
on Three Years' Data." Journal of Geophysical Research, 88(C13):8401-8414.
5. R.A. Rasmussen and M.A.K. Khalil, 1984. "Atmospheric Methane in the Recent and Ancient Atmos-
pheres: Concentrations, Trends, and Interhemispheric Gradient." Journal of Geophysical
Research, 89(D7):11599-11605.
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Although there is no doubt that the concentration of greenhouse gases is
increasing, the future rate of that increase is uncertain. A recent report by
the National Academy of Sciences (NAS) examined numerous uncertainties
regarding future energy use patterns, economic growth, and the extent to which
CO emissions remain in the atmosphere (Nordhaus and Yohe 1983). The
Academy estimated a 98 percent probability that CO concentrations will be
at least 450 parts per million (1.5 times the year-1900 level) by 2050 and a
55 percent chance that the concentration will be 550 parts per million. The
Academy estimated that the probability of a doubling of CO concentrations
by 2100 is 75 percent. Other investigators have estimated that a doubling is
likely by 2050 (Wuebbles, MacCracken, and Luther 1984).
If the impact of the trace gases continues to be equal to the impact of
CO , the NAS analysis implies that the "effective doubling" of all
greenhouse gases has a 98 percent chance of occurring by 2050.l An
international conference of scientists recently estimated that an effective
doubling by 2030 is likely (UNEP 1985). However, uncertainties regarding the
emissions of trace gases are greater than those for C0_. Although the
sources of chlorofluorocarbon emissions are well documented, regulatory
uncertainties related to their possible impact on stratospheric ozone
depletion make their growth rate — currently about 5 percent--difficult to
forecast. The current sources of methane, nitrous oxide, and other trace
gases have not yet been fully catalogued.
1 Studies on the greenhouse effect generally discuss the impacts of a
carbon dioxide doubling. By "effective doubling of all greenhouse gases" we
refer to any combination of increases in the concentration of the various
gases that causes a warming equal to the warming of a doubling of carbon
dioxide alone. If the other gases contribute as much warming as carbon
dioxide, the effective doubling would occur when carbon dioxide concentrations
have reached 450 ppm, 1.5 times the year-1900 level.
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Considerable uncertainty also exists regarding the impact of a doubling of
greenhouse gases. Physicists and climatologists generally agree that a
doubling would directly raise the earth's average temperature by about 1°C if
nothing else changed. However, if the earth warmed,, many other aspects of
climate would be likely to change, probably amplifying the direct effect of
the greenhouse gases. These indirect impacts are known as "climatic
feedbacks."
Figure 3 shows estimates by Hansen et al. (1984) of the most important
known feedbacks. A warmer atmosphere would retain more water vapor, which is
also a greenhouse gas, warming the earth more. Snow and floating ice would
melt, decreasing the amount of sunlight reflected to space, causing additional
warming. Although the estimates of other researchers differ slightly from
those of Hansen et al., climatologists agree that these two feedbacks would
increase the expected global warming. However, the impact of clouds is far
less certain. Although recent investigations have estimated that changes in
cloud height and cloud cover would add to the warming, the possibility that
changes in cloud cover would offset part of the warming cannot be ruled out.
After evaluating the evidence, two panels of the National Academy of Sciences
concluded that the eventual warming from a doubling of greenhouse gases would
be between 1.5° and 4.5°C (3°-8°F).
A global warming of a few degrees could be expected to raise sea level in
the future, as it has in the past. The best understood mechanism is the
warming and resulting expansion of sea water, which could raise sea level
one-half meter in the next century (Hoffman, Keyes, and Titus 1983). Mountain
glaciers could melt and release enough water to raise sea level ten to thirty
centimeters (four to twelve inches) (Meier 1984). Revelle (1983) estimates
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FIGURE 3
ESTIMATED GLOBAL WARMING DUE TO A DOUBLING
OF GREENHOUSE GASES: DIRECT EFFECTS AND
CLIMATIC FEEDBACKS
O
o
>*f
03
D
•**
CO
i_
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that Greenland's glacier meltwater will raise the sea another twelve
centimeters in the next century, while Bindschadler (1985) estimates the
impact to be between ten and twenty-five centimeters. Antarctica could
contribute to sea level rise either by meltwater running off or by glaciers
sliding into the oceans. Although no one has estimated the impact of melting,
Thomas (1985) estimates that Antarctic ice discharges are likely to add
another twenty-four centimeters to worldwide sea level.
In 1983, two independent reports estimated future sea level rise. In the
National Academy of Sciences report Changing Climate, Revelle (1983)
estimated that the combined impacts of thermal expansion, the Greenland
icesheet, and mountain glaciers could raise sea level seventy centimeters (two
and one-third feet) in the next century. Although he also stated that
Antarctica could contribute two meters per century to sea level starting
around 2050, Revelle did not add this contribution to his estimate.
In a report by the Environmental Protection Agency entitled Projecting
Future Sea Level Rise, Hoffman, Keyes, and Titus (1983) stated that the
uncertainties regarding the factors that could influence sea level are so
numerous that a single estimate of future sea level rise is not practical.
Instead, they consulted the literature to specify high, medium, and low
estimates for all the major uncertainties, including fossil fuel use; the
absorption of carbon dioxide through natural processes; future emissions of
trace gases; the global warming that would result from a doubling of
greenhouse gases (the NAS estimate of 1.5°-4.5°C); the diffusion of heat into
the oceans; and the impact of ice and snow. They estimated that if all of the
low assumptions prove to be correct, the sea will rise 13 cm (5 in) by 2025
and 38 cm (15 in) by 2075 over the 1980 level. If all of the high assumptions
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are correct, the sea will rise 55 cm (2 ft) by 2025 and 211 cm (7 ft) by
2075. However, because it is very unlikely that either all the high or all
the low assumptions will prove to be correct, the authors concluded that the
rise in sea level is likely to be between two mid-range scenarios of 26 to 39
cm (11 to 15 in) by 2025 and 91 to 136 cm (3 to 4-1/2 ft) by 2075. Figure 4
and Table 1 illustrate the EPA and NAS estimates. Although neither of these
studies examined options to limit the rise in sea level by curtailing
emissions, Seidel and Keyes (1983) estimated that even a ban on coal, shale
oil, and synfuels would only delay the rise in sea level expected through 2050
by twelve years.2
Recent analysis by the Polar Research Board of the National Academy of
Sciences indicates that glaciers in Greenland and East Antarctica, as well as
those in West Antarctica, could eventually release enough ice into the oceans
to raise sea level two or three centimeters (about one inch) per year (Meier
et al. 1985). However, current thinking holds that such a rapid rise is at
least one hundred years away. Moreover, a complete disintegration of the West
Antarctic Ice Sheet--and the resulting six-meter (twenty-foot) rise in sea
level--would take several centuries (Bentley 1983; Hughes 1983). It is
possible that snowfall accumulation could partially offset the effect of
melting ice.
The East Coast of the United States is slowly sinking (Hoffman, Keyes, and
Titus 1983). Thus relative sea level rise along the Delaware estuary will be
twenty-five to thirty centimeters (ten to twelve inches) per century greater
than global sea level rise, as shown in Table 1.
2 Computer printout underlying calculations from Seidel and Keyes 1983.
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FIGURE 4
GLOBAL SEA LEVEL RISE SCENARIOS:
LOW, MID-RANGE LOW, MID-RANGE HIGH, AND HIGH
Inch**
100
LEVEL
RISE
50-
25
0-
CMS
250-
200_
1 SO
100
50-
HIGH
MID-RANGE
HIQH
MID-RANGE
LOW
• MAS
LOW
2075
Sources: HOFFMAN, J.S., D. KEYES, and J.G. TITUS, 1983. Projecting Future
Sea Level Rise. U.S. GPO #055-000-0236-3. Washington, D.C.:
Government Printing Office; REVELLE, R., 1983. Probable Future
Changes in Sea Level Resulting From Increased Atmospheric Carbon
Dioxide. In Changing Climate. Washington, B.C.: National Academy
Press (Revelle estimate does not include Antarctica).
-------
-14-
TABLE 1
SCENARIOS OF GLOBAL SEA LEVEL RISE: 1980-2100
(centimeters)
Current Trends
2000
2.0-3.0
2025
4.5-6.8
2050
7.0-10.5
2075
9.5-14.3
2080
10-15
2100
12.0-18.0
EPA Scenarios
High 17.1 54.9 116.7 211.5
Mid-range high 13.2 39.3 78.9 136.8
Mid-range low 8.8 26.2 52.6 91.2
Low 4.8 13.0 23.0 38.0
NAS Estimate-
70.0
345.0
216.6
144.4
56.2
SCENARIOS OF SEA LEVEL RISE AT LEWES, DELAWARE: 1965-2100
(centimeters)
Current Trends
2000
13
2025
22
2050
31
2075
41
2080
43
2100
50
EPA Scenarios
High 27.7 71.7 140.0 290.8
Mid-range high 23.7 56.1 102.0 166.1
Mid-range low 19.3 43.0 76.7 120.5
Low 15.3 29.8 46.0 67.3
NAS Estimate*
100.6
380.6
251.5
180.0
91.8
* Excluding Antarctic contribution.
Sources: HOFFMAN7, J.S., D. KEYES, and J.G. TITUS, 1983. Projecting Future
Sea Level Rise. U.S. GPO #055-000-0236-3. Washington, D.C.:
Government Printing Office; REVELLE, R., 1983. Probable Future
Changes in Sea Level Resulting From Increased Atmospheric Carbon
Dioxide. In Changing Climate. Washington, D.C.: National Academy
Press (does not include Antarctica); HICKS, S.D., H.A. DeBAUGH, and
L.E. HICKMAN, 1983. Sea Level Variation for the United States
1855-1980. Rockville, MD: National Ocean Service.
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-15-
III. SALINITY IN THE DELAWARE ESTUARY
A rise in sea level of even thirty centimeters (one foot) would have major
impacts on coastal erosion, flooding, and saltwater intrusion. Until this
effort, no one had estimated the saltwater intrusion expected to result from
an accelerating rise in sea level due to the greenhouse effect. However,
previous EPA studies have examined the impacts of erosion and flooding, as
well as possible responses (Earth and Titus 1984). Ongoing EPA studies are
investigating the potential impacts on coastal sewerage systems, wetlands, and
seawalls.
The Delaware River Basin Commission (DRBC) has considered the implications
of recent sea level trends in its policy making since the late 1970s.
Accordingly, the DRBC already had the necessary model and data for assessing
accelerated sea level rise. This section provides background information on
the Delaware estuary, and presents estimates of saltwater intrusion likely to
result from sea level rise in the next century due to the expected global
warming.
Saltwater Intrusion
Salinity in an estuary ranges from that of sea water (at the mouth) to
that of fresh water (near the head of tide). The salinity at a particular
point varies over the course of a year, depending primarily on the amount of
fresh water flowing into the estuary. Mixing and advection caused by tidal
currents and wind can also change the salinity at a particular point.
In the Delaware estuary, tidal effects extend as far upstream as Trenton,
where the tidal range is more than twice that of the ocean boundary. Although
-------
-16-
the net flow of the estuary tends to carry salt water toward the ocean, tidal
currents carry salt water upstream, where it mixes with fresh water.
Differences in the densities of salt water and fresh water also contribute to
saltwater intrusion; heavy salt water on the bottom tends to move upstream
when adjacent to lighter fresh water, forming a wedge.
A rise in sea level generally results in increased salinity, assuming
other factors remain constant. In this respect the impact of sea level rise
is similar to the impact of reduced flows during a drought. The former
increases the saltwater force, whereas the latter decreases the freshwater
force. Salinity levels generally respond to changes in tide and river flow
within a matter of minutes or hours.
In the past eighteen thousand years, sea level has risen one hundred
meters (three hundred feet), converting freshwater rivers into brackish
estuaries (Bonn, Farrand, and Ewing 1962). Chesapeake Bay and the Delaware
estuary are examples of such drowned river valleys. The Delaware estuary is
probably the first estuary for which the salinity effects of future sea level
rise have been studied (Hull and Tortoriello 1979). 3 The salinity of this
estuary, as affected by the impacts of river diversion and flow regulation
projects, has been the subject of study—and litigation—since the early 1930s,
The Delaware Estuary
The Delaware River Basin covers an area of thirteen thousand square miles
in New York, Pennsylvania, New Jersey, and Delaware. It is located in the
heart of the megalopolis that stretches from Boston to Washington, D.C., on
3 Louisiana is also experiencing salinity increases from sea level rise
(Haydl 1984).
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-17-
the eastern seaboard of the United States. The Delaware River reaches from
the Catskill Mountains of southern New York to the head of Delaware Bay. The
river is tidal from Trenton, New Jersey, to the bay; the tidal river and bay
form the Delaware estuary, which is 215 kilometers (133 miles) long. The
boundary between the estuary and the ocean is a line between Cape May, New
Jersey, and Cape Henlopen, Delaware. Major cities on the estuary include
Trenton and Camden, New Jersey, Philadelphia, Pennsylvania, and Wilmington,
Delaware. The lower reach of the tidal river is physically connected with the
northern part of Chesapeake Bay by the Chesapeake and Delaware Canal, which
runs from Delaware City, Delaware, westward about twenty-seven kilometers
(seventeen miles) to the Elk River in Maryland. Figure 5 shows the general
location of the Delaware Basin; Figure 6 is a map of the estuary.
The Delaware estuary is one of the most extensively used tidal waterways
in the world. From the ocean, past Philadelphia and almost to Trenton, the
estuary has a navigable depth of at least twelve meters (forty feet) and is a
major port for ships of all nations. Sport and commercial fishing are
important uses of Delaware Bay, where oysters are the major shellfish
harvested. Many industries along the banks of the estuary use fresh or
brackish water for cooling and other processes. The estuary also serves the
region by assimilating or transporting to the sea the residual wastes
discharged from its tributaries as well as from about one hundred municipal
and industrial wastewater treatment plants located along the estuary.
Based on data published by the U.S. Geological Survey (Bauersfeld et al.
1985), we estimate that the average flow of fresh water into the Delaware
estuary from its tributaries is 609 cubic meters per second (21,500 cubic feet
per second). The nontidal Delaware River, which drains about half the
-------
-18-
FIGURE 5
GENERAL LOCATION OF THE DELAWARE BASIN
NEW
JERSEY
LAWARE
90 Miles
-------
-19-
FIGURE 6
THE DELAWARE ESTUARY
PENNSYLVANIA
rlv«r mil* 120/
Torresdale
rlv«r mil* 100
Philadelphia
Burlington
Chester
Marcus Hook
/ Wilmington
Reedy Island
DELAWARE
N
A
Trenton
Atlantic Ocean
Cape Henlopen
-------
-20-
basin, has an average flow rate of 332 cubic meters per second. The
Schuylkill River drains about 15 percent of the Basin and conveys an average
flow of 84 cubic meters per second. The Christina, which drains 5 percent of
the Basin, has an average flow of 24 cubic meters per second. Smaller
tributaries provide most of the remaining freshwater input, with smaller
contributions from aquifers and direct rainfall onto the estuary.
The waters of the tidal river at Philadelphia and northward are normally
fresh, and several municipalities, including Philadelphia, obtain portions of
their public water supplies directly from this part of the river. Other
cities take ground water from aquifers that are recharged in part by the tidal
portion of the river.
The many consumptive uses of water throughout the Delaware Basin reduce
the flow of fresh water into the estuary. Basin-wide withdrawal of fresh
water is estimated at 351 cubic meters per second (8 billion gallons per day),
of which 24.9 cubic meters per second (568 mgd) is used consumptively (i.e.
evaporated or otherwise removed from the Basin instead of draining back into
the estuary). Community water systems withdraw approximately 51.7 cubic
meters per second (1,180 mgd), of which approximately 10 percent is consumed.
The average daily per capita water use in the Basin is 0.617 cubic meters per
day (163 gpd), compared with the mean rate of 0.606 cubic meters per day (160
gpd) for the United States (Seidel 1985). In addition, diversion of Delaware
River water to New York and northeastern New Jersey are authorized up to 35
and 4.4 cubic meters per second (800 and 100 mgd), respectively (Supreme Court
1954). Basin-wide consumption is projected to rise to 52.2 cubic meters per
second (1,191 mgd) by the year 2000 (DRBC 1981).
-------
-21-
Saltwater Intrusion and the DRBC
The water resources of the Delaware River Basin are under the regulatory
control of the Delaware River Basin Commission (DRBC), a regional
federal-interstate compact agency established in 1961 to represent the federal
government and the states that share the Basin. The five commission members
are the U.S. Secretary of the Interior and the Governors of Delaware, New
Jersey, New York, and Pennsylvania. The DRBC cooperates with state and
federal agencies to ensure that the water resources of the Basin are protected
and developed to meet the growing demands for all reasonable uses.
One of the most important responsibilities of the DRBC is to monitor and
control salinity in the estuary. Excessive concentrations of ocean salts at
water intakes would create public health risks, increase the cost of water
treatment, and damage plumbing and machinery. High salinity could also upset
the ecology of the estuary.
The DRBC tracks the levels of both sodium and chloride ions in the
estuary. To protect public health, the Commission attempts to control
salinity so that sodium levels of potable supplies do not exceed 50 milligrams
per liter, based in part on New Jersey's 50-mg/l drinking water standard. For
a variety of purposes, the DRBC also tracks the 250-mg/l isochlor (the line
across the estuary where chloride concentrations equal 250 mg/1). Although
this isochlor represents more than detectable levels of sea salts, it is
commonly known as the "salt front." This level also represents the EPA
drinking water standard for chlorides and the concentration at which water
tastes salty to many people.
DRBC seeks to attain its salinity goals by keeping the chloride and sodium
concentrations at river mile 98 below 180 mg/1 and 100 mg/1, respectively.
-------
-22-
These limits were designed primarily to protect the public groundwater
supplies pumped from aquifers upstream of mile 98, which have a good hydraulic
connection with the estuary. Over one-half of the water entering these
aquifers is supplied by the estuary. DRBC has estimated that as long as the
river mile 98 objective is met, sodium levels in most wells tapping the
aquifers will remain below 50 mg/1. Moreover, the Philadelphia water intake
at Torresdale (river mile 110.4) will be supplied with water with sodium
concentrations less than 30 mg/1.
Because the flow of fresh water opposes salt water migrating upstream, the
highest salinities in the estuary occur during droughts. Thus, the DRBC keeps
salinity from reaching unacceptable levels both by limiting consumptive uses
of water and by releasing water from various reservoirs during periods of low
streamflow.
When reservoir releases are needed for salinity control in the estuary,
the DRBC directs the U.S. Army Corps of Engineers to release water from
DRBC-financed impoundments operated by the Corps. Table 2 lists reservoirs on
which the DRBC currently relies, and scheduled increases in reservoir
capacity. The Corps has constructed two multipurpose impoundments in the
Basin: Beltsville Reservoir on a tributary of the Lehigh River, and Blue
Marsh Reservoir on a tributary of the Schuylkill River. Two reservoirs
originally designed for flood control (Francis E. Walter Reservoir on the
Lehigh River and Prompton Reservoir on the Lackawaxen River) have also been
operated for salinity control during drought emergencies. The U.S. Congress
has authorized modifications of these facilities for water storage purposes;
the DRBC plans to fund these modifications. Augmentation of low flows is also
provided by many small reservoirs that are not listed in Table 2. Although
-------
-23-
these other reservoirs were designed for local community water supplies, they
sometimes augment freshwater flows into the estuary, incidentally, during
critical low-flow periods.
Efforts to decrease consumptive uses of water require the Commission to
address both the diversion of water to other basins and consumptive uses of
water in the Delaware River Basin. The City of New York diverts fresh water
from the upper part of the Basin, as authorized by the U.S. Supreme Court in
1954. The court decreed that the City release water from its reservoirs
during low-flow periods to compensate downstream interests for the water that
is diverted or stored at other times. The current (1986) Basin Comprehensive
Plan incorporates an agreement among the parties to the 1954 decree--New York
City and the Four Basin States — calling for special drought operation of the
City's Delaware River Basin reservoirs to meet downstream needs for salinity
control while conserving and storing water against the possibility of an
extended drought. Water is also diverted through the Delaware and Raritan
Canal to northeastern New Jersey, with similar provisions to curtail diversion
during droughts.
Some of the most important consumptive users of water in the Basin are
steam-electric power plants. Because scheduled publicly owned reservoir
capacity in the Basin will not be sufficient to meet increased consumption of
water projected to the year 2000 (the DRBC's current planning horizon), the
DRBC has required these utilities to develop storage capacity to provide
freshwater flows into the estuary to offset their consumption.
The most severe drought of record in the Delaware River Basin was that of
the 1960s. For a four-month period the average flow at Trenton was only one
quarter the long-term average flow, and during the worst month the flow was
-------
TABLE 2
EXISTING AND SCHEDULED RESERVOIRS THAT CAN CONTROL
SALINITY IN THE DELAWARE ESTUARY a/
Name of Facility or Project
( 1)
Ex i st i ng
Cannonsville Reservoir
Pepacton Reservoir
Lake Wai lenpaupack e_/
Mongaup River System e/
Neve rs ink Reservoir
Francis E. Walter Reservoir f/
Beltzville Reservoir
Nockamixon Reservoir
Blue Marsh Reservoir
Scheduled
Cannonsville Reservoir
Mod if icat ion h/
Prompton Reservoir
Mod i f icat ion J_/
Merri 1 1 Creek
Reserve! r j/
Francis E. Wa 1 ter h/
Reservoir Modification
87
Trexler Reservoir J./
Location of dam, stream
( River-mi le q/)
(2)
W. Br. Delaware River
(330.71 - 18.0)
E. Br. Delaware River
(330.7 - 33.3)
.
Wa I lenpaupack ureek D/
(277.7 - 16.4 - 1.4)
M o n C| 3 u p Rivsr
(261 . 1 - 12.0)
Neve rs ink River
(253.64 - 41 .9)
Lehigh River
( 183.66 - 77.8)
Pohopoco Creek c/
( 183.66 -41.5- 5.2)
Tohickon Creek
( 157.0 - 1 1 .0)
Tulpehocken Creek d/
(92.47 - 76.8 - 6.5)
West Branch,
De 1 awa re R i ve r
(330.7 - 18.0)
West Branch
Lackawaxen River
(277.7 - 27.1 - 4.9)
Merri I I Creek
(177.4 - 7.8 - 3.8)
Lehigh River
(183.7 - 77.8)
Jordan Creek
(183.7 - 36.3 - 4.6)
Lonq-Term Ac
I
Cubic Meters
(3)
6
373 x 10
6
560 x 10
0 /
e/
0 /
e/
6
I35 x 10
0 f/
6
49 x 10
6
49 x 10
6
18 x 10
6
49 x 10
6
38 x 10
6
57 x 10
6
86 x 10
6
49 x 10
tive Storage (
Bill ions
of Ga I I ons
(4)
98
I 48
...... 0 /
G/
„ /
H/
36
0 f/
13
13
4.8
13
10
15
23
13
Capacity a/
Ac re- Feet
(5)
302,000
454,000
_______ 0 /
H/
_______ QI
109,200
0 f /
39,830
39,815
14,620
39,800
30,900
46,000
70,000
40,000
-------
TABLE 2 (continued)
FOOTNOTES
a/ These reservoirs are operated or designed to serve multiple purposes, including salinity
control, or will tend to provide salinity control as an incidental benefit when operated for other
purposes. Only part of the storage capacity in these impoundments will be available for salinity
control. The operating plan for these reservoirs is based in part on offsetting the projected
rise in sea level to the year 2000 based on the recent trend, but does not take into account any
future acceleration of that trend related to the greenhouse effect.
b/ Tributary of the Lackawaxen River.
c/ Tributary of the Lehigh River.
d/ Tributary of the Schuylkill River.
e/ Privately owned hydroelectric power system. Storage available for salinity control only
after DRBC declares a drougtit emergency.
£/ Federal flood-control reservoir. Storage capacity available for salinity control only
after DRBC declares a drought emergency.
g/ Statute miles, measured along the axis of Delaware Bay and River from the mouth of the
bay. Second and third mileages indicate distances above the mouths of tributaries.
h/ Scheduled for completion in 1990; designed primarily to maintain conservation releases in
upper Delaware River, with secondary use to support downstream flow objectives and diversions to
New York City within limits of U.S. Supreme Court decree of I95U.
±/ Scheduled for completion in 1995.
j/ Construction began in 1985. Planned completion date is 1988.
i
N>
-------
-26-
only 13 percent of the average. In late 1964, the salt front advanced up the
estuary as far as river mile 102, just above the Benjamin Franklin Bridge in
Philadelphia. (The salt front's average location is near river mile 69.) The
drought continued through 1966. Because of the threat to water systems
depending on the estuary, the DRBC declared an emergency, as authorized by the
Delaware River Basin Compact (DRBC 1981). Under its emergency powers, the
DRBC regulated the river flows to control salinity and conserve water. The
emergency was in effect for many months. Several impoundments in the Basin in
1965 made it possible for the DRBC to call for water releases at strategic
times to control salinity in the estuary, thereby preventing major harm to
water users that draw upon the estuary for their supplies. However,
significant economic damages associated with the higher salinities were
reported by some water users. Some industries in the reach below Philadelphia
were forced to switch temporarily to a municipal system that imports water
from the Susquehanna River Basin. Shellfish production was subject to
abnormal stresses related to the high salinities.
The DRBC uses a mathematical model to study salinity changes. The
Delaware estuary salinity model, developed for the DRBC by Thatcher and
Harleman (1978),* relates freshwater inflows, tides, and ocean salinities to
chloride distribution in the estuary. (Technical details of the model are
* The model is a deterministic, one-dimensional time-varying model that
simulates saltwater intrusion in the tidal system extending from the head of
tide at Trenton to the Atlantic Ocean. A one-dimensional model was developed
because the Delaware estuary is well mixed vertically, especially in the tidal
river above Delaware Bay, and even the bay is vertically homogeneous during
low-flow periods when salinity intrusion is likely to be a problem. The well-
mixed character of this estuary is related to strong tidal currents and
shallow average depth. The normal range of tides at the mouth of the bay
varies from 3.95 feet in December to 4.3 feet in August. At the head of the
tidal river at Trenton, the tidal range varies from 7.6 feet in January to
8.55 feet in June.
-------
-27-
explained in Appendix A.) The dynamic nature of the estuary is reflected in
the model outputs, which can show the average chloride concentration every few
minutes at one hundred equally spaced cross sections along the 214-kilometer
(133-mile) estuary. Typical simulations have produced maximum and minimum
chlorinity readings at key locations for each tidal cycle over a period of a
year or more (Thatcher and Harleman 1981). These simulations have been used
to analyze the effects of various methods of reservoir operation on year-round
salinity distribution in the estuary. The model has also been used to predict
the effects of rising sea level on estuarine salinity, as discussed next.
The salinity distribution of an estuary affects sedimentation and
shoaling. Thus, changes in salinity could change the geometry of the
estuary. Although maintenance dredging for navigation would tend to maintain
the present dimensions of the main channel in the tidal Delaware River and
Bay, changes in salinity-related sedimentation and shoaling outside the
channel accompanying a very large rise in sea level might alter the geometry
and thus the dispersion characteristics of the estuary. In modeling the
changes in sea level and salinity intrusion, we have not attempted to take
into account possible changes in shoaling characteristics. This is not a
serious modeling flaw for a rise less than one meter. Additional research in
this aspect of the problem would be useful for more accurate projections of
the impact of a large rise in sea level.
The DRBC (1983a) uses the 1961-1966 drought as the basis for planning a
dependable water supply. Thus, for assessing most salinity problems, the
model is calibrated for the drought conditions of 1965, the driest year of
record in the Delaware Basin. The model is adjusted to reflect post-1965
changes in reservoir capacity, depletive uses of water, and sea level.
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-28-
Estimating Impacts of Sea Level Rise on Salinity
Current Sea Level Trends. Although worldwide sea level has been rising 1
to 1.5 millimeters per year (4 to 6 inches per century), the measured rise
along the east coast has been greater, because of local subsidence. Hicks
(1978) reported an average rise of 3.7 millimeters per year at Lewes,
Delaware, for the period 1921 through 1975. Hicks, DeBaugh, and Hickman
(1983) report a rise of 2.6 millimeters per year at Philadelphia, as shown in
Figure 7.
The DRBC was the first government agency to investigate the potential
effects of recent sea level trends on salinity in a particular estuary (Hull
and Tortoriello 1979). In 1979, the current DRBC planning horizon was the
year 2000, and the DRBC wished to know what estuarine salinity changes would
result from the projected change in sea level from 1965 to 2000. Considering
only historical trends, not accelerated sea level rise from the greenhouse
effect, Hull and Tortoriello (1979) estimated a 35-year rise of 13 centimeters
(0.42 feet), and analyzed this rise with the Delaware estuary salinity model.
The model was first exercised for 1964-1965 drought conditions, including
observed sea level, but with flow of the Delaware River at Trenton regulated
by reservoirs to maintain an average flow of three thousand cubic feet per
second for the low-flow season. A fifteen-month period (1 October 1964
through 31 December 1965) was simulated. The minimum, mean, and maximum
chlorinities for each tidal cycle, as well as the running sixty-day averages,
were simulated over the fifteen-month period. These data were produced for
locations spaced along the axis of the estuary, with spacing close enough to
allow easy interpolation between locations.
-------
-29-
FIGURE 7
CHANGE IN YEARLY MEAN SEA LEVEL FROM 1923
TO 1980 AT PHILADELPHIA
24 r
18
(0
S
ULJ
I
O
!
12
6
I
I
I
I
I
I
1930
1940
1950
YEAR
1960
1970
1980
Source: HICKS, S.D., H.A. DeBAUGH, and L.E. HICKMAN, 1983. Sea Level
Variation for the United States 1855-1980. Rockville, MD: National
Ocean Service.
-------
-30-
Next, a model simulation was carried out for year-2000 conditions,
assuming a recurrence of the 1964-1965 drought flows but with sea level
adjusted upward by 13 centimeters (0.42 feet) to reflect the projected sea
level rise. Other model inputs were held at the values used for 1965.
The maximum sixty-day average chlorinities for 1965 and 2000 were compared
to show the effect of the thirty-five-year sea level rise. Figure 8 shows the
increase in the maximum sixty-day average chlorinities as a function of river
miles. The chlorinity increase due to the simulated sea level rise was most
pronounced at river mile 60, where the sixty-day average increased by about
210 mg/1. The average position of the salt front moved two to four kilometers
(one to two miles) upstream. The salinity impact of the projected sea level
change decreased with distance seaward and landward of river mile 60, with no
measureable effect above mile 120.
Using a series of year-2000 simulations with various degrees of streamflow
regulation, Hull and Tortoriello (1979) found that the salinity increase
caused by the projected thirty-five-year rise in sea level could be offset by
a level of year-round river-flow regulation that augmented the summer flow by
150 cfs. This augmentation could be provided by a moderately sized reservoir
(about fifty-seven million cubic meters, or forty-six thousand acre-feet) in
the Delaware Basin. These findings have been used in the formulation of plans
for water resources development for the Basin (DRBC 1981).
Accelerated sea level rise. Because of limited resources, we investigated
only two scenarios of accelerated sea level rise. Because the magnitude of
the future rise is uncertain, a conservative approach is to pick a wide range
so that our results are most likely to encompass the actual situation. We
finally settled on 73- and 250-centimeter (2.4- and 8.2-foot) rises over 1965
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-------
-32-
levels at Lewes, Delaware. (For drought conditions the DRBC Salinity Model
requires inputs relative to 1965 sea level, which was 6 centimeters lower than
1980 sea level.)
We hope that the reader will not attribute excessive significance to these
scenarios. Nevertheless, it is useful to understand when a 73- or 250-
centimeter rise is likely to take place. Because relative sea level at Lewes
is rising about 2.5 millimeters per year more rapidly than the global average,
these estimates do not correspond directly to published estimates of worldwide
sea level rise. The 73-centimeter scenario is consistent with the National
Academy of Sciences estimate for 2050, while the 250-centimeter case is
consistent with the NAS projection for 2125.5 The 73-centimeter scenario is
also consistent with the EPA's mid-range low estimate for 2050, as well as
EPA's high estimate for 2025. The 250-centimeter scenario is consistent with
the EPA mid-range high estimate for 2100 and the EPA high estimate for 2075.
Although our understanding of future sea level rise is incomplete, the
73-centimeter scenario appears to be a more realistic near-term possibility
than the 250-centimeter scenario. Nevertheless, when considering responses to
sea level rise in the next fifty to seventy-five years, one should not
completely ignore the rise that may occur in subsequent years.
The earlier DRBC simulations (Hull and Tortoriello 1979) involved only a
relatively minor change in mean sea level, 13 centimeters (0.42 feet), which
did not require any modification of the salinity model. However, in the study
reported here, it was necessary to consider changes in the geometry of the
estuary itself, as well as in the mathematical representation (model) of the
5 Revelle estimates 70 centimeters per century due to factors other than
Antarctica and 2 meters per century after 2050 due to Antarctic deglaciation.
-------
-33-
estuary. For sea level increases of 60 centimeters (2 feet) and more, not
only would the depth of the estuary increase, but the width would also
increase. The techniques used in these model-geometry modifications are
described in Appendix B.
Table 3 and Figure 9 compare the maximum thirty-day average chloride
levels at different river miles for a recurrence of the 1964-65 drought at the
1965 sea level and rises of 73 and 250 centimeters over that level. We
estimated that a 73-centimeter rise would increase the maximum thirty-day
chlorinity at river mile 98 from approximately 135 mg/1 to 305 mg/1. The
thirty-day average location of the salt front would advance to mile 100,
compared with mile 93 for such a drought occurring in 1965. Although the salt
front would be well below Philadelphia's Torresdale intake on average, the
78-mg/l isochlor would be at river mile 109, just below the intake at mile
110.4. A 250-centimeter rise would bring the salt front up to river mile 117,
well above Torresdale.
Further analysis of the simulations of saltwater intrusion using the
modified geometry yielded statistical information for comparing the numbers of
tidal cycles during which chloride levels exceeded a particular value. Figure
10 presents these comparisons for river mile 110.4, Torresdale. This figure
shows the effects of post-1965 sea level rises of 73 and 250 centimeters in
terms of the percent of tidal cycles during which a given chloride
concentration would be exceeded by the maximum and minimum concentrations
calculated for every tidal cycle (total of 705 cycles in the simulation
period). For example, a sea level rise of 250 centimeters would cause the
78-mg/l chloride value to be exceeded during more than 50 percent of the
cycles, while a 73-centimeter rise would result in exceedance of that
-------
-34-
TABLE 3
MAXIMUM 30-DAY CHLORIDE CONCENTRATION (MG/L)
AT DIFFERENT RIVER MILES FOR A RECURRENCE
OF THE 1960s' DROUGHT
1965 Sea Level
River Mile Base Case 73-Centimeter Rise 250-Centimeter Rise
61.9 4,660 5,357 7,726
64.6 4,010 4,718 7,127
67.3 3,358 4,084 6,571
70.0 2,760 3,474 5,943
72.7 2,243 2,928 5,373
75.4 1,778 2,427 4,873
78.1 1,390 1,998 4,398
80.8 1,065 1,619 3,914
83.5 800 1,295 3,426
86.2 588 1,021 3,021
88.9 431 804 2,652
91.6 301 605 2,289
94.3 210 450 1,967
96.9 151 335 1,655
98.0 a/ 136 b/ 305 b/ 1,560 b/
99.6 114 262 1,421
102.3 82 193 1,211
105.0 57 135 1,001
107.7 38 89 789
110.4 26 56 608
113.1 19 35 437
115.8 15 23 301
118.5 14 17 196
121.2 13 14 110
123.9 13 13 55
&/ DRBC salinity-control point, where objective is to prevent 30-day
average chloride concentration from exceeding 180 mg/1.
b/ Interpolated value.
Note: Model inputs are based on observed tides during calendar year 1965.
-------
-35-
FIGURE 9
MAXIMUM 30-DAY CHLORINITY VS. RIVER MILE FOR A
RECURRENCE OF 1960s' DROUGHT:
(a) 1965 SEA LEVEL; (b) 73-CENTIMETER RISE;
(c) 250-CENTIMETER RISE
RIVER REACH
14
13
12
11
10 I 9
8
7
6
5
4
3
2
(MILE 98--DRBC SALINITY
CONTROL POINT.)
55 60 65 70
80
86 90 95
RIVER MILE
100 105 110 115 120 125 130
-------
-36-
chlorinity about 15 percent of the cycles. The base case (1965 sea level)
never showed chloride concentrations in excess of 78 mg/1; the maximum
calculated chlorinity at Torresdale was 62 mg/1. Similarly, the calculated
chloride concentration exceeded 250 mg/1 about 42 percent of the tidal cycles
for the 250-centimeter rise, but did not reach the 250-mg/l level for the
73-centimeter rise, which resulted in a maximum chlorinity of about 129 mg/1.
Implications
A rise in sea level of several feet would substantially exacerbate today's
salinity problems in the Delaware estuary. The upper estuary above the
Schuylkill River in Philadelphia, now a source of fresh water for both
municipalities and industries, would become too salty for most uses,
necessitating a switch to alternative supplies--at great expense.
Philadelphia's water supply intake at Torresdale, now in the freshwater reach
of the estuary, would be subject to occasional invasions of sea salts, which
would sometimes leave the water unacceptable for the City's many water
customers. Industries now using fresh water from the upper estuary would,
after the sea level rise, find brackish water at their intakes during dry
periods. Those industries now using brackish water from the middle and lower
reaches of the estuary would experience much higher salinities than those for
which their systems were designed, which would damage pipes, tanks, and
machinery and increase water-treatment costs. In some cases these industries
would have to shift permanently to alternative water supplies.
Oysters. In the upper, narrow reach of Delaware Bay are found natural
oyster beds, which are managed by the oyster industry with supervision by the
State of New Jersey to provide seed oysters for planting in leased growing
-------
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o.
L
;C\
i\>
•v \ *
* \
\
\
\
\.
^t
\
»
^
\
V
\
>A
•- <&
\
&
\
%
^
a
\
v% °
\
-------
-38-
areas in seaward, more saline areas of the bay. Because of their location in
less saline water, the natural seed-oyster beds provide havens for the young
oysters from some of their natural enemies that require higher salinities for
survival. Oyster biologists believe that increased salinities over the
natural beds at critical periods in the annual life cycle of oyster predators
and competitors would afford an advantage to these oyster enemies (Corps of
Engineers 1982). Although the highest salinities generally occur during
summer droughts, experts have expressed concern that the increases in
springtime bay salinities resulting from increased depletive use of fresh
water, or from storage of springtime runoff in reservoirs, would harm the
natural beds and deprive the bay's oyster industry of its seed-oyster source
(Raskin 1954; Gunter 1974).
Hull and Tortoriello (1979) presented evidence that for the historical
period of decline in oyster production in Delaware Bay, the observed gradual
rise in sea level was a more likely cause of increasing bay salinities than
depletive use or storage of fresh water. If the relatively small rise in sea
level--less than thirty centimeters (one foot)--during the period for which
observations are available could damage oyster beds significantly, the much
greater rise considered herein could severely threaten the bay's oyster
industry. The natural seed oyster beds near the head of Delaware Bay would
tend to shift up the estuary. Such a shift would reduce yields both because
the estuary is much narrower above the bay and because shifting upstream would
bring the oyster beds closer to upstream sources of pollution.
General ecological impacts. Potential impacts of increasing salinities
on other estuarine plants and animals have been matters of concern expressed
by ecologists (Corps of Engineers 1982). The magnitude of salinity increases
-------
-39-
found in the DRBC model simulations of postulated accelerated rises in sea
level would be expected to produce major changes in the ecology of the
Delaware estuary. There would be an up-estuary advance of marine and
estuarine species and a retreat of freshwater species. Some species now
thriving in the relatively clean waters of the lower estuary would migrate
into the more polluted areas of the upper estuary, closer to wastewater
outfalls and other hazards. Water craft using the now freshwater reaches of
the upper estuary would be subject to problems caused by marine fouling
organisms. These marine organisms would also infest water systems that take
water from the tidal river in reaches now free of this problem.
Although this report focuses on salinity, other environmental impacts of
rising sea level may be important and should be investigated. Higher water
levels could drown much of the approximately 830 square kilometers (320 square
miles) of wetlands along the estuary. These wetlands, which provide critical
habitats for many species of birds and fish, are partially protected from
current human interference by federal and state laws. Although these
ecosystems could migrate landward with rising sea level, such migration would
be inhibited if development just inland of the marsh is protected by
bulkheads, levees, and other structures; there are currently no environmental
programs to ensure that development and other human activities permit this
migration in the future (Titus, Henderson, and Teal 1984; Titus 1985). By
removing one of nature's cleansing mechanisms, a loss of wetlands could
increase pollution loadings in the estuary. Although long-term management of
the estuary will have to consider these impacts, they are beyond the scope of
this report.
-------
-40-
IV. IMPACT OF INCREASED RIVER SALINITY
ON NEW JERSEY AQUIFERS
Perhaps the most serious potential implication of increased river salinity
would be saltwater contamination of adjacent aquifers. Many water users in
the lower Delaware River Basin adjacent to the estuary depend on groundwater
supplies, which are recharged in part by the river. Some New Jersey wells
used for public water supply have already been shown to produce water with
high concentrations of sodium, which, according to the State Health
Department, represent a public health hazard (Braun and Florin 1963; Korch,
Ramaprasad, and Ziskin 1984). The increasing salinities in the Delaware
estuary that would accompany a large rise in sea level would severely
aggravate the existing saltwater intrusion problems of aquifers in the
Delaware Basin, primarily in New Jersey and Delaware. Some aquifers now
heavily used would probably become too salty for drinking water and would have
to be abandoned or limited to agricultural and industrial uses.
This section focuses on the impact of increased estuary salinity on the
Potomac-Raritan-Magothy aquifer system, which supplies much of the water used
in southern New Jersey. Although other aquifers are hydraulically connected
to the estuary, this aquifer is the only major system with a connection to the
part of the estuary likely to become salty as a result of future droughts or
sea level rise.
The Relationship Between Sea Level and Aquifer Salinity
The only portion of an aquifer likely to be salty is the part below sea
level. In coastal aquifers, a layer of fresh water floats on top of the
heavier salt water. The salt water generally forms an intrusion wedge such
-------
-41-
that the farther inland (the higher the water table), the farther below sea
level is the boundary between fresh and salt water, as shown in Figure 11.
According to the simplistic Ghyben-Herzberg relation, for aquifers where the
water table slopes toward the ocean, this boundary is forty meters below sea
level for every meter above sea level the freshwater level in the aquifer
lies. As sea level rises, the freshwater/saltwater boundary shifts inland and
upward, with a time lag depending on how far that boundary is from the coast.
Pumping wells cause water levels to fall below sea level, and if the
withdrawal rate is too high, the equilibrium saltwater line will move far
inland. The time lag is the major reason that many heavily pumped coastal
aquifers are not yet salty.
Many aquifers such as those in the Potomac-Raritan-Magothy aquifer system
release water into rivers in their natural state. If such an aquifer is
pumped so that groundwater levels fall below mean sea level, it will be
recharged by nearby rivers. As discussed in Section 3, estuary salinity could
respond to sea level rise or changes in precipitation quite rapidly. Thus,
should the river become salty even temporarily, salt could infiltrate into
such an aquifer and persist for a long time. The Potomac-Raritan-Magothy
aquifer system is both a coastal aquifer and an aquifer recharged by a river.
The Potomac-Raritan-Magothy Aquifer System
The Potomac-Raritan-Magothy aquifer system is the principal source of
water for the population and industrial centers in the coastal plain of
southern New Jersey (Luzier 1980). The aquifer extends along the coast from
North Carolina to Long Island. In New Jersey, the Potomac-Raritan-Magothy
lies directly on top of bedrock, is confined above by a relatively tight clay
-------
-42-
FIGURE 11
SALTWATER INTRUSION IN A COASTAL AQUIFER
Ground
Surface
Note: H = 40 x h
-------
-43-
layer, and has a poor hydraulic connection to other aquifers far offshore.
The Delaware River flows along the outcrop of the Potomac-Raritan-Magothy from
Trenton, New Jersey, to Wilmington, Delaware (Figure 12), and there is a good
hydraulic connection between the river and aquifer system, especially above
river mile 98 (Camp Dresser and McKee 1982).
Vowinkle and Foster (1981) calculated the inflow into the aquifer for the
river reaches shown in Figure 12 using a groundwater model developed by Luzier
(1980) for 1973 and 1978 groundwater levels. The data showed that the
greatest inflow occurs between river miles 101 and 106.5--adjacent to wells in
the vicinity of Camden City-where water levels are significantly below mean
sea level.
Even without a rise in sea level due to the greenhouse warming, saltwater
intrusion into the aquifer will worsen in the future. The existing saltwater
boundary to the south of Camden (Fig. 14) reflects a sea level that was
fifteen to thirty meters (fifty to one hundred feet) lower than the present
sea level, implying an ongoing adjustment to the one hundred meter rise that
has taken place over the last eighteen thousand years (Meisler, Leahy, and
Knobel 1984). As ground water is removed and the aquifer approaches
equilibrium with current sea level, the salt front will move farther inland
from the Atlantic Ocean.
Figure 13 illustrates the water levels in the Potomac-Raritan-Magothy
aquifer system based on 1973 field data. Figure 14 illustrates a prediction
that water levels will be more than 37 meters (120 feet) below mean sea level
in Camden County by the year 2000, if the rate of groundwater withdrawal
increases by 1.7 percent per year. As a result of deep saltwater movement
from offshore, the saltwater line in the aquifer will advance to the location
-------
-44-
FIGURE 72
Source;
«« ^sey: U^CeS^^^
-------
-45-
shown in Figure 14, far enough inland to render the Potomac-Raritan-Magothy
ground water in Atlantic, Cape May, and Cumberland Counties brackish or salty.
impact of a Drought on the Aquifers--Current and Future Sea Level
Salinity levels in the ground water are monitored at selected locations by
the United States Geological Survey and other agencies (see, for example,
Schaefer 1983). Low salinity levels are normally found in the Potomac-Raritan-
Magothy aquifer system adjacent to the Delaware River above river mile 98
because of freshwater inflows. However, when the salt front moves up the
estuary during droughts, the high-salinity recharge water from the Delaware
River increases salinity in the ground water, as shown in Figure 15.
Table 4 shows the maximum thirty-day average chloride concentrations at
the center of each reach for each of the three sea level scenarios for a
recurrence of the 1964-65 drought. Because the DRBC is primarily concerned
with protecting the Potomac-Raritan-Magothy aquifer system above river mile
98, we focus on reaches 1 through 8 (river mile 98 through 131).6
During the 1961-66 drought, the salt front moved up the Delaware estuary
and allowed salt water to recharge the Potomac-Raritan-Magothy aquifer
system. Increased salinity was observed in many wells adjacent to the
Delaware River (Figure 15). In Camden County wells, for example, chloride
concentrations increased 10 to 70 mg/1 from background levels (5 to 10 mg/1)
(Camp Dresser and McKee 1982). Elevated chloride levels persisted more than
6 There are practical limits to the control of salinity in the Delaware
estuary by reservoir regulation. Although it is recognized that some recharge
of aquifers by the estuary takes place seaward of mile 98--some as far down as
the Delaware Memorial Bridge—it is not practical to control salinity to
provide drinking-water quality at all points along the estuary where recharge
occurs. On the other hand, regulation at any point on the estuary, say at
mile 98, does provide some control of salinity throughout the estuary.
-------
-46-
F1GURE 13
POTENTIOMETRIC SURFACE OF THE POTOMAC-RARITAN-MAGOTHY
AQUIFER SYSTEM, NOVEMBER-DECEMBER 1973
Contour show* altitude of the
aquifer's water surface In feet
(1 foot » .305 meters).
Datum Is NQVD (sea level) of
1929. Contour Interval variable.
•n
Location and number of
observation wells.
MILES
KILOMETERS
New jersey
Pennsylvania
RM 110
Torresdale
RM 98
RM 69-
ATLANTIC OCEAN
Cone of Depression
at canter Exceeds
-SO Feet NQVD
Source: LUZIER, J.E., 1980. Digital-Simulation and Projection of Head
Changes in the Potomac-Raritan-Magothy Aquifer System, Coastal
Plain, New Jersey. U.S. Geological Survey Water Resources
Investigations, 80-11.
-------
-47-
FIGURE 14
POTENTIOMETRIC SURFACE OF THE POTOMAC-RARITAN-MAGOTHY
AQUIFER SYSTEM AT YEAR 2000 WITH
ANNUAL EXTRACTION GROWTH RATE OF 1.7 PERCENT
Contour shows altitude of the
aquifer's water surface In feet
(1 foot * .305 meters).
Datum Is NQVD (sea level) of
1929. Contour Interval 20 ft.
•n
Location and number of
observation wells.
--— 250 mg/L -»
Chloride concentration near
bottom of aquifer system
In milligrams per liter.
saiBMSVi 2SO mg/L •••••
Chloride concentration near
top of aquifer system In
milligrams per liter.
New Jersey
Pennsylvania
RM 110
Torresdale
RM 98
RM 69-
MILES
KILOMETERS
ATLANTIC OCEAN
Source:
LUZIER, J.E., 1980. Digital-Simulation and Projection of Head
Changes in the Potomac-Raritan-Magothy Aquifer System, Coastal
Plain, New Jersey. U.S. Geological Survey Water Resources
Investigations, 80-11.
-------
-48-
FIGURE 15
CHLORIDE CONCENTRATIONS AT WELLS IN THE CAMDEN CITY VICINITY
LEGEND
Oalalr 3 Wall Nama
R.M. 104.6 Rlvar Miles from
Mouth of
Dalawara Bay
t-%1 Indicates tha flva
b*ul yaara of drought
(1961-1966)
k
hlorlde Concentration (mg/llter)
-» r» w 4
o o o o <
| 1
•Camden C
•R.M. 97.5
•g=
•—— <
tV 7
y
i
g
/
f
/
.v.v
vlfc'I
;^^
/•'.<•'• X
".•'.•'•'.•
I'i'i'i
••]••>'
*\
\
\
\
Mor/la 1, 6
u7'^.A.--^- ^ •-
\|j**-Dalalr 2,^3. [ *
^Puchack 2, 3>4_- \
^Camden City 7, 11
y^X
/( ^-^•^-S
I j Dalawara Rlvar
LOCATION MAP
-Dalalr 2
-R.M. 104.6
0 '30 '40 '60 '60 '70 1980 '30 '40 '60 '60 '70 1980 '30 '40 '80 '60 '70 1980
40
-Puchack 2
-R.M. 104.7
30
20
10
10
30 '40 '60 '60 '70 1980 '30 '40 '60 '60 '70 1980 '30 '40 '60 '60 '70 1980
hlorlde Concentration (mg/llter)
10 u> t
o o o o e
* Puchack 4
- R.M. 104.
—••^M
— -"
vXv
•'•II-'
•il-l:
l^^*"H_
: '-'^^^
.•'.•'.:•
40r
-Morris I
-R.M. 106. 1
30
20
10
'30 '40 '60 '60 '70 1980 '30 '40 '60 '60 '70 1980 '30 '40 '50 '60 '70 1980
Source: CAMP DRESSER and McKEE, INC., 1982. Groundwater Management Plan for
Study Area 1: Coastal Plain Formations. Prepared for Delaware River
Basin Commission, West Trenton, New Jersey.
-------
TABLE 4
RECHARGE OF DELAWARE RIVER WATER INTO POTOMAC-RARITAN-MAGOTHY AQUIFER SYSTEM
AND MAXIMUM SALINITY LEVELS OF RECHARGE
River Reach
1
2
3
4
5
6
1
8
9
10
1 1
12
13
14
Length of
Reach
(Mi les)
8.0
6.2
5.0
2.5
2.7
2.9
2.8
2.8
2.8
4.5
2.5
6.5
5.4
5. U
River Mi le
123 - 131
117 - 123
1 12 - 1 17
109.5 - 1 12
106.5 - 109.5
104 - 106.5
101 - I0t
98 - 101
95.5 - 98
91 - 95.5
88.5 - 91
82 - 88.5
76.5 - 82
71 - 76.5
Est ima ted
Reach Recharge
to Aquifer
System
(cfs)**
2. 1
6.6
1 .3
4.0
23.0
19. 1
9.4
4.9
13. 1
1 1 .9
1 1 .8
1.4
5. 1
Percent of
Total River
Recharge to
Aquifer System
2. 1
6. 1
1 .3
3.5
20.0
16.6
8.2
4.2
\ 1 .14
10.4
10.5
1 .3
4.6
Maximum 30-Day Average
Chloride Concentration img/l)
No Rise*
-
-
-
-
50
50
100
100
150
250
350
600
1 100
2250
73-cm Rise*
50
50
100
200
250
350
450
700
1050
1650
2950
250-cm Rise*
100
200
350
500
650
900
1200
1450
1700
2000
2500
3050
3950
5400
* Post-1965 rise in sea level superimposed upon a recurrence of the 1964-65 drought flows.
** Total 1978 river recharge was approximately 43% of total water entering the aquifer system (Luzier 1980).
-------
-50-
ten years; once introduced into the aquifer salinity contamination tends to
remain (Camp Dresser and McKee 1982) .
From such observed data, aquifer salinity distributions can be generated.
Simulating the salinity distribution in the aquifer for the sea level
scenarios requires a predictive numerical model. However, a first-order
approximation can be deduced by considering (1) the estuary's salinity
distributions for selected sea level rise scenarios (see Figure 9); and (2)
the distribution of inflow into the aquifer (see Table 4).
Table 5 shows the penetration distances during the time that chloride
concentrations exceed 250 and 78 mg/1, respectively, for the fifteen-month
drought simulation. Although we simulated only fifteen months of the
five-year 1961-1966 drought, these fifteen months were the worst part of that
drought, with the lowest river flows and the highest estuarine salinities.
Therefore, the computed chloride concentrations of recharge water would be no
greater if we simulated the entire five-year drought. The estimates in Table
5 are based on groundwater velocities near the advancing edge of the saltwater
front, estimated for each river reach based on 1978 water levels from Walker
(1983) and aquifer properties affecting water velocities from Luzier (1980).
The inflow rate obtained by Vowinkel and Foster (1981) was divided by the
available cross-sectional area and porosity, providing an alternative method
of computing groundwater velocities. The velocity ranges were extended to
include both these estimates.
For the baseline scenario (recurrence of the 1964-65 drought flows with no
sea level rise), the thirty-day average 250-mg/l isochlor in the estuary
penetrates into reach 10 (river mile 91.0 to 95.5) with chloride concentrations
in the estuary in excess of 50 mg/1 extending up into reach 5 (river mile
-------
-51-
106.5 to 109.5). Although the 250-rag/l line would not penetrate to reach 8,
penetration distances of over ninety meters (three hundred feet) are predicted
for the 78-mg/l line in reaches 6, 7, and 8 (Table 5). If in subsequent years
the salinity in the recharge water decreased again to normal levels, the slug
of high-salinity water would continue to move toward the area of lower water
levels, that is, toward the center of the major cone of depression in Camden
County (see 1973 water levels in Figure 13). As this slug slowly moves,
however, the chloride concentration would decrease because of diffusion,
dilution by lower salinity recharge water (including precipitation), and
withdrawal from the aquifer. Nevertheless, levels in excess of the New Jersey
drinking water standard (50 mg/1 sodium, corresponding to 78 mg/1 chloride)
could occur for several years in areas within a mile or two of the river.
For the 73-centimeter sea level rise scenario, water with chloride
concentrations slightly in excess of 250 mg/1 (corresponding to a sodium
concentration of 145 mg/1) would begin to recharge the aquifer system in the
vicinity of reach 8 (river mile 98 to 101). The dilution and diffusion of the
salt water as it moves through the aquifer would undoubtedly reduce the
chloride concentration below 250 mg/1 within a very short distance of the
Delaware River. Above reach 8, the chloride concentrations are predicted to
be below 250 mg/1. Thus, like the baseline case, no significant region of the
aquifer adjacent to the Delaware River above river mile 98 should experience
sustained chloride concentrations above 250 mg/1. Sodium concentrations
greater than 50 mg/1 would be present in the recharge water as far as reach 4
and would penetrate several hundred feet in reaches 6, 7, and 8.
For the more severe 250-centimeter sea level rise scenario, a significant
zone (reach 3 and seaward) of the aquifer system would be recharged by water
-------
TABLE 5
VELOCITIES, CONTACT TIMES, AND PENETRATION DISTANCES FOR
GROUND WATER WITH ELEVATED CHLORIDE CONCENTRATIONS
R i ve r
Reach
1
2
3
4
5
6
7
8
9
10
1 1
12
13
14
Est imated
( 1978)
Ground-Water
Velocity Range
(meters/year)
0-90
0-90
90-150
30-60
90-180
270-580
460-550
240-370
120-150
180-400
300-370
90-150
0-30
60-90
Time that 30-Day Average
Chloride Concentration
Exceeded 78 mg/ 1 and
250 mg/l (days)
78 mq/l
BASE
-
-
-
60
1 10
140
160
170
180
190
200
230
250 mq/
73 cm BASE
30
100
130
150
160
170
180
190
200
210
250
100
120
140
160
180
73 cm
,
100
120
140
160
180
200
220
250 cm
120
130
140
150
160
170
180
190
200
400
400
400
Maximum Aquifer Penetration
Distance for Duration of
Contact Time
meters)
78 mq/l
BASE
-
-
-
-
-
95
165
140
65
185
180
80
15
55
73 cm
5
50
205
225
160
70
195
190
80
15
60
250 mq/
BASE
-
-
-
-
-
-
-
-
-
1 10
120
60
15
45
73 cm
-
-
-
-
-
-
-
100
50
150
160
75
15
55
250 cm
50
20
70
240
240
170
75
205
200
165
35
too
Ui
N3
I
Note: A chloride concentration of 78 mg/l corresponds to a sodium concentration of 50 mg/l, the New
Jersey drinking water standard.
-------
-53-
from the river with thirty-day chloride concentrations in excess of 250 mg/1.
The slug of high-salinity water would move significant distances before
dispersing to insignificant background levels.
In summary, a recurrence of the 1960s1 drought with a higher sea level
would cause increased sodium and chloride levels in parts of the
Potomac-Raritan-Magothy aquifer. These increased levels would persist for
long periods--probably several decades—as the high-chloride water dispersed
and propagated toward pumping wells. For many years, some wells would
experience elevated sodium levels that could make the water unfit for many
purposes, including human consumption, in which case, water from alternate
sources could be required.
Improved Estimates
Although we used the DRBC salinity model to estimate surfacewater impacts,
no similar model was available for assessing groundwater impacts without an
investment of resources exceeding what was available for this study. To more
adequately evaluate the impact of the estuary salinity distributions on the
groundwater system will require a solute transport and dispersion model, such
as the one presented by Konikow and Bredehoeft (1978). A significant field
investigation should be conducted, including an in-depth review of existing
field data. Because of the complex hydrogeology, a numerical model is
required. The model must contain such features as salinity concentration at
the boundaries, which can vary in time and space. Although a two-dimensional
representation may prove adequate, a three-dimensional model may be
necessary. During drought conditions, high-chloride water will recharge the
aquifer far up the estuary for a limited period of time. The output of a
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numerical model will allow tracking of the slug of high-chloride water as it
propagates and moves through the aquifer in the down-gradient direction.
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-55-
V. RESPONSES TO SALINITY INCREASES
In spite of the severity of projected salinity increases, the major
impacts are far enough in the future to be incorporated into planning by the
DRBC, state governments, and the private sector. The options fall primarily
into two categories: preventing increased salinity or adapting to it. This
section briefly discusses such options. A determination of the most
appropriate responses to be undertaken is outside the scope of this report.
Preventing Salinity Increases
Increasing river flow can offset salinity increases. The DRBC currently
maintains capacity to release fresh water from reservoirs and has regulatory
authority to decrease consumptive use of water during droughts.
Hull and Tortoriello (1979) determined that the thirteen-centimeter
(five-inch) rise in sea level expected for the period 1965-2000 (based on
recent trends) would require an increase in reservoir capacity of fifty-seven
million cubic meters (forty-six thousand acre feet). The DRBC's comprehensive
plan provides for such an increase in capacity.
A conservatively low extrapolation of the results from Hull and
Tortoriello (1979) implies that for the thirty-centimeter (one-foot) rise in
sea level expected through 2025, the required additional reservoir capacity
would be approximately 140 million cubic meters (110 thousand acre feet),
about one fourth the capacity that would be provided by the proposed Tocks
Island reservoir. Table 6 lists reservoirs that are currently in the DRBC's
long-range comprehensive plan, with a combined reservoir capacity of 730
million cubic meters (592 thousand acre-feet). These reservoirs would augment
streamflow during droughts enough to offset salinity increases caused by sea
-------
TABLE 6
UNSCHEDULED RESERVOIRS IN DRBC'S PLAN THAT COULD BE
USED TO OFFSET SALINITY INCREASES IN THE DELAWARE ESTUARY a/
Name of Project
( 1)
Tocks Island Reservoir
Aquash ico 1 a Reserve I r
Maiden Creek Reservoir
Evansburg Reservoir
Newark Reservoir
Icedale Reservoir
Tota 1
Location b/
(2)
Delaware River
(217.2)
Aquashicola Creek
(183.66 - 36.3 - 4.6)
Ma iden Creek
(92.5 - 86.7 - 9.6)
Skippack Creek
(92.47 - 32.3 - 3.0 -
1.0)
White Clay Creek
(70.7 - 10.0 - 12.0)
W. Branch Brandywine Cr.
(70.73 - 1.5 - 20.0 -
25.6)
Increment of Long-Term Storage Capacity
Cubic Meters
(mill ions )
(3)
525
30
91
29
37
18
730
Bill ions
of Ga I Ions
(4)
139
8
24
8
10
5
194
Ac re -Feet
( thousands)
(5)
425.6
24
74
23.5
30
14.6
592
a/ These multipurpose projects are currently (1985) in the DRBC's Comprehensive Plan, but
this does not guarantee ultimate construction, which will depend on further analysis of water
demand, environmental impact, and financial feasibility. Such analysis may result in
deauthorizat ion of some of these projects. They are generally in reserve status—for development
after year 2000, if needed.
b/ Statute miles, measured along the axis of Delaware Bay and River from the mouth of the
bay. Second and subsequent mileages indicate distances above the mouths of tributaries.
i
Ul
o\
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-57-
level rise and increased water consumption well inuo the 21st century.
However, most of these dams have not yet been scheduled for construction.
Although reservoirs are generally not built before they are needed,
incorporating future reservoirs into the Comprehensive Plan long before
construction can help to limit eventual costs. Otherwise, the best sites may
be developed for other uses, increasing the cost of purchasing the land,
perhaps to the point where a dam at that site becomes economically infeasible,
which could necessitate selection of an alternative reservoir site that is
less environmentally or economically attractive.
The advantage of adding reservoir capacity is that such an approach fits
within the current policy framework. The limitations, however, must also be
considered. Although dams can mitigate environmental disruption caused by
consumption of water, environmental disruption can result from the dams
themselves, a factor of no small importance in the opposition to the proposed
Tocks Island Lake, the consideration of which has been deferred until after
the year 2000. Moreover, the capacity of reservoirs must keep pace with
increased consumptive use of water, as well as sea level rise. Finally, each
additional dam tends to cost more than the previous one, as the least costly
sites are usually developed first. Thus, even ignoring environmental
questions, there is a limit to the ability of reservoirs to counteract
saltwater intrusion in a cost-effective manner.
Increased private storage capacity could augment public reservoirs. As
mentioned in Section 3, electric utility companies in the Delaware Basin are
already required to develop enough storage capacity to offset their new
consumptive uses during low-flow conditions. Actions could be taken to
encourage other users to develop storage or decrease consumption.
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Decreasing the depletive use of water from the river would also prevent
salinity from increasing. The DRBC has used its special powers during several
drought emergencies since 1965 to curtail diversions to New York City and
northeastern New Jersey and other depletive uses. In 1983, the DRBC (1983b,
1983c) adopted regulations that automatically cut back consumption within the
basin and diversions out of the basin during droughts.
Decreasing depletive uses of water has been one of the DRBC's tools for
combatting saltwater intrusion. Nevertheless, there are practical and
physical limits on the ability to offset salinity increases caused by a large
rise in sea level. Although conservation has been exploited to a high degree
within the basin, consumptive use is expected to grow with population.
Curtailing diversions of Delaware River water to New York City and other areas
may impose increasing hardships on these areas as alternate supplies such as
the Hudson River also become saltier. Moreover, even if all depletive uses of
water were eliminated, a substantial rise in sea level would eventually
increase salinity in the estuary, as it has since the last ice age.
Adapting to Increased River Salinity: Surfacewater Users
If measures are not undertaken to prevent a salinity increase, water users
will have to adapt to it. The City of Philadelphia could adapt to increased
salinity by moving its intake upstream. This approach was actively considered
as a temporary measure during the 1960s1 drought, when the Torresdale intake
was threatened by saltwater intrusion (Hogarty 1970).
Although Philadelphia will almost certainly continue to rely on the
Delaware River for part of its water supply, other users may be able to shift
to alternative supplies. The Chester (Pennsylvania) Municipal Authority has
already done so. Formerly taking its water supply from the tidal Delaware
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River below Philadelphia, the Authority was forced to abandon this source in
1951 because of frequent high salinities related to low river flows. The
Authority now obtains its water supply from the Susquehanna River Basin.
However, the Susquehanna River flows cannot be reduced without limit to help
Delaware Basin water users avoid increasing salinity; the Susquehanna has its
own problems, including the need to maintain adequate low flows for salinity
control in upper Chesapeake Bay (Schaefer 1931; Susquehanna River Basin
Commission 1973).
Some industries along the Delaware estuary may eventually find it
impossible to obtain adequate freshwater supplies. Such industries may be
forced to relocate to areas where fresh water is available. Others may be
able to survive at their present locations by shutting down river pumps during
periods of high salinity and switching to municipal water distribution systems
with access to fresher sources. This has happened in past droughts in the
area along the Delaware estuary served by the Chester Municipal Authority.
However, alternative sources may be prohibitively expensive.
Although water conservation measures could make only a limited
contribution toward preventing salinity increases, they could also play a role
in adapting to decreased availability of fresh water. Nevertheless, they
would face institutional barriers that could substantially delay an effective
response. Additional regulations of water use would require identification of
additional activities to be controlled. Although higher prices could
theoretically induce an economizing shift toward conservation, public agencies
would find it difficult to raise water prices, particularly for those whose
water is supplied by wells on their own property.
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Finally, companies and individuals may adapt by using water with higher
salinity. Companies that use water for cooling may experience increased
corrosion of pipes and machinery, or may invest resources in
corrosion-resistant materials. Some individuals may shift to bottled water
during droughts,7 while others may choose to drink water with elevated salt
content rather than go to the expense of distilling water. Health-conscious
people may respond to salt-laden drinking water by reducing salt intake from
other sources. Nevertheless, the health hazard of elevated sodium in water
ingested by persons subject to hypertension and other diseases requiring
low-sodium diets is an argument for avoiding high salt content in public
drinking-water supplies, so that susceptible persons will not be forced to
save money by sacrificing health.
Adapting to Increased River Salinity: Groundwater Users
Groundwater users can adapt to increased salinity in ground water by many
of the same methods by which surfacewater users can respond. In addition,
efforts may be undertaken to prevent the river from recharging the aquifers
with salt water. The methods include physical barriers, extraction barriers,
freshwater injection barriers, and increased recharge from sources other than
the estuary. Modified pumping patterns could also be employed.
Physical barriers. Subsurface physical barriers, such as sheet pile
cutoff walls, clay slurry trenches under earth dams, and impermeable clay
walls, are routinely used by engineers to control the movement of water and
7 A large fraction of citizens in New Orleans use bottled water or
purchase home distillers; the salt-intrusion problem in Louisiana probably
will continue to be more severe than that in the Delaware River Basin.
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other liquids, including hazardous waste materials. It is also possible to
inject materials that form a zone of low permeability.
Extraction barriers. Extraction barriers consisting of a line of pumping
wells parallel to shore have been used in various locations in order to
prevent or reduce saltwater intrusion (Stone 1978). Extraction barriers may
withdraw some fresh water that would otherwise be useful and thus may not be a
viable option where water supplies are scarce.
Freshwater injection barriers. Figure 16 illustrates a typical injection
barrier in operation to control the saltwater intrusion for cases where the
sea level is in excess of freshwater levels. In contrast to the extraction
barrier, with an injection barrier, fresh water is injected into the aquifer
through a line of wells along the shoreline. The higher groundwater levels
along the injection barrier prevent saltwater intrusion.
Increased recharge. In many coastal locations in the United States,
sufficient amounts of fresh water are available for recharge during periods of
high precipitation. Although some water is captured during these periods and
stored in surface reservoirs, very little water is artificially recharged to
groundwater reservoirs for use during droughts. This extra water, which is
"wasted" to the ocean, could be used to replenish the aquifer, build up
groundwater levels, and slow or stop saltwater intrusion.
Modified pumping patterns. For aquifers where moderate pumpage already
occurs and the effect of a sea level rise is projected to be important, a
phased shutdown of wells can be designed as the monitored saltwater intrusion
progresses. Instead of a disorganized search for alternate water as the
chloride concentrations increase, logical permitting of new wells or new
economical surfacewater distribution schemes can be implemented. Because a
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-62-
FIGURE 16
INJECTION-TYPE SEAWATER INTRUSION BARRIER
wEW MEAN
SEA LEVEL
PIEZOMETRIC SURFACE
INJECTION BARRIER
WELL
IMPERVIOUS BED
WATER SUPPLY
WELL
AQUIFER
Source: SORENSEN, R.M., R.N. WEISMAN, and G.P. LENNON, 1984. Control of
Erosion, Inundation and Salinity Intrusion Caused by Sea Level Rise.
In Greenhouse Effect and Sea Level Rise: A Challenge for This
Generation, edited by M.C. Barth and J.G. Titus. New York: Van
Nostrand Reinhold.
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-63-
saltwater slug will pass through the aquifer even when the drought that caused
the high river salinity has passed, the well could be reopened after the
aquifer has become fresh again. However, such natural purging of a
contaminated aquifer may require decades, if not centuries.
Although it is technically possible to use physical, extraction, or
injection barriers to prevent saltwater intrusion in the Potomac-Raritan-
Magothy aquifer system, the large expense probably would not be justified.
Harbaugh, Luzier, and Stellerine (1980) present technical information on how
an injection barrier could be employed in the aquifer system to reduce the
existing saltwater intrusion. However, Camp Dresser and McKee (1982) provide
cost estimates showing that the implementation of such a groundwater barrier
is not feasible because of the large area needing protection. Although these
types of barriers may be considered, they probably cannot be justified
economically.
Increased recharge in the aquifer's outcrop could be employed at a
reasonable cost, as could modified pumping patterns, which would shift the
pumping away from the critical areas. The State of New Jersey is currently
studying alternative water systems for the critical area of excessive drawdown
in Camden County. Among alternatives being considered is the improvement of
the water distribution system, which would transfer water to the area of heavy
drawdown from other sources, thus relieving pumping stress in the critical
area.
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VI. NEXT STEPS
Considering Climate Change
Although this paper focuses on the impact of sea level rise on salinity,
other consequences of the greenhouse effect may accelerate or delay the
consequences of sea level rise. For example, if droughts become more severe
in the future, the resulting reduction in river flow would also allow salinity
to increase. Although projections of drought conditions cannot currently be
made for specific regions, general circulation models suggest that drought
frequencies may change substantially.
Rind and Lebedeff (1984) examined model calculations of the change in
drought frequency, caused by a doubling of atmospheric CO,., for four regions
of the continental United States, one of which included the Delaware River
Basin. Two of the regions would change slightly, one would experience half as
many droughts, while the other would experience ten times as many. Although
the Delaware River Basin is largely in the latter region, the authors strongly
warn that their model does not accurately project climate for particular
regions.
This report focuses on rising sea level because our ability to project it
is far superior to our ability to predict future precipitation change.
Nevertheless, planning for hydrologic shifts may be more important than
planning for sea level rise. It is possible to plan around a gradual rise in
sea level; even waiting until the 1990s for a confirmation of the predicted
global warming would allow time to prepare for the most severe consequences.
By contrast, a drought can occur suddenly, and several droughts may have to
occur before people know that their area is more prone to drought than it was
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in the past. Thus, successful planning for changes in the hydrologic cycle
will probably have to start before those shifts are well understood.
Chen, Boulding, and Schneider (1983) have thus argued that in this
situation, water resource officials should rely on "robust" strategies--
policies that are less vulnerable to large changes in conditions and can
accommodate a shift in either direction. In the case of the Delaware River
Basin, two types of policies readily come to mind. Reservoirs provide more
water storage for increased drought frequency, but they can also be used to
prevent flooding that would occur from an increased frequency of extremely wet
periods. Market mechanisms can also help for shifts in either direction
because they encourage individuals to adapt quickly to new information rather
than to wait for the government to formulate its response.
Although policies have been identified that would reduce the vulnerability
of the water supply in the Delaware River Basin to future climate change, it
would be infeasible and unwise to implement these policies until a
comprehensive assessment of the likely impacts and possible solutions has been
undertaken.
The DRBC's long-range comprehensive plan includes numerous measures that
would reduce the vulnerability of the region's water supply to salinity
increases resulting from rising sea level or changes in climate.
Comprehensive assessments of the likely impacts and possible solutions should
be undertaken to provide adequate lead time for implementing these measures if
and when they become necessary.
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Necessary Research
The highest priority is to determine the impact of various climate change
scenarios on river salinity and the streamflow modification required to
maintain acceptable salinity levels in the face of climate change. An
examination of the costs and benefits of various response options should then
be undertaken for each of these scenarios. By examining each option for a
variety of possible sea level and precipitation changes, it may be possible to
identify which solutions are likely to be robust and which are likely to be
clearly inferior. A particularly important question for such an analysis is
what amount of resources could be saved by planning in the 1980s, compared
with delaying the planning until the 1990s or later.
A second research priority that concerns other parts of the nation as well
as the Delaware River Basin is to develop better estimates of future sea level
rise and climate change. In addition to undertaking the research, it is
essential that the results be made available to decision makers and the public
at large. For the private sector to make locational and design decisions that
are consistent with expected water availability, people must become informed
about future conditions.
Improvements in the models for estimating salinity changes will also be
necessary. The model used in this report to estimate river salinity would
benefit from a more in-depth assessment of the impact of sea level rise on
shoaling and the estuary's width and cross-sectional geometry. Increasing
salinity of the Potomac-Raritan-Magothy aquifer system is already a research
priority of the U.S. Geological Survey. Current efforts should be
supplemented with analysis of the implications of rising sea level on that
system.
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Conclusion
The expected rise in sea level and climate changes caused by the
greenhouse effect are likely to have profound impacts on the quality and
availability of water in the Delaware River Basin. Although the greatest
impacts are decades in the future and cannot be predicted precisely,
assessments of how to respond should start now. Public officials responsible
for water quality will have to decide whether to adapt to salinity changes or
attempt to prevent them. Such assessments may require lengthy public debates,
after which planning, design, and implementation may take decades.
Furthermore, even current trends may necessitate management changes by the
year 2000.
An important impediment to implementing the farsighted policies that will
be necessary is the relatively short planning horizon of 15-20 years generally
used by the DRBC, as well as other agencies. This time horizon has been
appropriate in the past because decisions have involved such phenomena as
economic growth and technology that did not require a longer lead time. But
given the longer-term impacts of climate change and sea level rise, the longer
lead time required to prepare for the consequences, and the potential
magnitude of the impacts, a longer time horizon is warranted.
We cannot rule out the possibility that our current understanding
overlooks factors that will substantially reduce the saltwater intrusion
expected from the greenhouse effect. Perhaps the Delaware River Basin will be
one of the regions that experience fewer droughts in the future. Should one
conclude that preparations are not necessary? Can we afford to gamble with
our water supplies on the hope that problems will not emerge in the future?
Such issues are outside the scope of a technical report and must be addressed
by policy makers and the public at large.
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APPENDIX A
TECHNICAL DESCRIPTION OF DELAWARE
ESTUARY SALINITY MODEL
Governing Equations: Knowns and Unknowns
The model is based on the general, one-dimensional equations of open
channel hydraulics and conservation of salt as coupled through an equation of
state that relates density to salinity and to temperature. The unknowns of
these equations are the surface elevation, the velocity (discharge divided by
area), and the salt concentration. Known quantities are the geometry, the
time-varying tributary inflows and tributary salinities, the temperature, and
the tidal elevations at the ocean boundary throughout the period being
modeled. The time scale of calculation is sub-tidal so as to properly account
for the estuarine dynamic response to the driving force of the ocean tidal
oscillation and to ensure that the mixing can be represented as accurately as
possible. The use of such a small time-scale is essential, as sub-tidal
variations play an important role in long-term and short-term salinity
concentrations (Najarian et al. 1983; Elliot 1978). The equations on which
this model is based are as follows.
Continuity equation:
9h 3Q
bi7 + 7T-q = 0
Longitudinal momentum equation:
3Q 3(QU) 3h Adc 3 Q|Q| (a-2)
+ — + gA + g —^ + g 1— = 0
3t 3x 3x p 3x AC2Rh
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Salt balance equation:
a(Ats) a(QS) a as
^- + = (A* E ) (a-3)
3t 3x 3x l 9x
Equation of state (relation between density, salinity, and temperature):
P = <*T + PTs O-4)
In the above equations: b = total surface width; h = depth from water surface
to horizontal datum; Q = cross-sectional discharge; q = lateral inflow per
unit length; U = longitudinal velocity averaged over the cross section of the
core area; A = core area; g = acceleration of gravity; d = distance from
c
the water surface to the centroid of the cross section; p = mass density of
water; R, = hydraulic radius of core area; C = Chezy coefficient; A = the
total area of the cross section, including that of storage areas; s = salinity
averaged over the total cross section; and E = the longitudinal dispersion
coefficient. All of the aforementioned quantities (except g) are functions of
longitudinal location (x), and time (t). The coefficients ay and &j
depend only on temperature.
The continuity equation (a-1) and the momentum equation (a-2) are
essentially in the form used by Harleman and Lee (1969) to describe the
unsteady tidal hydraulics of estuaries and canals. Thatcher and Harleman
(1972) incorporated an additional term into the momentum equation that
includes the effect of the longitudinal density gradient due to salinity.
This term, gA(d /p) 3p/3x, couples the two hydraulic equations to the
C
salt balance equation through the equation of state (a-4).
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The one-dimensional salt balance equation (a-3) can be obtained by
spatially integrating the three-dimensional convective-diffusion equation for
turbulent flow, as demonstrated by Okubo (1964) and Holley and Harleman (1965).
Longitudinal Dispersion Relationship
The longitudinal dispersion coefficient, E, represents the combined effect
of internal circulation induced by saline density gradients and mixing due to
boundary shear. The boundary shear effect is assumed to be represented by a
dispersion relation similar to that formulated by Taylor (1954). The density
gradient effect is assumed to be proportional to the absolute value of the
local, longitudinal salinity gradient. This assumption implies that
density-induced circulation is greatest in the region in which the
longitudinal salinity gradient is largest. This relationship, assumed by
Thatcher and Harleman (1972), is
E(x,t) = K
1 9x
mET (a_5)
in which K is a stratification parameter that depends on the degree of
o
vertical stratification in the estuary. The value s is defined by the
relation s = s/so , s0 being the maximum salinity at the ocean boundary.
The value x is defined by the relation x = x/L, L being the length of the
estuary from the ocean boundary to the head of tide, and E-j- is the
Taylor-type dispersion coefficient applicable in the upstream freshwater
portion of the estuary where 3s/3x = 0. The coefficient m is used to
account for channel irregularities and other effects not caused by
density-induced circulation. The coefficient ET is defined for free surface
flow by
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-71-
ET= 77UnRh5/6 (a.6)
(where n is Manning's n)
The local salinity gradient is expressed in dimensionless form and the
stratification parameter, K, has the dimensions of a dispersion coefficient,
!2/t. The stratification parameter, K, is assumed to be independent of
distance and time within a particular tidal period. However, K may vary from
one tidal period to the next, depending on the degree of stratification. In
order to make the model more predictive, it is necessary to relate this
stratification parameter to the gross estuary properties that determine the
degree of stratification. This is done by a correlation between K and a
densimetric estuary number as defined by
P FF
M^D
(a-7)
in which P is the tidal prism, calculated by integrating the flow at the
ocean boundary during the flood portion of the tidal cycle; Qf is the
freshwater inflow upstream of the salt front; and T is the duration of the
tidal period. The densimetric Froude number, fF-, is defined by
FD= — (a-8)
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in which Ap represents the density difference between fresh water and
ocean water; p the ocean density, and U and h are the maximum tidal
velocity and the depth, respectively, at the ocean boundary.
By using salinity data from laboratory flumes, estuary models, and field
studies, Thatcher and Harleman (1972) found a correlation between the
normalized stratification parameter, K/(U L), and the estuary number, as
shown in Figure A-l. The densimetric estuary numbers, tEn, extend over two
orders of magnitude ranging from relatively stratified conditions in the
Rotterdam Waterway to fairly well-mixed conditions in the Delaware estuary.
Within this range, the ratio K/(U L) varies only by a factor of 5. The
longitudinal dispersion coefficient is thus determined by equations (a-5) and
(a-6) as a function of local parameters and global estuary parameters.
Numerical Solution of Governing Equations
Initial conditions and boundary conditions being specified, equations
(a-1) through (a-4) can be solved by numerical techniques for the dependent
variables, elevation, h; discharge, Q; and salinity, s; as functions of
distance, x; and time, t. The finite difference scheme used to solve the
tidal-dynamic equations is similar to that of Harleman and Lee (1969), and is
a staggered explicit scheme wherein surface elevations and discharges are
calculated at alternative locations in the space and time mesh. This scheme
has been shown to be efficient in calculation, but the choice of Ax and At
must be made in a manner that does not violate the approximate stability
criterion,
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-73-
FIGURE A-l
CORRELATION OF STRATIFICATION
PARAMETER WITH ESTUARY NUMBER
-2
10 -
"i '—I—i i i i u
103
u L
o
ROTTERDAM
HUDSON
O WES 10
A DELAWARE 3
l***"-^4_
DELAWARE 2*
DELAWARE 1
id4
J 1 I I I I I I I I I I I I I I
0.1
10
100
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-74-
At < .
U + c (a'9)
where, Ax and At are the longitudinal distance and time increments of
calculation, U is the average cross-sectional velocity, and c = /gh is the wave
speed at the same location as U.
The finite-difference scheme employed for solution of the salt-balance
equation (a-3) is a six-point implicit scheme based on the minimum-error
investigation of Stone and Brian (1963). This scheme has a truncation error
with terms proportional to (Ax)2 and (At)2 and thus is of second
order. The truncation error contains no term proportional to 32s/3x2,
which means it has no numerical dispersion term as found in some first-order
schemes (Bella and Grenney 1970).
Calibration Parameters
Three parameters are treated as calibration coefficients for this model.
The first is the friction parameter used to determine the Chezy coefficient.
This is achieved through a Manning's "n" distribution for the estuary. Using
observed values of high and low tidal elevations throughout the estuary, the
calibration distribution of Manning's "n" was determined. This distribution
(Figure A-2) was then used in all subsequent calculations.
The second and third parameters relate to the longitudinal dispersion
relationship of equation a-5, specifically to the determination of parameters
K and m. The slope of the K/(U L) vs. E relationship (Figure A-l) was
assumed to be that shown, e.g., -(1/4). The intercept of this relationship
was treated as a calibration parameter. Sensitivity analyses and comparison
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-75-
FIGURE A-2
CALIBRATION DISTRIBUTION OF MANNING'S "nf
Manning's
n
.05
.04
.03
.02
.01
50 100 150 200
Distance from ocean, meters X 10~3
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-76-
of a three-month calculated set of salinities with observed values provided
calibration values of
= 0.0015
U0L
m = 35
Additional Sources of Information
Further details about the model's development and application can be found
in the two-volume report made to the Delaware River Basin Commission by
Thatcher and Harleman (1978). This report includes data sources for
calibration and verification as well as the results of sensitivity studies.
Further applications by the Commission have been documented in DRBC reports.
Thatcher and Harleman (1981) have presented some of the model results in a
paper published in the Journal of the Environmental Engineering Division of
the American Society of Civil Engineers. This paper was discussed by Fischer,
August 1981, and by Hull, December 1981. The authors' closing response can be
found in the February 1983 issue of that journal.
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APPENDIX B
MODIFICATION OF SALINITY-MODEL GEOMETRY
FOR LARGE RISES OF SEA LEVEL
The salinity model represents estuarine geometry by a schematization to a
double rectangular section as shown in Figure B-l. The flow-carrying
rectangle is referred to as the core area and the adjoining rectangle (if any)
represents the storage area. The schematization in use by the Delaware River
Basin Commission is based on the U.S. Army Corps of Engineers "Table of
Mid-Tide Volumes." This source of geometry is accurate, but is limited to
near present conditions of sea level because of the rectangular schematization
in terms of a constant width. As shown in Figure B-2, to schematize a
significant rise in sea level, Az, would require new widths, whereas the
schematization based on the Table of Mid-Tide Volumes can accommodate only new
depths. A procedure for modifying this rectangular schematization for a sea
level rise of approximately seven or eight feet is presented as follows.
Figure B-3 shows the basic relationships leading to a second rectangular
schematization of a typical estuary cross section that would be filled to the
elevation of inundation. To create a rectangle whose width is the distance
between contours of inundation, and whose area is the sum of the present core
area plus the additional inundated area, would result in a new depth that
would be very shallow in comparison with reality. This is because the shallow
inundated region can be very wide compared to the present width of the
estuary. To use such a shallow depth would distort the model calculations
because the speed of propagation of the tidal wave is primarily dependent upon
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FIGURE B-1
ESTUARINE GEOMETRY SCHEMATIZATION
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FIGURE B-2
TYPICAL ESTUARY CROSS SECTION SHOWING INCREASE
IN WIDTH RESULTING FROM SEA LEVEL RISE
ELEVATION
OF INUNDATION
W
core
FIGURE B-3
TYPICAL ESTUARY CROSS SECTION SHOWING
ADDITIONAL CROSS-SECTIONAL AREA (RIGHT-HAND SIDE)
RESULTING FROM SEA LEVEL RISE
ELEVATION OF INUNDATIONy
Awfi
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the depth. To maintain this most important parameter, depth, a second
approach is presented.
The bottom elevation of the core area (sometimes called "conveyance area")
will be maintained. Outside the core area, the additional cross-sectional
area of inundation, AA, will be calculated in terms of the newly inundated
width, AW, by assuming triangular cross sections adjacent to the rectangular
core area. With reference to Figure B-3, this area is
A A I AW
AA =
where, AA = the sum of left-hand area AAy and right-hand area AAR,
AW = the sum of left-hand width AW, and right-hand width AW-,
ii K
Az = elevation of inundation - mean sea level, and
Az' = elevation of inundation - mean high water.
Note that the change in width, AW, was from mean high water to the
elevation of inundation. These parameters come from topographical data
related to mean high water. A linear slope was assumed, and this slope was
utilized to calculate the additional area for a prescribed sea level rise.
These calculations yielded a new Effective Width, W*
w*= Acore + *A = w + _ AA _ (b.2)
dcore + Az dcore + Az
where, A and d are the area and depth of the core.
core core
The storage widths of the original schematization are maintained. As the
measurement of width between contours of inundation was along the main stem of
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the estuary, this assumption is conservative. It is conservative because it
is probable that new, significant storage areas would be opened up adjacent to
the main stem under the sea level rise condition.
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