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better coastal management with a longer “memory” that accounts 
for extreme events and monitors the activity in the shore zone closely. 
Figure 6.18 shows a 1.3 m rise in mean water levels on Lake Michigan-Huron 
between 1934 and 1952, followed by a 1.4 m drop from 1952 to 1964. This is a 
much larger fluctuation than the annually expected 0.5 m fluctuation. In general, the 
total water level fluctuation along the Great Lakes (adding the annual and long-term 
change) is of the order of 2 m. Periods of major shore zone damage can be directly 
related to periods of high water levels, such as 1929, 1952, 1973, 1986 and 1997 for 
Lake Michigan-Huron. These high water levels allow the large waves to come 
closer into shore for several months to several years. When such water levels 
combine with short-term storm surges, structures are destroyed and protective 
beaches disappear This exposes the shore, which mainly consists of glacial till 
bluffs, to direct wave action and severe erosion. Extreme low water levels (such as 
1934 and 1964 on Lake Michigan-Huron) also cause problems. Wells run dry, there 
is insufficient water for navigation and power generation, and pleasure craft cannot 
enter or leave marinas. 
6.7.2 Eusiutic (Sea) Level Change 
The term eustutic refers to a global change in ocean water levels, resulting from 
melting or freezing of the polar ice caps and thermal expansion of the water mass 
with temperature change. Detailed descriptions may be found in Carter ( 1 988) and 
Bird (1984). The sea levels 25,000 years ago were 150 m below the present level. 
Between then and 3,000 years ago, water level rose at about 7 m d y r to almost the 
142 Introduction to Coastal Engineering and Management 
present water level. The present average rate of eustatic rise is small and therefore 
difficult to measure. The best estimates are 1 to 1.5 mm/yr. This relatively small 
rate of rise, nevertheless, submerges the ocean shores and is at feast partly 
responsible for the fact that most beaches around the world are eroding over the long 
term. 
6.7.3 Isostatic (Land) R ~ b ~ u n d a n ~ ~ u b s i ~ e n c e 
0 300 - 
I 5 2 3 km 
0 75 
Figure 6.19 Relative Rates of Crustal Movement (mmfyr) 
(after Clark and Persoage, 1970) 
The common natural cause for isostatic (land) elevation change is a result of the 
adjustment of the earth’s crust to the release of pressure exerted by the 1 to 2 km 
thick ice sheet that covered it during the last glaciation. Typically, the earth’s crust 
was severely depressed by the ice and a rise (forebulge) was formed in the earth’s 
crust ahead of the glaciers. When the ice retreated, the earth’s surface rebounded 
(upward) where the glaciers had been and lowered where the forebulge had 
occurred. This process still takes place today, but at a much-reduced rate. Most 
Chapter 6 - Tides and Water Levels 143 
areas in the higher latitudes experience isostatic rebound and areas at more 
intermediate latitudes experience some subsidence. Figure 6.19 shows the isostatic 
rebound over the Great Lakes and Fig. 6.20 shows a Northeast-Southwest line 
through the Northern United States, indicating both rebound and subsidence with a 
hinge line near Kingston (N.Y.). 
3 
0 
c) 
m 
.I 
$ 0 
s 
- 1 
0 100 200 300 400 
Kilometer 
Figure 6.20 Isostatic Adjustment in Northern United States ( m d y r ) 
(after National Research Council, 1987) 
In general, isostatic rebound decreases the impact of eustatic sea level rise, or even 
reverses it. For example, the measured rate of relative sea level rise (water level rise 
with respect to the land) at San Francisco is 1.3 m d y r while at Juneau, Alaska the 
sea level drops at 13.8 m d y r (National Research Council, 1987). On the Great 
Lakes, the effect of isostatic rebound is not quite so simple. All the land rises, but 
the relative rise of the land with respect to the water is controlled by the difference 
between the local rate of rebound and the rebound at the outlet of the lake. From 
Fig. 6.20 it may be seen that along Lake Michigan-Huron, the rate of rebound is 0 to 
144 Introduction to Coastal Engineering and Management 
2.5 m d y r . The outlet rises at about 0.5 d y r while most of the land rises at a 
greater rate and hence most of Lake Michigan-Huron has an emerging shore. 
Conversely, for Lake Ontario, the outlet rises at 2.5 m d y r while the shore rises at 
0.75 to 3.0 m d y r , thus forming a submerging shore over most of the lake. Clearly 
the morphological development in these two lakes is totally different. 
Although subsidence does occur naturally, often it is man-made. Pumping 
groundwater, petroleum and natural gas are common causes. Subsidence 
exacerbates the effects of eustatic sea level rise since the relative sea level rise with 
respect to the land will now be greater. The earlier example of Venice clearly 
demonstrates the effect of subsidence. The delta on which Venice is located was 
sinking at a small annual rate and the sea level was rising as everywhere else. In this 
century, however, pumping of both water and natural gas caused an accelerated rate 
of subsidence. As a result, the city and its Mediaeval monuments are subjected 
more and more regularly to 'Aqua Aha' or high water. An international effort is 
underway to save Venice and its monuments at great expense. The leading idea is to 
use storm surge barriers. Large gates are to be built in the tidal entrances between 
the offshore islands that separate the Venice Lagoon from the Adriatic Sea. 
Normally these gates will lie on the bottom, permitting unobstructed navigation, but 
at times of storm surge, these gates will be raised to isolate the city temporarily from 
the sea and protect it from storm surge and seiche. The southern part of the 
Netherlands is protected by such a series of storm surge barriers, built as part of the 
Delta Project and designed to counteract storm surge flooding such as occurred in 
1953. 
6.7.4 Global Climate Change 
The final and potentially most dangerous water level change results from trends in 
global climate. In the discussion of eustatic sea level rise, we have already seen that 
global warming after the last glaciation has resulted in a sea level rise of 100 to 150 
m through melting of the polar ice caps and thermal expansion of the water in the 
ocean. The present rate has slowed down to an estimated 1 to 1.5 m d y r , but any 
additional warming would increase this rate of sea level rise. 
Concern is centered around the production of the so-called greenhouse gases. These 
combustion products act as an insulating blanket over the earth, decreasing the net 
longwave radiation from the earth back into space and thus trapping the sun's heat to 
cause global warming. It is a controversial subject and indeed there is a contingent 
of respected scientists that disputes the whole idea. It is estimated (National 
Research Council, 1979) that a doubling of carbon dioxide (COz) would result in an 
Chapter 6 - Tides and Water Levels 145 
average global temperature rise of 1.5 to 4.5 'C. At the poles the temperature rise is 
estimated to be two to three times the average. Monitoring stations such as Mauna 
Loa, Hawaii indicate an increase in C02 concentration from 315 to 340 parts per 
million (ppm) between 1958 and 1980 (National Research Council, 1983). Tree 
ring data show that from 1850 (prior to major industrialization) to 1950, there has 
been a 50 ppm increase in C 0 2 concentration. Estimates of future concentrations 
vary greatly, but there is a 75% probability that by 2100, the pre-industrial COZ 
concentration will have doubled. 
Global climate change models study how such an increase in greenhouse gases 
translates into temperature and water level rise. Such numerical models have 
produced several widely varying scenarios. Predicted rise in water level for the year 
2025 varies from 0.1 to 0.2 m. For 2050, the estimates vary from 0.2 to 1.3 m and 
for 2100 the