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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