3. WATER Next Chapter
3.1 Forms Of Inundation And The Threats They Pose
Inundation of coastlines from the ocean comes in a number of forms, some more immediate and threatening than others, and I will briefly examine them here.
3.1 (a) Flood
By far the most prevalent of historical accounts referring to inundation of some form is the legend of the Deluge that is mentioned in the Bible. Found, in many various yet strikingly similar forms, in such diverse countries and cultures as the Middle East, Asia, Africa, the Pacific Islands, North and South America, there are more than 500 deluge legends known around the world. While some can be shown to have stemmed from the Bible in its present form, the specialist researcher Dr. Richard Andee concluded that a major portion of the Deluge legends were entirely independant of the Mesopotamian and Hebrew accounts. (Filby, 1970)
The threat posed by such an event as described in the Bible and other documents, should it possibly occur in the future, would be of such proportions that it would render the topic of this paper useless - it is beyond the scope of this project. However, there is a growing body of present day research into the matter, as can be seen in the publications of Zangger (1992), Wood & Campbell (1994), Flem-Ath & Flem-Ath (1995), Allan & Delair (1995)
3.1 (b) Rising Ocean Levels.
It has now been established beyond doubt that the level of the oceans is rising. Globally, the sea-level has risen 10 - 25 cm over the last century, and is projected to rise by 15 to 95 centimetres by 2100. (Schroeder and Bassett, 1998) This is due to the release of water previously trapped in frozen form in the polar ice caps into the Earth's hydrological cycle, an effect of Global Warming due to an increase in the "greenhouse effect", where chemicals and particles in the Earth's atmosphere insulate the planet from the loss of heat received from the sun. Mean surface temperatures have increased by 0.3 to 0.6 degrees C over the last century, and are projected to increase by about 1 to 3 degrees C by 2100. (Schroeder and Bassett, 1998) There is evidence of rapid 20m changes in sea levels about 400,000 years ago, found in connection with sudden appearances of coral reefs due to a deposition of calcium carbonate in the water. (New Scientist, 31 May 1997).
3.1 (c) Land Subsidence
Closely connected to Rising Ocean Levels is Coastal Land Subsidence, which combined together form what is known as passive submergence. Subsidence is caused by two factors, gradual tectonic movement, and natural wave action, the latter found to have less effect than the former. The western seaboard of the US has been particularly noteworthy of land and property losses due to subsidence, caused by active plate movements. This is a problem that has been recognised within State governments concerned - the Massachusetts Office of Coastal Zone Management, for example, has estimated that 65 acres of upland per year are lost to the sea. (www.whoi.edu 1994) It is generally a slow process, which varies from region to region, but is measured in amounts of less than a centimetre per year. Seismic plate movements cause land subsidence on a greater scale, but such events generate wave, or tsunami effects that are experienced synchronously but with more effect than subsidence itself.
Although the subject of rising sea levels and land subsidence has important consequences for world climate, hydrological systems and loss of land in the future, it is beyond the scope of this paper. Residents and authorities will have sufficient warning when threats arise to relocate or take preventative measures such as diking or land reclamation
3.1 (d) Storm Surges
Giant waves travelling up the mouths of rivers and estuaries as a result of hurricane influence, has only been recently recognised. A number of port regions around the world experience "king tides", which are simply very large changes in tide heights, and some river systems, particularly in South America, experience unusual wave effects due to a mixture of tidal changes with unique bathymetric and geographical structures.
Storm surges are destructive waves caused by hurricanes, and can be as high as 5-6 metres. They are a large dome of water that sweeps into the coast as much as 5 hours before a hurricane makes its landfall, and cause most of all hurricane related deaths. The dome is a large body of agitated water that is driven at the surface by the extremely high winds of the hurricane. It also appears among rough seas of increasing intensity as a single massive wave followed by a decrease in intensity, and tends to match normal onshore speeds of breaking waves. When coinciding with high tide, their waves can be extremely destructive: over 6000 people were killed in the Galveston, Texas Hurricane of 1900, most by storm surge, and Hurricane Hugo in 1989 generated a 20-foot storm tide in South Carolina. (www.fema.gov 1997) As many as 300,000 perished when a storm surge caused by a hurricane in the Bay of Bengal swept across the Ganges Delta on 12 November, 1970 (Times Publication, 1989 )
Studies conducted by Purdue University (wxp.eas.purdue.edu) have found that estimates of storm surge wave heights can be based on hurricane strength.(see Fig.1)
Fig. 1 1997 Hurricane/Tropical Data for Atlantic
Type Category Pressure Winds
For storm surge waves to represent any threat, they need to be generated by very strong hurricanes. Category 4 or 5 hurricanes, which take days to grow, form waves that only begin to be comparable with wave heights of tsunami's, and thus there is plenty of early warning indicators to alert authorities of impending danger. The mechanics of storm surges are very similar to tsunami's, albeit on a smaller scale, and a model to describe the action and effects of either would be very similar.
3.1 (e) Catastrophic Wave events
Impact waves caused by meteor strike, and those caused by ice-sheets breaking off from the polar ice-caps, are yet to be seen in recent history, but there is considerable we only have geological and archeological evidence for their occurence in the past. Meteor strike, similar to that which is thought to have caused the demise of the dinosaurs, has also been thought to have formed the Gulf of Mexico, while some researchers, A.T.Wilson of Victoria University, New Zealand for instance, believe that the Biblical Deluge could have been caused by a slippage of an ice sheet into the sea (Sitchin, 1976 p. 402-406)
Such events, despite their potential destructive power, happen too rarely for serious study to be undertaken, although separation of glaciers from the polar ice caps is an event of growing concern due to the rising temperatures of the sea, and indeed there have been reports of a 300km ice flow finally breaking off from the North Pole ice cap in the last year. Added to that the fact that there is, at present, too little wave data available for a thorough study of this phenomenom.
The tsunami, after theDeluge, has the most prominent recognition of inundation events within a cultural tradition. Despite it being localised to only a physically small but densely populated island nation, Japan, their occurence and devastating effect on the culture has been so great and prolonged that they have become a part of Japanese heritage. The Japanese were the first to recognise earthquakes as being the cause of tsunamis, having historical records exist of tsunamis as early as 2000 years before present (Nakata & Kawana, 1995). The word tsunami has been created to describe the phenonmenom, they have been depicted in folk art (The cover graphic to this paper shows a 19th Century print by Hokusai), and have even been instrumental, along with earthquakes, in determining the structural design and construction of traditional Japanese buildings. Japan, being on the join of the Pacific and Asian tectonic plates has probably experienced more tsunamis than any other coastline in the world. They have become part of the Japanese psyche. Other cultures have not experienced them enough to have them become part of their historical tradition - for example, the very same community at Aitape was struck by devastating tsunami at the beginning of this century, yet no-one in the community remembered, though Alaska and the Aleutians have also experienced many tsunami's as evidence shows.
For further information, see a printout of the NOAA searchable database on earthquakes and tsunami's
Statistically, earthquakes occur on a regular basis. For smaller quakes around the world it is on a daily basis, for larger quakes in certain areas of the world, they occur less occasionally, on average every 3-4 years. For the South China Sea region, 28 earthquakes of magnitude 7.0 or greater, which is the average minimum for tsunami potential, have occurred in the 94 years from 1900, as measured by the NOAA.(see Fig. 2)
Fig. 2 Graph by J. Wotherspoon based on NOAA data for period 1900 - 1994, magnitude greater than 7.0, in area 0.0 - 20.50 deg. N by 110 - 125 deg. E, 1998.
Although it is difficult at present to predict with accuracy the occurence of future earthquakes, some pattern is discernible from the graph. The most obvious trend is a decrease in magnitudes over time. This is not necessarily a good indication, as after closer inspection, one notices that there appears to be an event of great magnitude followed soon after by events of lesser magnitude, eg. 1905 - 1955, followed then by a much larger quake, or if after a period of inactivity, a cluster of events, eg 1970 and 1990. After a long slow downward trend, it could be argued that a very large event of manitude 8.0 or greater is due to occur within the next 5 years, or a cluster of events after a period of 10 years. Periods of inactivity tend to build up pressure, which leads to larger and more forceful releases of energy when events do occur.
The following map is a very good visual indicator of not only the high frequency of earthquake occurences, but it also shows the plate boundaries by marking the most active seismic zones.
Source: ERMOS, created online in response to query, 17th September, 1998
|The accompanying map shows
the seismicity for the South East Asia region and was generated automatically
in response to a web query to the IRIS
online database on 17th September, 1998, showing all magnitudes for the
period of 5 years..
Of interest is the magnitude 5.5 earthquake in the South China Sea directly north of East Malaysian Kalimantan, which occurred in September 1998.
It's epicentre is roughly 1000kms from The Zhujiang Delta.
Waves caused by undersea avalanches and land slippages may or may not be asscociated with earthquakes. With tsunami's there is a seismic event which releases a shockwave into the water that transfers into a tsunami. An undersea avalanche or land slippage may not necessarily be associated with a seismic event, they could be a resettling of the sea bottom, known as "slope failure"(Dawson et al, 1993) as a result of centuries of slow build up of ocean deposits. The shockwave in these cases is caused by sudden changes in the sea floor, which causes rapid movement of a large body of water, and a tsunami like wave will result. Of course, such an event will leave a seismic signature on testing stations, but it will appear differently to that of earthquakes.
3.2 Decide which type of inundation are important for the project model.
By a process of elimination of the more impractical and inappropriate types of inundation, and due to the wealth of physical data available today, and due to the prevalence of the particular type of wave occurence, I will choose the tsunami event as the inundation type for this project.
3.3 Examine the mechanics of the chosen inundation type.
Earthquakes of magnitude greater than 7.0 at depths of 50-100km are considered to cause tsunami risk, however "tsunamigenic" earthquakes of much less magnitudes have been known to cause destructive tsunamis. The propagation of energy in a tsunami radiates through what is described as linear wave dispersion theory. (Gonzalez & Kulikov, 1993). Tsunamis are amplified through resonation caused by decreases in bottom depth, narrowing of bays and inlets, and by coinciding with high tide periods present in the inlets. (Henry & Murty, 1993) Prior to a tsunami's landfall, sea levels decrease dramatically like a rapidly receeding tide, leaving the sub-littoral zones exposed. This is followed immediately by the first of usually 3 tsunamis.
In linear wave theory, directional changes occur through simple geometric reflection, therefore transferral of wave energy is reflected by sea bottom relief and bolstering from sides of inlets etc. When the sea floor depth shallows to 0 m, a wave is produced as the end product of the transferral of shockwave energy, which is finally dissipated on land. The effect is the same as normal beach break waves, only greatly magnified.
The tsunami continues inland until the forward transferral of energy is dissipated, then the sea water will succumb to the pull of gravity, and seek the lowest elevations. The resistance of buildings and structures to the force of a tsunami depends upon load ratings of the construction materials in simple equations of fluid dynamics.