4. LAND  Next Chapter

4.1 Topography Types Susceptible to Inundation

The shape of the land under sea determines the strength and direction of the tsunami toward the coastline, while above ground it determines the run-up. Since the generation of a wave is defined by transferance of energy, or force in simple mathematical terms, the decrease in sea floor depth has the proportional effect of focusing and speeding up the movement of the wave. Thus a wave generated from a large, deep body of water will produce larger waves than those produced from a shallower body of water.

As shown by Adams & Lewis, (1979) offshore coastline has a modifying effect on the wave shape as it breaks on land. The most destructive waves are where the force of the wave is focussed as it breaks on land. Beaches, with their concave sea floor relief are thus more susceptible are than promontaries to damage from a tsunami of equal force, because the convex relief of the sea floor deflects the wave off centre and out to the side.

Beach breaking tsunamis have potentially more threat for two reasons. Land behind beaches tends to be lower than land behind promontaries, therefore the penetration by a wave would be greater. Secondly, populations tend to congregate around beaches for reasons of leisure, and practicality of access for fishing vessels.

The tidal period will also affect the height and run-up of a tsunami. Low tides, where the water's edge recedes off shore due to lowering of the sea level have a mitigating effect of reducing the threat of tsunami. Combined with a high tide period where the sea level is higher and closer in shore, tsunamis pose more of threat.

A tsunami hitting an estuarine environment will also have devastating effects for several important reasons. Since estuarine environments are not subject to normal wave action from the sea, only indirect tidal influences, the coastline topography can be quite different, particularly with large delta regions such as Zhujiang. Most of the land mass around the shorelines of a delta region is flat, and barely above the water line, and is somewhat static. In this type of environment, populations can and do congregate very close to shorelines, due to the lesser apparent wave threat (bearing in mind that people are thinking of ocean waves, not tsunamis), and also because of the easy access to plentiful food supplies. Most of these coastal communities have a lifestyle that revolves around water, fishing, boating, washing and irrigation are all activities associated with estuarine lifestyle. This familiarity with close proximity to water would lead people to have less fear of water, and hence possibly, less caution.

Coastal estuarine communities of developing countries also generally build with traditional construction techniques and materials. Simple wooden structures designed for ease of construction, from readily available materials, such as wood, twine and leaves are the norm. Communities such as the Papuans, the Javanese, still Phillipino still follow traditional building methods to this day. These types of structures have absolutely no rsistance when it comes to meeting the force of a tsunami. The Aitape tsunami of July, 1998 is a prime example of this type of situation. The coastal village communities built virtually on the beach, and around the lagoon at Sissano, and the 10m tsunami simply wiped the shorelines clear of all structures.

Secondary to tsunami effects on coastlines, is the effect of the wave as it travels up rivers. Henry & Murty (1995) described the process of "resonance amplification", where the wave grows as it travels higher upstrem. Another effect is wave shear, where the river banks and their features are "scraped" by trapped "edge waves" as it travels upstream. This effect is almost identical in mechanics to tide surges and storm surges, and has been mentioned by Carrier in a paper on transverse waves travelling parallel to coastlines (Carrier, 1995)

Another feature of topography that adds to tsunami threat is the presence of hills or slopes behind built structures lying on coastlines. Slopes cause a very destructive tsunami phenomenon known as run-up, where the wave will wash in over land, and the inertia will carry the water inland and push it up the slopes of a hill or mountain range, where it will then wash down, back into the sea. Historically, some absolutely incredible elevations have been reached by run-up. The July 9 tsunami in 1958 at Lituya Bay, Alaska recorded scouring by waves at the height of 1720 ft above normal sea level, (Halacy, 1974). This case was unique as Lituya Bay is shaped like a cul-de-sac with a ring of high and steep mountains around the foreshores. Nevertheless, the force necessary to create such a wave was tremendous, the triggering earthquake recorded a magnitude of 7.9, and bathymetric conditions present only compounded the effect. Other events have recorded run-up heights as much as 100m and 50m, however the frequency of such events, fortunately is quite low.

Another effect of run-up is run-down, where the water naturally runs back to the sea. This has a doubling action against built structures, attacking them from both sides, but it also has the effect of washing people and structures out to sea. In the 1998 PNG event, only 2000 bodies from 8,000 missing were recovered. While many were left to rot in Sissano Lagoon due to the overwhelming situation, many were washed out to sea. According to the Sydney Morning Herald, 5 weeks after the event, reports came from Indonesia of bodies being washed ashore (www.smh.com.au)

4.2 Reasons For Selecting The Zhujiang Delta For The Project

This project is built on the premise that there will be a very large tsunami event in the South China Sea in the not too distant future. The area is not free from earthquakes, events have been recorded as recently as September 1998 (see IRIS) While the Zhujiang delta is not under as serious threat from tsunami's as is Japan or Hawaii, there are many good reasons for my selection of the area for such a project.

Japan and Hawaii have an efficient and effective tsunami warning and mitigation system in place, albeit in the early stages. The system has worked, as already the Tsunami Warning Centre has had to issue evacuation warnings to residents, which has had the effect of preventing loss of life. Japan, with it's long history of tsunami's, has naturally become wary of the dangers of living close to the coast, and have established similar warning and mitigation systems to that of Hawaii. The Zhujiang delta on the other hand, does not. The communities are only just coming out of development stage to modernisation, and undoubtedly no thought has yet been paid to tsunami warning or mitigation. At least, no published work is available to date.

The coastline of Guangzhou is still subject to some measure of threat, much more so than many populated coastal regions around the world. It lies less than 1,000 kms from the "Ring Of Fire", the edge of the Pacific Rim to the east, and has bathymetric conditions in the South China Sea that are typical of the majority of events. The low-lying plains and shorelines of the estuary are becoming heavily populated; and the shape of the delta itself has the conditions to create a funneling effect in focusing a wave impinging upon the shorelines. Potentially, there is great cause for concern for the region.

The delta region is very representative of the type of coastal development of populations around many parts of the world, especially developing countries, or those countries becoming modernised. China's huge population is growing at an enormous rate, and many coastal regions along the Chinese mainland are following the trend found at Guangzhou. We have the advantage of having much more data available on the Guangzhou region than we do other coastal regions of China, which enables us to make a deeper examination of the problem than we could if we were looking elsewhere.

Next Chapter