7. THE HYPOTHETICAL MODEL Next Chapter
7.1 Set The Parameters For A Worse Case Scenario.
An earthquake of magnitude 8.5 occurring 100kms below the Earth's surface is recorded just south east of the Macclesfield Bank, in the central South China Sea. It caused a massive land slippage 250kms offshore, where a large body of silt deposition from the Pearl River Delta dislodged and resettled, causing a 100m drop in the sea floor over an area of almost 20,000 sq.kms.
It's 1.00 o'clock on a Monday morning. Calls are placed to Hong Kong, Macau and Guangzhou to warn local authorites of the recorded event, but they are unsure how to respond.
Twenty minutes later, a 20m wall of water stretching 60kms along the coast of Guangdong is approaching the Pearl River Delta. The tide is rising, and the outflow of water from the Pearl River delta has slowed due to the incoming tide. The wave enters West Lamma Channel between Hong Kong and Lantou, breaking on the New Territories just west of Kowloon. It travels up the inlets of the Pearl, Xi, and Tan Rivers, increasing in height due to compression of the river bottoms and sides, and breaking on all shorelines facing south. When it reaches the mouth of the Zhujiang River, the wave is 30m tall. The main wave travels up the Pearl River, past Huangpu and surges as far as the city of Guangzhou. Secondary waves travel up the Xi and Tan Rivers, with similar effects. Most of the encroaching water finds low ground and floods the region, while the rest recedes as run-down, back to sea.
20 minutes later, a second, larger wave generates off the coast, and makes its way inland in the same way, penetrating further than the first. A third and final wave, of lesser size than the first two arrives another 20 minutes later
7.2 Effects on Land And Populations
7.2 (a) Dynamics of the wave action against land.
The East Side.
At the entrance to the Zhujiang estuary, the 20m wave is just forming. It breaks on the southern shores of the islands of Lantou, Hong Kong, Lamma, and Po Toi on the west side. Lantou and Hong Kong Islands shield the city of Kowloon and most of the New Territories, but a smaller wave of several metres passes through the Lamma channel and breaks on land at Stonecutters Island, and Kao of the New Territories. The city of Shenzen is not affected, but the towns of Baishizhou and Rongshujiao experience smaller waves of 5m. There is extensive flooding along the low-lying land of the Shenzen River as the 5m surge travels upstream towards Shenzen. The towns of Xin'an and Ziyou along the shore north to Shiwei, all on land under 6m in elevation, experience 10-15m waves that break 100m inland after travelling 1km across the freshwater breeding plots, and encroach as wash up to 1km inland. The road from Nantou to Humen is damaged in many places. One of the worst hit shorelines is the promontary at the mouth the Pearl River itself, where there is a large town on the southern shore, and several towns, on the other side including Humen. Here the wave had grown to 30m, and since the land is all under 6m at this point, the wave breaks and washes over the area to 1.5km inland. The wave travels 20kms up the Pearl River as a 30m surge, having a shearing effect on the banks of both sides of the river. The land on the east bank is more than 1-3m in elevation, and the soft irrigated soils that support the rice paddies get washed away into the surge. The wave spreads through the intricate network of river systems towards Dongguan, inundating the whole low-lying paddy region with a muddy mixture of estuarine and salt water. This surge and shear effect was also replicated up the Xi River as far as the city of Jiangmen, and the Tan River as far as the city of Xin. As the surge wave reaches Huangpu and Xintang, the Pearl River turns to the west. Here the forward momentum would carry most of the water onto land where it would break on another large town, also on the banks of the river. The Guangzhou railway experienced flood damage between Xintang and Huangpu, effectively cutting off rail access. Since the wave, as it travelled up the river lost a lot of it's size and force due to dissipating into the side rivers and inundating the low-lying paddies, the resonance amplification effect is not as pronounced as the surge that travelled up the Xi River to Jiangmen. Guangzhou experienced a minor surge where water breached some of the roads and pavements.
The West Side.
On the west side the 20m wave breaks on the southern shorelines of Hengqin, Coloane and the Gaolan islands. As these islands have sloped shorelines, as do Lantou and Hong Kong, they experience run-up effects to the height of more than 50m. These islands shield the city of Macau proper, but not Gongbei, Zhouzai and Zhuhai east of Macau. Tiangjia, in the center of a small bay experiences a 25m wave that completely washes over Tonggu Point. The estuarine island of Qiao experiences a 25m wave also, with a run-up of up to 50m. Townships along the western shorelines from Xiazha to Linx 20kms north, experience only a minor change in wave heights due to their aspect, but Xinken, Xin'an, Tongxing and Wangchinsha bear the brunt of a 30m wave, depite an elevation of 15m. The wave runs up the slope on the southern tip, and washes over into the towns situated among the paddies. The landsat image shows an island in the stream south of Nansha that does not appear on the MapInfo maps. This is probably reclaimed land converted to paddies, at barely 1m elevation. The wave washes over this 5km island and breaks on Nansha and Nanheng, which experience run-up to 50m. The tributary arm that runs into Heng Men (entrance) experiences minor changes in water height as it is protected at the mouth by an island, and runs perpendicular to the direction of the wave. The river entrances at Minhe and Nansha, however, do catch the force of wave, and it surges up stream past Xinan and Nansha, inundating the large basin at Shunde and south of Nanhai.
7.2 (b) Effects on Population.
Effects of a major tsunami on a population should be measured
in terms of (1) immediate effects, (2) short term effects, and (3) long
Immediate effects are deaths and injuries in the first few hours, as a direct result of the wave, such as drowning and physical trauma caused by impact with water borne objects. A series of three waves would have a combined effect much worse than 3 separate waves at different times, because survivors of the first wave, weakened by their struggle to survive, would probably not last through a second and third wave. Short term effects would be experienced in 1-3 days subsequent, where stranded survivors, too weak to move and in need of urgent medical attention, are unable to withstand the elements before help arrived. Long term effects will be found 5 days - 2 weeks afterwards, as homeless and hungry survivors struggle to stay alive with the limited assistance of the emergency authorites. In the PNG tsunami, death tolls were rising up even 2-5 weeks after the event, where survivors were dying from hunger, exposure, infection and disease (Phillips, 1998, and BBC News, 1998). Also in the long term, is the effect on the food supply of the region, as this area is one of the country's main producers of rice. Inundation by salt water will make the soil untillable for a long time, unless efforts are made to divert the flow of rivers to continually flood the land in order to leach the soil of the salt.
A few locations pinpointed around the delta will be examined to estimate effects on populations. At the mouth of the Zhujiang River, on the east bank, where a triangular point of no more than 6m elevation sticks out into the estuary, there is a large (unnamed) town of at least 10,000 south east of Shajiao. The 1990 population densitiy figures for this area show up to 1000 persons per sq.km over an 7.3 sq km section. The wave is estmated to wash up to 1.5 kms inland, which would destroy the town and residences along the coast. Here the immediate toll could be as much 10,000. Across the river mouth to Nanheng and Nansha is another area of concentrated population, roughly 15 sq. kms with 500 -1000 persons per sq. km. Upstream to a point midway between Huangpu and Xintang, is a large town on the banks of the river just as it starts to narrow, and turn west. A lot of water would wash over this area, as the wave compressed as it surged upstream. This township, shown on the Landsat, but not the MapInfo maps, appear to occupy an area equal to the city of Macau proper. There are perhaps 20,000 persons within a 3km arc from the tip of the point. This town is backed by mountains at least to 100m, so run-up and run-down wil be experienced.
7.2 (c) Effects on the Natural and Built Landscape
Only structures that lay in the path of the wave itself, and the backwash would be destroyed outright. Photographic documentation of building types in intensive subsistence regions such as the Zhujiang Delta show few wooden structures, but mostly brick and cement rendered houses with tiled roofing. Brick houses have some measure of protection against the action of water, but should the structure give way, injury to occupants is great due to the nature of the building materials. Wooden structures, such as those mostly employed by the PNG residents, are flimsy and easily washed away, but due to their flimsy nature, are not as great a threat to the occupants. Structures that may have only been weakened by the first wave yet still remained standing, may then be demolished by the second and third waves. Structures ruined due to water damage or flooding would be rendered unusable until repairs and clean-up efforts could be made. Space for makeshift medical facilities and emergency housing serviced by road access needs to be created
Roads do not stand up well to water damage; the scouring action of flowing water erodes the surface rapidly, and a smooth surface is needed for them to be practical. As most of the population, being surrounded by so much water, would travel by small boat, transport will be major concern for emergency services in reaching survivors with aid. Boats will be good forshort distance travel, but road travle will be necessary to service all areas of the Zhujiang Delta effectively.There are a number of bridges evident on some maps, and the Landsat image, but data on these is not available. Transport will be crucial for emergency services to reach devastated areas, and service them
Destruction of the rice paddies would be a major concern. Inundation by a mixture of salty and estuarine water, as well as the scouring action of the waves which would upset all channeling and landscaping work, would have a major impact on the food producing potential of the region. As the region relies heavily on intensive farming production, upset to the normal day to day routines that maintain the upkeep of the food growing areas will cause undamaged areas to be unattended, thus raising the possibility of losing perfectly good crops.