6. Map Analysis Click here first for Interactive Map Presentation Next chapter
The area chosen for this study is a rectangular region bounded by the longitudes 112.5 deg to 114.3 deg. East, and 21.75 deg. to 23.5 deg North, an area of roughly 400 by 550 kms. All maps, (except where noted) were created by MapInfo Professional, v.4, using copyright data from ACASIAN. The images were then processed in PaintShop Pro v.5. The overlaying maps were automatically registered to latitude and longitude in MapInfo, while the Landsat and Population Density maps were registered manually, as they came from different souces. The Interactive Map Presentation was specifically designed to use the "frames" facility of Java enabled broswers, to quickly and accurately compare maps showing different data.
Landsat Image (Click to view map)
The LandsatTM Image was adapted from a map produced in 1992 by Geocarto International Centre using Landsat data processed by RSGS China. The colours are digitally enhanced to emulate the light frequencies as seen by the human eye, ie. it is an attempt to create a true colour image. It was produced on a different projection to the other maps, hence the 13 degree rotation from vertical, and the slight error in registration to the other maps. It offers a superb "photo-realistic" aerial view of the region, in colours that are quite accurate to the ground features, and provides physical details not present in the maps produced by vector graphics software. Most importantly, it offers a very accurate and detailed topographic picture of the region not found in the other maps. By taking note of the shadows and light source, it is possible to recognise individual mountain ranges, land contouring and relief features which will assist in determining the areas susceptible to tsunamis.
Features (Click to view map)
The Zhujiang Delta, or the mouth of the Pearl River, is the ocean outfall for practically all the major river systems of Guangdong Province. The estuary is 35 - 40kms wide and narrows to Zhujiang River 60kms inland, and has a depth of no more than 20m. A further 50 kms up river lies the province level administrative centre, the city of Guangzhou, with more than 3 million inhabitants. On the western heads is Macau, on the eastern is Hong Kong, and along its shorelines lie at least 25 towns with populations greater than 10,000 registered inhabitants. Between 10 and 20 kms offshore lie the small island systems of Wanshan and Dangan, and 20 kms inland lie the islands of Nei Ling Ding, and Qiao which occupies about 12 sq. kms. The region lies centrally at the top of the South China Sea, boxed in by the Phillipines to the east, Hainan and Vietnam to the west, and Kalimantan, more than 2000kms to the the south.
Two motorways, one running up the coast 12kms inland on the west side, and one 6kms on the east side, connect Macau and Hong Kong to Guangzhou in the north. At Humen the motorway crosses the Zhujiang R. on a bridge that appears to be several km in length. There is no data available on the bridge height or construction, but it appears the engineering is taking advantage of 2 hills of greater than 30m either side of the river mouth. Between the west shoreline and the Macau-Guanzhou motorway lies a road classed as highway in the Atlas (CCPH, 1989), though whether this is accurate remains to be seen.
Land Use (Click to view map)
This region is one of the major rice producing regions, as well as one of the most productive economic zones of China. Very little land that is not hilly or mountainous remains untilled. The predominant land use of the delta region is rice farming, in the form of plain and terraced paddies and diked ponds, which lies under 20m. These areas occupy the low-lying flat and nutrient rich land formed by estuarine deposits of the rivers, and are very labour intensive; most of the work would be done by hand, with the odd mechanical hoe finding use. On the higher areas, above 30m, the land is mostly left to forest and woodland. To the north near and around Guangdong is vegetable plots and orchard farming. The region is very densely populated, with practically all the land under 15m being inhabited by at least 300 - 500 persons per sq.km., but mostly 500 - 1000 per sq.km.
Along the shores of the delta, for roughly 50 kms on the east bank from Nantou to Songgang, and 40kms on the west bank from Tangjia, is muddy flats, which would be exposed during low tides, and covered during high tides. The whole region is dotted with small townships only 2-10 kms apart. Adjacent to the muddy flats near the mouth of the estuary are several areas of saltwater breeding plots.
Elevation (Click to view map)
The elevation data used for the creation of the topographic map was provided free by internet download from the EROS Data Centre website, and processed in ArcView and MapInfo GIS systems at ACASIAN. Since the information is available to the public over the web, it is of a lower resolution than commercial or military use data sets, being provided at 30 arc second intervals, as opposed to 5 second intervals that EROS can resolve to. Each square pixel seen in the map represents a 1 sq. km area of the Earth's surface, and the elevation given is measured at the centre of the square.
The artificial colouring of the map has been grouped into 3 different hues, reds, yellows, and greens, to correlate to various important elevations related to tsunami heights. According to run-up data in the NOAA databases, an average maximum run-up height for tsunamis is 30m, so elevations at 30m and above have been coloured various shades of green, to indicate relative safety. Elevations of less than 10m have been coloured shades of red, a colour easily recognisable as danger. Simple single storey residential and urban structures are generally less than 5-6m in height, and the frequency of tsunamis of 10m height is much higher than waves of greater than 10m.
The mouth of the estuary is protected by mountainous headlands on the east and west banks, with Wu Gui Mtn. north of Macau at 530m, and Lantou Mtn on Lantou (H.K.) at 935m. 3km inland from Houjie, on the east bank an unnamed mountain reaches 376m, while 15km inland from Humen on the NE banks lies another mountain at 530m. Guangzhou, a river city with elevations no more than 15m, lies almost 50kms upriver at the foot of a small range of mountains, varying from 372m to 433m.
Two very large basins, which lie on either side of the Zhujiang R., are less than 10m in elevation. These are the rice paddies, which are low-lying, flat and featureless tracts of land, are generally inundated with water most of the year round. They are intersected with a very complex and constantly changing network of water channels, (see Run-Up Map) The area to the east of Guangzhou in particular, occupying more than 250sq.km is very low land, less than 3m.
Along the east bank of the estuary, from Nantou through Humen to Xintang, ranging from 6 - 12 kms inland, the shoreline elevations are less than 7m. Across the river, lie 2 arms extending out from the shoreline, at Nansha, Wangqinsha and Xinken, also on low land.
The Wangshan Islands at offshore from the mouth of the estuary, are generally 30m to 75m in elevation, while the Dangan Islands are very low lying, at roughly 3m, save for a few mountainous peaks to about 75-100m.
Shenzen, buffering the New Territories and mainland China, despite lying in very low land, is protected from the ocean by Lantau Island, while the western banks of the estuary from Xinsha north to Nanlang are protected from the sea by the Promontory at Tangjia. The island of Qiao isn't so fortunate...although it has 2 hills over 30m, the southern shore and central inland is less than 15m, which is where most of the population is concentrated. The aspect and topography of Qiao will make it incredibly vulnerable, with quite high run-up heights, due to the relative steepness of foreshores.
50kms west of Macau, the Tanjiang R. runs through very low land, near the towns of Xin, and Jiangmen where the land is 1-2m. This low land lies at the head of the Ya Delta, and would replicate the effects found in the Zhujiang Delta.
County Populations (Click to view map)
County and Town population figures are taken from the 1990 census of urban residents, ie. those inhabitants who are registered as living in the censused area. These figures do not take into account population growth of the past 8 years (no data is available), nor does it accommodate the "floating" population, those inhabitants who are not registered as urban residents. These are itinerant workers, people on the move, visiting relatives, and persons inhabiting the region without registration...which is quite common. Although there is no data for these floating populations, estimates are that the percentages are quite high.
By calculating total populations from the towns map and comparing it with the county populations figures, it will be noticed that the towns map can only account for little over half of the stated county populations. This is because only towns of greater than 10,000 inhabitants are shown. What is not shown, are the hundreds of small villages and hamlets, as well as residences on private plots of land that are interspersed among these towns. No data is available for these villages either, but estimations will have to be made based upon photographic evidence and first hand experience of rural landuse and residential distributions in these areas.
Six counties immediate to the waters of the Zhujiang estuary will be vulnerable; Zhuhai Shi, Zhengshan Xian, Panyu Xian, Dongguan Shi, Bao'an Xian, and to a lesser extent Shenzen Shi. The southern shores of Hong Kong will also be vulnerable, though theyare somewhat protected by Dangan Islands.
Dongguan Shi. This county has about 20 towns of at least 10,000 person, and a large city of at least 290,000 persons. This would account for more than 520,000 persons, but the county register has a population of 965,470. How do we account for the remaining 445,000? Even if those 20 towns held 20,000, we still cannot account for 215,000 persons. There must be large numbers people living outside the towns. If we refer to the Landsat map, we can see that there are built up areas not indicated on the Landuse map, Towns map, or the Features map. Take, for example, the triangular promontary of land with Humen near the mouth of the Zhujiang R. The Landsat Image reveals a rather large township on the southern shore of the promontary. It's not Shajiao, as it lies on the north western side of the promontary, south west of Humen. This built up area is on the water line and occupies perhaps 4 sq.kms. By comparing its size with a town of equal area, say Donglicun on the southbank of the Hong Men R., we can see that it would have a population of around 10,000 as well. So the Landsat Image is indispensable in helping us form a realistic picture of the state of the Zhujiang delta.
Town Populations (Click to view map)
The towns map and features map both lack detail for showing the number of towns and buildings, but interspersed among these large townships of 10,000, lie smaller towns, and between them, the countryside is dotted with small farm residences and houses on private plots of land at 50m distance apart. Along the shoreline of the estuary from Nantou and Macau upriver 20kms to Xintang, more than 25 towns of at least 10,000 inhabitants lie less than 4 kms inland. Most of these towns are situated on land less than 10m, except for a stretch of about 35kms from Tiangjia to Minhe, west of Xinken, which is 10-15m in elevation, yet fairly protected from the ocean, anyway.
Population Densities (Click to view map)
This map was adapted from the Population Atlas of China, and registered manually to the other maps created in MapInfo, and is accurate to about 1km. The coloured squares are bounded by a longitude interval of 1' 52.5", and a latitude interval of 1'15", giving boxes of roughly 7.34 sq. kms maximum at the top of the map. The vertical line down the centre of the map is a discolouration caused during the scanning process by merging two large map scans together. All attempt has been made to adjust the brightness and hues to a standardised index in the legend.
The population density figures are based upon data available in 1987 so it would be safe to use the maximum figures in the ranges contained in the legend, possibly even to inflate them somewhat. Coastal regions are more desirable to live, as the weather tends to be more temperate, and it is close to a plentiful food supply. The delta land, being rich, flat and close to a source of water makes it ideal for heavy cropping. Moreover, according to a report from the American Association for the Advancement of Science, nearly 100 million Chinese are thought to have moved from the poorer provinces in the central and western regions to coastal areas in search of better economic opportunities for themselves and their families. At any given time, somewhere between 20 and 40 million Chinese are on the move, a population the size of Spain. The bulk of this large, "floating population" is concentrated in coastal provinces, precisely those areas with the highest economic growth rates. (www.aaas.org/ 1995)
Ocean Bathymetry (Click to view map)
The map of the South China Sea bathymetry has been adapted from a special map produced by the NOAA. The pixel resolution is not known, and the depth legend has been prepared from several sources (The Times Atlas of the Sea, 1989, Atlas of China, 1989) South China Sea is roughly trapezoid, being 2000kms in length, and a little over 1000kms wide. The eastern boundary of the Sea lies with the Phillipines, extending south to Kalimantan, placing it on the edge of the "Ring Of Fire", the earthquake zone which marks the edge of the Pacific Plate. There is no virtually no continental shelf on the west coast of the Phillipines, whereas adjacent to the Guangdong coastline, the shelf extends about 230kms. The shelf at 200m depth drops away relatively evenly parallel to the coastline to a fairly flat and even floor at 5000-6000m. 500 kms directly south to south-west of the Zhujiang lie two ocean rises, the Xisha and Qunsha Qundao, or Macclesfield Bank. This small rise comes to at least 200m and possibly has some reefs. This site was near the epicentre of a magnitude 5.5 earthquake within only 15 days of the 17 September 1998 IRIS event query made by the author (see IRIS Map) 1200kms south-south-west lies another rise north of the nearby Kalimantan coast, the Nansha Qundao, which is more open and scattered than the previous two. It also rises to 200m and displays some coral reefs. If one draws a line perpendicular to the coast and extending south east out to sea, at the continental shelf one can see a steep corridor roughly 200kms wide with protruding buttresses of shallower slope on each side of the corridor, at depths of around 500-1000m. Adams & Lewis have labelled this sea floor shaping as "flaring cosine relief" (Adams & Lewis, 1979) This would have a focusing effect on a tsunami wave. At 100-200m however, the silt deposits of the river outfall have created a minor protruding slope, which would also have a modifying effect on the tsunami wave. This, Adams & Lewis label as a "tilted cosine relief" (See below)
Coast Bathymetry (Click to view map)
The coastline of Guangdong is relatively straight for about 1200kms, and has a relatively even and perpendicular sloping sea floor above the edge of the continental shelf, that drops away 20m roughly every 35kms giving a 1:1750 slope. The long slope would have the effect of slowing down the tsunami, while a steeper slope would have the effect of speeding it up. From the continental shelf, the sea floor drops away quite steeply from 200-6000m in roughly 210km, giving a 1:36 slope. This demarcation of flaring relief to tilted relief will surely have a modifying effect upon an incoming tsunami.
Wave Propagation (Click to view map)
The wave propagation shown in the accompanying map is a rough estimation of the projection and radiation of a tsunami caused by an earthquake in the South China Sea. As mentioned previously, wave energy radiates following by simple laws of fluid dynamics. The long waves radiate out from the epicentre, and as they approach the coastline, compress and flatten out to follow the relief of the coast. Small islands in the mouth of the delta would not receive as high a wave as the coast proper, as the transferral of energy is still being contained under the sea level. There would be minor, localised waves on shore as the shockwave of the tsunami passed by. As Adams & Lewis (1979) have shown, these small islands have little modifying effect on the tsunami, they would rather contribute to some measure of noise in the shape of the long wave.
As the long waves bounce off the continental shelf back to sea, the tide appears to recede. Once the long wave reaches the coast and mouth of the delta, the shallows compress the water into a rising wall and the shockwave starts to cause water to move forward. The banks of the delta at the mouth of the estuary will then compress the wave inwards on a horizontal plane by reflection, much the same way normal beach waves break around rocky headlands, and curve inwards to the beach.
This sideways compression, combined with compression from the decreasing estuary bottom, can only cause an increase in forward momentum and wave height, and thus the tsunami effect is under way. Bearing in mind that the 1998 PNG tsunami struck a 30km stretch of coastline, the 40km wide mouth of the delta would absorb the entire force of such a wave. This huge body of water will then be compressed into a space only 18 kms wide, and less than 10m in depth. The inertia would carry the wave up several of the river mouths at the reach of the estuary, and would probably extend many kilometres up the rivers, much like a hurricane driven storm surge. Henry & Murty (1995) describe this effect, known as "resonance amplification" observed in the March 1964 Alaska Earthquake, where an earthquake generated wave travelled up Barkley Sound on Vancouver Island, 65kms inland to Port Alberni. This compression by the inlet caused a 3-fold amplification of the measured tsunami height at the head of the Sound.
Run-up (Click to view map)
In the accompanying map, run-up is shown as bright red, and the areas likely to experience subsequent flooding are shown as pink. Blue represents the hydrology of the Delta region, which is derived from ACASIAN data and processed in MapInfo. This map is produced as an estimation of tsunami hazard, based on Elevation, Topography, and Wave Propagation data shown in their respective maps. The red colouring also will indicate run-down, where the run-up gets pulled by gravity downhill and back out to sea. It also indicates the areas likely to experience a wash over by the wave, where the water finds low ground inland and causes flooding. The south coastlines of Hong Kong and Macau show little red, because their shores are steeply sloped with mountains close to the water line. Run-up effects will be immediate and dramatic. Places that will receive full force of the tsunami are those areas that lie parallel to the face of the wave. In this case, the wave will have a generally east-west orientation, so coastlines and shorelines facing south will be the worst hit. Shorelines that run perpendicular to the face of the wave will experience a less destructive effect of the tsunami, which will have a "shearing" action as it moves along the banks of the rivers upstream.