Estuary Processes and Managment - River Dynamics and Integrated River Management - page 475

The subsidence is primarily caused by excessive groundwater withdrawal. Other reasons for the subsidence
include the oil withdrawal and the neotectonic movement. The common characteristics of land subsidence
are slow, accumulative, and irreversible. Adjustment of the aquifer exploitation practice is a subsidiary
way to control land subsidence, but it cannot solve this problem completely (Xue et al., 2005).
Built on coastal sand and clay that lie 70 m below the ground surface, Shanghai is suffering from
creeping subsidence. In addition to the overuse of underground water, the mushrooming of skyscrapers in
central Shanghai has also contributed to the city's creeping subsidence. In the period 1957-1965, the city
was sinking about 9 cm a year. At that rate, parts of the city would have been flooded by 1999. Control of
ground water withdrawal and ground water recharging are effective measure to reduce subsidence. In the
period 1966-1971, the subsidence was controlled and the ground level had been enhanced by a rate of
0.3 cm per year because of ground water recharging (China Daily, 2003). Nevertheless, the rate of subsidence
increased again due to ground water withdrawal from the 1970s to 2000. The subsidence rate was 1.6 cm
in the 1990s. Paying great attention to the problem, the city was installing subsidence and leakage monitors
inside the many subway tunnels now under construction to keep track of the problem. The city reduced
use of groundwater and is using increasing amounts of river water and pumping water back into depleted
aquifers. These measures have been proved effective. The rate of subsidence for Shanghai City in 2000
was 1.21 cm and in 2006 was 0.75 cm.
8.2.2 Tsunami and Hurricane
Ts u na m i — The term tsunami comes from the meaning harbor and wave in Japanese. Tsunamis are common
throughout Japanese history; approximately 195 events in Japan have been recorded. A tsunami has a
very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at
sea, forming only a slight swell usually about 300 mm above the normal sea surface. A tsunami can occur
at any state of the tide and even at low tide will still inundate coastal areas if the incoming waves surge
high enough (http://en.wikipedia.org/wiki/Tsunami). Tsunamis are among the most terrifying natural
hazards known to man and have been responsible for tremendous loss of life and property throughout
history. In the Pacific Ocean where the majority of these waves have been generated, the historical record
shows tremendous destruction. In Japan tsunamis have destroyed entire coastal populations. There is also
a history of tsunami destruction in Alaska, in the Hawaiian Islands in South America and elsewhere in
the Pacific, although the historic records for these areas do not go back sufficiently in time. Historical
records also document considerable loss of life and destruction of property on the western shores of the
North and South Atlantic, the coastal regions of north-western Europe, and in the seismically active regions
around the eastern Caribbean (George, 1985).
Tsunamis may be generated when an earthquake occurs causing the floor of the ocean to vertically
displace the water column—one part "rises" whilst the other part "sinks". This occurs in seconds and huge
volumes of sea water have incredibly large levels of energy transferred into them. This energy radiates
outwards in a series of low frequency and therefore long wavelength waves which generally have an
amplitude of about 300 mm above the normal swell of the ocean and are rarely observed as a result. It
should be noted that the displacement forms a trough and a peak as it moves away from the zone of
activation and will usually come ashore in the same manner. The trough causes "drawback" whilst the
peak arrives as a sudden surge. Tsunami possible may be caused by landslides and explosive volcanic
action, which rapidly displace large volumes of water, as energy from falling debris or expansion is
transferred to the water into which the debris falls at a rate faster than the ocean water can absorb it.
Historically speaking, tsunamis are not rare, with at least 25 tsunamis occurring in the last century. Of
these, many were recorded in the Asia—Pacific region—particularly Japan. On 1st April, 1946 a Magnitude
7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which