Osaka, the second largest city in Japan, has developed on an alluvial plain of ancient sediment from the Rivers Y o d o and Yamato. The alluvial layer consists in the upper part of a sand stratum several meters thick, originally deposited as the top bed of a recent delta, and in the lower part as an alluvial clay. The average thickness of the clay layer is about 15 m, but it becomes thicker as it goes nearer to the coastel zone. Below the Alluvium there is a very thick Diluvium deposit of Pleistocene age, which consists of alternating layers of sand and clay.
In early years, the ground water level in the City was very high and it is reported that even artesian wells could be found in some parts of the City. However, the use of ground water was gradually increased and land subsidence due to the withdrawal of the ground water began to appear.
In the period from 1885 to 1928, when pumping of underground water was not so heavy as in later years, the rate of subsidence in the City was very slight, holding at the almost uniform rate of 6 to 13 m m per year. This slight subsidence is considered to be the result of the natural movement of the earth’s crust and the natural consolidation of the newly deposited alluvial clay. After 1928, however, the rate of subsidence increased markedly with increasing use of underground water. Investigating the oíd data of leveling, the presage of the subsidence due to withdrawal of underground water can be seen in about 1928. This date almost coincides with the time when the use of underground water for industrial purpose became active. Subsequently, a remarkable rate of subsidence began to appear in about 1934, From this period, the precise leveling for the wider ,part of the City was frequently performed by the Municipal Authorities, following the suggestion of 106
Year
Annotat ion : OP: meanr tha lowst law wotrr IwaI observed In Osaka Port in 1- and this watrr level Is wad 01
the Standard Irvcl in tho O m k a Arra.
FIGURE 1-1. Annual Variation of ground water level and amount of land subsidence in Western Osaka
Prof. M. Imamura. Besides such leveling, the consolidated amount of the soil layer and the artesian head in the aquifer at 0.P.l-176 m were observed by the apparatus which had been installed by Dr. K. Wadachi at Kujho-Park in the City in 1938. Figure 1-1 shows the amount of land subsidence of various bench marks in Western Osaka and the annual variation of the artesian head at the Kujoh-well stated above. From this figure, the annual amount of land subsidence in the early period amounts to more than 15 cm. Figure 1-2 shows the monthly variation of the quantity of the water pumped in Osaka City, the variation in artesian head, and the rate of land subsidence measured at the Kujho-well.
In figure 1-2.1, the quantity of industrial water supplied by the newly established Industrial
1. O.P. (Osaka Peil) means the lowest low-water level observed in Osaka Port in 1885 and this level is used as the standard datum in Osaka area.
S. M u r a y a m a
a
YearFIGURE 1-2.1. Monthly variation of water quantity pumped from wells and variation of water buantity supplied by industrial waterworks in Osaka City
Year
FIGURE 1-2.2. Monthly variation of ground water leuel obseroed at Kujoh Park in Osaka
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E E YearFIGURE 1-3.3. Monthly variation of land subsidence observed at Kujoh Park in Osaka
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Water Works of Osaka is also illustrated. The peak points in figure 1-2.1 and figure 1-2.2 lie in 1961, while that in figurz 1-2.3 lies in 1958. In spite of some unconformity in the details, however, the general variation in the artesian water level and that in the rate of subsidence show a close correspondence and suggests the existence of strong relationship between both phenomena. In figura 1-2.3, the period when land subsidence stopped corresponds to the period of recovered level of the underground water (fig. 1-2.2). This was caused by pumping bsing stoppsd due to the destruction of the City by the heavy bombing in the World War II. The total amount of subsidence during thirty five years from 1935 to 1968 in Osaka is shown by the isopleth in figure 1-3. The figure shows that the subsidence is 1arg:r nearzr the coastul zone, and that a zone of almostno subsidence is left in the hilly part in the middle of Osaka, where the very thin alluvial layer is covered.
The pumping wells hava various depths, but most wells are shallower than -200 m.
The use of underground water fluctuates every day, every week, and every season. This
(unit : C F )
FIGURE 1-3. Isopleth of amount of land subsidence in Osaka City from 1935 to 1968
S. M u r a y a m a
is reflected in the artesian water level-levels during the night, Sunday and winter were mostly higher than usual.
D u e to the remarkable land subsidence stated above, the ground height of a part of Western Osaka has lowered below sea level and the City is exposed to the danger of flooding caused by a high tide in Osaka Bay. Figure 1-4 shows the ground height of Western Osaka in 1961.
The flood feared was unfortunately realized in 1934 by the Muroto Typhoon, one of the biggest typhoons which have ever attacked Osaka. As a result an area of about 49 sq k m was flooded by the high tide of O. P. + 4.20 m.
FIGURE 1-4. Ground height of Osaka City in 1961
'1 10
In order to prevent such a disaster in the future, more satisfactory dikes have been repaired and built. Besides these prevention works, the use of underground water has gradually been regulated in phase with the completion of the industrial water-supply works planned as the substitute of the underground water. (see fig. 1-2.1).
Owing to the regulation against the use of ground water in Osaka City land subsidence in the City gradually decreased and is almost stopped at present. This result can be recognized by comparing figure 1-3 and figure 1-5; the latter shows the isopleth of the amount of land subsidence from 1963 to 1968.
In spite of the success in preventing the land subsidence in Osaka City, however, the land subsidence in Eastern and Northern Osaka has increased remarkably during'the last few years because these regions have been developed lately and many factories using much
FIGURE 1-5. Isopleth of amount of land subsidence in Osaka area from I963 to 1968 (unit: c m )
underground water have been built. Figure 1-6 shows the amount of land subsidence in Eastern and Northern Osaka measured since 1965 at several bench marks. This figure shows that the subsidence at Nagase observation station come sup a remarkable 220 IMI
during only three years. Figure 1-7 gives the monthly variation of ground water level and
S. M u r a y a m a
the rate of land subsidence of the several observation stations in Eastern and Northern Osaka. From figures 1-7.1 and 1-7.2, it seems that the shallower the ground water level is, the less the rate of land subsidence becomes.
In order to prevent land subsidence in Eastern and Northern Osaka, the use of under- ground water for industrial purposes has been regulated by law since 1965 in the Northern
! 3 4 5f
Yeoi
@@- :i
I
FIGURE 1-6. Amount of land subsidence in Northern and Eastern Osaka
region and since 1966 in the Eastern region. The industrial water supply service to substi- tute for the underground water has been proceeding and the preventive effect should be realized in the near future.
The annual land subsidence from 1967 to 1968 is illustrated in figure 1-8. This figure shows that the land subsidence in Osaka City has almost stopped except for the
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FlGURE 1-7.1. Monthly variation of ground waier level in Northern and Eastern Osaka
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coaste1 zone arround the Port of Osaka. However, the land subsidence in Eastern Osaka is still active and there is a large area of land subsidence whose m a x i m u m amount is 16 cm.
Besides these land subsidences, it can be found that a new area around Kishiwada City in the Southern Osaka is developing. Though the size of the area is still very small, it m a y be important to pay close attention to it.
Y e a r
FIGURE 1-7.2. Monthly variation of land subsidence in Northern and Eastern Osaka
In order to protect the City from high tides, as described previously, high-tide preventive dikes with a height 5 m above O.P. were constructed along the bay coast and both sides of rivers running through the City by the Osaka Prefectural Government and the Osaka Municipal Office. The total length of those dikes amounts to about 124 kilometers.
Furthermore, in order to withstand a severe high tide caused by an extra large typhoon, the Osaka Prefectural Government n o w has a project to raise the height of the existing high-tide preventive dikes up to 6.6 m above O.P. and is constructing high-tide preventive locks across the rivers at the most effective down-stream sites.
2. G E O L O G I C A L C O N S T I T U T I O N AND SOME P H Y S I C A L P R O P E R T I E S OF