NIIGATA GROUND SUBSIDENCE AND GROUND WATER CHANGE
6. STANDARD LEVELS OF THE GROUND WATER
The above statement concerns with the analysis of the data shorter than one year. But the data for about 10 years from the beginning of the severe subsidence are the object of discussion hereafter.
In figure 10 are shown monthly mean values of the subsidence speed per day and the height of ground water level of the 610 m well together with those of barometric pressure and mean sea level, the former-two changing gradually in parallel with each other, that is, the subsidence speed decays with rising ground water level. It is also observed that one year period of subsidence speed can be clearly traced in parallel with the barometric pres- sure without any phase lag larger than one month. The ratio of the undulation in the subsidence speed to that in the barometric change is roughly estimated as 0.08 mm/day.mb.
Although the ratio of ground water level to the mean sea level with respect to the change with yearly period is roughly 0.2-0.3, it is not appear distinctly as in the case just stated above, because the range of the change of the ground water level is extremely large. A similar character may be put to the behavior of the variation in subsidence measured as referred to the 380 m well.
11059 il960 li961 11962 11703 Il964
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FIGURE 10. Subsidence speed and ground water level oj the 610 m well at Yamanoshita with the other elements. Monthly mean values areplottedfor every elements. Rs show the times of enforcement of the pumping regulations
Takuzol Hirono
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El5
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N o w , along the curve of the ground water level in figure 10, are seen the signs of R at 5 places. They indicate the times when the regulation on the pumping of ground water were enforced and w e see a marked decrement of the subsidence speed just after every moment when the regulation is put to effect. It took about a few months for the 610 m well to attain the stationary state after the regulation is effected for the first three cases and about six months for the fourth case.
Figure 11 shows the relation of the subsidence speed versus the ground water level, in which (lia), (llb) and (llc) are for the cases of the 610 m, 380 m, 260 m wells respect- ively. Series of points in every diagram should be thought to shift with the time lapse generally from upper right to the lower left in the figure. Except for the case of the 260 m well, close relation is clearly seen between rise of the ground water level and decay of the subsidence speed. The ratio k of the subsidence speed to the ground water levei for the 610 m well is about 0.02 mm/day.m., with the ground water level of -20 m, while it is 0.25 and 0.05 respectively when the ground water level is about
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30 m and -40 m.The cause of this variety of the ratio may be the influence of the contraction of the upper layers controlled by the water of the other aquifers. One of the main aquifer shal- lower than 610 m is that of 380 m, and the value of k with regard to this aquifer may be taken as constant, say, about 0.008 mm/day.m. (-cf. llb). O n the contrary, in the case of the 260 m well, the subsidence speed did not change appreciably for a while in spite of the monotonous rising of the ground water level due to the pumping regulations. But after the fourth regulation of July, 1958, which involved entire suspension of pumping within Niigata city, the situation is reversed. The level of the ground water stopped changing while the speed of subsidence continued diminishing. This may prove that the shallow layers above 260 m have stopped shrinking. It is a matter of course that similar phenomena have occurred in the cases of the 380 m well and the 610 m well after the fourth regulation (cf. lla, llb).
From figure 1 le,
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3 m may be taken as an upper limit of the level of the ground water for the 260 m well to stop subsidence. The curve expressing the tendency of the relation between the subsidence speed and the level of the ground water suggests-
12 m as an upper limitfor the 610 m well, but taking the changed condition just mentioned which occurred after the fourth regulation into consideration, a deeper level, say,-
15 m, may be safely taken as a limit. Similarly, for the 380 m well, though the limiting point seems to be-
2 m from generai tendency in figure 1 lb, it may be considered to be enough to be taken as deep as-
6 m for it.Figure 12 shows the relation between the speed of subsidence at the 610 m well and that at the 380 m well. In this case, too, the ratio of the former to the latter varies depend- ing on the stage of the regulation. Before the third regulation, the speed of subsidence at
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FIGURE 12. Relation between two subsidence speeds of the 610 m and 380 m wells. A n y group of data with similar marks occupies c o m m o n interval of any two successive regulations
Takuzo Hirono
the 610 m well diminishes more rapidly than the other but finally attains to a constant ratio of 2:l to the other, showing that only the layer between 610 m to 380 m was recover- ing from the contraction before the third regulation. After that, the contraction of the layer above 3 8 0 m stats to slow d o w n and tends to decay after the fourth regulation.
The points in figure 12 m a y be devided into four groups according to the intervals of the regulations. The points in each group are apparently on a line passing through the origin showing that the barometric pressure affects the subsidence speeds at both wells in proportion to the speed. It suggests that the atmospheric pressure affects homogeneously the whole layers.
Figure 13 shows the relation between the subsidence speed at the 610 m well and that at the 1200 m well. In this case, too, the ratio of subsidence speed at one well to the other is different according to the stage of the regulation. W h e n the speed was large the ratio was about 5:7, and when small, it is 1:l. This figure indicates that the major part of Niigata ground subsidence is due to the contraction of the layer lying between 380 m to 610 m.
FIGURE 13. Relation between two subsidence speeds of the 610 m and 1200 m well
7. C O N C L U S I O N
The ground subsidence occurring in the Niigata plain is extremely larger in scale and depth than those in Tokyo and Osaka. It is of some different nature compared with similar phenomenon in other regions, probably due to the difference in soil character. The layers are composed of sandy and silt-like soil, poor in clay, and contain ground water with soluble gas. This m a y be one of the reasons why the periodic components in the ground subsidence appear subsequent to the periodic change in the barometric pressure.
The complex structure of the layers in this district had made it difficult to obtain any positive results from the analysis of the data for short interval of time, say, less than a year.
The sufficiently long term data n o w available, however, made it possible to distinguish a number of small disturbances, natural or artificial, from the general trend and to ascertain the validity of the results obtained with the short interval of data. Thus, the facts displayed in the present papers approve us to conclude that the subsidence in the
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Niigata district is also be attributable to the same cause as in Tokyo and Osaka. In these cities, the proof of the theory that the subsidence is caused by the overpumping of the ground water, was given when the factories in the cities were destroyed by the War. But in the case of Niigata it was given by a series of regulations on pumping of ground water.
The regulations have resulted in a large slow down of subsidence, although there are still some locations outside of the city where people suffer from subsidence as in the Uchino and Shirone areas.
There are some problems left in future to be solved. One is that concerning the efficiency of the recharge of ground water. Since 1961, the pumping regulation has been enforced with a net value of pumping equal to the total amount of the pumped ground water substracted by the recharged amount. But its efficiency has not been assured yet.
Another problem is to examine the possibility of the rise of the ground surface by supplying sufficient ground water pressure. In the case of Tokyo and Osaka it was recognized difficult to raise it, but in the case of Niigata, where the nature of soil is somewhat different from those of the other regions, it is worth while to extend examination in this direction. Study in these problems, which relate to each other, will be a clue to better methods to prevent further subsidence and to improve the conditions.
ACKNOWLEDGMENT
The author thanks Dr. N. Miyabe for inspecting this paper. The author also thanks the authorities of the Ministries of International Trade Industry, Agriculture and Forestry, Construction, and Transportations for the data presented through the Special Committee for Niigata Land Subsidence under the superintendence of the Science and Technology Agency and the Land Subsidence Subcommittee, Conservation and Disaster Prevention Committee, STA.
This research was supported in part by the Science and Technology Agency, Niigata Prefecture and Niigata City.
DISCUSSI0 N
Intervention of Dr. Kiyoo WADATI (Japan) Question:
The land subsidence of the Niigata region is mainly caused by the withdrawal of ground- water from deeper layers when compared with the cases of Tokyo and Osaka. Did the close relation between the subsidence rate and groundwater level exist even for a short period such as 5 or 10 days as it was reported in the investigation of Osaka and Tokyo?
Answer of DrTHIRoNo:
The linear relationship is observed in long term series of more than one month, however, w e could not establish it for short periods of several days. This, I think, is attributable to the strong disturbances due to other influences such as atmospheric pressure, tides, etc.
as well as the thickness of the sand layers of Niigata. In other words, I wanted to point out that for a long enough time period, the rate of subsidence corresponds to the change of water level. As to the detailed report on the short term change of several days, I would like to refer to m y present paper.