Recent trend of land subsidence in Japan

Transcription

1 Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). IAHS Publ. no. 234, Recent trend of land subsidence in Japan SOKI YAMAMOTO Tokyo Seitoku College, Jujodai, Kitaku, Tokyo 114, Japan Abstract Many areas of Japan are still affected by land subsidence. The author reviews and summarizes the history of land subsidence in Japan. Most areas of land subsidence are located on alluvial plains and hit the headlines as a result of natural disasters such as earthquakes, typhoons and droughts. Recently, new incidents of subsidence have occurred in areas where groundwater is used to melt thick snow on roads. An intergovernmental committee has been asked to propose solutions to this problem area by area. INTRODUCTION There are many land subsidence areas in Japan. According to the Environmental Protection Agency of Japan (1995), principal subsiding areas are distributed in 37 prefectures with 62 regions. The 11 areas where subsiding exceeds 20 mm year" 1 amount to 276 km 2. Although in general subsidence is slowly decreasing, some remarkable new incidents of subsidence have occurred in some areas. Last year (1994) Japan was suffered a severe drought and most of the reservoirs dried up. The shortfall in water resources for drinking and irrigation was supplied by groundwater, and this accelerated some remarkable incidents of land subsidence. It noteworthy that recently Japan introduced the centimetre (cm) as a unit of subsidence amount instead of the millimetre (mm). HISTORICAL REVIEW OF LAND SUBSIDENCE IN JAPAN Deep well drilling started at the beginning of The drilling company "Nissaku" which adopted the western type drilling method was established in Taking advantage of the economic boom during the World War I, deep well drilling became popular in Japan. The great Kanto earthquake which hit Tokyo in 1923 drew attention to land subsidence in Tokyo. Then attention was drawn to the subsidence at Osaka around Hirono & Wadachi (1989) pointed out that land subsidence was caused by the withdrawal of groundwater. Generally speaking, land subsidence in Japan is brought to our notice through natural disasters such as typhoons, earthquakes, heavy rainfalls and droughts. Comparing the distribution of the areas of land subsidence in 1965 and 1994, no change is found, but differences can be seen in the size and amount of maximum subsidence (Fig. 1). The historical changes of subsidence are shown in Fig. 2. This

2 488 Soko Yamamoto Shonan " Tokyo km I.... I 1 = 1 > 4 cm/year V Jg Subsiding area (0 Alluvium Fig. 1 The distribution of land subsidence areas and alluvial plains in Japan. figure shows there are three types of change: slow, rapid and slowing-down components of sinking. The decline in water level is parallel to this sinking curve with the exception of two rises, one at the end of World War II and the other at the invocation of the subsidence regulation law. In response to these two rises, an interruption or slo wingdown of subsidence can be seen.

3 Recent trend of land subsidence in Japan '20 '40 '60 '80 1, ' ' ' C/5 c IO g20- o \» \ \ \ \ V \ \\ \ \ \. 0 OSAKA i\\ \\ \y \J \\ \\ y\ SAGA V \ v \ NIIGATA V \ ^ N ^\ \ \ \ v ""~ TOKYO \ OOSAKA \\\ \ A \\ ' / M \\NAGOYA \\\ x A \\ ' A \ \ \\K \ /NAGOYA \\\ V ^~- 40- \ TOKYO Fig. 2 Subsidence and change in piezometric head in selected areas. There have been many discussions concerning the cause of land subsidence. The denial that the principal cause of subsidence in Niigata was due to gas water withdrawal was a famous one. But the gas water discharge in Kujukuri was approved with its iodine product in the sparsely populated region and recharge work of separated water. Later in the Niigata region, gas extraction was permitted on the condition that separated water was recharged into the aquifer when gas was extracted from the deep Tertiary formation. Industrial Water Law (1956) and Building Water Law (1962) prevented land subsidence, and the establishment of the Japanese Environmental Protection Agency also contributed to stopping land subsidence in Japan. Present disastrous subsidence is occurring on agricultural land and regions of drinking water shortage because of an increase in groundwater withdrawals. NEW ASPECTS OF LAND SUBSIDENCE Recently the practice of using groundwater for melting snow is increasingly being used in Japan. Along the Japan Sea coast a thick snow-covered zone exists. Snow covers as deep as 7.3 m are recorded in the eastern part of Nagano. In winter time in this region, first-floor windows are used as doorways! It is very important to remove snow on the roads.

4 490 Soko Yamamoto n30 OOOm Groundwater m Surface water Snow-covered >30days/yr. Fig. 3 Length of snowmelting pipes in Japan. As a snow removal method, snowmelting pipes are installed under roads. Through nozzles located 10 cm apart in the pipes, water is sprinkled to remove the snow. The distribution of the snow removal pipeline is shown in Fig. 3. The length is considerable along the Japan Sea coast. Groundwater is more suitable than surface water for this purpose and groundwater is pumped intensively for four months in winter for this purpose. Recently remarkable subsidence due to groundwater withdrawal has been observed at Muikamachi in the southern part of Niigata Prefecture. The maximum subsidence amounts to 73 mm year" 1 and area affected is estimated to be as much as km 2. This is one of the worst incidents recently recorded in Japan. As Muikamachi is a thick snow-covered area, a tremendous amount of groundwater is used for melting snow. The total discharge of groundwater in this town is 126 x 10 3 m 3 day" 1 during winter time. The drawdown of water level at Bunkakaikan's well was 13.0m and shrinkage of the clay layer amounted to 57 mm in The total amount of subsidence over 17 years reached 450 mm. Muikamachi town is located on the alluvial valley of a tectonic origin. The maximum depth of the alluvial deposits, consisting of sand and gravel layers with two clay

5 Recent trend of land subsidence in Japan 491 I ' '80 I [ I I I 85 I I I I I'90 I I I Fig. 4 Subsidence and water level changes at Minami-Uonuma. 0 1 Km Fig. 5 Geological cross section at Minami-Uonuma.

6 492 Soko Yamamoto layers, is about 150 m. The upper clay layer is situated at a shallow depth of 40 m and the other one of 50 m thickness is deeper than 80 m and underlain by sands and gravels (Fig. 4). The popular depth of wells ranges from 40 to 80 m, and most wells tap the upper confined aquifer of m depth. The compaction of the upper clay layer is occurring. Land subsidence of Muikamachi town is produced by the heavy pumping of groundwater for a limited period in winter time. It is noticed that the groundwater level recovers quickly after pumping has stopped, but that while subsidence continues to accelerate with no recovery. The town authorities are now considering the introduction of legal regulations to remedy this situation (Fig. 5). RECENT NEW POLICY TO COPE WITH LAND SUBSIDENCE Japan has two laws which regulate groundwater utilization for building and industrial use but none for drinking and irrigation use. Most local governments have municipal ordinance for groundwater. Recently our government organized an "Intergovernmental Committee for Land Subsidence". The Committee was composed of nine Ministers as members and nominated three specific regions - Nobi Plain (Nagoya), Chikushi Plain (Saga) and North Kanto (northeastern part of Tokyo) - where disastrous subsidence was still happening over large areas. Several specialists were appointed as regional committee members. Discussions are underway to determine the tolerable cutback for each region and from computer analyses to decide the techniques of optimal pumping operation with regard to safety and permissive groundwater head. In addition to this effort, the author would like to see artificial groundwater recharge introduced in Japan. REFERENCES Environmental Protection Agency (1995) Brief Summary of Land Subsidence in Japan. Hirono, T. & Wadachi, K. (1989) Land Subsidence in Western Osaka, 1. Report of the Disaster Prevention Research Institute.