Land Subsidence Management in Japanese Deltas July 28, 2017 Tomoyuki OKADA Director for International Coordination of River Engineering River Planning Division, Water and Disaster Management Bureau Ministry of Land, Infrastructure, Transport and Tourism (MLIT), Japan Ministry of Land, Infrastructure, Transport and Tourism Outline 1. Land subsidence problems in deltas 2. Mechanism of land subsidence 3. Groundwater monitoring 4. Measures to prevent land subsidence 5. Conclusion 1 Tokyo “Zero-meter” Area below Sea Level Tokyo metropolitan has a large storm surge hazard area including areas below high tide and low tide. 5 km Ayase River (Adachi) Chiba Prefecture (Kita) (Katsushika) (Arakawa) (Sumida) (Bunkyo) ■ (Taito) (Chiyoda) (Chuo) (Edogawa) (Minato) Arakawa River (Koto) & Naka River Contour lines show elevation (m) from Old Edo River Arakawa River Peil, which is 1.13m below Tokyo Peil (Average Tokyo Bay Sea Level). Potential storm surge flooding areas Source: Tokyo Metropolitan Government above high tide level Construction Bureau Areas below high tide Level Tokyo Bay Areas below low tide level 2 Evidence of Land Subsidence Original ground level when the well was bored Land subsidence caused by the contraction of confining clay layer Ground level in 1970 Source: Tokyo Metropolitan Government Environment Bureau 3 Groundwater Level and Land Subsidence Rate Groundwater levels and land subsidence rate change in parallel. Kujo Reference Point in Western Osaka 30.3m deep well Groundwater level (m) 60.6m deep well 175.7m deep well Land subsidence rate (mm/5 days) Precipitation 1940 1939 year Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May Jun. Jul. Ground water levels of each deep well were based on the average sea level of Osaka Bay. (Source: Kiyoo WADACHI in 1940) 4 Mechanism of Land Subsidence in Alluvial Lowland Pressures of the water of confined When the water pressure of aquifer and the confining clay layer confined aquifer falls, the water in are balanced. the clay layer is squeezed out. top of the well pumping up slipped upward from underground pumping up wells Land subsidence static water level ground water level wells ground water level confining clay layer dynamic water level confined aquifer (unconfined) n e Water is squeezed-out e r c into aquifer s l l e subterranean stream w bedrock Original natural condition when pumping began. Ground water level falls and land subsides. Source: Masaru MORITA, 2012 5 Groundwater Flow in Musashino Plateau • A spring depletes when an unconfined groundwater level falls below a spring discharge level. • Rainfall infiltration can’t catch up with unconfined water leakage into lower confined aquifer. Infiltration of precipitation Shallow well Unconfined aquifer Run off Unconfined groundwater level Spring water Groundwater flow Confined aquifer 1) Groundwater originally came from precipitation infiltration(recharge) 2) Horizontal groundwater inflow in the unconfined aquifer (here, from right to left) 3) Run off, Spring water form cliff surface 4) Pumping up from shallow wells 5) Leakage into a lower confined aquifer Source: Masaru MORITA, 2012 6 Obstructed Groundwater Flow by Structure A linear underground structure blocks an unconfined groundwater flow and causes negative impact. structure wells dry up ground water land level rises subsidence original groundwater level depletion of spring pond and marsh dry up ground water groundwater flow level drops oxygen deficiency salinization groundwater flow interception aquiclude A long and successive underground structure (from front to back in this figure) blocks groundwater flow (white arrow from left to right). 7 Source: Masaru MORITA, 2012 Trend of Groundwater Level in Tokyo • The line shows a record of groundwater level at Tokyo University from 1900 to 2000. • The groundwater level dropped sharply during the era of post-war economic growth. • After the effective pumping regulation, the groundwater level has recovered. Industrial Revolution in Japan 1945 the end of WW II ) m ( l e v e l the Post-war Economic Growth r 1935 e The first book about the land t a subsidence in Tokyo was published. w d n u o r G Groundwater pumping regulation started. Year (The ground water level is converted based on the average Tokyo Bay Sea Level) Source: Masaru MORITA, 2012 8 Land Subsidence and Groundwater Level • Land subsidence in Tokyo lowland started in the 1920s. • In the 1960s, the groundwater level dropped 2.5 m/year, and land subsidence advanced 10 cm/year. • The pumping regulation of 1970s restored the groundwater level, but the subsided land is not recovered. 1945 the end of WW II ) Cumulative land ) m Subsidence (Adachi-ward) ( m e ( c Cumulative land l n e v e Subsidence (Koto-ward) e d l i s r b e t u Ground water level of a s the deep well at Tokyo Univ. w d d n n a l u e o v r i g t a Ground water level (Koto-ward) d l e u n i m f u n C o C Ground water level (Adachi-ward) Year The ground water levels are converted based on the average Tokyo Bay Sea Level. Source: Tokyo Metropolitan Government Civil Engineering Laboratory 9 Who Owns Groundwater? River water in Japan Groundwater in Japan River Act Civil Code • Article 2 • Article 207 “A river is public property.” “Ownership in land shall extend to above and below the surface of the land, • Article 23 subject to the restrictions “Any person who intends to prescribed by laws and use the water of a river shall regulations.” obtain permission of the river administrator.” 10 Pumping Regulation in Japan Industrial Water Act (1956) Objectives 1) Ensure the rational supply of industrial water 2) Conserve sources of groundwater in designated districts and prevent land subsidence 1. Industrial zones where land subsidence and other groundwater damage occurred shall be designated. 2. Wells with pump outlet of 21 cm2 (currently 6 cm2) and over shall get governor’s permission. 3. Boring new wells that do not meet the standard is prohibited. 4. Where the construction of unqualified well is prohibited, a substitute water source is provided. 11 Code of Preventing Land Subsidence The Code of Preventing Land Subsidence (1985-) aims to prevent land subsidence caused by groundwater pumping and to preserve groundwater. Outline of the Code Three Target Delta Areas • Objective • Current situation • Target area • Limit of groundwater pumping • Prevention measures of land subsidence • Monitoring and survey Osaka ● Tokyo • Prevention of damages and recovery ● • Promotion of the Code ● Saga 12 Conclusion Restoration of healthy hydrological cycle • Expand the recharge (permeable) area • Prevent groundwater flow obstruction • Prevent unconfined ground water leakage into the lower confined aquifer Groundwater as a future water resource • First priority of water resource is river water. • Groundwater supplements river water. Groundwater level is an indicator to measure the amount of water pumping. 13 Nuinoike Pond Reference: Groundwater Tells – Crisis in Hidden Resources (in Japanese) Dr. Masaru MORITA, June 2012 14.