Rivers case study

• Topography of Kanto plain


  • Very flat with lots of rivers (high drainage basin density)
  • High chance of flooding
• Heavy rainfall from Typhoon Hagibis


  Typhoon's trajectory was directly over the Kanto plains
• Kanagawa Prefecture saw 922.5 mm of rain that day alone, 11 cm per hour which is 3 times as much as the total for October of an average year
• Heavy rain from Typhoon 18 fell before Typhoon Hagibis

  Water levels were already relatively high
• Ground was already saturated from rain, antecedent moisture, from Typhoon 18


  Water couldn't be stored as the infiltration capacity of the ground is exceeded
• Water couldn't be stored


  Increases overland flow that would collect into rivers
• Urbanisation in Kanto - more impermeable surfaces and building on the floodplain near rivers increases risk
• Urbanisation of Tokyo has been increasing at an average annual rate of 2.4% since 2000
• Many rivers concreted - speeds up flow and chance of floods downstream (e.g. Ara river)
• Less 'green' spaces (parks, forests) to intercept and store water
• Urbanisation has caused Tokyo to sink due to the weight of buildings and underground water extraction
• Some parts of the city have dropped more than 3 metres since the 1930s
• Levees along the Tama River were exceeded by the highest-ever recorded water level of 6.24 metres
• Between 2010 to 2022, Tokyo has experienced deforestation of 0.15%
• Japan's forests are being lost at an alarming rate, with the country losing almost 5 million hectares of forest from 1995 to 2015
• 23% of domestic water supply comes from groundwater, which is over-exploited in parts of the country
• The Tama River started overflowing from a low-lying bank, flooding several houses and condominium buildings facing the waterfront of Futako-Tamagawa
• In the Noge area of Setagaya Ward, Tokyo, 151 buildings, including a hospital, were damaged by inland flooding
• Rugby world cup match cancelled between England and France due to travel concerns
• People asked to evacuate from areas near rivers
• Lack of food in supermarkets as people 'overstocked'
• 10 people dead, 16 missing and more than 100 people injured
• When Typhoon Hagibis made landfall on Japan's mainland and wreaked havoc in October 2019, the water level of the Arakawa peaked well after rainfall ceased and evacuees returned home
• The typhoon, which killed more than 100 and collapsed levees on more than 70 rivers, saw the Iwabuchi sluice gate in Kita Ward close for the first time in 12 years to prevent water from the Arakawa entering the Sumida River, whose embankments are lower than the Arakawa and thus more vulnerable to flooding
• Arakawa's water level reached the third highest level recorded since World War II and alarmingly close to the 7.7 metres considered a dangerous threshold
• Sports fields along Tama river flooded - implication on biodiversity
• Methods of flood management had been implemented such as flood zoning and the use of levees
• CCTV Cameras - River office has real time access CCTVs located in 3900 locations
• AmeDas Stations - The Automated Meteorological Data Acquisition System (AMeDAS) is a collection of Automatic Weather Stations (AWSs) run by Japan Meteorology Agency for automatic observation of precipitation, wind direction/speed, temperature and humidity to support real-time monitoring of weather conditions with high temporal and spatial resolution
• The AMeDAS system currently includes as many as 1,300 rain gauges at average intervals of 17 km nationwide and around 840 stations also observe wind direction/speed, temperature and humidity in addition to precipitation
• The Tropical Rainfall Measurement Mission was launched into space from 1997-2015 as a joint mission between JAXA and NASA. It has microwave imaging and lightning sensors so it can detect rainfall and provide forecasts
• The 5-level warning system was also used so that civilians could determine whether they needed to evacuate or not
• Flood plains around Tama river allow for some flooding → Construction on the area surrounding the river has been limited and is used as a recreational area consisting of baseball pitches, football fields and marathon courses to minimise damages in case of floods
• Underground discharge channel located in Saitama is the world's largest underground flood water diversion facility, built to mitigate the overflowing of the city's major waterways and rivers during rain and typhoon seasons
• The tunnels fill up during times of flood preventing less water joining the Arakawa and then pump water back into the Edo river after the flood levels have dropped
• The Tokyo Metropolitan Government has a new project to construct a giant subterranean reservoir that will cost 24.5 billion yen
• The 3.3-km-long tunnel-shaped reservoir, which is 7.5 m in diameter, will be able to handle 135,000 cu. metres of water, enough to fill 54 Olympic-size swimming pools
• When heavy rain causes a river's water level to rise, water flows from a weir built into a revetment down into the reservoir, thus reducing the amount of water flowing downstream
• So far, 28 such facilities have been constructed, collectively holding a total of 2.56 million m3 of water
• When a typhoon dropped record rainfall of up to 32 mm per hour on Tokyo in 2019, the reservoir retained about 490,000 m3 of water, or 90% of its total capacity