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Disaster Prevention Map:Protecting Lives from Tsunami and Floods

Flood Disaster Prevention Map Outline

What is the Flood Disaster Prevention Map?

In recent years, torrential rains exceeding the capacity of rivers and sewers have resulted in large-scale flood damage. Earthquakes present an additional risk, as experts believe there is a 60% to 70% probability of a south or southeast sea earthquake occurring within the next 30 years. In Osaka, such an earthquake would generate tremors of grade 5-plus on the seismic scale (up to grade 6 in some areas), as well as 2- to 3-meter high tsunamis arriving about two hours after the quake.
The City of Osaka has created this Disaster Prevention Map in preparation for heavy rains exceeding the estimated capacity of our rivers and sewer systems, and for post-earthquake tsunamis. The Map indicates the areas vulnerable to flood damage and gives information on evacuation and safety.
To reduce flood damage, we need to be prepared in our day-to-day lives. This includes identifying the degree of flood risk present in your area and formulating a plan for when an emergency situation arises. We urge you to take advantage of this Disaster Prevention Map and discuss with the people nearest to you what to do in case of an emergency, including the roles of each person, how to contact one another, and so on.

Osaka City is geographically vulnerable to flooding

Osaka City is surrounded by the Yodo-gawa, Kanzaki-gawa, Yamato-gawa, and Neya-gawa Rivers, as well as the sea. With up to 90% of its urban area on flat lowlands, natural water drainage is a challenge and the city is extremely vulnerable to flooding due to heavy rains and tsunamis.
The City of Osaka is presently upgrading its sewage system to handle up to 60 mm/h of rainwater.

Flood damage predicted in Osaka City

In Osaka City, the river administrator predicts flooding in the event of heavy rains or of a tsunami following a south or southeast sea earthquake.
Three types of flooding are predicted: river flooding, inland flooding, and tsunami flooding.

1 River flooding

River flooding occurs when the river swells above its banks and when the bank collapses. In Osaka City, flood damage is predicted for the Yodo-gawa, Yamato-gawa, Kanzaki-gawa, Aigawa, Neya-gawa, Dainineya-gawa, and Hirano-gawa Rivers, the Hirano-gawa River Diversion Canal, and the Higashiyoke-gawa River.
  1. Flooding of the Yodogawa River
    In the event of flooding of the Yodogawa River, flood damage is expected in the following 16 Osaka city wards: Kita, Miyakojima, Fukushima, Konohana, Chuo, Nishi, Naniwa, Nishiyodogawa, Yodogawa, Higashiyodogawa, Higashinari, Ikuno, Asahi, Joto, Tsurumi, and Nishinari Wards.
  2. Flooding of the Yamato-gawa River
    In the event of flooding of the Yamato-gawa River, flood damage is expected in the following 12 Osaka City wards: Chuo, Tennoji, Naniwa, Higashinari, Ikuno, Joto, Abeno, Suminoe, Sumiyoshi, Higashisumiyoshi, Hirano, and Nishinari Wards.
  3. Flooding of Kanzakigawa and/or Aigawa Rivers
    In the event of flooding of the Kanzaki-gawa and/or Aigawa Rivers, flood damage is expected in the following 3 Osaka City wards: Nishiyodogawa, Yodogawa, and Higashiyodogawa Wards.
  4. Flooding of the Neya-gawa, Dainineya-gawa, and/or Hirano-gawa Rivers, and/or the Hirano-gawa Diversion Channel
    In the event of flooding of the Neya-gawa, Dainineya-gawa, and/or Hirano-gawa Rivers, and/or the Hirano-gawa Diversion Channel, flood damage is expected in the following 6 Osaka City wards: Higashinari, Ikuno, Joto, Tsurumi, Higashisumiyoshi, and Hirano Wards.
  5. Flooding of the Higashiyokegawa River
    In the event of flooding of the Higashiyoke-gawa River, flood damage is expected in Hirano Ward.

2 Inland flooding

Inland flooding occurs when rain in an urban area exceeds the sewer systemís drainage capacity. Water accumulates in the area, causing flooding. Inland water refers to rainfall in districts where rainwater cannot be drained to rivers without pumping.
  1. Inland flooding
    In the event of inland flooding, flood damage is expected in the following 24 Osaka City wards: Kita, Miyakojima, Fukushima, Konohana, Chuo, Nishi, Minato, Taisho, Tennoji, Naniwa, Nishiyodogawa, Yodogawa, Higashiyodogawa, Higashinari, Ikuno, Asahi, Joto, Tsurumi, Abeno, Suminoe, Sumiyoshi, Higashisumiyoshi, Hirano, and Nishinari Wards.
When river or inland flooding is predicted, an evacuation advisory or order will be announced on television, radio, emergency speakers, and public announcement vehicles.
  • Persons living in multistory apartments should evacuate to floors with no risk of flooding.
  • Persons living in detached houses should evacuate to the nearest evacuation shelter.
  • If evacuating to a shelter is difficult, take refuge in a sturdy building on a floor with no risk of flooding.

3 Tsunami Arrival

  1. Tsunami Arrival
    The City of Osaka is building up the seawalls from 5.7 meters to 7.2 meters above the lowest water level of Osaka Bay (Osaka Peil, hereafter O.P.). This height, which is equal to the high tide level of 2.1 meters above O.P. plus the maximum tsunami height of 2.9 meters, exceeds the 5.0 meters above O.P. stipulated in the Tsunami Preparedness Plan. By ensuring that the tide gates are tightly closed, tsunamis can be prevented from making landfall.
    However, should south and southeast sea earthquakes (at magnitude 8.6) occur concurrently, and should some of the tide gates built into the seawall, such as those left open overnight, fail to close, tsunami flood damage is predicted during high tide for the following 5 wards: Minato, Taisho, Nishiyodogawa, Suminoe, and Nishinari.
    In the event of a tsunami warning, persons in areas where flooding is predicted should evacuate promptly to an open evacuation area or to a temporary emergency evacuation site outside the potential flood damage zone.
    Tsunami warnings are issued from 3 to 5 minutes after the occurrence of an earthquake.
    However, should it prove difficult to evacuate to a location outside the predicted flood damage area, or if there is not enough time to evacuate to such an area in the two hours between the earthquake and arrival of the tsunami, you are urged to take refuge in the nearest shelter or in a sturdy building, on a floor with no risk of flooding, after checking that it is safe.
    In the event of a tsunami warning, take the following precautions during evacuation.
    1. Tsunamis occur in multiple waves. The second wave may be larger than the first.
    2. The movement of the water surface due to a tsunami may continue for a prolonged period of time. Even after the tsunami warning has been lifted, keep away from rivers and coastal areas.
    3. The tsunami will travel upriver. Keep away from rivers and coastal areas.
    4. During evacuation, watch for collapsed and collapsing buildings, concrete walls, and other hazards.
    5. Even small earthquakes may cause tsunamis. If you feel a tremor, quickly move away from coastal areas and rivers.

How to read the Flood Disaster Prevention Map

There is a Flood Disaster Prevention Map for every one of Osaka Cityís 24 wards. Each Map contains the following information:

  1. Flood risk diagram
    The map is color-coded to indicate the risk of flooding and the expected depth of floodwater for each situation: River flooding, inland flooding, and tsunami flooding.
    The colors indicate the height of water above the road surface.
        Under 0.1 meters
      0.1 to 0.5 meters
      0.5 to 1.0 meters
      1.0 to 2.0 meters
      2.0 to 3.0 meters
      3.0 to 4.0 meters
      4.0 to 5.5 meters
      5.5 to 6.0 meters
    The depths shown are based on computer simulations and may vary in reality.
  2. Shelter Information
    When flooding is predicted, evacuation advisories or orders will be announced on television, radio, emergency speakers, and public announcement vehicles. If you live in a high-flood-risk area, promptly evacuate to an evacuation area provided you can do so safely.
    Evacuation sites will be opened in accordance with the situation of the disaster.
    The Disaster Prevention Map shows the following information on evacuation areas and shelters:
        : Evacuation shelter (All floors safe from flood risk)
      : Evacuation shelter (Flood risk on 1st floor. 2nd and above safe from flooding)
      : Evacuation shelter (Flood risk on 1st and 2nd floors. 3rd and above safe from flooding)
      : Temporary emergency evacuation site
      : Open evacuation areas
      : Ward office, Health and Welfare Center, police station, fire station, or other.
  3. Precautions
    • Decide in advance which route to take to the evacuation area, and walk through it beforehand.
    • Use the Disaster Prevention Map to identify the flood risk of your neighborhood, and check out safe evacuation areas beforehand.
    • Depending on the topography and type of rainfall, the flooding may exceed forecasts. Flooding may be more extensive or severe when more than one river overflows.
    • Check out the latest information on the radio, television, and other sources.
  4. Other symbols
        : Tide gates expected to be open (managed by Osaka Prefecture)
      : Tide gates expected to be open (managed by the City of Osaka)
      : Tsunami Height (in cm)

For more information:

For information or questions on the Flooding Disaster Map, contact the City of Osaka Office of Emergency Management (TEL: 06-6208-7388).


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