Japan Flood Map Update 2022

JBA’s team of flood experts have recently updated JBA’s Japan Flood Map with new data, advanced methods, and the latest science across the whole country - providing you with the most comprehensive insights available at national-scale for your property-level flood risk assessment.

We’ve been the proud provider of flood maps for Japan since 2014 as part of our global flood mapping capability, empowering the re/insurance, financial, NGO, charitable and public sectors in flood risk management.

Below, we explore the new update, including the latest terrain data, improved methods, and validation against multiple recent flood events in Japan.

Japan's flood risk

Due to Japan’s climate and geographical location, it faces flood risk from numerous sources.

Tropical cyclones and typhoons pose a significant risk each year to communities throughout Japan, particularly during spring and summer and can bring heavy rainfall alongside strong winds (Japan Meteorological Agency (a)). In 2019, Typhoon Hagibis was the nineteenth typhoon of the season and the worst storm to hit the country in 60 years. It brought unprecedented rainfall and flooding to eight prefectures across Japan’s main island, Honshu, as well as landslides (JBA Risk Management Event Report, 2019).

Alongside risk from typhoons and cyclones, and often compounded by these events, Japan is host to several major river systems which can bring widespread flooding following heavy rainfall and storms. During Typhoon Hagibis, at least 142 rivers flooded in Honshu, including the major Arakawa, Chikuma and Tama rivers (JBA Risk Management Event Report, 2019). During the July 2020 floods in Kyushu, which caused at least 72 fatalities, several rivers overtopped their banks to flood surrounding areas, including the Kuma River and Chikugo River (JBA Risk Management Event Report, 2020).

Japan’s climate and location also make it particularly prone to this heavy rainfall. From subtropical climates in the south of the country to its position on the eastern edge of monsoon Asia and the Baiu front, many areas of Japan are subject to extreme rainfall over varying seasons (Swiss Re, 2020; Japan Meteorological Agency (b)). Additionally, with many steep sided valleys and hills throughout the country creates rainfall-runoff into flatter and more densely populated areas which can cause widespread flood risk.

As a result of this unique flood risk profile, comprehensive flood data is key to managing exposure.

2022 japan flood map update

incorporating the latest terrain data

We’re committed to incorporating the latest science and methods in our flood data to enable the most effective flood risk assessment.

Due to the development of new data from external partners over recent years, the latest Japan Flood Map includes new high-resolution terrain data, with 100% of the country now covered by bare earth terrain data.

This new terrain data, coupled with our sophisticated machine learning methods and meticulous reviewing and cleaning of the data by our technical team, enables us to define river channels, narrow urban streets, infrastructure, and topography more clearly. In turn, this enables improved representation of water flow paths over the ground and subsequent flood extents.

Figure 1: A comparison showing digital surface model data, which captures the Earth's surface including trees and buildings; in comparison to bare earth digital terrain model data used in the 2022 mapping (right). Through using bare earth terrain data, we see the river channel and surrounding topography much more clearly. DSM image source (NASA Shuttle Radar Topography Mission (SRTM)(2013). Shuttle Radar Topography Mission (SRTM) Global) ), DTM image source (Airbus WorldDEM DTMlite).

Figure 2: JBA river flood map for 2016 (left) and river flood map for 2022 (right) for the Mogami River basin. Improved terrain data enables better representation of ground height, with higher parts of the terrain now modelled as not at risk to flood. World Imagery - Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.

Learn more about the difference in terrain data and the effects on resulting flood maps in our blog.

bespoke methods for japan

The latest update also includes improvements to methods including new bespoke methods for Japan.

The flood mapping now uses a new land cover dataset, which is derived from the Impact Observatory, Microsoft and ESRI Sentinel-2 land cover information.

With the inclusion of more land use classifications, specific to Japan’s individual topography, our technical teams can improve rainfall-runoff methods and assumptions to better represent spatially varying land roughness, which influences how water spreads across the ground and where water pools to cause flooding.

Figure 3: New land cover dataset with 10 land use classifications for Japan, enabling improved representation of land use and water flow paths. Image created by JBA Risk Management using ESRI data.

Alongside this, JBA’s technical team have improved classification of river and surface water flood types through new data and methods, now incorporating all small rivers into our river flood mapping to better help define this type of flooding against surface water sources.

Across our global flood mapping, we use bespoke methods for each flood type to account for their unique hydrological characteristics.

For example, rainfall totals are generated for different hourly periods to represent storms of different durations. Shorter, intense storms are likely to cause more flooding in steep sided valleys and smaller river catchments, while longer, slower moving storms will have greater impact in flatter regions.

This new data and methodology is incorporated into our hydrological model to enable improved mapping of Japan’s specific risk profile, bespoke to its topography.

new national flood defence information

Defence information has been updated across the whole of Japan, with a Defended Areas layer demonstrating the mitigating effects of flood defences where present. This enables an improved view of the realistic flood risk in an area.

There are now over 2,000 defended areas in place in the Japan Flood Map, with over 1,400 new defence areas added in the latest update. Alongside this, existing flood defences have been reviewed, edited, and improved where appropriate.

Figure 4: Change in defended areas for the city of Ishinomaki between 2016 JBA flood mapping (left) and 2022 flood mapping (right). World Imagery - Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.

All, some or none of the defence information can be considered when assessing the risk, enabling a different view depending on your risk appetite – for example, turning all defences on would enable a best-case view of risk, whereas turning all the defences off would provide a worst-case view.

Validation against major flood events

Japan faces major flooding on an almost annual basis as a result of its topography, climate and geographical location. In order to assess the accuracy of our updates, JBA’s team of specialists validate the maps against observed extents from some of these previous floods in the country. Figures 5 and 6 demonstrate the performance of the 2022 Japan Flood Map against recent flood events in the country.

Figure 5: Comparison of observed flooding in the Mogami river basin, west of Sendai, during the summer 2020 flooding compared to JBA’s 2022 RP100 river flood map for the same area. Satellite imagery – Source: Geospatial Information Authority of Japan; World Imagery - Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.

Figure 6: Comparison of observed flooding in Sakura during Typhoon Hagibis in 2019 compared to JBA’s 2022 RP500 river flood map for the same area. Satellite imagery – Source: Geospatial Information Authority of Japan; World Imagery - Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.

using and accessing jba's japan flood data

Image: JBA fluvial flood mapping for Japan. World Imagery - Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.

JBA’s flood data can be used by re/insurers, mortgage providers, banks, asset managers and other investment organisations to effectively assess the extent and severity of flooding. This enables improved decision-making across property and asset screening, risk selection, underwriting, pricing, portfolio optimisation and funding and investment opportunities. The data can also be used by public bodies for land use planning, disaster preparedness and more.

The latest Japan Flood Map is complemented by our catastrophe flood model for Japan, enabling probabilistic quantification of flood losses throughout the country, and is part of our wider global flood data capability.

We're committed to ensuring our clients can access our data in a way that suits them. Our Japan Flood Mapping is platform agnostic and available:

  • In-house directly from JBA
  • Via our GIS visualisation web application
  • Via our wide range of resellers
  • As part of a consultancy service to help identify exposure hotspots, run per-risk flood assessments and manage accumulations.
  • Via API service from Q4 onwards, including the ability to assess and quantify current and future risk under climate change

For more information on any of our data or our bespoke consultancy service, get in touch today.

The Japan Flood Map update is the first of JBA’s 2022 global flood mapping updates. We will also be releasing updates to our flood mapping in China, Hungary, Canada and the UK this year.

References

Swiss Re. 2020. Mitigating Flood Risk in Japan - When Strong is Not Enough. [online] Available at: https://www.swissre.com/en/japan/news-insights/articles/natural-catastrophes/mitigating-flood-risk-in-japan.html [Accessed 19/05/2022].

Japan Meteorological Agency (a). General Information on Climate of Japan. [online] Available at: https://www.data.jma.go.jp/gmd/cpd/longfcst/en/tourist.html#:~:text=Eastern%20Japan%20has%20hot%20and,have%20a%20subtropical%20oceanic%20climate [Accessed 19/05/2022].

Japan Meteorological Agency (b). Overview of Japan's Climate. [online] Available at: https://www.data.jma.go.jp/gmd/cpd/longfcst/en/tourist_japan.html [Accessed 19/05/2022].

NASA Shuttle Radar Topography Mission (SRTM)(2013). Shuttle Radar Topography Mission (SRTM) Global. Distributed by OpenTopography. https://doi.org/10.5069/G9445JDF. [Accessed 22/04/2022].

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