Mitigating Flood Risk in Japan – When Strong is Not Enough

A year ago, we looked at the series of natural disasters that struck Japan in 2018, which brought the "kanji of the year" 災 ("sai" meaning "disaster"). What we didn't know then was what 2019 would bring and prove how vulnerable Japan is to disasters. An earthquake registering "6 Lower" seismic intensity struck Kumamoto in January; a major aftershock hit the Iburi district of Hokkaido in February; and an earthquake off the coast of Yamagata prefecture in June recorded a "6 Upper" intensity in Murakami, Niigata prefecture.

As the seasons changed, the storms came. Typhoon 15 (Faxai) made landfall near the city of Chiba in September. High winds including maximum gusts of 207 km/h (128.6 m/h) swept across the southern Kanto region causing a major blackout that affected 930,000 households, and damaged 75,000 homes mainly in the Chiba prefecture1.

Then came Typhoon 19 (Hagibis), which made landfall at Izu Peninsula in October. Total rainfall resulting from the storm reached 1,000 mm in Hakone, Kanagawa prefecture, and exceeded 500 mm in 17 locations, primarily in eastern Japan.

Typhoon 19's torrential rains caused 192 rivers to exceed their flood-danger water levels – embankments were breached in 140 locations on 71 rivers. Even embankments on major rivers such as the Tone River and the Arakawa River were in danger. The disaster caused 91 deaths and damaged 96,500 homes, primarily on the Pacific side of the Tohoku region and in the Kanto area2. There were also damages to major infrastructure and numerous companies' businesses. The Swiss Re Institute's sigma report estimated that the insured losses were USD 8 billion.

Unforgiving Geographical and Climatic Conditions

Japan is a hilly country, with steep mountains and many rivers on short, steep gradients. Around 70% of its land is considered hilly and mountainous, and half of Japan's total population today live on alluvial plains that constitute only 10% of the land mass. Nearly three-quarters of the country's total assets reside on those plains and the highly-populated Tokyo, Ise and Osaka Bays have below sea-level areas.

Japan is also highly exposed to volatile climatic conditions. Positioned at the easternmost edge of monsoon Asia, one of the world's leading high-rainfall zones. Japan receives double the global average rainfall of 1,718 mm annually, with much of it concentrated during the rainy and typhoon seasons. For example, the average monthly rainfall in Tokyo in September is 208.5 mm, five times in volume as compared to 39.6 mm in December.3

In recent years, climatic conditions have made seas warmer and precipitation patterns more unpredictable. The Japan Meteorological Agency (JMA) related the impact of global warming to an individual natural disaster for the first time in an announcement on the torrential rainfall of July 2018: "Probable contributing factors include both the long-term trend of rising air temperature and increasing water vapor content in the air due to global warming."4

Last year, JMA also identified three factors that contributed to the unprecedented heavy rains that Typhoon 19 brought: (1) the influx of a large quantity of water vapor from the approach of a large typhoon with very strong, sustained force; (2) the formation of a persistent ascending air current resulting from a strengthening local front and geographical features; and (3) the passing of rain clouds near the center of the typhoon5.  

Two climate factors also contributed to Typhoon 15's violent wind conditions. Higher-than-average sea temperatures along the path traveled by the typhoon and the fact that Typhoon 13 (Lingling) had just passed through the northward path days earlier prevented southwards flow of dry air from the Asian continent.

A Strong Foundation in Flood Control

Japan is no stranger to flood risks and has established a strong foundation in flood control over history. The first large-scale flood control projects date back to the 16th and 17th centuries. In 1896, Japan enacted the River Act, followed by the Sand Control Act and Forest Act in 1897, establishing the foundation for modern flood control and provided the fundamental principles for river control in Japan. Since the 1950s, subsequent government initiatives have also strengthened the country's flood control infrastructure.

Rapid urbanization that came with economic growth have diminished the water-retaining and flood-retarding capacities of river basins. While efforts to strengthen flood protection in urban areas continue, recent catastrophe events offer valuable insights on these measures' effectiveness in mitigating climate change.

Typhoon 19 caused river embankment breaches and overflows, inland flooding, and landslide damage across Japan, resulting in huge losses. But the heart of Tokyo was spared of any large-scale flooding.

Key flood-control infrastructure saved Tokyo from the typhoon's wrath — the massive Lake Saiko Reservoir in the Arakawa River basin, and the Metropolitan Area Outer Underground Discharge Channel, one of the world's largest underground flood water diversion facilities. The discharge channel can store up to 670,000 cubic meters of water from five rivers in the vicinity, provided Edogawa River does not rise to a level that prohibits discharge from the surge tanks. During Typhoon 19, the three-day cumulative drainage from the surge tanks into the Edogawa River was 11.51 million cubic meters, the equivalent of approximately 7,673 Olympic-size swimming pools.

Other dams and underground systems helped too, but they were very close to capacity. The new Yamba Dam on the Tone River had just begun holding water on October 1and Typhoon 19 filled it almost to its full capacity of approximately 75 million cubic meters of water in one night.

Taking Climate Change into Account

According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), the warming of the Earth's climate system is an undisputable fact. JMA had forecasted a nationwide average rise of 4.5 °C by the end of the 21st century, and the number of extremely hot days (of at least 35 °C) will rise each year. If greenhouse gas emissions continue unabated, current occurrences of short, intensive downpours of over 50 mm/hour may double6.

In April 2018, the Ministry of Land, Infrastructure, Transport and Tourism, which has jurisdiction over flood control measures, launched the Technical Review Board for Climate-Change-Based Flood Control Planning. In October 2019, the board proposed a National Climate-Change-Based Flood Control Plan, which calculated the amount of expected rainfall by the end of the century based on the most stringent targets in the Paris Accord—RCP2.6 (equivalent to a rise of 2 °C). The amount of rainfall large enough to consider for flood control plans for Class A rivers was estimated to be 1.1 times greater on average across Japan than a century earlier, with double the frequency, and the proposal encourages a reflection on the figures for flood control plans' target run-off flow volumes and aims to expand the range of flood control options. The proposal also aims to accelerate the pace of river improvements to ensure targeted levels of flood control safety are achieved as climate change impact on water disaster risks7. In November 2019, following the impact of Typhoon 19, a new sub-committee was established to prepare for the implementation of that proposal with a forward-looking perspective on the climate.

Embracing Climate Change Impact in Insurance

Typhoons in Japan are no surprises and huge investments in coastal and inland flood defences after devastating typhoons since the 1950s and 1960s have helped to mitigate some Japan's flood risks.

However, climate change has altered the game, influencing the variability and uncertainty of weather events that result in a greater risk of insufficient flood protection in the future. Observations from recent typhoon events have prompted questions on the adequacy and resilience of the country's flood defences. Today, Japan's flood control measures have helped mitigate the impact of Typhoon 19, but by no means entirely. A recalibration of flood risk assessment in the country is much needed, particularly within the context of a warming climate, to mitigate future risks and keep weather events insurable.

Just as Japan's government agencies are embracing new measures that take into account the impact of climate change, the insurance industry needs to actively embed and dynamically track the effects of the warming climate, adapting risk assessments to a profoundly changing landscape. Some of Japan's non-life insurance companies have also begun engaging in a range of activities related to climate change mitigation and adaptation, in addition to fulfilling their main function of providing compensation through insurance.

The heightened intensity of recent natural disasters highlights an uncomfortable truth: when the next major disaster strikes we may not be able to prevent or recover from nature's destructive force. The risk landscape is dynamic and to avoid falling behind the curve, insurers need to actively track socio-economic developments, scientific findings on climate change effects, and the status of local risk mitigation measures in order to keep weather risks insurable.

This year, COVID-19 has left an indelible mark on history. As the world grapples with its impact on the economy and our societies, risks such as climate change and extreme weather events should not be forgotten. These will not go away and will require action to safeguard our future. As an industry, we need to act now, and act together – the national or local government, re/insurers, businesses and the general public, working as one to start adapting to climate change.

1 Fire and Disaster Management Agency: Damage from Typhoon 15 of 2019 and Status of Response by Fire Departments, etc. (Report No. 40) (Japanese only)
2  Fire and Disaster Management Agency: Damage from Heavy Rains Due to Typhoon 19 of 2019 and Weather Front and Status of Response by Fire Departments, etc. (Report No.66) (Japanese only)

3 Ministry of Land, Infrastructure, Transport and Tourism: On Flood Control Measures (Japanese only)

4 JMA: Primary Factors behind the Heavy Rain Event of July 2018 and the Subsequent Heatwave in Japan from Mid-July Onward

5 JMA: Typhoon 19 of 2019 and its Accompanying Heavy Rains and Other Features and Causative Factors (Prompt Report) (Japanese only)

6 JMA: Forecasts for Climate Change in Japan (Japanese only)

7 Ministry of Land, Infrastructure, Transport and Tourism: Technical Review Board for Climate-Change-Based Flood Control Planning
 (Japanese only)