Japan is no stranger to natural disasters, and each one offers a lesson on the need to be as prepared as possible.

A July 3 mudslide in Atami, Shizuoka Prefecture, killed at least 21 people and damaged around 130 buildings in the town’s Izusan district. An investigation is underway into how the disaster came about, but the district was designated by a national government hazard map as being at risk from landslides.

As recovery work there continues, Tropical Storm Nepartaki is expected to make landfall as early as Tuesday. The resulting rains in the Kanto area, including Tokyo, and the Tohoku region are creating concerns about possible flooding.

As a result, there is a renewed focus on hazard maps for areas at risk from floods, landslides and other natural disasters.

What information do hazard maps give?

The maps provide estimates of how extensive damage could be if an area is hit by heavy rains, floods, landslides or damage from volcanoes or typhoons. They also contain information about evacuation routes and sites. Their purpose is to help residents evacuate quickly and efficiently if a disaster hits, and avoid areas at high risk from secondary disasters, such as flooding and landslides following heavy rains.

Hazard maps are created based on an area’s topography and land conditions at the time of the map’s creation. However, natural or human-made changes to land conditions and topography can occur after a hazard map is drawn up. Such changes can alter the degree of risk to an area without residents knowing, which is why hazard maps and recommended evacuation procedures require regular updating.

Self-Defense Forces search for missing people at a landslide in Atami, Shizuoka Prefecture, on July 5. | AFP-JIJI
Self-Defense Forces search for missing people at a landslide in Atami, Shizuoka Prefecture, on July 5. | AFP-JIJI

What kind of hazard maps does Japan have?

There are about a half dozen types of maps for areas at risk from different kinds of natural disasters.

The government has a national portal site for hazard maps, which includes maps for floods, landslides and tsunamis.

Local governments often provide flood hazard maps that include not only how bad flooding could be, but also evacuation routes. Maps for river flooding show areas likely to be flooded after heavy rains as well as projected water depth.

Landslide or mudslide hazard maps, meanwhile, indicate both the areas at risk and where to evacuate. Earthquake hazard maps show ground areas that could be subject to both ground liquefaction and large-scale fires.

Volcano hazard maps include indications of where a crater could appear, the predicted direction of lava and pyroclastic flows, the areas where volcanic ash would be expected to fall and where mudflows would likely reach.

Tsunami hazard maps show areas that could be inundated by a massive wave, and which would be closed to traffic if a tsunami hits.

As typhoon season is approaching and flooding is a major concern, what’s the situation with hazard maps for rivers and streams?

The land ministry keeps track of the nation’s rivers and river systems and which ones have hazard maps for flood estimates. It broadly defines the water systems as either “flood-predicted rivers” or “rivers with known water levels,” and separates them into nationally managed rivers and those overseen by prefectural governments.

Flood-predicted rivers are defined as having a large basin area that may cause serious or substantial damage to the national economy due to flooding. This category includes well-known rivers where evacuation is ordered once the water reaches a certain level. The other category includes smaller rivers and river systems, and evacuation warnings are also given once the water level reaches a certain point.

Hazard maps for foreign residents in Joso, Ibaraki Prefecture | KYODO
Hazard maps for foreign residents in Joso, Ibaraki Prefecture | KYODO

As of March, there were a total of 448 nationally managed rivers, including 298 classified as flood-predicted rivers and another 150 with known water levels. There were 1,710 prefecturally managed rivers and river systems, including 130 flood-predicted rivers and another 1,580 as rivers with known water levels.

In addition, of the 1,403 cities, towns, and villages nationwide that are required to have hazard maps, 1,365 had drawn up general flood maps as of that month. However, the maps of only 1,080 local governments, 77% of the total, were drawn up based on predicted maximum rainfall.

Do people really rely on hazard maps?

An NHK survey earlier this month showed that 67% of respondents had consulted a hazard map to confirm places in their neighborhoods where there are dangers of flooding or mudslides. Nearly half of respondents, 48%, said they placed top importance on information from television and radio during a time of a natural disaster, while 24% gave top priority to loudspeaker announcements from local governments.

Only 17% said that they placed top priority on information from social media and the internet, and 7% said they relied on word-of-mouth from family and acquaintances.

Color-coded hazard maps are often distributed in paper form to local residents or posted on municipalities’ websites. Some also release information about rising water levels through apps such as Line.

But disaster prediction is not an exact science. There are instances where areas that had been designated as safe ended up becoming flooded due to rains that were heavier than expected, or changes to the area’s topography after the maps were drawn up resulted in flooding or subsequent mudslides in areas that had been deemed safe.

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