1. What the "flood danger water level" means and why water has not yet spilled at that point
According to the Japan Meteorological Agency's forecast terms, the flood danger water level is defined as a water level where there is a risk of flooding that could cause significant damage, such as the submersion of many houses [1]. It also serves as a guideline for municipal heads to decide whether to issue evacuation information [1].
This means it is not the level "where flooding occurred," but the level "where the risk of flooding has emerged." River water levels are divided into four stages: the flood control team standby level, the flood caution level, the evacuation judgment level, and the flood danger level [1]. The level beyond this, Level 5, relates to cases where flooding has occurred or is imminent [1]. So, how is the height at which this risk emerges determined?
Safety Note: The Japan Meteorological Agency defines the flood danger water level as a level where flooding from rivers or internal water could cause significant house submersion, serving as a guideline for mayors to issue evacuation orders [1]. The four levels, from lowest to highest, correspond to Danger Levels 1 through 4. Level 5 relates to actual or imminent flooding, so the flood danger level is set before that point [1].
2. How the danger water level is set using levee design or the ground level of houses
According to Ministry of Land, Infrastructure, Transport and Tourism (MLIT) materials, the method for setting water levels is divided into three types based on the condition of the levees [2]. For rivers with completed levees, the "planned high water level" is used. This is not the top of the levee itself, but a position calculated by subtracting a "freeboard" (a safety margin for waves caused by wind) from the levee's height [2]. This margin ensures that water does not reach the very top of the levee [2].
For rivers under construction where lower sections remain, the height is calculated by subtracting the freeboard from the height of the lowest part of the levee. For rivers without levees, such as enclosed channels, the standard is the ground level behind the river, meaning the height of houses and land [2]. The Kanto Regional Development Bureau explains that if there are no levees, the standard is near the ground level of houses; if the levee is low, it is the levee height minus the freeboard; and for completed levees, the planned high water level becomes the flood danger water level [3].
Even for the same "flood danger water level," what is being protected differs by river: either the "floor of a house" or the "margin of the levee."
Safety Note: The freeboard is understood as the height added to the levee to ensure it remains safe even with wind waves when water reaches the planned high water level [2]. Therefore, the danger level is not the "height of the levee" itself, but a position that already has the margin subtracted. The Kofu River and Road Office (Fuji River) explains the same structure: for completed levees, the planned high water level is the flood danger water level, and if there are no levees, the standard is near the ground level of houses [3].
3. Why the time people need to escape is subtracted when setting the danger water level
If the level were set only by the height of the levee, there is a worry that by the time people are notified of "danger," they may already be too late to escape, depending on how fast the water is rising. To address this, the concept of "lead time" (the time available for evacuation) is used.
In the upper Arakawa River, the flood danger water level is set at the level where overflow is feared 2 to 3 hours later at the relatively lowest point of the levee. The lead time is 3 hours for the main Arakawa River and 2 hours for its tributaries. The amount the water level rises per hour is determined based on past flood situations [4]. The 2 to 3 hours is an example for the upper Arakawa and is not a nationwide standard.
As a national policy, the flood danger water level is set at the lower of either the planned high water level or the water level derived from the lead time (April 2014 notification) [2]. Choosing the lower value ensures that people are warned long before the water actually spills over. Additionally, the evacuation judgment level, which is a stage before the danger level, is set with a 1-hour lead time from the flood danger water level in Arakawa materials [4].
Safety Note: If the standard is only the levee height, in rivers where water rises quickly, evacuation may not be possible by the time the danger notice is issued. In the upper Arakawa, the flood danger water level is set as the level where overflow is feared 2 to 3 hours later at the lowest levee point, with a lead time of 3 hours for the main river and 2 hours for tributaries [4]. The evacuation judgment level is set with a 1-hour lead time from the flood danger water level [4]. The national policy (April 2014 notification) states that the flood danger water level is "the lower of the planned high water level or the water level set from the lead time" [2].
4. How the height of dangerous spots on a levee is converted into water levels at observation stations
Low spots on levees and dangerous areas are scattered throughout long sections of a river. However, water levels are measured at fixed points called water level observation stations. Therefore, the height calculated for dangerous spots is converted to the water level at the observation station for setting the warning [4][3]. Arakawa materials caution that even if the water level is a single value from an observation station, it must be viewed as information for the entire section [4].
Water levels are expressed as a height relative to a reference height (zero point) determined for each observation station. Therefore, the numbers are different for each river and station. In the flood control standard water level table from the Tone River Upper Basin Office, the flood danger water level is 9.20 m at Kurihashi, 9.70 m at Koga, and 8.70 m at Otome, showing different values for each station [7].
You cannot simply apply a number like "X meters is dangerous" to a different river.
Safety Note: For this reason, the height calculated at dangerous spots is converted to the water level at the observation station for setting [4][3]. In the Tone River Upper Basin Office's flood control standards, the flood danger water level differs by station: 9.20 m at Kurihashi, 9.70 m at Koga, and 8.70 m at Otome [7]. While the detailed handling of the zero point is left for another note, what can be said here is that comparing whether a number is dangerous across different rivers has no meaning.
5. Why the old, scattered water level names were hard to understand
Previously, there were several water levels with different purposes, such as the caution water level and notification water level (guidelines for flood control activities), the danger water level (risk of flooding), the planned high water level (design level), and the special caution water level (judgment for evacuation). MLIT materials identified this as a problem because it lacked uniformity and was very difficult for the general public to understand [5].
Therefore, in a River Bureau Director's notification dated October 1, 2006, a policy was shown to unify danger levels into 5 national levels and to organize the names so that the danger level and the action to take were clear [2][5]. The old “notification water level” is now called the flood control team standby level, the caution water level to the flood caution level, the special caution water level to the evacuation judgment level, and the danger water level to the flood danger level [5].
However, reviews continued. The April 2014 notification cited that the previous flood danger information was difficult for municipal heads to use for evacuation judgments and had not effectively led to resident evacuation actions, leading to a reorganization of the water level positions [2]. Furthermore, in the May 2021 revision of the Basic Act on Disaster Management, evacuation advisories and evacuation orders were unified. The flood danger water level is now a guideline for the mayor's decision to issue an evacuation order (for everyone), and the evacuation judgment level is a guideline for evacuation of the elderly and others [4].
Safety Note: Before the 2006 review, multiple water levels with different purposes (caution, notification, danger, planned high, special caution) existed, which lacked uniformity and was very difficult for citizens to understand [5]. A policy was shown to unify danger levels into 5 national levels and to name water levels based on danger and action. From the materials, notification water level corresponds to flood control team standby, caution to flood caution, special caution to evacuation judgment, and danger to flood danger [5]. This organization was shown in the October 1, 2006 notification [2], and positions were reviewed in the April 8, 2014 notification [2]. The reason for the review was that previous information made it difficult for mayors to judge evacuation orders and did not lead to resident actions [2]. In the May 2021 law revision, advisories and orders were unified, making the flood danger level a guideline for evacuation orders and the evacuation judgment level a guideline for elderly evacuation [4].
6. How to look up the danger water level of your town's river
Try looking for the name of the nearby river and the name of the water level observation station on your local flood or emergency map or the river's water level information page. If you find them, look at the water level numbers displayed for each observation station.
For rivers with water level data, try comparing the flood danger water level number with nearby rivers or other observation stations. Try to work out which of these factors in this article—"levees," "lead time," or "zero point per station"—can explain why the numbers are different. Stay away from rivers when water levels are rising.
Safety Note: MLIT's "Water Level Notification Rivers" are rivers designated by the national or prefectural government that are not flood forecast rivers but could cause significant economic damage from flooding. When they reach the special caution water level (flood danger water level), flood control managers are notified, and the public is informed if necessary [6]. It is good to try explaining why flood danger water levels differ between multiple observation stations using criteria, lead time, and zero point.