[Fukuoka University] The Reality of Heavy Rains and Disasters - The Key is Not to Let Them Flow into Sewers

In recent years, frequent heavy rains across various regions have had a significant impact on our lives. However, opportunities to correctly understand their mechanisms and dangers are limited.

Therefore, in this project, five faculty members from different specialized fields will convey the reality, risks, and countermeasures of heavy rains and disasters through the university's initiatives. "To know is to prepare." We hope this will empower individual judgment and actions.

For the first installment, we spoke with Professor Ryoichi Watanabe (River Basin System Laboratory) from the Department of Urban and Environmental Engineering, Faculty of Engineering.

<An Excellent Drainage System That Protects Cities from Water Damage, Which We Know Little About>

When it rains, rainwater that falls on roofs flows down gutters and is directed into sewer pipes. The sewer pipes collect rainwater and discharge it into rivers, which then flow into the sea. Rainwater falling on roofs is drained into nearby rivers within 15 minutes at the latest.

"The main player in protecting cities from water damage is the sewer system," says Professor Watanabe.

The sewer system is a very convenient mechanism that collects all the rainwater and removes it from the city. During heavy rains, the sewer pipes, with a total length of over approximately 7,300 km spread throughout Fukuoka City, collect a large amount of rainwater and rapidly discharge it into rivers, protecting the city from water damage.

However, in recent years, situations where water overflows onto roads and fields are submerged have occurred frequently with each heavy rain.

Why do such situations occur in Japan, where the drainage system using sewer pipes is almost complete?

<How to Handle Heavy Rains That Exceed the Capacity of Sewer Pipes>

Fukuoka City has so far developed its sewer system based on rainfall that occurs about once every five years (52.2 mm per hour). "During heavy rains, I've heard on the news that it rains over 100 mm per hour...?".

Some of you may have realized this. In fact, in August 2025, 110 mm of rain per hour was recorded in Munakata City, where a linear rain band occurred, causing significant damage. Reference: Yomiuri Shimbun "Linear rain band in Fukuoka Prefecture, 110 mm of rain per hour in Munakata City... possibility of increasing risk in various areas from the morning of the 11th"

One commonly proposed solution to deal with such situations is to widen the diameter of sewer pipes to improve their processing capacity. However, this presents two major problems.

The first is that it leads to river flooding. The rainwater drainage function must be considered in conjunction with rivers. Even if the sewer system can handle heavy rains exceeding 100 mm per hour, if the river's flow capacity cannot keep up, the rainwater flowing in from the sewer system cannot be fully discharged to the sea. This ultimately leads to river flooding.

The second is that sewer systems are already in place in urban areas. In Fukuoka City, 99.7% (Note 1) of the sewer system is already complete, and replacing all the sewer pipes, which exceed approximately 7,300 km within the city, would require enormous costs and time. Note 1: Sewerage coverage rate (from Fukuoka City website)

For these reasons, the idea of replacing the sewer system is not realistic. It is necessary to consider alternative methods while utilizing the existing sewer system.

The problem with heavy rain damage is the excessive concentration of rainwater. In other words, if rainwater can be temporarily stored or processed elsewhere so that it does not flow into the sewer system immediately during heavy rains, the amount of rainwater entering the sewer system can be reduced.

<You Are Also a Stakeholder! "Watershed Flood Control" Requires the Actions of Each and Every Resident>

In 2022, the Ministry of Land, Infrastructure, Transport and Tourism introduced a new flood control policy, "Watershed Flood Control." This approach shifts from flood control led by the national and local governments to an approach where "all people living in the watershed" take the lead in flood control measures. This has enabled the creation of flood control facilities on private land as public works.

Near Fukuoka University, the Nanakuma River flows and merges with the Hii River downstream. The university's campus area accounts for about 10% of the Hii River watershed. Therefore, if a storage facility were built on campus, it could be expected to have a flow control effect. Professor Watanabe believed that building a facility at Fukuoka University, located upstream of the Hii River, would be highly significant and has been in repeated consultations with Fukuoka City.

<"Store When It Rains Heavily. Use as a Ground When the Rain Stops" - Fukuoka University's Initiative>

In June 2026, Fukuoka University and Fukuoka City will sign a partnership agreement to strengthen preparedness for water-related disasters. A facility will be installed on the ground west of the rugby field on campus, capable of storing rainwater equivalent to approximately 7,000 household bathtubs (about 2,000 cubic meters).

Generally, sports grounds are higher in the center and have a gentle slope towards the edges. Drainage ditches are provided at the edges, designed to quickly drain rainwater into the sewer system. In the newly installed facility, the ground will be surrounded by a wall 40 cm high, and the entrance to the sewer pipe where rainwater flows in will be narrowed. This will allow for the storage of 2,000 to 3,000 tons of rainwater within the ground that has nowhere else to go if the rainfall exceeds the set amount.

<Achieving Both by Improving the Ground Soil to "Toesu Soil">

What makes the coexistence of rainwater storage and infiltration facilities and sports grounds possible is "Toesu Soil" (to-su-do), developed by Professor Watanabe. Toesu Soil is made by mixing polymers and solidifying agents into the soil to create "granules" (making soil particles into small clumps). This creates moderate gaps, allowing rainwater to quickly infiltrate into the ground.

The primary purpose of the facility is as a sports ground, so it must be quickly drained and ready for use after the rain stops. If water is stored on a typical ground, it cannot be drained immediately and becomes unusable for a while. However, if it is improved with Toesu Soil, the surface dries quickly after drainage and can be used as a ground.

Toesu Soil has a proven track record, having been used on the Fukuoka University soccer field and the grounds of Jonan High School in Fukuoka Prefecture (Note 2) for about 20 years.

"According to his graduates who belong to his lab, Jonan High School's ground is known for its good drainage, so club activities are rarely canceled due to rain," Professor Watanabe says with a smile. Note 2: Constructed not for flood control purposes but to utilize infiltration effects for dust suppression.

Professor Watanabe explaining the comparison of drainage properties between Toesu Soil (left) and general soil (right).

<What Each of Us Can Do Now to Prevent Water Damage>

Q. What can we do now?

The best thing is to create a spot in a corner of your garden with soil that has infiltration properties, like "Toesu Soil," and direct rainwater from the downspout to infiltrate underground. It would be very effective if everyone participated. Starting with a rainwater tank is also recommended. The important thing is to prevent rainwater from entering the sewer pipes.

Q. Are there any problems with the ground if everyone infiltrates rainwater into the soil?

Currently, most of the rainwater that falls in the city does not infiltrate the ground. Most of it is discharged into rivers through sewer pipes, so there is no time for it to soak in. Therefore, the groundwater level in urban areas is currently very low. There is still capacity for water to enter the ground. The current situation is that the groundwater level is so low that seawater is intruding into the ground inland. When seawater enters inland, well water becomes unusable. If everyone starts infiltrating rainwater into their gardens, the city will gradually become cooler. This is because rainwater stored in the ground evaporates on sunny days.

In Professor Watanabe's laboratory, students create pamphlets containing data scientifically verifying the infiltration of rainwater and river water levels. They have also distributed these to households in Jonan Ward.

Rain Garden Guidebook created by students.

"I think few people recognize that 'Watershed Flood Control is for all residents' and most people do not feel that they are a stakeholder. Unless everyone, including those living in elevated areas with a low risk of damage, makes small efforts, the damage cannot be reduced.

First, I would like you to observe how much rainwater accumulates when it rains. As a result, I hope that initiatives to infiltrate rainwater into the ground or temporarily store it will spread.

Plants are planted in areas where rainwater infiltrates the garden, creating a biotope. The laboratory has actually created a rain garden.

◎ Mini Information ◎

Many people may be wondering if it's okay to drain bathwater on rainy days.

In most areas of Fukuoka City, a "separate system" is used, where rainwater and sewage are treated in separate pipes. Bathwater and rainwater follow different paths, so there is usually no problem.

On the other hand, in some parts of the city and in areas outside the city, a "combined system" is used, where rainwater and sewage are treated in the same pipe. In such cases, caution may be necessary depending on the situation. It is reassuring to check the system in your area.

[Related Links] Faculty of Engineering, Department of Urban and Environmental Engineering Website