Ajigawa Floodgate is an important tidal flood-control facility located at the mouth of the Ajigawa River. It was built to protect the Osaka area from storm surges caused by typhoons and strong winds blowing in from Osaka Bay, helping safeguard the lives and property of local residents.
The Osaka Plain is an alluvial plain with relatively low elevations, making it particularly vulnerable to typhoons and storm surges. Osaka City suffered severe flooding and storm surge damage from major typhoons, including the Muroto Typhoon in 1934, the Jane Typhoon in 1950, and the Second Muroto Typhoon in 1961.
As Osaka’s population and industrial activity increased, land subsidence also became a serious concern. The repeated disasters demonstrated the urgent need for large-scale coastal flood-control infrastructure.
In 1965, Osaka established the Permanent Osaka Storm Surge Protection Plan. The plan was designed on the assumption that a typhoon comparable to the Isewan Typhoon would directly strike Osaka.
The planned storm surge level was set at O.P. +5.2 meters, based on Osaka Bay’s mean low-tide level. This figure consisted of an average high-tide level of 2.2 meters plus an additional 3.0 meters of water-level rise caused by a major typhoon.
Because the Ajigawa River is an important navigation route for ships, an arch-type floodgate was adopted. Compared with conventional floodgates, this design provides advantages in maintaining navigable waterways as well as in earthquake and wind resistance.
Similar arch-type floodgates can also be found at the Shirinashi River Floodgate and Kizugawa River Floodgate.
The main floodgate consists of a steel gate measuring 66.7 meters wide and 11.9 meters high. Its total weight is approximately 530 tons.
The gate is made up of two sections connected by a hinge at the center. It is opened and closed using a winding mechanism operated by wire ropes.
The auxiliary floodgate is a swing-gate type structure measuring 17.1 meters wide and 11.55 meters high. It is mainly used to regulate water levels.
The gate weighs approximately 107 tons and is operated using hydraulic cylinders.
The supports for the main gate are installed on reinforced-concrete piers. These piers are supported by caissons extending approximately 44 meters underground to reach stable ground.
An underground passage connecting the two piers is also provided as part of the facility’s infrastructure.
When the floodgate needs to be closed, the operation is carried out in a specific sequence:
1. A siren is sounded and the signals are changed to red.
2. Buffer chains are stretched to close the waterway.
3. The dust screens are opened and the main floodgate is closed.
4. After the main floodgate has been closed, the auxiliary floodgate is closed.
While the floodgate remains closed, water flowing downstream from the upper reaches is discharged into the Shin-Yodo River using pumps at the Kema Pumping Station.
When the floodgate is reopened, the same operations are carried out in reverse order.
Closure of the floodgate is carried out by the director of the West Osaka Flood Control Office based on instructions from the Governor of Osaka Prefecture.
The floodgate is closed when a storm surge warning is issued and the water level exceeds O.P. +2.5 meters.
The Ajigawa Floodgate also faces several operational challenges. Its operation depends on an electric power supply, meaning that the system could potentially stop functioning in the event of a power outage or damage to fuel-related piping.
In addition, some parts of the structure are not designed to withstand the full impact of a tsunami. Further disaster-prevention measures therefore remain an important consideration.
August 1975: Typhoon No. 6
September 1994: Typhoon No. 26
September 2018: Typhoon No. 21
Gate types: Arch-type gate for the main floodgate; swing gate for the auxiliary floodgate
Main floodgate: 66.7 meters wide × 11.9 meters high
Auxiliary floodgate: 17.1 meters wide × 11.55 meters high
Completion: March 1970
Ajigawa Floodgate plays an essential role in protecting the Osaka area from storm surge disasters. Its distinctive arch-shaped design helps maintain a navigable waterway while providing structural resistance against earthquakes and strong winds.
At the same time, its dependence on electric power and other operational limitations highlight the continuing challenges involved in maintaining coastal disaster-prevention infrastructure.
As an important part of Osaka’s flood-control system, the Ajigawa Floodgate continues to require careful maintenance and appropriate measures to protect the city from future natural disasters.