Disaster-prevention target raised from 86 mm to 101 mm per hour

Ulsan upgrades benchmark as heavy downpours grow more frequent

New standard to apply to urban planning and disaster-mitigation facilities

Taehwagang National Garden, built along the banks of the Taehwa River — the lifeblood of Ulsan — was inundated on July 19, 2025, after 101.9 mm of rainfall was recorded in a single day, according to the Korea Meteorological Administration. [Reader-submitted photo]
Taehwagang National Garden, built along the banks of the Taehwa River — the lifeblood of Ulsan — was inundated on July 19, 2025, after 101.9 mm of rainfall was recorded in a single day, according to the Korea Meteorological Administration. [Reader-submitted photo]

Ulsan has raised its "disaster-prevention performance target" — the benchmark rainfall that drainage and flood-mitigation infrastructure must be able to handle — and announced the new standard Friday. The target sets the hourly rainfall capacity for facilities such as sewers, drainage systems and retention basins, and serves as the basis for flood-prevention planning.

The revision follows a Ministry of Interior and Safety directive to raise the nationwide design standard from a once-in-30-years rainfall event to a once-in-50-years event. Under the updated targets, the thresholds for Jung-gu, Nam-gu, Dong-gu and Buk-gu — the city's urban districts — have been raised from 86 mm to 101 mm for a one-hour event, from 123 mm to 147 mm for two hours, and from 150 mm to 178 mm for three hours.

Rainfall exceeding the previous targets has grown increasingly common in Ulsan as climate change intensifies. This year alone, Dong-gu recorded 99 mm of rainfall in a single hour on Aug. 30, far surpassing the 73.5 mm recorded on Aug. 25, 2014. On Oct. 5, 2016, hourly rainfall across Ulsan reached 104.2 mm.

Ulsan faces particular flood risk because rivers including the Taehwa, Dongcheon, Hoeya and Oehwang run through the heart of the city. During heavy downpours, the city is vulnerable to a cascade effect: rainwater rushes in from surrounding hills, overwhelming urban storm drains, raising river levels, slowing drainage and ultimately flooding low-lying areas and below-ground spaces.

The city said it will apply the new targets to disaster-impact assessments for future urban planning and development projects, natural disaster risk improvement zone projects, comprehensive flood-resilience lifestyle zone improvement projects and local flood-prevention projects.

Authorities will also use the higher standard to reassess the flood-handling and waterlogging-response capacity of existing facilities. Where shortfalls are identified, the city plans to build new infrastructure, scale up or expand existing facilities, improve drainage systems, and install stormwater runoff reduction facilities and upland drainage channels. Specific infrastructure covered includes sewer pipes, retention ponds, storage basins, small-stream embankments, drainage pump stations, rainwater pump stations, sewage storage facilities, drainage channels, roadside ditches, stormwater runoff reduction facilities and upland drainage channels.

"This upward revision of the disaster-prevention performance target reflects the anticipated increase in rainfall due to climate change," a city official said. "We will concentrate our administrative efforts on building a flood-prevention system that protects the lives and property of our citizens."


cityblue@heraldcorp.com