Osong 2023 | DA-Team4

Overview

An incident in which large-scale casualties occurred on the morning of July 15, 2023, when the Gungpyeong 2 Underpass in Osong-eup, Heungdeok-gu, Cheongju-si, Chungcheongbuk-do was flooded due to torrential rain and the collapse of the Miho River embankment.
Several vehicles were trapped inside the underpass, and rapid flooding occurred within a short period of time, leading to a major disaster.

Scale of damage

July 15, 2023: Flooding occurred at Gungpyeong 2 Underpass in Osong, Cheongju, North Chungcheong Province
Approximately 17 vehicles submerged, 14 fatalities reported
Underpass interior flooded rapidly within minutes
Damage caused by road flooding, landslides, and river overflows across the Chungcheong region

Official Press Release from the Korea Meteorological Administration

In July 2023, a large amount of water vapor continuously flowed into Korea along the edge of the North Pacific High, and atmospheric instability became very strong due to a trough accompanied by cold air located to the north. During this process, a stationary front that had moved southward became active, resulting in heavy rainfall nationwide, with torrential downpours occurring particularly in regions south of the Chungcheong region.
The nationwide average precipitation was approximately 506.4 mm, which was significantly higher than the normal range (245.9–308.2 mm), and recorded one of the highest levels of precipitation on record. The precipitation ratio map also showed that most regions across the country received more precipitation than the normal range, with significant precipitation deviations observed particularly in the Chungcheong region and some parts of the southern area.
In mid-July (particularly around the 14th to 19th), very heavy torrential rains occurred as a stationary front overlapped with a massive supply of water vapor. Examples of daily precipitation in major regions include Nonsan (372.8 mm), Cheongju (251.5 mm), Seosan (208.1 mm), and Iksan (195.1 mm); the time-series graphs also clearly show the concentration of precipitation during this period.

AWS/ASOS observation data

Maximum hourly precipitation (Bokdae-dong, Heungdeok-gu, Cheongju-si, Chungcheongbuk-do)

: 24.8 mm/hr

Cumulative Precipitation (Bokdae-dong, Heungdeok-gu, Cheongju-si, Chungcheongbuk-do)

: 256.8 mm (24-hour cumulative on July 15)

171 mm (24-hour cumulative on July 14)

Radar Observation

A linear precipitation band running in a southwest-northeast direction has begun to develop, centered on the Chungcheong region.
Precipitation cells are continuously forming due to strong water vapor flowing in from the West Sea.
This is the initial stage of widespread precipitation expanding into the Chungbuk region.

A very long linear precipitation band persisted in the Chungcheong region, exhibiting a distinct train echo pattern.
Heavy rain fell repeatedly in the same area, causing a rapid increase in accumulated precipitation.
At the point when continuous torrential rains persisted in the Osong and Miho River basins.

The precipitation band remains centered on the southern Chungcheong region, but the overall precipitation area is gradually weakening.
The cumulative precipitation across the entire basin has become very high due to prolonged rainfall.
It can be viewed as a stage where flood damage has expanded due to the rise in the Miho River water level and the collapse of levees.

Key Causes

At the time, the monsoon front was stationary over the Chungcheong region, and a large amount of water vapor was continuously flowing in from the West Sea.
As the incoming water vapor strongly converged, a highly developed convective precipitation band formed. Atmospheric instability was maximized as strong updrafts were further intensified by the influence of the upper-level jet stream and upper-level trough.
In particular, torrential downpours persisted, characterized by a train echo precipitation pattern where heavy rain fell repeatedly over a narrow area. (A pattern very similar to the 2022 Gangnam case)
In the Cheongju area, very heavy rain of approximately 60–70 mm or more fell per hour, and in some areas, cumulative precipitation exceeded 300 mm.
Due to the continuous torrential downpours, the water level of the Miho River rose rapidly. As the inflow exceeded the design standards of the levee, the levee eventually collapsed.