ShaunTyphoon Ragasa, the world’s most powerful tropical cyclone this year, barreled towards tens of millions of people in southern China on Wednesday, September 24, after killing at least 14 people in Taiwan, leaving scores missing and lashing Hong Kong with ferocious winds and heavy rains.
Many also went missing in Taiwan’s eastern Hualien county, after a barrier lake overflowed and sent a wall of water into a town, the Taiwan fire department said, with Ragasa’s outer rim having drenched the island since Monday, September 22.
How The Barrier lake formed and grew?
The lake was first discovered in July, having been formed when earlier typhoons caused landslides blocking a valley.
Barrier lakes are formed when rocks, landslides or other natural blockages make a dam across a river, normally in a valley, blocking and holding back water, hindering or even stopping natural drainage. This lake is in a remote part of the mountains above the town of Guangfu on a tributary of the Matai’an Creek.
A Reuters analysis of satellite imagery by Planet Labs shows that the lake first started forming sometime between July 17 and July 25. The surface area of the lake increased by about five times between July 25 and mid-September.
How big was the lake?
A Reuters analysis of satellite imagery shows the lake grew to a size of 0.92 square km by mid-September.
Before it overflowed this week, the dam forming the lake stood approximately 120 metres (390 ft) high, with the water behind it covering an area 500 metres (1,600 ft) long, and 1,650 metres (5,400 ft) wide, according to the government.
It has since shrunk in size by about 75%. The government said the overflow released about 60 million metric tons from a barrier lake that held an estimated 91 million metric tons—enough to fill about 36,000 Olympic-sized swimming pools.
The water hit like a “tsunami”, said Guangfu postman Hsieh Chien-tung, who was able to flee to the second floor of the post office just in time. Later, he got home to find his car had been swept into the living room.
Typhoon Ragasa brought heavy rainfall
The mountainous region of Taiwan saw heavy rainfall, according to satellite analysis data.
The lake’s remote, hazardous location has hampered authorities’ efforts to lower the water level and clear the rubble.
Some parts of the mountains in Hualien remain unstable after a 7.2 magnitude quake hit the region last year, Taiwan’s biggest earthquake in at least 25 years.
Strongest storm of the year
Ragasa formed over the Western Pacific last week. Fuelled by warm seas and favourable atmospheric conditions, the tropical cyclone rapidly intensified to become a Category 5 super typhoon on Monday with winds exceeding 260 kph (162 mph).
It has since weakened, but was still powerful enough to bring down trees and power lines.
“Authorities have taken lessons from Hato and Mangkhut, which both caused billions of dollars in damage in 2017 and 2018,” said Chim Lee, a senior energy and climate change specialist at the Economist Intelligence Unit.
More than 100 people were injured in Hong Kong, where authorities imposed the highest typhoon signal 10 for most of Wednesday.
Prior to Ragasa’s arrival, authorities handed out sandbags on Monday for residents to bolster their homes in low-lying areas, while many people stockpiled daily necessities, leading to bare supermarket shelves and surging fresh vegetable prices.
In the gambling hub of Macau, next to Hong Kong, casinos were forced to shutter their gambling areas. One user on China’s Xiaohongshu app showed videos of doors being sealed at a casino resort for protection against gales and debris.
Strong winds reached up to 280 kph
More than 2 million people were evacuated across Guangdong and the ministry for emergencies dispatched tens of thousands of tents, folding beds, lighting equipment and other rescue supplies, Chinese state media reported.
Storm surges in Hong Kong, southern China
In Hong Kong, where huge waves crashed over areas of the Asian financial hub’s eastern and southern shoreline, some roads and residential properties were submerged.
At the Fullerton Hotel on the island’s south, videos on social media showed seawater surging through glass doors. No injuries have been reported, the hotel said.
China’s marine authority issued its highest red wave warning for the first time this year, forecasting storm surges of up to 2.8 metres (9 feet) in parts of Guangdong province, as Ragasa headed towards the densely populated Pearl River Delta.
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