Heavy Rains Cause Major Floods, Mudslides in China: AIR Worldwide

July 12, 2013

AIR Worldwide reports that “days of heavy rain across the mountainous Yunnan and Sichuan provinces of southwestern China have caused widespread flooding and triggered landslides. Official reports state that 99 centimeters (39 inches) of rain fell in the city of Dujiangyan (Sichuan) in just 40 hours, between Monday, July 8 and Wednesday, July 10, which is the heaviest rainfall the region has experienced since record keeping began in 1954. Thousands of homes have been destroyed, and more than 81,000 acres of crops have been inundated by the floodwaters.”

Dr. Yucheng Song, senior scientist at AIR Worldwide noted: “On Sunday evening, July 7, heavy rains began to fall across southwest China. According to some reports, these rain storms are the most extensive the region has seen for fifty years, with 39 inches of precipitation falling in just 40 hours in some locations. However, the record-breaking rainfall is only the latest in a series of events that have struck Sichuan province since mid-June.

“First, from June 18 to 22, all 16 cities (43 counties) of Sichuan were affected by heavy rains. Then, from June 29 to July 2, all communities in Sichuan except for Ya’an city experienced further rainfall. Next, most of the western Sichuan Basin received significant rainfall from July 3 to July 5. Since the evening of Sunday, July 7, heavy rains have fallen across the whole of Sichuan province and neighboring Yunnan province.”

AIR said that as of yesterday, July 11, “cumulative rainfall in the region has generally ranged from 45 to 80 centimeters (17.7 to 31.5 inches). However, it has been notably heavier in some locations; for example, nearly 100 centimeters (39 inches) of rain has fallen in the town of Dujiangyan (Sichuan province). In several places, rainfall has reached record-breaking levels. Specifically, in the communities of Pengzhou, Mianzhu, Beichuan, Deyang, Jiange, Pixian, Chongzhou, and Zhongjiang, rainfall has been 2 to 3 times higher than normal. In addition, Dujiangyan, Shifang, and Dayi have experienced rainfall 3 times higher than normal.”

Dr. Song explained: “Meteorological analysis suggests that this unusually large amount of precipitation can be attributed to the interaction of four factors. First, southerly winds (summer monsoon) carried large amounts of water vapor into the Sichuan Basin. Second, the atmospheric instability of the basin was increased by a continuous supply of cold air flowing into the basin. Third, the topography of the region—mountainous terrain to the west and flatter terrain to the east—exacerbated the uplift of water vapor in the atmosphere. Finally, the atmospheric circulation patterns associated with the western Pacific subtropical high set the stage for continued heavy rainfall in the area.”

According to AIR, China has a diverse commercial/industrial building stock, which continues to change as older structures are replaced with ones that are engineered for wind and water resistance. AIR said that in general commercial and industrial buildings are “more resistant to wind and water damage than residential buildings. Also, residential wood-frame buildings are expected to sustain more damage than residential masonry buildings. Concrete construction is less vulnerable to flood than steel or masonry. Commercial and apartment buildings usually have stronger foundations than residential buildings, and are thus better able to resist flood loads.”

According to AIR, “flood vulnerability also varies by building height. Because damage is usually limited to the lower stories of a building, high-rise buildings will experience a lower damage ratio—the ratio of the repair cost and the total replacement value of the building—than low-rise buildings because a smaller proportion of the building is affected.”

In addition AIR’s analysis noted that “the flooding is also expected to take its toll on agriculture. As of today, July 11, more than 81,000 acres of crops have been affected by the floods. The current wet conditions are affecting the harvesting of early double-cropped rice, the planting of late double-cropped rice, and the harvest of the early corn crop. While crop insurance is now offered across all provinces and for many crop types, participation remains relatively low because many challenges remain in the pricing and administration of insurance products and in effective risk diversification.

“Local experts suggest that small-to-medium reservoirs in Yunnan and Sichuan may also be at risk from the floods, and state these structures should be strengthened. Additional flood defenses in mountainous regions may also require upgrading.

“On Tuesday, July 9, heavy rain triggered a landslide in the city of Dujiangyan, Sichuan. The landslide covered an area of 2 km (0.8 square miles) with 1.5 million cubic meters (329 million gallons) of mud, rock, and debris, according to a Chinese news agency. (Deforestation has rendered parts of China prone to landslides after heavy rains.) The landslide also destroyed several buildings in Dujiangyan.

“As of Thursday, July 11, thousands of homes have been destroyed across Yunnan and Sichuan, and three bridges in Sichuan have collapsed. Chinese officials state that at least 1.6 million people have been affected by the flooding. In some parts of Sichuan, roads and bridges remain closed.”

Dr. Song concluded: “According to local forecasts, rain is expected to continue across southwest China, including the regions already affected by floodwaters this week, for the next two days. Although rainfall across the western Sichuan Basin is anticipated to gradually weaken during this time period, more rain is expected to fall in the region early next week on July 16 and 17.

“Cumulative rainfall in most of the affected region on July 16-17 will likely range from 40-90 mm [1.56 to 3.5 inches], with parts of the western Sichuan Basin experiencing total rainfall of between 100 and 200 mm [3.9 to 7.8 inches] and because the soils are already saturated, additional rain may worsen flooding in the region.”

AIR also noted that it is “closely monitoring Super Typhoon Soulik, the seventh named storm of the 2013 Pacific season. Currently with maximum sustained winds of 45 m/s (186 km/h [116 mph]), Soulik is moving at 20 km/h [12.5 mph] in a westerly direction toward Taiwan, according to the Japan Meteorological Agency. It is forecast to either make landfall on or pass closely by Taiwan sometime early Friday, local time, and then head toward coastal provinces of China. AIR will provide updates on Super Typhoon Soulik as warranted by events.”

Source: AIR Worldwide

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