AIR Analyzes Fallout from Heaviest Rainfall to Hit Beijing since 1951

July 24, 2012

According to catastrophe modeling firm AIR Worldwide, on Saturday, July 21, a heavy storm in China delivered as much as 18 inches [45.72 cms] of precipitation to parts of the country in and near the capital of Beijing (population 20 million).

“Almost seven inches [17.78 cms] of rain fell in the capital—the most since records were first kept, in 1951 and losses from this storm would have been greater had the soils been fully saturated,” AIR explained; adding that “should another storm hit this area soon, the soil’s inability to absorb water could become a serious issue. According to a senior engineer from the Ministry of Transport, it rained as much in a single day in Beijing as it normally rains in six months.”

Dr. Peter Sousounis, senior principal atmospheric scientist at AIR Worldwide, explained that the “heavy rain was caused in part by a slow-moving cold front that was partly fed by moisture from Typhoon Vicente, a storm located well to the south, near Hainan province. According to the Japan Meteorological Agency’s 1800 UTC advisory today, Vicente has maximum sustained winds of 105 miles per hour [168 km/h] and is roughly 85 miles [136 kms] south of Hong Kong, in the open ocean.

“The heavy rainfall over the weekend was also a result of a favorable Madden-Julian Oscillation (MJO)—identified on satellite imagery as enhanced regions of convection, or cloudiness; favorable MJOs can further enhance precipitation,” he continued. “The 20-hour storm started July 21 and lasted until the early morning hours of the following day, Sunday, July 22. The heavy flooding also flooded 700 homes, collapsed over 30 roads or bridges, and disrupted transportation at Beijing’s Capital International Airport.”

Notably, the large-scale flow responsible for this slow moving front is not related to the one responsible for the floods in Japan last week.

AIR said the areas hardest hit by the weekend storm were “in the outskirts of the capital, in the district of Fangshan, a suburb about 40 kilometers [25 miles] from downtown Beijing. Fangshan received 16 inches [40.6 cms] of precipitation at popular tourist spots such as its scenic Shidu Nature Park.

“Meanwhile, Fangshan’s Qinglonghu village, home to a large population of migrant workers from surrounding provinces, experienced the heaviest deluge of all: 18.4 inches [46.74 cms] of rain. In the town of Zhoukoudian, 60 kilometers [37.5 miles] southwest of downtown Beijing, several bridges were washed out by floodwaters.”

The record-breaking flooding caused by the heavy rain has called into question the quality of the capital’s flood drainage systems, on which the weekend’s storm put tremendous stress.

Dr. Sousounis pointed out: “Since the late 1940s, China has continued to strengthen flood control measures. To that end, more than 280,000 km [175,000 miles] of embankments, 86,000 reservoirs, and 97 key flood retention areas have been completed.

“The Chinese government claims that major rivers and lakes are capable of withstanding a 100-year return period flood event, while major sea dikes defend up to the 50-year level. Medium and smaller rivers are said to be able to withstand normal floods (Ministry of Water Resources, the People’s Republic of China). As a result, China is better protected against the truly catastrophic floods of the past; however, flooding is increasing along the smaller rivers and lakes because of China’s increasing population, whereby urbanization without strict land management continues to cause significant flood losses.”

He also noted that “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. Commercial and industrial buildings are generally more resistant to wind and water damage than residential buildings.”

According to AIR, “residential wood-frame building is expected to sustain more damage than a residential masonry building. 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.

“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.

“The flooding is also expected to take its toll on agriculture. As of July 23, the crop area affected in Hebei province (the neighbor to Beijing) is 132,730 hectares [app. 328,000 acres]; the total damaged area is 12,540 hectares [app. 31,000 acres].”

Source: AIR Worldwide

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