China Daily Global Edition (USA)
Ending the reign of flooding
Academic hopes to combat the scourge by predicting the most likely areas to be inundated by rainwater, Fang Aiqing
Cities like Beijing, Shanghai, Hangzhou and Wuhan understand only too well the devastation that follows a violent rainstorm. Roads and pathways are inundated, buildings damaged and tragically, lives are lost.
One of the heaviest rainstorms in more than 60 years hit Beijing and its surrounding areas on July 21, 2012. Ding Zhijian, 34, became trapped in his vehicle, when rising water levels caused a 4-meter-deep surge under Guangqumen flyover on the East Second Ring Road. Such was the pressure and intensity of the water that Ding was unable to smash the vehicle’s windows or open a door. He drowned before rescuers could reach him.
That storm accounted for 79 lives and destroyed more than 10,000 residences, officials said in August that year.
Last summer, continuous rainfall hit eastern and southern China. Some places in Chongqing municipality, Anhui, Jiangxi, Hubei, Guangdong provinces and Guangxi Zhuang autonomous region experienced their worst floods in decades.
A particularly violent storm on May 22 caused 443 incidences of flooding in Guangzhou, the provincial capital of Guangdong, and four people lost their lives. Several metro stations were flooded and services had to be suspended.
The scourge of flooding, and the suffering it causes, has spurred Hou Jingming’s determination to make cities safer. The Xi’an-based scholar, 39, and his team of more than 30 members, are working to enhance city drainage systems and flood defenses.
He and his team want to create a “Sponge City” that can flexibly adjust to environmental changes.
The water-modeling professor from Xi’an University of Technology, Northwest China’s Shaanxi province, mainly deals with a numerical model that involves hydrological and hydrodynamic processes like precipitation, infiltration, evaporation, surface runoff and flood of the area.
The model is able to deduce the predicable effectiveness of different measures to improve a city’s capability to deal with potential flooding and suggest the best measures to combat it.
Coastal cities situated near flood plains mean that they are potential victims of flooding.
While climate change has brought more extreme weather, intensified construction of roads and buildings and population growth in cities has resulted in the widespread loss of natural surfaces that allow rainwater to seep into earth or flow into rivers, lakes and reservoirs. With the natural water flow disrupted, flooding occurs.
“A decrease in urban green space makes it difficult for water to seep down, and the paved surfaces accelerate the water flow,” Hou says.
The trouble is that construction methods have outpaced advances in drainage systems. There is increased construction, bigger buildings, more residents, but the same antiquated drainage system. The result? More flooding. With nowhere for excess water to go, river levels rise, flooding surrounding areas and rainwater leaves roads and streets inundated.
It partly explains why northwestern cities like Guyuan, located in the Ningxia Hui autonomous region’s southern mountainous “Xihaigu area”, one of the most water-scarce regions in China, could experience such a problem.
According to Hou, who hosts one of his research programs in Guyuan, there’s an annual precipitation of less than 500 millimeters. This falls, he says, mainly through July to September. It also varies drastically from year to year.
Therefore, apart from drainage problems, it’s also urgent for the city to raise the utilization rate of rainwater, based on rain-sewage diversion — the separation of rainwater and foul water from household drainage systems — to help alleviate the local water shortage.
Specifically, Hou’s numerical model takes landform, rainfall intensity and the infiltration rate of the underlying surface into account and simulates urban flood situations during storms.
The model is able to identify potential locations of flood inundation, as well as the possible depth and volume of bodies of water.
Although the model can be seen as a universal tool in research into both mountain torrents and mudslides as well as urban flood scenarios, Hou spent most of his time and energy on the latter, with a focus on renovating old town areas where pipeline networks might be dated and in a state of disrepair.
He developed the model with his mentors during his studies at the Technical University of Berlin in Germany and Newcastle University in Britain.
On the basis of his predecessors’ achievements, he made the simulation more stable and precise when analyzing complicated surfaces, and sped up the computation significantly using high-powered Graphic Processing Units.
After graduating in Britain in 2016, where he completed his postdoctoral studies, Hou came back home with a driving ambition, looking to seize the opportunity as China was galloping along the urbanization trail and seeking answers to questions raised by urban growth as cities expanded.
While Danish and British models have been widely applied in China for decades, Hou believes his model is competitive in providing better solutions.
Hou’s model not only helps improve drainage capacity, but is also potentially able to contribute to risk awareness, traffic line optimization, and the maintenance of relative facilities. This is according to Ma Yue, assistant to the director of the Sponge City technology center of Fengxi New City at Xixian New Area, located between the cities of Xi’an and Xianyang, Shaanxi province.
The center, run by the local government, has provided Hou’s team with high-precision topographic information and monitoring data about pipelines, outfalls of residential zones and other infrastructure that is needed to run the simulation.
Since 2014, the country has been exploring replicable ways to improve city capability of rainwater storage, seepage and purification.
A 2015 guidance released by the State Council points out that the measures aim to minimize the impact of construction programs on urban ecological environments.
The guidance announced a quantified goal that no less than 70 percent of rainfall should be consumed locally.
The 30 designated pilot Sponge Cities around the country include Guyuan and the Xixian New Area where rain gardens and green roofs have been built to control rainwater. The greenery is able to filter out pollutants, while the collected water can be used in cleaning roads and irrigating the city’s green spaces.
Although it costs 30 percent more for such renovations, it also helps improve the ecological environment, reduces the risk of natural disasters and saves natural water resources.
“After all, it’s a comprehensive project that kills more than two birds with one stone,” Hou says.
According to Ma, the water table (the underground water level) has risen, and guaranteed the city’s capacity to hold rainwater. Moreover, the improved ecology has had a positive effect on land value and investment attraction in the pilot area.
Xixian New Area is now working to extend Sponge City facilities to cover the whole area. In the future, they want to popularize the lessons that have been learned through years of effort to neighboring cities, Ma says.
In January, Hou launched a cooperative program with a team from his German alma mater. They will share innovation on urban flood modeling and prediction, and green infrastructure that mitigates urban flooding.
According to Hou, he’s planning to advance the current model further, for example, to integrate the influence of curbs, subway lines and underground shopping malls, to calculate suitable percentage distribution of gray infrastructure and green areas, and to identify key zones that contribute the most to urban floods so that targeted measures can be taken.