Unlike many photography enthusiasts, 63-year-old Yu Heping took an interest in a very "earthy" subject matter: a city's sewage system.

Over the past few years, the retiree from a bank took thousands of photos documenting how his hometown Pingxiang, in eastern China's Jiangxi Province, has transformed from a frequent urban flooding victim to an exemplary "sponge city."

"In the past, whenever heavy rains hit Pingxiang, it flooded like an ocean," said Yu. "Once, I even witnessed the collapse of a bridge that was under construction because of flooding."

Surrounded by mountains and with several rivers running through the city proper, Pingxiang used to be very vulnerable to urban flooding during rainy seasons. A large part of Yu's photos featured waterlogged shops and cars when floods struck.

To address such flooding, which plagues many Chinese cities, the central government decided to build "sponge cities" to absorb excess water through upgraded sewer systems, filtration pools, wetlands and permeable public spaces.

In 2015, Pingxiang was chosen as one of 16 pilot cities for the "sponge city" project. The municipal authorities first set up a 33-square-km experimental zone for the project, before expanding it to the whole city area.

In the upper streams of the river running through the city, authorities built more tunnels to divert water into other waterways. In the middle streams, they relied on lakes to reduce flood peaks and retain water. Downstream, more pump stations were built to enhance the drainage capacity of the city.

"Rather than simply tunneling excess water into watercourses, we also focused on making the city work like a sponge," said Liu Min, deputy director of natural resources and planning bureau of Pingxiang.

To this end, authorities preserved lakes and ponds and built vast areas of lawns and permeable pavement in the new district of the city. In the old district, they upgraded the city sewer systems to enhance their capability to absorb urban precipitation.

Years of efforts to build a "sponge city" in Pingxiang have paid off. Yu could easily tell the change by comparing the photos he took before the project with those after.

"After 2017, there has been no urban flooding in Wanlongwan, one of the more vulnerable districts in the city," said Yu. "Over 40,000 residents have been spared the damage of urban flooding."

Building a "sponge city" in Pingxiang has also brought new business opportunities in the related sectors, with their annual output topping more than 8 billion yuan (about 1.18 billion US dollars).

Jiangxi Longfa Industrial Company Ltd. has benefited from the project by producing a new type of permeable brick.

"These bricks can absorb water like a sponge," said Huang Yong, general manager of the company, as he demonstrated how a brick quickly absorbed a whole bottle of water. In 2018, over 70-million-yuan worth of such bricks were sold.

"We used to be a traditional firm making industrial ceramics. The'sponge city' project has brought us new business opportunities," said Huang.