China has been hit by devastating floods in recent weeks, inundating cities and causing deaths and infrastructural damage, as well as raising questions about the effectiveness of its 2015 "sponge city" initiative aimed at reducing urban flood risks.

The initiative was launched to boost flood resilience in major cities and make better use of rainwater through architectural, engineering and infrastructural tweaks.

But cities remain vulnerable to heavy rain. In July alone, floods and related geological disasters caused 142 deaths and disappearances, destroyed 2,300 homes and caused direct economic losses of 15.78 billion yuan ($2.19 billion), China's emergency ministry said on Monday.

Following is an explainer about the sponge city scheme.

WHY WAS THE INITIATIVE LAUNCHED?

China has long sought to improve the way it handles extreme weather, and make highly populated cities less vulnerable to flooding and drought.

The "sponge city" initiative was designed to make greater use of lower-impact "nature-based solutions" to better distribute water and improve drainage and storage.

Those solutions included the use of permeable asphalt, the construction of new canals and ponds and also the restoration of wetlands, which would not only ease waterlogging, but also improve the urban environment.

Breakneck urbanisation has encased vast stretches of land in impermeable concrete, often along banks of major rivers that traditionally served as flood plains. With wetlands paved over and nowhere for surplus water to settle, waterlogging and flooding was commonplace.

According to 2018 data, 641 out of 654 large- and medium-sized cities in China were vulnerable to flooding and waterlogging, with 180 facing flood risks every year.

WHAT HAS BEEN DONE SO FAR?

Studies show that many of the local pilot initiatives launched so far have had a positive effect, with low-impact projects like green roofs and rain gardens reducing run-offs.

But implementation has so far been patchy. A total of 30 pilot sponge cities were selected in 2015 and 2016. By last year, only 64 of China's 654 cities had produced legislation to implement sponge city guidelines, researchers said in January.

The researchers said the government had so far paid "minimum attention" to sponge city construction, and called for national legislation to be drawn up as soon as possible.

WHAT ARE THE LIMITATIONS OF SPONGE CITIES?

Even if sponge city measures had been implemented in full, they would have been unable to prevent this year's disasters.

Zhengzhou in Henan province was one of the most enthusiastic pioneers of sponge city construction, allocating nearly 60 billion yuan to the programme from 2016 to 2021. But it was unable to deal with its heaviest rainfall in history in 2021.

Experts believe sponge city infrastructure can only handle no more than 200 millimetres (7.9 inches) of rain per day. At the height of the rainstorms that lashed Beijing at the end of July, rainfall at one station reached 745 millimetres over three and a half days. In July 2021, Zhengzhou saw rainfall in excess of 200 mm in just one hour.

Authorities are also playing catch-up to climate change. This year's heavy rain hit cities in the normally arid north, where sponge city development is less advanced.