With a maze of pipes and tunnels already under the city, Shanghai is running out of space to upgrade its drainage system, exposing the city to flooding, officials warned yesterday.

Experts say drainage pipelines should ideally be built between 2.5 meters and 6 meters underground.

However, with the construction of the Metro, telecom and power facilities, it is becoming increasingly difficult to find suitable places in downtown for drainage pipelines.

The Shanghai Flood Control Headquarters said it has to intensify maintenance of the drainage system to prevent large-scale floods during torrential storms.

Currently, Shanghai's drainage system in urban areas can cope with downpours of 36 millimeters per hour - which usually happens once a year. In key areas, such as airports and commercial zones, the capacity is 50 to 56 millimeters per hour, which occurs about once every three years.

However, compared with other metropolises - such as Paris which can cope with freak storms that may occur once in 10 years - Shanghai still has a long way to go.

"The biggest problem facing the city is not simply upgrading the drainage system but a lack of space for this work downtown," said Zhang Zhenyu, director of flood control.

"Upgrading involves many departments and is not easy to coordinate."

Under the city's development plan, by 2015, Shanghai's Metro network should exceed 700 kilometers.

However, many stations are built near intersections, and their paths often force drainage pipes to be altered, reducing drainage ability.

"In some areas, we have to change 1-meter diameter pipes into two 50-centimeter ones," Zhang said. "And when a torrential downpour comes along this is much less efficient."

Another problem is that there is no uniform standard for drainage systems in different urban areas, as many of them were developed around 100 years ago by different countries in their concessions, said the Shanghai Water Authority.

In recent years, Shanghai had renovated 255 drainage systems across the city, covering more than 550 square kilometers. But 30 percent still require upgrading, Zhang said.

Torrential rain in Beijing last Thursday caused serious floods in which two people drowned and road and air links were paralyzed.