More divers would be used to help in dredging work on city pipes this summer as many construction sites clogged up the system, water authorities said yesterday.

In the flood season, water collected rapidly when rainstorms hit if the pipes were not dredged properly, said the Shanghai Water Bureau.

A diver was called to clear out slop and stones from a pipe on Siping Road in Yangpu District on Thursday. The Metro Line 10 is under construction on the road.

"The mud from construction sites has flowed into the system and may dry soon," said Zhuang Minjie, director of the flood control and facility section under the bureau. "Common dredging methods cannot clear this mud."

Zhuang said more divers were expected to be on call amid the great amount of construction leading up to the 2010 World Expo.

The bureau said it would tell construction companies not to let slop and mud into the city's sewage system.