Underground utility work related to the construction of the new Multnomah County courthouse caused a water main to break and flood downtown Portland streets Sunday.

Sewer work done days earlier under the 16-inch cast-iron water main put pressure on the pipe, leading it to burst around 11:45 a.m. at Southwest First Avenue and Madison Street, said Jaymee Cuti, a Portland Water Bureau spokeswoman.

It's not clear how much water was released onto the streets, Cuti said. But within 15 minutes, several inches of standing water flowed along First Avenue between Madison and Taylor streets.

The Hawthorne Bridge was temporarily closed because of the nearby flooding. Water service was shut off to five commercial buildings, including World Trade Center 3 near First Avenue and Salmon Street, where water poured into the basement.

The broken pipe was first installed 59 years ago and was "otherwise stable" before the construction in the area, Cuti said. The average age of Portland pipes is 65 years, she said. The water main was repaired by 4 a.m. Monday.

Underground work for the county's new $300 million central courthouse began in February, including relocating sewer lines at the west end of the Hawthorne Bridge to start excavation. The courthouse is scheduled to open in 2020.

Before the water main broke, construction crews hadn't been working at the new courthouse site since Thursday, said Multnomah County spokesman Mike Pullen. Installing a storm sewer line near Southwest First Avenue and Naito Parkway is among the underground work that still needs to be done.

There were no warning signs that the construction work would cause any issues, Pullen said. Crews have had an inspector with the Portland Bureau of Environmental Services on site during the utility work and they raised no red flags, he said.

-- Everton Bailey Jr.

ebailey@oregonian.com

503-221-8343; @EvertonBailey