A second sewer leak in three weeks at Lincoln Park convinced King County engineers to replace the sewer line along the popular park's shoreline and to lay a temporary above-ground bypass sewer pipe.

Work on the four-month project began this past weekend. Most of the bypass sewer pipe is now in place alongside the shoreline pathway.

The latest leak in the sewer pipe occurred Feb. 4 about 500 feet north of Colman Pool. Part of the trail around the north beach cove of the park had to be excavated by track hoe, which revealed steel rebar protruding from the bottom of the 30-inch concrete sewer pipe.

Soon after the first leak happened Jan. 17 beneath the shoreline path at the southern end of the park, workers from the King County Wastewater Division used a remote-control television camera to inspect about 1,700 feet of the sewer line along the beach. That convinced engineers the problem extends beyond the two recent leaks. They've decided to replace about 6,250 feet of the sewer pipe from the Fauntleroy Ferry Terminal, where the Barton Street pump station is located underground next door, north to Lowman Beach Park, where the Murray Avenue pump station is.

A 6,000-foot-long temporary above-ground pipe now runs from the Barton Street sewage pump station next to the Fauntleroy Ferry Terminal northward to a manhole just south of Lowman Beach Park The temporary sewer pipe is 18 inches in diameter.

Ever since the first leak on Jan. 17, tanker trucks have been taking loads of wastewater from the Barton Street pump station to the Alki treatment plant. The trucks couldn't capture all of it however, so an estimated 1 million gallons of raw sewage and other wastewater flowed into Fauntleroy Cove every day since mid-January through an outfall next to the ferry terminal, said Annie Kolb-Nelson, spokeswoman for the King County Wastewater Division. When the temporary bypass sewer line is operating, there will be no need for the trucks nor the outfall into Puget Sound, she said.

A decision has not been made about whether to close the beach at Lincoln Park, said Dewey Potter, Seattle Parks and Recreation spokeswoman. Parks employees will monitor construction to decide whether to close all, part or any of the trail. Meanwhile the rest of Lincoln Park will remain open throughout the four-month project.

County wastewater officials are scheduled to attend the Feb. 15 meeting of the Morgan Community Association to discuss the situation with the public. The meeting is set for 7 p.m. in the community room at The Kenney, 7125 Fauntleroy Way S.W.

In March, construction crews will begin digging two trenches along Lincoln Park's beach. One trench will go down to the existing 30-inch sewer line. A new polyethylene pipe with a 24-inch diameter will be slipped inside the length of the old pipe. That way the old pipe won't have to removed.

Because the smaller diameter will reduce the volume of the line, a parallel trench will be dug and another 24-inch polyethylene sewer pipe will be installed in it. Work will start next to the Fauntleroy Ferry Terminal and progress northward to Lowman Beach Park.

Both new sewer lines will be "forced mains," meaning the pipes will be pressurized by pumps as was the old line.

Recent heavy rains poured record amounts of storm water into the sewer system shortly before the leaks occurred. The volume of water was so great it dislocated the more-than-50-year-old sewer pipe, Kolb-Nelson said.

The Jan. 17 leak occurred when the weight of the sewer pipe laid heavily on a sharp piece of granite in the surrounding soil. The granite chunk pierced the sewer line as the pipe settled on it, Kolb-Nelson said.

When heavy rain hits Seattle, the storm drainage system can only hold so much water. Once it is full, the excess rain water goes into the sewer system in what is called a combined sewer overflow. The overflow can contain rocks, sticks, nails and other debris washed into the pipe from the streets. Under pressure, all that grit eventually erodes the interior wall of the pipe, Kolb-Nelson said.

"After decades, it's like a sandblaster," she said. First the mortar lining on the inside wall of the pipe erodes and exposes the steel rebar, which begins to corrode. In turn, the corroded rebar weakens the outer layer of the pipe, Kolb-Nelson explained.

From the late 1800s through the 1940s, sewage and storm water were collected in one pipe and delivered to nearby bodies of water, according to the King County Wastewater Division website.

In the 1950s, sewer systems started to be built separately so the sewage could be treated. Storm water continued to be directed to natural bodies of water during inclement weather.

By the late '50s, storm water was being cleaned too.

The Alki sewage treatment plant was converted into a storm-water plant in the early 1990s. Now nearly all West Seattle sewage is treated at the West Point treatment plant next to Discovery Park.

Tim St. Clair can be reached at tstclair@robinsonnews.com or 932-0300.