East Corte Madera $2.6M flood project planned

Corte Madera is moving forward with an estimated $2.6 million rehabilitation of a pump station pipe that helps prevent flooding on the east side of town.

The San Clemente pump station, one of the largest of nine flood control systems in Corte Madera, is situated at Madera Del Presidio Drive at Paradise Drive. The station's 54-inch diameter outfall pipe runs underneath the roads. It collects water from a 410-acre watershed and carries it to San Clemente Creek, which feeds into the bay.

The Town Council voted 4-0 on Tuesday, authorizing staff to solicit contractors to bid on the project that will rehabilitate the 805-foot long pipe that has corroded and caused several sinkholes in recent years. Mayor Charles Lee did not vote because he owns property within 500 feet of the project site.

"The San Clemente pump station is a critical piece of Corte Madera's flood control infrastructure," Vice Mayor Eli Beckman said after the vote. "With drought-deluge cycles growing more intense, it's absolutely crucial that we make sure this pump station is ready to prevent flooding in the decades ahead."

The pump station was built in 1987 to address the flood risk on the east side of town, Chris Good, the town's senior civil engineer, told the council on Tuesday.

The outfall pipe is made of welded steel and reinforced concrete, Good said. The creek head where the outfall pipe discharges water is near the Central Marin Fire Department's Station 13 at 5600 Paradise Drive.

In 2010, the town replaced the pump station's discharge manifold, which is a chamber that splits into multiple pipe openings, due to corrosion damage, Good said.

The corrosion ate holes through the pipe, leading to leaks that caused sinkholes near the pump station in 2016 and 2021. There have also been two other sinkholes near Paradise Drive due to leaks in the concrete pipe section that required emergency repair, Good said.

"That's what alerted us to the issue, and once we started investigating, we found a lot of other issues," Good said.

Staff discovered that an interior pipe liner had completely worn away, allowing the corrosion to spread, Good said.

A geotechnical team was brought in to bore into the ground. They found there had been 27 inches of ground settlement near the pump station, Good said. The movement in the ground caused cracks on the pump station walls and is putting stress on the damaged pipe system.

It's anticipated that there could be another 11 inches of settlement near the pump station and up to 25 inches of settlement on Paradise Drive.

"So the problem will continue to get worse over time if it's not addressed," Good said.

Lastly, the pipe components, including valves and expansion joints, are all at the end of their lifespan, Good said.

The project would replace the existing steel manifold and pipe with a high density polyethene, or HDPE. There would be three flexible expansion joints installed that can accommodate up to 20 inches of settlement and lateral movement. The HDPE material is resistant to corrosion and doesn't require a protective coating.

The new manifold would be placed above ground rather than buried so that it's easier to access for repairs. This will require the removal of three 10- to 12-foot trees in the area. The new manifold will be protected by a fence structure, Good said.

These replacements are estimated to cost about $2.3 million, Good said.

Staff is also recommending 65 internal pipe joint seals to be installed within the existing reinforced concrete pipe section to prevent future leaks as a bid alternative. Bid alternatives are for specific work that is above the base bid of the project. This component of the project is estimated at $325,000.

The project is expected to be paid for with Measure F sales tax revenue. The town has $2.9 million from that fund available for the project.

Some of pipe materials require a 12-month advance order. While staff plans to bid the project immediately, construction is not expected until the summer of 2024, Good said.

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