City seeks to control flooding

Experts have identified several sites for detention facilities, some of which are on public land and some on private property, including Notre Dame de Namur University, Carlmont Drive/Hidden Canyon Park, Twin Pines Park and Church of Immaculate Heart of Mary.

“These plans are a blend of technical, financial and political, and that last part will be a tricky one,” Mayor Doug Kim said. “We’ll have to balance what the experts say versus what the public will accept.”

Conversations to find out what the public and stakeholders will accept are underway, but experts have especially high hopes that Notre Dame de Namur will agree to allow underground detention basins on its soccer and softball fields, among other sites on campus. The proposed basin for the softball field encompasses .9 acres 10 feet below ground while the potential soccer field basin would span 1.7 acres, also 10 feet underground.

“[The Notre Dame sites] offer the best solution for capturing peak from a major flood or rain event,” said Erika Powell, program manager for San Mateo County’s Flood Resiliency Program.

Powell said the proposed Notre Dame basins could handle more than 300 cubic feet of water per second, which is the unit by which storm runoff is measured. She said the “perfect world” goal of the project is to add enough detention facilities so the creek can handle 500 CFS, but that number would be hard to achieve without the Notre Dame ones, which on their own could handle more water than all other proposed sites combined.

Powell said the cost of these particular basins would be in the $10 million to $15 million range, adding that similar projects in the Bay Area have been funded by Caltrans.

As with any of these locations, property owners will have to balance the long-term benefits of capturing runoff upstream with the temporary impacts of construction and water storage, such as debris and soggy grounds in the aftermath of storms.

The project, which involves officials from Belmont, San Carlos and San Mateo County, is not an all-or-nothing affair, and may entail an ongoing, piecemeal approach.

“I think we can get to a 25-year storm with just a couple detention facilities and anything on top of that is additional benefits,” said Belmont Public Works Director Afshin Oskoui, adding that the project can always be added onto in the future.

These upstream detention facilities will likely be paired with other flood management strategies downstream, such as widening a by-pass culvert, or underground pipe, along Harbor Boulevard. Powell said the scope and need for downstream projects like that depends on the degree to which water is stored upstream.

“The more water you retain in the watershed, the less infrastructure you have to build downstream,” she said, adding that installing two 10-foot-by-10-foot culverts along Harbor Boulevard, as originally proposed in 2014, would be especially challenging given the prevalence of underground utilities in that location.

But with some upstream detention, she said, a single 8-foot-by-10-foot culvert there may be sufficient.

Other potential downstream efforts include improvements to the Belmont channel such as stabilization measures to prevent the banks from eroding and the removal of non-native vegetation clogging the flow of water.

Many of these strategies will bring the added benefit of water quality enhancement, Powell said. Fortifying banks, for example, will limit sediment pollution. And some detention basins could feature an open bottom rather than a pumping system so that runoff can percolate underground, which also improves water quality, she said.

Originating at Water Dog Lake, the creek runs through Belmont’s downtown underneath Sixth Avenue and El Camino Real and jogs south around the Caltrain tracks to flow parallel to Harbor Boulevard before it empties into a slough in Redwood Shores. Due to the flatness of the area where the creek crosses the Caltrain tracks near Old County Road, sediment has collected there and created the conditions for flooding during heavy storms.

Oskoui said Belmont Public Works has had to dredge those areas nearly every year.

“The creek has been flooding as far back as we can remember,” Kim said. “The time has come for us to decide whether to keep applying Band-Aids or get it right.”

zachary@smdailyjournal.com

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