Last week’s atmospheric river brought a brief but torrential downpour to Half Moon Bay and unincorporated parts of the Coastside, flooding roadways and leaving officials thinking about how to improve local drainage systems for the next round of storms, which have already rolled into town.
On Highway 92, flooding from a swollen Pilarcitos Creek washed across the roadway and swamped La Nebbia Winery on Dec. 13. The winery, which announced last week it would remain closed through the end of the year and possibly into 2022, said it needed time to repair its damaged outdoor patio space.
At nearby Ox Mountain Sanitary Landfill, which is run by Half Moon Bay’s waste management contractor Republic Services, said that the flooding on Highway 92 was the result of debris from Corinda Los Trancos Creek, a 1.5-mile tributary to Pilarcitos Creek. The debris reportedly clogged a culvert operated by Caltrans. Caltrans, which had a crew on the scene Monday morning, did not immediately respond to requests for comment.
Staff from San Mateo County Environmental Health Service’s Solid Waste Inspection Program, which regulates the landfill operations, visited the Ox Mountain Landfill several times last week to assess the erosion and possible contamination caused by the rain, according to Heather Forshey, program director. The county reports minor erosion from the rain, but not to the extent that it contaminated Pilarcitos Creek.
Reports of localized flooding in and around Half Moon Bay during the storm earlier this month go well beyond flooding on Highway 92. Half Moon Bay Public Works Director John Doughty said staff is examining its drainage system capacity where flooding occurred last week, including at Railroad and Miramontes avenues, and on Golden Gate Avenue in between Highland and Silver avenues. Doughty acknowledged that these areas have repeatedly had drainage issues during major storms and that the city’s stormwater system was overwhelmed by last week’s weather.
“This was a large and unusual storm, even in the world that we’re looking at with the potential for these types of storms more often,” he said.
Doughty noted that last week’s torrential downpour was shorter but more intense than October’s atmospheric river that set record rainfall levels around the Bay Area. The National Weather Service reported on Dec. 14 that Half Moon Bay received 4.87 inches of rain over the previous 72 hours. The city’s average rainfall for December is 5.17 inches, according to Golden Gate Weather Services.
“Clearly, our drainage and stormwater system was overwhelmed,” Doughty said. “It was not ever designed, nor would we probably ever design, for that type of storm, because you just can’t afford to do that.”
The city’s blueprint for managing stormwater comes in the form of the 2016 Storm Drain Master Plan Update, which determines how stormwater capital improvement projects should be created and funded. Doughty said that over the last several years the city has been slowly transitioning from the plan’s first phase that identifies issues and infrastructure to the second phase that delves into cost and implementation.
Half Moon Bay’s Capital Improvement Program that was adopted in June forecasts the city will spend $11.3 million on stormwater projects in the next five years, including in the Kehoe neighborhood and with Seymour and Roosevelt ditch improvements. It states that there is a $9.5 million shortfall between funding and necessary expenditures, with the Kehoe neighborhood project accounting for the bulk of that gap. The estimated cost of that project alone is $7.8 million over time.
“It’s hard because some of these projects will take many years to get to the point of permitting, and then we have to figure out funding,” Doughty said.
Doughty noted that the city is dealing with infrastructure installed over the last five decades, and there are regulatory challenges from agencies that want to transition away from “gray” infrastructure, like pipes or tunnels, to “green” infrastructure that utilize plant or soil systems to manage stormwater. Doughty said if there was enough support, one option could be for neighborhoods to form a Benefit Assessment District. It’s a mechanism for residents to pay back loans or bonds along with their property taxes to fund public improvements and services.
Doughty said, ultimately, major upgrades will require a cost-benefit analysis from city officials and residents. If these types of stronger weather events become more common, the city government and local communities will likely have to weigh how much to spend on dependable drainage.
“If the predictions continue to play out with more frequent, higher intensity and shorter duration events, obviously we’ll have to look at those in light of the changing environment,” he said.