Last week’s atmospheric river brought a brief but torrential downtown to Half Moon Bay and unincorporated parts of the coast, flooding roads and leaving officials to ponder how to improve local drainage systems for the next set of storms, which have already swept through the city.
On Highway 92, flooding from a swollen Pilarcitos creek crossed the roadway and inundated La Nebbia winery on December 13. to repair his damaged outdoor patio.
Near the Ox Mountain sanitary landfill, which is operated by Half Moon Bay waste management contractor Republic Services, officials said the Highway 92 flooding was the result of Corinda debris Los Trancos Creek, a 1.5 mile tributary of Pilarcitos Creek. The debris is believed to have clogged a culvert operated by Caltrans. Caltrans, which had a crew on the scene Monday morning, did not immediately respond to requests for comment.
According to Program Director Heather Forshey, staff from the San Mateo County Environmental Health Department’s Solid Waste Inspection Program, which regulates landfill operations, visited the Ox Mountain Landfill several times the last week to assess erosion and possible contamination caused by rain. The county reports minor erosion from rain, but not to the point of contaminating 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 Manager John Doughty said the Staff was reviewing the capacity of its drainage system where flooding occurred last week, including at Railroad and Miramontes Avenues and on Golden Gate Avenue 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 has been overwhelmed by the weather over the past week.
“This was a large and unusual storm, even in the world we see 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 Dec. 14 that Half Moon Bay had received 4.87 inches of rain in the previous 72 hours. The city’s average rainfall in December is 5.17 inches, according to the Golden Gate Weather Services.
“Obviously our drainage and stormwater system was overwhelmed,” Doughty said. “It was never designed, and we probably never would, for this type of storm, because you just can’t afford it.”
The city’s master plan for stormwater management comes in the form of the 2016 Storm Sewer Master Plan Update, which outlines how stormwater capital improvement projects are to be created and funded. Doughty said that over the past few years, the city has slowly moved from phase one of the plan, which identifies issues and infrastructure, to phase two, which looks at cost and implementation.
Half Moon Bay’s capital improvement plan, passed in June, calls for the city to spend $11.3 million on stormwater projects over the next five years, including in the Kehoe neighborhood and with improving the Seymour and Roosevelt ditches. It says there is a $9.5 million shortfall between funding and necessary spending, with the Kehoe Neighborhood project accounting for most of that gap. The estimated cost of this project alone is $7.8 million over time.
“It’s difficult because some of these projects will take many years to get to the point of licensing, and then we have to figure out the funding,” Doughty said.
Doughty noted that the city is dealing with infrastructure that was installed over the past five decades, and there are regulatory challenges from agencies wanting to move from “grey” infrastructure, like pipes or tunnels, to a “green” infrastructure that uses plant or soil systems. to manage stormwater. Doughty said if there is enough support, one option could be for neighborhoods to form a benefits assessment district. It is a mechanism for residents to repay loans or bonds and their property taxes to fund improvements and utilities.
Doughty said that ultimately major improvements will require 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 reliable drainage.
“If the predictions continue to materialize with more frequent higher intensity, shorter duration events, we will obviously have to review them in light of the changing environment,” he said.