Last week’s atmospheric river brought a brief but torrential downtown to Half Moon Bay and unincorporated parts of the coast, inundating roads and leaving officials thinking about how to improve local drainage systems to the next round of storms, which have already swept through the city.
On Highway 92, flooding from a swollen Pilarcitos stream crossed the causeway and inundated the La Nebbia vineyard on December 13. The winery, which announced last week that it would remain closed until the end of the year and possibly until 2022, said it needed time to repair its damaged outdoor patio.
Near the Ox Mountain sanitary landfill, operated by Republic Services, the Half Moon Bay waste management company, officials said the flooding on Highway 92 was the result of debris from Corinda Los Trancos Creek , a 1.5 mile tributary of Pilarcitos Creek. The debris would have blocked a culvert operated by Caltrans. Caltrans, who had a team on site Monday morning, did not immediately respond to requests for comment.
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 last week to assess erosion and possible contamination from rain, according to Heather Forshey, program director. The county reports minor erosion from the rain, but not to the point of contaminating the Pilarcitos stream.
Reports of localized flooding in and around Half Moon Bay during the storm earlier this month extend far beyond the flooding on Highway 92. Half Moon Bay Public Works Director John Doughty said that staff were reviewing the capacity of its drainage system where the 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 experienced drainage problems during major storms and that the city’s stormwater system has been overwhelmed by weather conditions over the past week.
“This was a large and unusual storm, even in the world that we are 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 the atmospheric October river which set record levels of precipitation around the Bay Area. The National Weather Service reported on December 14 that Half Moon Bay had received 4.87 inches of rain in the previous 72 hours. The city’s average precipitation for December is 5.17 inches, according to the Golden Gate Weather Service.
“Obviously our drainage and storm water system has been overwhelmed,” Doughty said. “It was never designed, and we probably never would have designed for this type of storm, because you just can’t afford to do it.”
The city’s master plan for stormwater management comes in the form of the updated Stormwater Drainage Master Plan 2016, which outlines how stormwater improvement projects are to be created and funded. Doughty said that in recent years the city has slowly moved from the first phase of the plan which identifies issues and infrastructure to the second phase which looks at costs and implementation.
The Half Moon Bay Capital Improvement Program that was passed in June calls for the city to spend $ 11.3 million on stormwater management projects over the next five years, including in the Kehoe neighborhood and with the improvement of the Seymour and Roosevelt ditches. He says there is a shortfall of $ 9.5 million between funding and necessary spending, with the Kehoe neighborhood project accounting for most of that gap. The estimated cost for this project alone is $ 7.8 million over time.
“It’s difficult because some of these projects will take many years to get the permit, and then we have to find funding,” Doughty said.
Doughty noted that the city faces infrastructure installed over the past five decades and that there are regulatory challenges on the part of agencies wishing to move from “gray” infrastructure, like pipes or tunnels, to a “green” infrastructure that uses plant or soil systems. to manage stormwater. Doughty said if there was enough support, one option could be for the neighborhoods to form a benefit assessment district. It is a mechanism by which residents can repay loans or bonds as well as their property taxes to finance improvements and public services.
Doughty said that ultimately major upgrades will require a cost-benefit analysis from city officials and residents. If these types of more severe weather events become more frequent, the city government and local communities will likely have to weigh how much to spend on reliable drainage.
“If the predictions continue to occur with more frequent events of higher intensity and shorter duration, we will obviously have to examine them in light of the changing environment,” he said.