Poplar Beach bluffs
San Mateo County officials have been closely monitoring the retreat of the Poplar bluffs. It is the site of an old landfill that could one day be exposed by erosion. Adam Pardee / Review

The bluffs overlooking Poplar Beach are as majestic as they are protective. They act as a barrier between the water and decades-old garbage buried beneath the soil near the bluff top.

Though there is no immediate threat of the contents of the long-retired landfill seeping into the ocean, the San Mateo County Public Works Department is taking proactive measures to avoid such an outcome.

This month, SCS Engineers Inc., a consultant hired by the Public Works Department, will bore five deep holes into the bluffs at Poplar Beach Park to study the soil’s makeup. This investigation will help the county determine how much longer the bluffs have before they retreat even farther inland and how susceptible the area is to earthquakes.

The project is expected to begin on Monday and take about seven days to complete. During that time, the city of Half Moon Bay said the project may impact public access to the Half Moon Bay State Beach and Poplar Beach parking lots, as well as the surrounding beach and coastal trail areas. A crew will be on-site rerouting traffic.

“You can tell bluff erosion will not stop, so we’re planning in advance so we make sure we’re protecting the landfill before we see any problems,” said Jim Porter, the county’s director of Public Works, shortly after the plan was presented to the Half Moon Bay Planning Commission in November 2020.

Two borings 30 feet below the ground are proposed at the toe of the bluffs on the beach. The other three borings are proposed near the top of the bluffs and will range from 60 to 75 feet below the ground.

The project site stretches across 22 acres of county-owned land south of Poplar Street and adjacent to the Coastal Trail. Though the area is now used as recreational open space, 14 acres once served as an active landfill beginning in 1951.

Disposed material was burned until 1958 as was common at the time. Then, beginning in 1962, operations took on the more modern method of keeping disposal to the smallest land area possible and covering the fill with a layer of soil after each day’s use. Finally, in 1978, the county retired the landfill by capping it with a soil cover.

The threat of coastal erosion was foreshadowed in the early 1990s when ocean waves eroded the bluff face and scraped off a portion of the landfill covering. The exposed area was secured with a concrete block and steel chain mat.

The county routinely monitors the site and budgets $150,000 in annual inspections.

A county report on sea level rise released in 2018 identified the area as highly vulnerable to coastal erosion, sea level rise, coastal scouring and potential exposure of landfilled debris.

Bluff erosion is imminent and the county has two plans in place. The short-term plan is to armor the shoreline. Armoring often deploys the use of seawalls, breakwaters and riprap.

“That’s a short-term solution because the sea will win that battle every day,” Porter said.

The long-term plan is to physically remove the landfill contents and transfer them to an active landfill like Ox Mountain.

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