RPV Ramps Up Landslide Defenses Ahead Of El Niño Winter: Report

RANCHO PALOS VERDES, CA — Rancho Palos Verdes is lining canyons, installing sandbags and running deep dewatering wells continuously as the city prepares its landslide complex for what officials say could be strong El Niño conditions this winter.

The preparations come after land movement across the area slowed dramatically from rates recorded during the height of the crisis in 2024.

The average movement rate across areas still moving within the active landslide boundary fell to 0.99 inches per week in early August, according to the city's land movement updates. That was a 9.8 percent decrease from July and marked the first time the overall average had fallen below 1 inch per week since October 2023.

By comparison, portions of the landslide complex were moving as much as 14 inches per week in mid-2024, according to city data cited by Spectrum News 1. With the rainy season approaching, the city's focus is now on preventing water from getting underground and potentially reversing that progress.

"We've got to make sure that water that enters our landslide complex is conveyed to the ocean and doesn't recharge the water table," City Manager Ara Mihranian told Spectrum Tuesday. Crews have been lining canyons with plastic material and sandbags while the city's dewatering wells continue operating.

"We've seen deceleration of the land movement," Mihranian told the outlet. "We've been able to protect our infrastructure, so our implementation measures are proven to be effective."

Preparing For The Rain

The work is part of a broader "winterization" effort underway across the landslide complex ahead of the rainy season. Crews have reset protective pond liner, cleaned and upgraded an outlet pipe and replaced sandbags in Klondike Canyon, while additional work has been underway in Upper Altamira, the Seaview graben, Kelvin Canyon and Portuguese Canyon.

Sections of pond liner are also being welded together in critical areas to better withstand land movement, rockfall and wind while improving stormwater drainage.

Near the shoreline at the toe of the Portuguese Bend Landslide, electrical cabinets and equipment serving the deep dewatering wells are being elevated to protect them from high surf. The city is also making drainage improvements along Palos Verdes Drive South to reduce the amount of pumping required when the roadway floods during storms.

Much of the winterization work was expected to be finished by the end of September, with all of it scheduled for completion by mid-October. The preparations build on mitigation measures that have substantially reduced movement in the Portuguese Bend Landslide.

Its average movement rate fell another 13.9 percent between July and August to 0.81 inches per week, according to the city. Rancho Palos Verdes said that represented a 92 percent reduction compared with the period immediately before installation of the deep dewatering wells began in 2024. About 74 percent of the Portuguese Bend Landslide within its historical boundary was moving at less than 1 inch per week.

The wells have also removed an enormous volume of water from beneath the area. The city reported in an August landslide update that more than half a billion gallons of groundwater had been extracted since the deep dewatering program began in fall 2024.

Despite the slowdown, movement has not stopped, and the approaching rainy season will provide another test of the mitigation measures now operating across the complex. For the city, Mihranian said, the objective is to keep rainfall moving through the area and into the ocean rather than allowing it to replenish the groundwater beneath the landslide.