DEL MAR, Calif. – Bluff erosion along the coastal city has shut down train travel and endangered beachgoers.

Now, a new test has been set up to understand how rainwater destabilizes the bluffs in Del Mar.

The San Diego Association of Governments, or SANDAG, is using red dye to color water and then flush it down the storm drains to see where the water runs out of the bluffs into the ocean.

“Water goes through, but how long it takes, how it dissipates, etc. is the first time they’ve tested this,” Del Mar City Councilmember Dave Druker said.

There are plenty of factors in play: Waves from the ocean crash at the foot of the bluffs which undercuts them and trains rumble across the top of the bluffs. Engineers want to understand what storm and irrigation runoff is doing to erode the cliffs as well.

Druker, a longtime local resident and the city’s former mayor, says engineers have studied many factors, but dyeing the water is a novel touch.

“All of these projects are after the bluff erosion,” he said. “There have been geotechnical studies where people have looked at it. Scripps is watching to see how the bluffs are eroding, but nobody has really studied the water movement.”

Drone video from February 2021 shows how the bluff collapsed in February 2021, collapsing south of 4th Street. It brought new fears and warnings to visitors of the area, particularly as many recall the deadly 2019 bluff collapse at Grandview Surf Beach in Encinitas that claimed the lives of three people.

Residents who live on the bluffs at 4th Street are curious to see what is discovered.

“Tracking the source of the groundwater, the runoff and the impact on the bluffs is critical for the erosion control,” said Rochelle Putnam a homeowner living on the bluffs.

Tests are expected to run for the next three weeks. Nontoxic red dye is expected to be visible in the area of 4th Street over the next coming days.