Erosion on the Central Coast

Relentless waves, pounding rain and eternal winds have eroded coastal bluffs into huddled sentries of rock guarding the grass-covered hillsides of the Central Coast.

Last week, an intense storm with hurricane-force winds generated gigantic waves in the Southern Hemisphere.

On May 9, a wave rider buoy moored about 375 miles south of New Zealand near Campbell Island measured a 78-foot wave, the highest individual wave ever recorded south of the equator.

Long-period — 20- to 25-second — waves from the storm will travel thousands of miles across the Pacific and arrive along our coastline Monday into Wednesday, producing higher surf and strong riptides along the southerly and westerly facing beaches as well as beach erosion.

We are blessed to live along the Central Coast. Our scenic coastline is composed of beautiful beaches, jagged rocky headlands and coastal bluffs, which produce nearly endless recreational opportunities.

The beauty of this treasured coastline is shaped by waves, currents and rains, which produce different amounts of erosion depending on the type of geological formation and the exposure to incoming waves along the shoreline.

About 20 years ago, I was hiking along the coastal bluffs of Montaña de Oro State Park at very-high tide. An extraordinarily long-period swell from the northwest with an interval over 25 seconds was slamming into our coastline.

The longer the period of the swell, the more energy it contains and the greater amount of erosion it can produce.

You could actually see sediment from the coastal bluffs eroding in the breakers, turning the water into a murky brown.

At Montaña de Oro, some of the rocky headlands have been eroded into fingers extending into the sea from steeply dipping beds of erosion-resistant basalt and less resistant shale of the Monterey Formation.

According to Bill Page, a geologist with Pacific Gas and Electric Co., some of the sediment I observed came from the eroding beds, but most was from the sandy, unconsolidated deposits that cover the bedrock here as well as along other parts of the coast.

He went on to say that the terrace that makes the wide apron in front of the Irish Hills was formed by wave erosion thousands of years ago when sea level was slightly higher.

At other places, the erosion-resistant igneous rocks that form the headlands along the Central Coast are deposits from ancient volcanoes, including the pillow basalts at Point San Luis.

They formed from hot lava spilling into the sea and cooling rapidly into pillow-shapes, similar to what is happening today to the lava that is pouring into the sea from Kilauea on the Big Island of Hawaii.

The jagged headlands north of Pismo Beach and the bluffs near Diablo Canyon Power Plant consist of volcanic tuff, a rock formed from volcanic ash.

This ash deposit has solidified into erosion-resistant rock. Morro Rock is the old neck of a volcano whose extrusive deposits have been eroded completely away.

Erosion can be accelerated by heavy rains that saturate the ground from above. The greater weight of the saturated ground combined with the long lever arm of an undercut bluff can result in a collapse.

Ten years ago, Larry and Judy Cobbs were walking through Dinosaur Caves Park in Pismo Beach when a large section of the coastal bluff collapsed into the ocean in front of them. They were able to scramble to safety farther inland, but their neighbors thought it was an earthquake or sonic boom.

Thankfully, most of our coast is only undergoing very slow erosion, but you should always be careful hiking around coastal bluffs that have been undercut by waves.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Ffollow him on Twitter @PGE_John.

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