Weather Watch

Erosion of bluffs can prove perilous to coastal hikers

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 10 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 into the sea, from steeply dipping beds of erosion-resistant and less resistant shale of the Monterey Formation.

According to Dr. 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 along the coast the erosion-resistant igneous rocks that form the headlands along the coastline of the Central Coast are deposits from ancient volcanoes, such as the pillow basalts at Point San Luis.

These formed from hot lava spilling into the sea and cooling rapidly into pillow-shapes, similar to what is happening today to the lavas that are 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 nuclear power plant consist of volcanic tuff, a rock formed from volcanic ash.

This ash deposit has solidified into erosion-resistant rock. Morro Rock is an 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.

In January, 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. 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 the coastal bluffs that have been undercut by waves.

This week’s forecast

A 996-millibar storm is expected to develop off the Northern California coastline this morning. The associated cold front will swap in from the Pacific and produce fresh to strong (19-31 mph) southeasterly winds, increasing clouds and a few scattered rain showers this morning.

The southeasterly winds will reach moderate gale to fresh gale (32-46 mph) levels by this afternoon with gusts reaching 50 mph along the coast.

Moderate and at times heavy rain is expected to start early this afternoon along the North Coast and spread southward with the cold front. A fairly strong southwesterly jet will take a position over San Luis Obispo County today and should enhance the atmospheric dynamics enough to produce April’s total average rainfall of 1.7 inches in less than 24 hours.

Rainfall amounts should range between 1 and 2 inches with more in the coastal mountains.

Frontal passage is expected to occur about 3 p.m. near Cambria, about 5 p.m. in the San Luis Obispo area and 7 p.m. in the South County.

The strongest winds and heaviest rains will occur during frontal passage. It will be unseasonably cool, with highs only in the 50s to low 60s.

Rain is expected to turn to showers late tonight and will continue through Monday. Snow levels will start out high, but will drop to 3,000 feet in the north to 5,000 feet in the south.

A 1,022-millibar area of high pressure will move toward the Central Coast and will produce dry and mild weather on Tuesday, Wednesday and Thursday.

The longer-range models show another wet and unseasonably cool weather system affecting our area next Saturday, with showers lingering into Sunday.

Surf and sea report

Increasing southerly winds will generate 4- to 6-foot southerly (190-degree shallow-water) seas (with a 3- to 5-second period) this morning, increasing to 8 to 10 feet by this afternoon through tonight.

These southerly seas will be followed by an 11- to 13-foot westerly (270-degree deep-water) swell (with an 11- to 15-second period) on Monday.

This westerly swell will gradually decrease to 7 to 9 feet and shift out of the northwest on Tuesday.

A 4- to 6-foot west-northwesterly (285-degree deep-water) swell (with an 11- to 16-second period) will develop on Wednesday and will continue at this height and period through Friday.

Arriving from the Southern Hemisphere:

Today’s 2- to 3-foot Southern Hemisphere (195-degree deep-water) swell (with a 15- to 17-second period) will remain at this level through Monday.

Preliminary analysis long-range forecast:

Today’s surface charts and model runs are still indicating a moderate-energy swell arriving along our coastline on April 20.

Seawater temperatures will range between 53 and 55 degrees through Monday, decreasing to 52 to 54 degrees on Tuesday.

Celebrate Earth Day

Come out and join me and other Pacific Gas and Electric Company employees on Saturday as we celebrate Earth Day at Montaña de Oro State Park by restoring some trails.

The event is one of several service projects sponsored by PG&E and the California State Parks Foundation throughout Central and Northern California.

If you are interested, please register at the California State Parks Foundation Web site: www.calparks.org/programs/earth-day

Be sure to dress for outdoor work with long pants, long-sleeved shirt, sturdy shoes, hat, gloves and sunscreen. Snacks and a light lunch will be provided.

Bring your own refillable water bottle. State Park rangers will provide tools and supervision.

John Lindsey is a communications representative for Pacific Gas and Electric Company. He is also a local weather expert and has lived along the Central Coast for more than 22 years. If you have a question, send him an e-mail at pgeweather@pge.com.

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