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Saturday, December 15 , 2018, 1:36 am | Fair 41º


Ken Macdonald: Why Are Our Beaches Shrinking?

Over this beautiful holiday I had time to pursue one of my favorite pastimes, a good long walk on the beach at low tide — at Hendry’s, at Leadbetter, at Campus Point, at Goleta Beach, just to name a few.

We are so lucky to have such great beaches right in our neighborhood. But they are shrinking fast, and even manmade sand berms (as at Goleta Beach) won’t stop this.

There is no cause for alarm; this is part of a natural seasonal variation.

The big wintertime surf carves away at the beach, moving hundreds of pounds of sand seaward with each big breaking wave.

When very high tides coincide with high surf, the sand moves seaward suddenly and in huge amounts. Recently, the sand level dropped a good 2 feet overnight when we had “king” high tides and big surf.

And when the big El Niño storms arrive (hopefully during January to March), the sand will be almost entirely gone and our beach walks will entail walking on bedrock and scrambling over boulders.

Where does this sand go? Those big breakers sweep the sand offshore 100 feet or more to a big sandbar. From there, the “longshore current” carries the sand east and south along the coast.

This “longshore current” extends all along the West Coast, moving sand from north to south (and locally west to east).

We see this most clearly where the longshore current is interrupted, for example at Santa Barbara Harbor. Here the sand is dumped at the harbor entrance, creating Sandspit Beach.

The sand would completely fill up the harbor if the government did not pay to have more than 280,000 cubic yards of sand dredged up every year and deposited back onto the longshore bar off of East Beach, where it continues on its way southward. This is equivalent to about 30,000 dump truck loads!

Our beach will finally return after the storm season is over, usually beginning in April.

The usual small waves, less than 2 feet high, work in a completely opposite way as the big surf. Instead of cutting away at the beach and hauling sand offshore, the small waves bring the sand back in and deposit it gently onto the beach.

It is not the same sand that was removed months earlier; that sand is way far south by now. The new sand has come from tens to hundreds of miles up the coast.

Restoring the beach is a much slower process, but by June or July we have our beaches back, and they continue to grow through September or even October. And then the cycle repeats with the first big storms of fall and winter.

My friend, Doug Inman, a marine geologist at Scripps Institution of Oceanography, made a 20-minute film that describes these processes very clearly and graphically. It is a bit dated (1967!) and therefore amusing in some places, but it shows very clearly how the big winter waves differ in the way they affect our beaches from the gentle spring and summer waves.

You will recognize Hendry’s Beach and the Santa Barbara Harbor in the movie. At about 12-13 minutes in, there is an ingenious wave tank simulation of wave action and how our harbor is affected. After watching the video, I promise you will never look at the beach the same way again!

— Ken Macdonald is an oceanographer and professor emeritus in UC Santa Barbara’s Department of Earth Science. He has been affiliated with Woods Hole Oceanographic Institution and Scripps Institution of Oceanography, and has led deep-sea dives up to 15,000 feet in the submersible Alvin. He is a naturalist for Channel Islands National Park and the Channel Islands National Marine SanctuaryClick here to read previous columns. The opinions expressed are his own.

(LSU Center for GeoInformatics video)

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