The Sand Spit and the Bay

Is the Estuary Filling With Blown Sand?

The Blowouts Are Stable

Addendum

 

THE BLOWOUTS ARE STABLE


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Aerial photographs were available for years 1937, 1947, 1957, 1973, 1986. The USGS map I used shows blowouts, and was compiled from aerial photographs taken in 1963. I spent about 30 days on the Sand Spit in 2004. The really striking thing is how stable the blowouts are.

Since the habitat is stable there may be unique creatures living in it. On one trip I saw a tiny spider. I took my eyes off it to grab my camera; I wasn’t able to find it again. Here is a beetle I did photograph:


 

THE SAND SPIT IS DYNAMIC

Very roughly 120,000 cubic yards of sand annually is washed ashore on the Sand Spit, which is at the center of the Estero Bay littoral sand cell, and blown across the Spit into the Bay. Some of that sand makes the trip repeatedly, being carried back out of the harbor mouth.

At the tip of the Spit the sand is blown into the water, and even across the Bay. In the middle of the Spit there are places where the wind makes dunes right at the water’s edge; the water then erodes this sand away. But by far the most interesting is the series of landslides that are filling in the south end of the Bay.  See the next two pictures:


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The top picture shows what the December 22, 2003 earthquake did to the 70 foot dune at the entrance to Shark Inlet. The bottom picture shows damage by the same event to a 100 foot high dune down the Inlet. The next few photographs will show this area in detail.

According to Bagnall in The Physics of Blown Sand (available at Cal Poly’s Kennedy Library) wind-blown sand forms sheets that are horizontally compact but vertically loose. When disturbed the sand sheets collapse, as shown in the following:


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When the collapse occurs on a dune slip face an avalanche occurs. At the bottom of the avalanche path there is often a fringe of chaparral around the edge of the Bay, with interlocking roots that give the turf considerable flexible strength. However, the mud below the water table liquefies in an earthquake, and also from the impact of the sliding sand. Therefore, the sand pushes under the chaparral, forcing the mud ahead of it.

 


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 Here’s where the sand went under the chaparral.
 


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And there’s the mud, beyond the chaparral. Notice that half the width of Shark inlet has been eliminated by this one event. In addition, at the narrows the bottom has risen 6 inches. Notice that the far edge of the chaparral on the right is also crescent shaped; more about this, Area “C”, later.


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Farther left a huge amount of sand slid, and the bottom was forced upwards 8 feet, obliterating 2/5 of the channel! Note that the mud in the center of the photo is rough; the mud on the right is smooth. The smooth mud is below the level of the high high tide. The rough mud is higher; it looks like this up close:


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Since this rough mud is above the water table, it will become covered with chaparral. That is precisely what happened in Area “C”. In the early 1900’s an earthquake occurred. The mud got pushed up 10 or 11 feet. In amongst the brush there are cracks three feet deep! 

During the December 22, 2003, earthquake the dunes settled enough that views of the ocean became possible for some Los Osos residents. My monitoring stations temporarily stopped showing encroachment. The dunes were “recharging”.

These processes have been going on some five thousand years. At first the Sand Spit must have been just a barrier island. 500 years ago Shark Inlet may have still connected to Estero Bay. But the work of the Estero littoral cell was relentless. It filled in the channel at a rate of two feet a year, and 110 feet deep. What we see today is just a vestige, a vermiform appendix, of what was!

 

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