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why we will have inaccurate tide readings this spring

 
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Riptide



Joined: 15 Jan 2011
Posts: 370

PostPosted: Tue Mar 29, 2011 10:40 am    Post subject: why we will have inaccurate tide readings this spring Reply with quote

Your tide book might misguide you this Spring season…

You may have noticed that the currents in your tide book don’t seem to be very accurate right now- particularly at Sherman Island and along Crissy field.

Why is that?

It’s a halocline!

As you might already know the Sierras have received record breaking snow falls this year. The snow pack is extremely deep as a result. Coupled with the warm spring conditions that we will have in the upcoming weeks, a tremendous amount of snow will melt. This will send torrents of fresh water into rivers, down into the delta and through the bay.

Salt water from the ocean and fresh water from the mountain snow melt have different densities. Salt water is more dense than fresh water. The fresh water can therefore ‘float’ upon the ocean’s salt water. Very surprisingly it takes quite a bit for the two layers to “mix” into a homogeneous state. A halocline develops as a result. During a rapid snow melt fresh water will flow down hill towards the Golden Gate. The tide still needs to rise and fall so the ocean water will “under cut” the fresh water drainage during flood tides. This is why it may continue to Ebb at the surface while the tide rises with in a flood tide. The following illustrations depict this quite well:

Ebb Tide: The salt water wedge and fresh water move in the same direction

Source: Encyclopedia Brittanica

Flood Tide: Salt water wedge undercuts the fresh water. The sea level rises, but the surface continues to Ebb.

Source: Encyclopedia Brittanica

To make things even more complicated, we need to take into account the complex shape of the bay. The example above is quite straight forward at a place like Sherman Island where the water flows one way, or the other, down a narrow channel. The central bay is far more complicated. For example, a very unique pattern developed on Sunday: The river of fresh water was flowing out of the north bay and all the way out of the gate. Meanwhile the tide was rising due to an undercutting salt water wedge. Keep in mind that the South bay does not have a fresh water sourse from the mountains feeding it. The water needs to get down there somehow in a rising tide. Some of that water may come from the north bay’s fresh water river. However much of it will still come from the ocean. We observed an intese upwelling of ocean water at Crissy Field just off of Anita Rock. A ripping flood developed and rolled along the marina green. This water wrapped around the city and flowed down to the south bay.

Crazy stuff!

So while the tides may rise and fall keep the halocline concept in mind if you are trying to figure out why the current in the central bay or up by Sherman Island seems to misbehave.

-Matthew Gregory

3-28-2011


for the full article with images

http://blog.windalert.com/?p=286
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jse



Joined: 17 Apr 1995
Posts: 1393
Location: Maui

PostPosted: Tue Mar 29, 2011 11:07 am    Post subject: Reply with quote

Interesting. Yesterday I was working on the 25th floor of the Steuart Tower, foot of Market St. in San Francisco. Around 2:00 I looked down on the bay and the rift line between brownish green and bluish green was razor sharp. The line went from around Pier 39, diagonally towards Oakland. Above the line the water was silty brown, below the line it was clear (relatively, we're talking S.F. Bay). When I got on the ferry, the entire bay appeared to be silted in. But from the boat (not the 25th floor) I could look south and see clearly the sliver of clear water on the horizon. The silty water had extended far into the south bay, but the rift line was still there such that I could see it from water level, when it was a few miles away.

Steve
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joespeed



Joined: 13 Jun 2000
Posts: 50

PostPosted: Tue Mar 29, 2011 11:11 am    Post subject: Reply with quote

how about third avenue ?, no source of snow melt there in the south bay but if there is all this extra water rushing in the bay from the Sacramento river, will this give us higher tides / slower floods ?

I'm looking at the tide chart for third avenue for this afternoon and 0.5 feet is kinda low but if all this extra water gives me another 0.5 - 1.0 feet , that would be most convenient

Thanks

Louis
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VinceSF



Joined: 05 Apr 2005
Posts: 247
Location: Marin County, CA

PostPosted: Fri Apr 01, 2011 12:58 am    Post subject: Reply with quote

makes sense, thank you for the aticle.
Much to my surprise I did notice alongside others the extreme 'freshness' of the sea water offshore from san Quentin.
I'd never thought there would be such a difference but indeed the extreme winter weather has an impact on water.

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