TBF Preparations (5), The Tides - A River Runs Through It

“In this mix of theory and empirical experience, theory works rather well for predicting tides at ocean ports, where the rebound of the tidal wave- bouncing off the continent – is relatively uncomplicated. Inside the San Francisco Bay and Delta, experience plays a larger hand: the tide wave bounds and rebounds from crooks and islands, and shallows. It meets the flow of sixteen rivers and, all together, they play some pretty tricks.”

Kimball Livingston, Sailing the Bay, 1st edition

______________________________

(B)log . . .

None for this post, I’m setting aside the journal entries to focus on the TBF Preparations
______________________________

(B)log comments

No Comment
______________________________

My Blog . . .

My preparation for the Three Bridge Fiasco (TBF) continues.

Part of this post is about my thoughts Saturday afternoon, January 10th. The Admiral and I took some time between the sailing excursion earlier that day and a very special moonlight sail planned for that night. We hung out in the north classroom and poured over Kimball's book, Chapman, and used the white boards to diagram the different forces that cause the Tidal Flows throughout the Bay and Delta.
The Tidal Currents of SF Bay take the form of a massive and swift flowing river; one whose existence is very temporal. It comes into being, lives and dies out over the course of less than an hour.

The Flooding tidal wave meets the flow of cold water coming down from the snow capped mountains to the East, mixed with the tidal Ebb currents.

An epic battle is waged, and the warmer pacific waters drive the colder mountain water to the bottom of the straights where it continues to flow westward surrounded by a confused sea that attempts to corral it.

These warmer waters only holding sway for a few hours or minutes.

Sometimes the cold bottom flow rises up and spreads out over the surface, forming a glassy, moving spectacle of its own. In general, the upwelling is clear and cold. Except when it’s not.

In the summer, the waters flowing down from the mountains are warm and muddy and the pattern is reversed, the cooler pacific waters forced to the bottom.

Sure, understanding the motions of the Earth, Moon and Sun in relation to each other is very helpful as background information when reading what Kimball describes as the “Most Misread Book in Town”. But that book only addresses when, how much, and in which direction the flow of water through specific points is predicted to occur.

To go beyond this generality, the first time I saw this spectacle of the river running through Raccoon Straights helped me visualize the massive amount of water spilling into and out of the bay from different directions: surging along the bottom and banks of the contours of the basin and gorges that lie below the surface.

Since those contours are hidden below the surface, and only suggested by the form of the land that breaks that surface, we are left with the patterns that this flow etches on the surface as it interacts with the flow of air along it. Reading the water is a skill that comes to the sailor on San Francisco Bay slowly. It only comes through constant observation when sailing along that surface and paying attention to it.

To the vast majority of boaters, it goes by unnoticed. To the dedicated bay sailor, it is the book that is read whenever the eye scans the wind, waves and water they transit, rain or shine, sun or moonlight, fog or starlight.

How to explain this as an answer to the Admiral’s questions about the action of the Earth, Moon and Sun?

We shut down the computers, close the books, gather up everything and head for the boat to meet the Full Moon at it’s closest approach in many years. The talk is that the moon of Jan. 10th, 2009, will be 30% brighter than normal.

We'll see.

Closer to when I'm posting this, I dug out my "Tidal Current Charts - San Francisco Bay". These used to be published by NOAA and NOS. They basically gave up. Miniature versions are sometimes included in Tide Logs and Tide Booklets.

The sales sticker on the front cover of my set reads: "Jun 77 $4.00".

I bought my set over 30 years ago!

I'm scanning in the appropriate parts for the TBF. In Microsoft Word, I'm calculating the expected Currents for each segment of the TBF. Text Boxes with the calculated values are being superimposed over the 'nominal' values.

So the situation is this. The Flood starts at the start of the race.

Which way to go?

I still think it will be determined by the weather. How much wind and where it's coming from. The weather will determine how long we are out there, and when we'll be going between Red Rock and Yerba Buena.

Either way, go to the Golden Gate first.

If we have wind, the whole race will be in Flood. The chart I'm posting tells the tale.

If it's a south or west wind, and there is plenty of it, go to Yerba Buena after The Golden Gate, and ride the slight Flood up to Red Rock with the spinnaker flying. There are lots of ways to handle the building and waning flood when there is a strong south or west wind, and get back to the cityfront. Going through Raccoon Straights and hugging the shore of Angel Island is one of them, (Knox will be slack around 5 PM), and then you dash across the strong current between Alcatraz and Aquatic Park and short tack along the cityfront to get to the line.

If we don't have much wind and are likely to be pushing the time limit, then it's ride what little Flood there is to Red Rock after the Gate. This will clearly be my strategy if it's a Northerly wind.

Fight it to Yerba Buena as a downwind leg, staying East of the ship channel, and then ride the start of the Ebb to the finish line.

At 4:50 PM, Three Hours after the Max Flood, the Ebb is established along the cityfront, the 'rebound' from the South Bay is in full swing and water is flowing South to North. At 5:50 PM, it is two hours before Max Ebb, and all of the water is now heading towards the gate.

The two charts above are shown without the calculated flows.

This is all just a theory, just a best guess. I still don't know exactly how to interpret the role that declination has in creation of a 'diurnal inequality', that 'do not permit the selection of the proper chart'.

So NOA has spoken in black and white on the inside cover of the June 28, 1973 chart sleeve.

Jan. 31st has the moon just one day away from it's first quarter, and the resulting neap tide. Does this mean that the moon is in the tropic of Cancer? I'm searching Wikipedia for the answer, but it doesn't leap out at me.

Has the warm weather of January increased the icy downriver flow? Will the Ebb make itself known earlier? Will the rain over the last several days add to the snowmelt and increase the downriver flow? The flow plays tricks.

"Watch the Water" is the only way to know for sure.

The Admiral and I went sailing during the most dramatic Full Moon in many years (the 3rd sail of the year), which produced a very dramatic Tide Range, and went out again for our 4th sail during a light wind day and to compare the ripples of our wake at about 1 knot of boat speed with the ripples of the wake of Bell Buoy #2 off of Treasure Island Three Hours after the Maximum Flood with a similar flow rate.

Preparing for the TBF is a fun way to experience the vast number of things that a serious sailor must understand and react to in order to get the most out of Sailing The Bay.