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Tag: tides & currents

Return of Bert, actual route

Step one: jump in. Step two: turn left and go off course.

The actual route for this swim does not match the shortest route. The shortest route, at 18.8 miles, heads north from the start through the East Passage. My route is over 2.6 miles longer, and goes left around Vashon Island.

Right now, I’m either regretting this decision, or laughing to myself that it is just crazy enough to work. Either way, here’s the logic behind it:

During Bert Thomas’s 1956 swim in the opposite direction, he swam about nine hours before getting stopped by the tide and finally finishing seven hours later. That’ll happen in the East Passage, where the ebb and flood of the Sound creates noticeable currents, and complicated eddies. While it sounds like a complicated, exciting challenge to hop between shores, tuck in behind spits of land to fight the tide and then burst out at the right moment, when one adds in the shipping lanes running through the East Passage, more likely it would become another Tappan Zee incident (see Stage 6) where I’d be constrained to unfavorable water and left to fight it out.

Instead, I’m going to avoid shipping traffic and, if I’m right, the tides altogether.

Now not far west from the start along the shore of Point Defiance, regular swims with Melissa have taught me that the current there always pushes west. And Melissa’s birthday swim taught me that a strong clockwise eddy lies just north in Dalco Pass, midway between Vashon and Tacoma shores. So I’ll ride that current west from the start, heading out to the mouth of the Narrows to meet a ripping ebb tide pushing north into Colvos. This part is a bit of a gamble; if I swim for the Gig Harbor Light fast enough, I should be pushed north into Colvos on the backside of Vashon.

And that is where it gets beautiful. Colvos they say, by some magic of oceanography, always has a north current. So I’m adding a few miles to the swim in hopes of avoiding a seven-hour delay like Bert had.

At the top of Vashon, we’ll ride the current as far north as possible, getting above the ferry lanes and preparing to swim due east against a flood tide pushing south.

shortest-route-planned

Haiku 8

Drifting

I am not just a
paper cup. To reach the bridge
requires much effort.

(Written 18 June 2014)

8 Bridges – Wind SSW

[Originally published 27 June 2014 at the 8 Bridges website: http://www.8bridges.org/wind-ssw/]

While we all rested on Sunday, the wind was hard at work. We had four days of favorable weather during the first half of 8 Bridges. On the fifth, as we arrived in the morning refreshed, ready to take on The Beast, a south wind was blowing.

Marathon swimmers rely on their training, kayakers, volunteers, and organizers, but they also rely on luck. Weather and currents can be forecasted and predicted, but not changed. Waiting in the narrow fjord beneath the Bear Mountain Bridge, the sky overhead was blue, but the water below was no longer flat. There was nothing else to do but jump in and hope for the best.

As four solo swimmers and a relay made their way downstream, the day looked promising. The current was running fast early in the course, spirits were high. As the river widened though, the wind was felt. It came in the form of short, choppy, irregular waves, head on. The kind of waves you look at from a boat and think nothing of, but as a swimmer you curse. These waves break your rhythm, and with it your spirits. Gulps of water come as frequently as gulps of air. Getting into the zone is difficult, and staying there is impossible.

As the day went on, the waves lengthened out into a more manageable, regular chop, something we could deal with. But during that time, another damage was being dealt by the wind, something more sinister than discomfit and a slight queasy feeling. The wind was slowing our current on the day we needed it the most. With the river at its widest during Stage 5, finding and using the ebb is critical in reaching the Tappan Zee before the flood.

By time the tide turned, no one was at the bridge. The waves were bigger now, and the lack of forward progress was demoralizing. Lighthouses did not fly by; bridges did not grow larger over time. An hour and forty-two minutes after the flood started, Andrew reached the Bridge. Shortly thereafter, the flood picked up to over two knots and halted the relay only six hundred yards from the finish. No one else made it.

Stage 6, another difficult day began where the last had left off. The same wind was blowing from the south as eleven swimmers splashed, and the ebb was again slow. Four finished before the tide turned, but all fought the same rough conditions for five to six hours. From the bow we watched our friends pushed backwards from the George Washington. It was sad to see so many not finish, especially when everyone gave a valiant effort.

But such is the sport we choose. For a few this event was the goal, but for most it was a part of something larger. For one swimmer who is training for the Ederle Swim, her mood was somewhat lightened to hear that her Stage 6 swim had been tougher than Ederle despite not finishing. Many other swam longer than planned, a feat in itself regardless of outcome.

And then the party afterwards. Sun beaten and weary, we pulled into Inwood. Swimmers, volunteers, kayakers, family, and friends mingled into the evening on the deck of La Marina. Sharing stories of the first six days, making plans for the future. The sunset across the Hudson couldn’t brighten the atmosphere more, though it tried. Fatigue waited patiently at the curb while swimmers reveled in the glory of one another.

Tuesday’s first swim

Melissa had a birthday on the Sunday past, and so on the same day, Melissa had a birthday swim.

Owens Beach in Tacoma looks north across Dalco Passage to Tahlequah, upon the southern tip of Vashon Island. The hills of Point Defiance rise behind you as you stand there and block the winds and currents that race through the Narrows. The water is usually calm owing to the small fetch from all directions but east towards Commencement Bay, and the current nearly always pushes west regardless of tide. Along the water for 1km, there is a running path maring the boundary between the tall northwestern pines and the gravelly northwestern beach. This is the Tacoma open water swim spot.

Between Owens Beach and Vashon is 2.4km, but not without its challenges. Through this 400ft-deep waterway runs any ship, tug, tow, log boom, and recreational boat transiting between the southern and northern Puget Sound. Currents in the Narrows around the corner reach 6kts and come spilling out into Dalco Passage. Also, sometimes it rains.

Sunday morning, Tuesday and I put in at the Tacoma public dock at the edge of Point Defiance and set out to find, map, and understand the currents immediately before the birthday swim. There are no NOAA current predictions for a lot of the Puget Sound (I suspect because of the complexity the landscape induces), but the models I could find suggest that on an ebb flow, the middle of the Passage flows strongly east and eddies back to the west along the Tacoma shore. As we slowly made our way across and back in the bright morning sun and still air, not a current could be found. After two hours of searching for anything, water movement, drifting of the boat, tide lines, debris lines, anything, we gave up and headed into the beach to meet Melissa and Alison.

Tuesday on Owens Beach

Melissa and Alison piled in and the three of us headed north to Vashon. When we reached the other side, where the swim would begin, we waited for the kayaker Heidi to complete her crossing. As we drifted near the beach, a moderate current pushed us to the west. Heidi reported some mid-channel water movement when she reached Vashon. These currents were not there thirty minutes earlier, I swear.

They jumped onto the sunny shore, smiled for a photo, and then jumped in the water and started the swim at 11:00am. The swim took a total of 52 minutes, and as swims go was straightforward. From my vantage, I was able to see many things a swimmer cannot. I could see the seals coming over to investigate, and watch the boats all safely pass us by. And I could finally see the currents. Where I’d previously thought perhaps I was no good at recognizing currents, I could immediately tell that I’d just been searching for them at a slack tide. The currents were ripping now.

About 1k from shore, we entered the strong eastbound current. The water racing out of the Narrows had made a hard right and was rushing past us. This torrent created boiling, swirling water, short choppy waves at the current interface, and a few standing waves with white caps. I was instantly jealous that I was not swimming. Melissa later told me that she could see the algae below her moving in different directions at different depths as the currents switched. And as suddenly as we entered the current, we were out the other side into a small, calm debris patch, as eddies roiled by behind us.

Swimming Dalco Passage

In the end, Melissa and Alison made it back to Point Defiance, and only 400m from where they planned to finish. The currents had done almost exactly what was expected (expectations set by this Tethys model), and so had we.

Back on the beach after, I was even more excited by this beautiful little boat that had just taken us there and back. She is very stable and handled well in some confusing water and I’m gaining confidence in my piloting and boat handling, and bigger things seem very possible. As an unexpected bonus, watching two friends swim for an hour made me want to swim, like really made me want to swim. It is that feeling that has been waning lately during the past few months of training, and it is nice to have it back.

Dalco Passage Swim

Happy birthday Melissa, from Andrew and Tuesday.

 

Strait of Juan de Fuca, tides & currents

[Originally published in Please Tap on the Glass at USMS Blogs on 17 June 2013.]

Heads up: there’s some math in this one.

If you think knowing the tides and currents is all there is to planning a swim, you’re wrong. But if you think you can plan a swim without knowing the tides and currents, well, good luck. Even on training swims, currents play a big part (see Fig. 1). The methods presented herein are my own, developed over the past year and largely untried in the real world. This swim will either be a joyful validation of my methods, or a long, cold, learning experience.

Figure 1: The effect of various currents on an out-and-back swim loop

Figure 1: The effect of various currents on an out-and-back swim loop

Adverse currents were cited as the reason for stopping in numerous historic articles about past swims. The reason why is clear when you look at a map. The Strait of Juan de Fuca, connecting the Puget Sound and Salish Sea to the Pacific Ocean, has currents ranging from 3kt flood to 3.5kt ebb swirling along the rocky shorelines, playing Plinko with the San Juan Islands. To get a good feel for the overall movement patterns, the Current Atlas (Atlas des Courants) published by Fisheries and Oceans Canada is extremely helpful (see Fig. 2). Unfortunately, its resolution, both spatial and temporal, is not sufficient for planning on the scale of a swim.

Figure 2: Excerpt from the Current Atlas showing some tricky currents.

Figure 2: Excerpt from the Current Atlas showing some tricky currents.

There is one resource that probably every American swimmer who has the slightest interest in currents has referenced: the NOAA tidal current prediction tables. The National Oceanic and Atmospheric Administration publishes current predictions on hundreds of stations across the US, providing times and velocities for maximum flood and, and slack times in between. They’re published well in advance so do not take wind or weather into account, but provide a reliable starting point for any maritime excursion. The downside is, they only provide a bunch of data points, not a curve.

To connect the dots, I’ve written a formula that fits each predicted high and low with a piecewise sine curve (Fig 3) and put it into an Excel spreadsheet, allowing me to calculate a current velocity at any given time. Since this equation does not take into account the predicted slack-current times, there is there is almost certainly some error. This error appears worse for some stations, but relatively good for the two stations I’ve based my model on. This unquantified “goodness” is assessed by matching up the predicted slack times with the plotted equation and seeing how closely they match (Fig. 4). Some have been as close as 6 minutes.

Figure 3: an equation to fit a sine curve calculating y at time t given a time range and y range (y = current velocity, tidal height, etc.)

Figure 3: an equation to fit a sine curve calculating y at time t given a time range and y range (y = current velocity, tidal height, etc.)

Figure 4: A calculated velocity profile showing NOAA-predicted slack current times as red triangles.

Figure 4: A calculated velocity profile showing NOAA-predicted slack current times as red triangles.

With a way to calculate currents and a feel for how the water sloshes, the course can be set. To make planning uncomplicated and conservative, I like to pick one heading for the duration of the swim and let the currents take me where they will. There is a bit of guess and check involved. In half-hour increments, I draw a line from the start along the fixed heading scaled to correspond with my anticipated speed, and then another matching the direction and velocity of the current just calculated. Repeat, repeat, repeat until the other shore is reached, or it becomes clear the other shore will not be reached (Fig. 5). By varying start times and headings, I’ve now got at least two routes planned for each day of my window.

Figure 5: Sample of route creation method showing 30-min steps

Figure 5: Sample of route creation method showing 30-min steps

One of the responsibilities of my swim manager will be to compare these predictions to our actual progress. By keeping a constant heading throughout the route planning, it should be easy to anticipate where a deviated heading will take us. My goal is to hit one of the two coves in Washington and end on a sandy beach. Fortunately, the coastline here is relatively straight, so messing up the currents should only mean a little extra swimming and/or ending on a rocky cliff.

The most important things in planning tides and currents are a reliable set of predictions and a good feel for how the currents operate. I admit I don’t really know the intricacies of the Strait the way I’d like to, but products like the Current Atlas help, but I think I’ve been conservative enough in my planning to account for a few reverse eddies near shore or a delay due to shipping traffic. I’m excited to find out if this works.