Strait of Juan de Fuca: Tides & Currents
by, June 17th, 2013 at 11:36 AM (5645 Views)
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
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.
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 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.
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.
Figure 5: Sample of route creation method showing 30-min steps
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.