Rob Casey is the owner of SUP school Salmon Bay Paddle in Seattle, co-founder for the PSUPA and is the author of two paddling guides.

Wednesday, December 2, 2015

Understanding Tidal Rapids

I teach and love paddling in the tidal rapids of Deception Pass which is 1.5hrs north of Seattle. Much like a whitewater river, there's eddies, eddylines, holes, standing waves and swift current up to 9 kts.  Unlike a river, these rapids change direction four times a day, saltwater and are located on Puget Sound.

There's never an issue of low water, no current or a specific season in which they run. Unlike rivers there's no hazards such as big holes or strainers and the energy is concentrated in a small area vs the length of an entire river.  There's many tidal rapids on the Sound, some very small which only go a few knots, and others in British Columbia, Canada that run twice as fast as Deception Pass!  Look up Skookumchuck on Sechlet Inlet, Seymore Narrows, Surge Narrows, etc.
Tidal rapids are also a super way to build your overall rough water skills and confidence as well as cross train for other pursuits such as overnight open water trips, surfing, downwinding and racing. We also use tidal currents to give us a free tide to get to our destination quicker. Learning how to use them makes for covering a lot of water in short
time. Not planning for currents can lead to a long day! On December 6th 2015 is the Deception Pass Dash a legendary 6 mile race through the Pass in current which has gone on for nearly a decade and open to all paddle craft.  A super fun race to do or watch!
Tidal Rapids - When tidal current pulls or pushes saltwater over a shallow reef or narrow passageway thus creating river like effects such as waves, whirlpools, boils, eddy lines and swift current.  Search 'Surf Skookumchuck' for a stronger version up in BC.
Current - Horizontal movement of water. We use Current Tables for planning DP trips. Tides - Vertical movement of water.  We use Tide Tables for planning trips in areas of no or little current.  We'll use both in areas of mixed tides/current to determine if there will be a beach to land on and/or which currents use to get to our destination quicker or to avoid. Boaters and paddlers alike often wait for currents to change to make better time.  
Ebb - Outgoing tide - We prefer the ebb in DP for beginning classes. Easier to work with, cleaner lines.Flood - Incoming tide - Stronger than the ebb in DP, more advanced current in specific spots. Slack - 10-15min period between the ebb and flood. Usually no or little current.  
Mixed Diurnal Tides - 2 ebbs and 2 floods during a 24hr period. These are common in Puget
Sound. Diurnal tides with just one ebb and one flood are more common in the Strait of Juan de Fuca or Pacific Coast. Most tidal cycles here are 6 hours long. 
Tidal Exchange - General term for describing a full tidal cycle (approx 6 hours) from low to high tide or reverse. Here in Seattle we get larger daytime tidal exchanges in the Spring
from a -3 tide in the morning to a +13 in the evening. A cycle of this range means there's a lot of water moving in or out thus stronger currents.  

Spring Tides - Large tidal exchanges that occur not because of the season but during New and Full moons.Neap Tides  - Smallest tidal exchanges during quarter moons. Neap is the Saxon word neafte meaning scarcity. 
Eddy - Not Eddy Vedder or Van Halen! A section of water that pushes upstream due to downstream current wrapping around an obstruction like a rock. We use eddies to enter current from and/or rest in. Often bull kelp will be in eddies. Eddylines - The division between moving current and the eddy. It can flip a board or boat if you hit it wrong!Ferrying - Crossing a river without losing ground. Aim your board at approx 45 degrees to the downstream current (facing upstream) and watch your destination, usually another eddy.  Less angle for faster current. 

Reading Current Tables - 015-11-07 Sat 12:42 AM PST -0.0 knots Slack, Ebb Begins2015-11-07 Sat 3:13 AM PST -5.9 knots Max Ebb2015-11-07 Sat 6:39 AM PST 0.1 knots Slack, Flood Begins

What does this mean? At 12:42, slack gives us time to paddle through the Pass with no or
minimal current. A great time to enter and see the Pass look like a smooth(ish) lake. Soon thereafter, the ebb current begins to build but first as a trickle then rapidly growing in 2-3hrs to a Max Ebb, the fastest of the cycle. Then it tapers off and drops in the next 2-3hrs back to minimal current, then slack. Then it switches direction and the Flood begins - builds, maxes, drops, slacks, then ebb...
When I take beginners to current into the Pass, we go ideally at slack before the ebb. Canoe Pass is the most tame section, easily accessible from Bowman Bay. Canoe also has less boating traffic in summer. For the flood we enter at Cornet Bay. It's fun to enter on the tail end of the flood, work it until it dies, slacks then turns into the ebb, so you see a sample of the entire cycle!


Resources -
I use Captain Jacks and online sites such as Mobile Graphics for planning my trips and classes. I'm a visual person and need a simple visual guide to the tides/currents. Fundamentals of Kayak Navigation is the best guide I've found for understanding currents and tides for paddling. Author David Burch is in Seattle and owns Starpath School of Navigation. He has many marine guides on navigation and weather. Another fun one is his Tidal Currents of Puget Sound which shows using arrows how currents move in Puget Sound. Planning a trip? U need these guides! 



Check out our SUP classes in Seattle - Beginning to advanced instruction including freighter and tug wave surfing, coastal surfing, rivers and racing, plus PSUPA Certification.