Currents, with Surge Narrows as an Example

When we talk about tides, we are interested in the vertical rise and fall of the water level. However, when we talk about currents, we are concerned about the horizontal movement of water. Of course, the currents are driven in a complex way by the tides, which are, in turn, driven by the sun and moon.

Currents can become quite strong in narrow passages. Some examples:
• Deception Pass (between Whidbey and Fidalgo Islands) – as high as 9 knots
• Dodd Narrows (near Nanaimo) – as high as 9 knots
• Seymour Narrows (north of Campbell River, west of Quadra Island) – as high as 16 knots
• Surge Narrows (north of Discovery Islands Lodge) – as high as 11.5 knots
• Saltstraumen (Norway) – as high as 22 knots

In our typical sea kayaks we might hit a top speed of 5 knots or so, which we would find difficult to sustain for long. So these higher currents will have their way with us. Further, there are eddies, overfalls, whirlpools etc. all hoping to capsize our kayaks. So we will go through Surge Narrows at slack i.e. the transition between flood current and ebb current.

We make use of the Surge Narrows Current tables. Such tables are approximate e.g. pretending that slack water or that the peak current extends throughout the narrows at a precise time. Although approximate, the tables are still useful for the kayaker.

Tidal currents in wider areas will be milder, but can make a difference by helping or hindering the kayaker. There is a yearly race, the Alert Bay 360, around Cormorant Island (off Port McNeill) . The flood and ebb tidal currents get up to 3 and 4 knots and race planning doesn’t care, you get what you get. But when planning a trip, it can be important to take advantage of the tides when they are at all significant.

The 50/90 Rule

From slack to the next slack is roughly 6 hours with the max current speed occurring halfway through i.e. from slack to max current is roughly 3 hours.
• Starting at slack, the current reaches 50% of max by the end of the first hour
• then 90% of max by the end of the second hour
• and 100% of max by the end of the third hour.

The current speed then begins to decrease
• reaching 90% of max by the end of the fourth hour
• then 50% of max by the end of the fifth hour
• and slack at the end of the sixth hour.

We emphasize that these speeds are estimates for the instantaneous current at the end of the associated hour. A mnemonic way to write the rule for those who think easily with numbers:
0:50:90:100:90:50:0

Of course, we said from slack to the next slack takes roughly 6 hours and you must adjust appropriately if it is different. For example, if it were 7 hours, each hour period in the description above would be replaced by 70 minutes. Duration of weak currents near slack

Near slack, what is the duration of relatively weak currents? Of course, about half that duration is just before slack, and the other half just after. Touring kayakers of modest skill (and without helmets) typically want to traverse a tidal rapid during this time of weak current. Unfortunately, the higher the max current, the shorter this time. Here are some rough estimates, choosing to define weak current as a half knot or less.
• max current = 2 knots, weak current duration = ~ one hour
• max current = 4 knots, weak current duration = ~ one half hour
• max current = 8 knots, weak current duration = ~ one quarter hour

We can get more mathematical. We'll start by defining slack as a current speed of less than 0.5 knots. The tables give the 'exact instant' of slack, call that slack0. Then we calculate the duration of slack around that time by figuring the amount of time when the current is less than 0.5 kt before slack0 and adding the amount of time when the current is less than 0.5 kt after slack0. The formula is, in each case,
• slack interval = (60/max_speed) minutes, where max_speed is in knots and is either at max flood or max ebb, whichever is appropriate

The formula above assumes 3 hours between slack0 and the adjacent max. If that is not close, the formula should be replaced by
• slack interval = (T/max_speed) minutes
where T is a third of the time in minutes between slack0 and the adjacent max.

Practical Example

Let’s assume we have decided to go to the Octopus Islands for one of our day trips, which means we’ll need to traverse Surge Narrows on the way there and about 6 hours later on the way back. Thursday is a good candidate because of when slack occurs i.e. looking at the current table for Surge Narrows for Thursday (2019-09-12) shows:
12:59 AM PDT 6.1 knots Max Flood
3:58 AM PDT -0.0 knots Slack, Ebb Begins
7:15 AM PDT -6.2 knots Max Ebb
10:05 AM PDT 0.0 knots Slack, Flood Begins
1:15 PM PDT 8.6 knots Max Flood
4:40 PM PDT -0.0 knots Slack, Ebb Begins
7:53 PM PDT -6.9 knots Max Ebb
10:50 PM PDT 0.0 knots Slack, Flood Begins

In particular, to fit our schedule nicely we find
• 10:05 AM PDT = slack0, flood begins … for the trip out (after breakfast)
• 4:40 PM PDT = slack0, ebb begins … for the trip back (before dinner)

Note that Surge Narrows
• Floods South (140° True)
• Ebbs North (320° True)

After breakfast we should get on the water in time to reach Surge Narrows (~20 minutes). Slack0 is at 10:05 AM and the current turns to flood. So if we’re late we fight the flood and if we’re a bit early, the remnant of ebb will help us. Hence we shoot for a bit early. To calculate the duration of slack (current less than 0.5kt) we have
(170/3)/6.2 minutes = 9.0 minutes before slack0
(190/3)/8.6 minutes = 7.3 minutes after slack0
for a total of 16.3 minutes to traverse Surge Narrows at the 10:05 AM slack time period. The precise number is very approximate, yet the result is still a useful ballpark figure. I’d say we have a comfortable 16 minutes to traverse the narrows. It takes about 10 minutes, if I recall correctly. We might start into Surge by ~ 9:50 AM to avoid the start of flood. If we start a bit earlier, it would be slightly more exciting.

Then we paddle to the Octopus Islands, have lunch, investigate the islands, and paddle back in time for the next slack0 at 4:40 PM. There is plenty of time, but we should stay aware of the clock. To calculate the duration of slack (current less than 0.5kt) we have
(205/3)/8.6 minutes = 7.9 minutes before slack0
(193/3)/6.9 minutes = 9.3 minutes after slack0
It’s roughly a 17 minute window. We might start into Surge at 4:25 PM to avoid the start of ebb. As before, if we start a bit earlier, it would be slightly more exciting.