To explain the importance of tides and currents, let’s look at a few examples. If you are paddling at a speed of 3 knots (average speed for a recreational paddler) against a 5 knots current, are you going forward or backward?
If you set camp at night and wake up with the water line 600 meters away from your boat in the morning, will you carry your kayak or just read a good book and wait for the water to come back?
As we see, tides and currents can get in our way, but by chance, water comes and goes, so if we are able to understand a bit more about the mechanism of tides and currents, not only will we avoid the down side, but we could have them work for us.
How about paddling at 3 knots with a 5 knots current in our back? A lot more fun!
Tides and currents are of course related but we will look at them separately because their timing is different and they don’t have the same effect on us as paddlers.
A few definitions:
- Tide - is the vertical movement of water (with the land as a point of reference)
- Current – is the horizontal movement of water (with the bottom of the ocean as reference)
So in order to use tides and currents to our advantage, we need to calculate the time when they turn, and when they will be at their highest or lowest point. To do that during the course, we will be using the Canadian Tide and Current Tables, a book edited by Fisheries and Ocean Canada every year for boaters to calculate their tide and current. There are different volumes for different areas, and the results are really accurate.
I won’t get into the actual calculation right now because it would be long and tedious to write, however, it is quite easy and fun to do in the class room.
But let’s get a bit of theory and vocabulary so things go smooth when we reach the mathematics.
We’ll take some theoretical trips on the chart below to illustrate our interest in the subject.
So how could the tide affect our paddling trip around this area? Let’s look at the green area (remember that Green on the chart is the colour for the inter tidal zone, meaning that at high tide the green area is covered with water and at low tide the area is dry) on the top (north) of Sidney Island. See the arrow pointing to the tiny island with the number (26) beside it? At low tide, we couldn’t get there at all. And if we set up camp on it, we would need to know when is the next high tide to be able to paddle away. Also, if we were trying to figure out where we are, can you imagine the difference of view from high to low tide? If we were just paddling in the bay, knowing if the tide is coming in or going out would tell us which way the current will be helping us.
Now let’s look around Sidney Island, imagine we are at point A. it is a low tide so there is land to stop and have lunch. But can we go for a hike. How long before the tide comes in and the kayaks float away? See the symbols on the Beige going around Sidney Island? Those symbols represent cliffs.
So now we need to know when slack tide is, or when the tide turns from Low to High, to figure out how long we have before land disappears and we are swimming.
Looking at that chart prior to the trip, I would know that if we are there during high tide, there wouldn’t be any landing areas around the island!
It is important to remember that high tides are not always at the same height, so some days at high tide we could still have quite a bit of land around. Tides are not regular around Vancouver Island, so the tide turns could come in 2 or 8 hours.
Now that you’ve come across the terminology related to tides, let’s make it clear.
Tides are influenced by the movement of the moon (and the sun to a lesser extent) around the earth. To put it simply, the moon attracts (or pulls) the water of our oceans towards itself. So when the moon is on the other side of the earth from us, we have less water and a low tide. On the other hand, when the moon is on our side of the earth, it is pulling thousand of millions of liters towards our shores, and it is high tide.
Let’s look at one full tidal circle:
- SLACK TIDE – the tide has reached its highest point. The water is not moving anymore. It’s the TURN.
- LOW TIDE – the tide now starts to GO OUT. It’s low tide from the moment the water starts to go out until it is at its minimum point. When it reaches its minimum, the water stops moving, and it’s the turn again.
- SLACK TIDE – the tide has reached its lowest point. The water is not moving anymore. It’s the TURN.
- HIGH TIDE – the tide now starts to COME IN. It’s high tide from the moment the water starts to come in until it is at its maximum point. When it reaches its maximum, the water stops moving, it’s the TURN.
What do we want to know and understand about currents? Current is the horizontal movement of water created by the tides coming in and going out.
When the tide changes and water starts moving one way or the other, it needs a path to reach the open water through the thousand of channels, inlets and narrows.
So we have two different currents:
- FLOOD - Is the current created by the water coming in at high tide
- EBB - Is the current created by the water going out during low tide.
Looking at one channel, the current will be going one way during flood and the other during ebb.
If you look at the chart, there are 2 arrows under the A in the square.
The arrow on the right has little feathers at the end and represents FLOOD (Feathers=Flood)
The other arrow represents EBB.
So why are we so interested in currents?
Well, as mentioned before, a current going faster than our own speed doesn’t help. Current calculations give us the current speed in the centre of the channel, but currents behave differently on the sides of a channel where we usually choose to paddle.
Sometimes you can hardly tell which way the current is going, and when going through back eddies the directions are inversed.
We are mostly interested in currents when the time comes to do a crossing, for example from SidneyIsland to James Island.
So if I were to cross from point A to point B when the current is flooding (bottom to top direction) at 3 knots, my course would be changed by the force of the current and I would end up at point C….or maybe miss James Island all together. It is hard to realistically calculate where I would end up, but it is for sure impossible to go in a straight line crossing a current (well, there are ways, but you’d need to know how strong the current is!).
Consider the symbols pointed out by the two big arrows in the channel. Those symbols from top to bottom represent tidal races and whirlpools, which can both be a hazard to us. We know that these can be avoided as they disappear when the currents are slow or non-existent.
Currents bring water in and out to shore from the open water. In doing so, they have to go through channels and around islands where they get funneled and pressured by the land and the different shape of the bottom of the ocean, creating all kinds of reactions at the surface.
Let’s look at a typical cycle of the current related to the tides:
SLACK TIDE (no movement of water)…
LOW TIDE STARTS (water is going out)
TURN TO EBB
THE WATER IS EBBING, STARTING SLOWLY
THE CURRENT SPEEDS UP
THE CURRENT REACHES ITS MAXIMUM SPEED, MAXIMUM EBB
THE CURRENT SLOWS DOWN
THE CURRENT DIES
SLACK TIDE (no movement of water)…
HIGH TIDE STARTS (water is coming in)
TURN TO FLOOD
THE WATER IS FLOODING, STARTING SLOWLY
THE CURRENT SPEEDS UP
THE CURRENT REACHES ITS MAXIMUM SPEED, MAXIMUM FLOOD
THE CURRENT SLOWS DOWN
THE CURRENT DIES.
When we time currents, we concentrate on the TURN and the MAXIMUM.
TURN because at that time there is no current.
MAXIMUM because at that time we don’t want to be crossing a channel!!!!
For the course we will use a Canadian Tide and Current Table and learn how to calculate the time for both, and then we will take our results on the water stapled to our chart.
Here is how it will look:
TIDES FOR COWICHAN BAY ON THE 24 DEC 2005
TIME HEIGHT (FEET)
0316 6,5 LOW TIDE
1056 11,1 HIGH TIDE
1856 5,2 LOW TIDE
CURRENTS FOR SANSUM NARROWS ON THE 24 DEC 2005
TURN TO MAXIMUM SPEED (KNOTS)
0430 0720 +3,0 FLOOD
0945 1405 -3,0 EBB
1825 2045 +3,0 FLOOD
Notice that the height for the tide is given in feet (or meters). To make sense of those numbers we need to understand what they are referring to. So we need to know what would be 0,0 feet at low tide.
0.0 is an arbitrary point chosen and accepted as reference in Canada (other countries have different reference points). The name of that point is CHART DATUM, and it represents the LOWEST NORMAL TIDE in Canada.
Chart Datum is, by international agreement, a plane below which the tide will seldom fall.
We get to that 0,0 height only a few times per year, usually during the summer equinox when we get “great” tide variations, and a few negative tides (for example -0.6).
Have a good look at those numbers, and if it doesn’t make sense and you feel a headache coming, don’t worry we’ll make sense of it together soon!