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Frequently Asked Questions

Every week we receive and read e-mail from fellow web surfers. We appreciate your feedback, and your sharpness in noticing our errors. We have made a list of frequently asked questions in the hope they will respond to your enquiries.

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What causes the tides?
It is the gravitational attraction of the sun and moon that cause waters of the ocean to raise and lower at different parts of the earth. Tides occur in oceans, and to a much smaller extent, tides also occur in large lakes, in the atmosphere, and within the solid crust of the earth. There are also non-astronomical factors, such as the configuration of the coastline, the local depth of the water, the ocean-floor topography, and other hydrographic and meteorological influences that play an important role in altering the range and interval between high and low water.
Why tides are important to humans?
The knowledge of the times, heights, and the extent of inflow and outflow of tidal waters is of importance in a wide range of practical applications for humans such as the following:
  1. Commercial and recreational Navigation through coastal waterways, and within estuaries, bays, and harbours.
  2. The establishment of chart datums for Hydrography, which are then used for demarcation of a base line or "coastline" for fixing offshore territorial limits, both on the sea surface and on the submerged lands of the Continental Shelf.
  3. For the furnishing of data useful to fishing, recreational boating, surfing, and a considerable variety of related water sport activities and tourism activities.
  4. Work on harbour engineering projects, such as the construction of bridges, docks, breakwaters.
  5. For the provision of information necessary for underwater military engineering uses.
Are there Tides in the Great Lakes?
Few inland waters are large or deep enough to be affected by the tidal forces of moon and sun, but a very small tide has been recorded on the Great Lakes. To the casual observer, the tide on the lakes is obscured by the more pronounced fluctuation of the water level in response to barometric pressure and wind, but gauges do show a tidal range of about 0.1 foot. This, of course, is too insignificant to affect navigation or harbour installations.
What is a seiche and when does it occur?

A seiche is a standing wave that occurs in an enclosed or semi-enclosed water body and is usually caused by strong winds and/or changes in atmospheric pressure. The seiche can continue, in a pendulum fashion, even after the cessation of the originating force.

Fig.1. Seiche

In Canada, strong seiche action can be observed on Lakes Erie and Ontario, in Sydney Harbour (N.S.), in the Bay of Fundy and Gulf of Maine.

What are Tidal Bores?

A tidal bore is the leading edge of the rising tide as it enters a river. It is a wave like phenomenon that moves up the mouth of rivers which are subjected to exaggerated tides.

Tidal Bore
Fig.2. Tidal Bore

Exaggerated tides in Canada can be seen in the Bay of Fundy on the East coast of Canada. The largest bore in the Bay of Fundy occurs on the Petitcodiac River. The bore or wall of water travels upriver at about 8 mph and is about 1 meter (3 feet) high during spring tides when the Moon is full or new. Tidal bores are most pronounced where river channels narrow.

What are "Reversing Falls"?

Reversing Falls are phenomena resulting from tidal action. At low tide, the inland waters empty into the sea over a rocky shelf in a waterfall. As the tide rises above the falls, the seawater forces its way against the river flow. The resulting turbulence, in the form of whirlpools, eddies and rapids, makes the falls appear to have actually reversed. Canada's three reversing falls are:

  • Saint John River, Bay of Fundy, New Brunswick
  • Wager Bay, Ford Lake, Northwest Territories
  • Barrier Inlet, Hudson Strait, Northwest Territories
What is a Tide Rip?

A tide rip, is readily apparent at the surface of the ocean. A rip is often a stretch of turbulent water at sea or in a bay or strait caused by conflicting tidal currents, or a tidal current moving over a rough bottom. Tide rips can appear as stretches of slightly choppy water running alongside glassy-calm water, or they might resemble whitewater rapids amid otherwise calm seas.

What is a Rip Tide?

A strong, sub surface tidal current that conflicts with another current or currents causing a violent underwater disturbance, usually in a direction contrary to that of the surface water is called a rip tide. Although rip tides may appear as dark or calm paths running through breakers, they can exist where there is no apparent surface commotion.

What are Spring and Neap tides?

The height of the tide varies from day to day but the fluctuations are predictable. They are mainly caused by: 1) the degree to which the sun and moon's influences are acting in the same direction; and 2) the varying distance between the moon and the earth.

Combination of lunar and solar equilibrium tides to produce spring tides at new and full moon and neap tides at moon's first and last quarter.
Fig. 3. Combination of lunar and solar equilibrium tides to produce spring tides at new and full moon and neap tides at moon's first and last quarter.

Twice each month, at the time of the new moon and the full moon, the gravitational influences of the moon and sun reinforce one another and cause the tides to rise to greater heights and fall lower than average tides. These are called spring tides from the Old English word "springan" which means to well up. At the time of the quarter moon, when the sun, earth, and moon form a right angle, the difference between high and low tide is less than average. These are neap tides, from the Old English "nep", as in nipped in the bud. Every 27 and a half days, when the moon reaches a point in its orbit closest to the earth (called perigee) the tidal range is increased. When perigean tides coincide with spring tides, extreme tides can be expected. Example, in the Bay of Fundy, these conditions may create tides as large as 16 meters (53 feet). Conversely, when the moon is at apogee, its farthest point from the earth, even spring tides are diminished.

What is a tsunami?

Japanese word that means "harbour wave". Tsunamis are huge ocean waves caused by an undersea disturbance generating a vertical motion to the water column. These disturbances are triggered by volcanoes that erupt under water or by earthquakes and landslides that occur underwater.

Generation of a tsunami
Fig. 4. Generation of a tsunami

The waves of the tsunami spread in a circular manner from the point of disturbance and can move large distances without a great loss of energy. Their speed can reach more than 800 kilometers per hour. In deep waters, tsunamis are less than a meter high. However, when they reach shallow waters or narrow bays, the waves slow down and pile up into a tall wall of water which causes devastation to things on shore. The first wave of a tsunami is often preceded by a lowering of the water level. When the tsunami hits the coast, it appears like a rapid rise in water level instead of a breaking wave. Earthquakes outside Canada have caused tsunamis which have damaged the Canadian coastline.

Where are the largest tides in the world? What causes them to occur there?

The largest tidal ranges in the world occur in the Bay of Fundy (more exactly Minas Basin) and in Ungava Bay (more exactly Leaf Basin) on the East Coast of Canada, where you can observe a 16 metre (53 foot) tide range. Tidal range varies during the month according to phase of the moon. They are largest at the new and full moons, and smallest at the quarter phases. Tidal ranges in the Bay of Fundy and Ungava Bay are the highest in the world because of an unusual combination of resonance (or seiche) and the shape of the bay. The Bay of Fundy and Ungava Bay are "V" shaped, so that water entering at their wide mouth at the open ocean end is funneled into less and less space as it moves into the head of the bays and the water can only pile up and form a large tide. The water in the Bay of Fundy and Ungava Bay also has a natural rocking motion called a seiche. You could compare this to the movement of water in a bathtub. Although the water in a bathtub sloshes from one end to the other and back again in a few seconds, it takes about 13 hours for the water in the bays to rock from the mouth of the bays to the head of the bays and back again. The Atlantic ocean tide rising and flooding into the bay every 12 hours and 25 minutes reinforces the rocking motion. The seiche in the Bay of Fundy and Ungava Bay are therefore sustained by a pulse from the ocean tides.

Other Large Tides

Other places in the world have large tides: the Port of Bristol in England (10 m); the Sea of Okhotsk northeast of Japan (10 m); Turnagain Arm in Alaska (12 m); the Gulf of St. Malo in France (14 m). However, tidal ranges at these locations do not reach the extremes that are reached in the Bay of Fundy and in the Ungava Bay (up to 17 m).

Where can I obtain Canadian Tidal predictions?

There are 7 separate volumes of the Canadian tide and current tables:

  • Volume 1 covers the Atlantic Coast and the Bay of Fundy;
  • Volume 2 covers the Gulf of St. Lawrence;
  • Volume 3 covers the St Lawrence and Saguenay Rivers;
  • Volume 4 covers the Arctic and Hudson Bay;
  • Volume 5 covers Juan de Fuca Strait and the Strait of Georgia;
  • Volume 6 covers Discovery Passage and the West Coast of Vancouver Island;
  • Volume 7 covers Queen Charlotte Sound to Dixon Entrance.
Canadian Tide and Current Tables index
Fig. 5. Canadian Tide and Current Tables index

You can purchase the Canadian Tide and Current tables, which provide one full year of predictions from over 800 authorized chart dealers across the country and internationally.

By calling 1-877-775-0790, you can find out vocal tidal prediction.

How to calculate the vertical clearance in non-tidal waters?