Tidal phenomena

Learn about tidal phenomena, including tides and tidal streams, effects effects and non-tidal influences.

On this page

• Tides and tidal streams
• Shallow water effects
• Non-tidal influences

Tides and tidal streams

The tidal stream is the horizontal motion (particle velocity) in a tide wave, which is said to flood and ebb. This is opposed to the vertical tide, which is said to rise and fall. The flood direction of a tide is:

• progressive if it’s that of the wave propagation
• a standing wave if it’s inland or toward the coast (upstream)

The flow is the net horizontal motion of the water at a given time from whatever causes. The word ‘current’ is frequently used synonymously with ‘flow.’ The term ‘residual current’ is used for the portion of the flow not accounted for by the tidal streams. A tidal stream is:

• rotary if its velocity vector traces out an ellipse
  • most tidal streams are rotary (except in restricted coastal passages)
  • the shape of the tidal ellipse and the direction of rotation may vary
• rectilinear if it flows back and forth in a straight line

Slack water refers to 0 flow in a tidal regime. The stand of the tide is the interval around high or low water in which there’s little change of water level. This doesn’t need to coincide with slack water.

Variations on predictions

The observed tide will never fit exactly any of our simple descriptions. This is because it consists of the superposition of many tide waves of different frequency and amplitude.

We can’t expect the heights of successive high waters (HWs) or of successive low waters (LWs) to be identical, even when they occur in the same day. Thus, the 2 HWs and 2 LWs occurring in the same day are designated as:

• higher and lower high water (HHW and LHW)
• higher and lower low water (HLW and LLW)

It’s likewise only the tidal stream associated with a single frequency tide wave that traces a perfect tidal ellipse. The composite tidal stream each day traces a path more closely resembling a double spiral, with no 2-day patterns identical.

Also, no tide is ever a purely progressive or a purely standing wave. Slack water shouldn’t be expected to occur at the same interval before HW or LW at all locations.

Shallow water effects

Tides in the open ocean are usually of much smaller amplitude than those along the coast. This is partly due to amplification by reflection and resonance. It’s more generally the result of shoaling.

As the wave propagates into shallower water:

• its wave speed decreases
• the energy contained between crests is compressed both into a:
  • smaller depth
  • shorter wavelength

The tide height and the tidal stream strength must increase accordingly.

If the tide propagates into an inlet whose width diminishes toward the head, the wave energy is further compressed laterally. This effect, called funneling, also causes the tide height to increase.

Tidal bores

A bore is created when the front of the rising tide propagates up a river. This churning and tumbling wall of water advances up the river like a breaking surf riding up a beach.

Creation of a bore requires:

• a large rise of tide at the mouth of the river
• some sandbars or other restrictions at the entrance to impede the initial advance of the tide
• a shallow and gently sloping river bed

The water can’t spread uniformly over the vast shallow interior area fast enough to match the rapid rise at the entrance. Friction at the base of the advancing front causes the top of the advancing front to tumble forward. This friction is paired with the resistance from the last of the ebb flow still leaving the river. Such activity sometimes gives the bore the appearance of a travelling waterfall.

Impressive bores

There are spectacular bores a metre or more high in several rivers and estuaries of the world. The best known bore in Canada is that in the Petitcodiac River near Moncton, New Brunswick. There’s another in the Shubenacadie River and in the Salmon River near Truro, Nova Scotia, all driven by the large Bay of Fundy tides. These are impressive (about a metre) only at the time of the highest monthly tides. However, they may be no more than a large ripple during the smallest tides.

Reversing falls

The reversing falls near the mouth of the St. John River at Saint John, New Brunswick, is caused by the:

• large Bay of Fundy tides
• configuration of the river

A narrow gorge at Saint John separates the outer harbour from a large inner basin. When the tide is rising most rapidly outside, water can’t pass quickly enough through the gorge to raise the level of the inner basin at the same rate. On this stage of the tide, the water races in through the gorge, dropping several metres over the length of the gorge.

When the outside tide is falling most rapidly, the situation is reversed. The water races out through the gorge in the opposite direction, again dropping several metres in surface elevation.

The water levels inside and out of the gorge are the same twice during each tidal cycle. This is when the water is placid and navigable. However, navigation through the gorge isn’t possible near peak flow time. This is when the surface of the water is violently agitated and the velocity of flow is rapid and turbulent.

The upper photo shows an aerial view at slack water and includes the:

• inner basin
• outer harbour
• bridge over that separates them

The photo in the lower left shows the inflow through the gorge at high water in the outer harbour (7.6 m above chart datum at time of photo).

The lower right image shows the outflow though the gorge at low water in the outer harbour (0.9 m above chart datum at time of photo). The recorded extreme high and low waters at Saint John are 9.0 and -0.4 m, respectively, above chart datum. At the time the photos were taken the flows would have been correspondingly greater.

Tide rip

A tide rip (overfall) is an area of breaking waves or violent surface agitation that may occur at certain stages of the tide in the presence of strong tidal flow.

They may be caused by:

• a rapid flow over an irregular bottom
• the conjunction of 2 opposing flows
• the piling up of waves or swell against an oppositely directed tidal flow

If waves run up against a current, the wave form and the wave energy are compressed into a shorter wavelength. This causes a growth and steepening of the waves. If the current is strong enough, the waves may steepen to the point of breaking, and dissipate their energy in a wild fury at sea.

Non-tidal influences

Non-tidal influences include:

• wind-driven currents and atmospheric pressure effects (storm surges)
• seiches
• tsunamis
• melting and freezing

Wind-driven currents, and atmospheric pressure effects (storm surges)

Wind-driven surface currents in the deep ocean are of major importance to navigation, while tides and tide streams have little to no effect on it.

A storm surge is the affect of atmospheric pressure and wind on water levels along:

• ocean coasts
• the shores of inland bodies of water

Storm surges are pronounced increases in water level associated with the passage of storms. Much of the increase is the direct result of wind set-up and the inverted barometer effect under the low pressure area near the centre of the storm.

Surge may become more exaggerated if the storm:

• depression travels over the water surface with a long surface wave travelling along with it
• path directs this wave up on shore and the wave steepens and grows as a result of shoaling and tunnelling (shallow water effects)

The term ‘negative surge’ is sometimes used to describe a pronounced non-tidal decrease in water level, which:

• is less extreme than storm surges
• can be associated with offshore winds and travelling high pressure systems
• can create unusually shallow water if they occur near the low tide stage (which can be of considerable concern to mariners)

The range is generally smaller than that of the tide on the coast. The importance may not be fully realized until an extreme of the non-tidal fluctuations coincides with a corresponding extreme (high or low) of the tidal fluctuation.

Tidal predictions, such as those in the Canadian Tide and Current Tables, contain no allowance for non-tidal effects. However, they still contain the average seasonal change in mean water level.

Seiches

A seiche is the free oscillation of the water in a closed or semi-enclosed basin at its natural period. Seiches are frequently observed in:

• bays
• lakes
• harbours
• basins of moderate size

They may be caused by:

• passage of a pressure system over the basin
• build-up and subsequent relaxation of a wind set-up in the basin

Following initiation of the seiche, the water sloshes back and forth until the oscillation is damped out by friction.

Seiches aren’t apparent in the main ocean basins. There’s no force sufficiently coordinated over the ocean to set a seiche in motion. Tides aren’t seiches.

If the natural period (seiche period) is close to the period of a tidal species, the constituents of that species (diurnal or semidiurnal) will be amplified by resonance more than those of other species. The constituent closest to the seiche period will be amplified most of all. The response is still a forced oscillation, whereas a seiche is a free oscillation.

A variety of seiche periods may appear in the same water level record because:

• the main body of water may oscillate:
  • longitudinally or laterally at different periods
  • both in the open and closed mode if the open end is somewhat restricted
• bays and harbours off the main body of water may oscillate locally at their particular seiche periods

Seiches generally have half-lives of only a few periods, but may be frequently regenerated.

The largest amplitude seiches are usually found in shallow bodies of water of large horizontal extent. This is likely because the initiating wind set-up can be greater under these conditions.

Tsunamis

A tsunami is a disturbance of the water surface caused by a displacement of the sea-bed or an underwater landslide. It’s usually triggered by an earthquake or an underwater volcanic eruption. The surface disturbance travels out from the centre of origin in a pattern similar to ripples from the spot where a pebble lands in a pond.

The waves may almost immediately dissipate their energy against a nearby shore in some directions. In other directions, they may be free to travel for thousands of kilometres across the ocean as a train of several tens of long wave crests. Being long waves, they travel at the speed (gD)½. This gives them a speed of over 700 km/h (almost 400 knots) when travelling in a depth of 4000 m. The period between crests may vary from a few minutes to the order of 1 hour. In a depth of 4000 m, the distance between crests might range from less than a hundred to several hundred kilometres.

The wave heights at sea are only the order of a metre. This doesn’t constitute a significant distortion of the sea surface over a wavelength of several hundred kilometres. However, when these waves arrive in shallow water, their energy is concentrated by shoaling and possibly tunnelling. This causes them to steepen and rise to many metres in height.

The tsunami waves are high and massive when they arrive on shore. They’re capable of tremendous destruction in populated areas. Because of the relative gentleness of tsunamis in deeper water, ships should always leave harbour and head for deep offshore safety when warned of an approaching tsunami.

The origin of the word tsunami is from the Japanese expression for ‘harbour wave.’ This name has been adopted to replace the popular expression ‘tidal wave,’ whose use is to be discouraged since there’s nothing tidal in the origin of a tsunami. Another expression sometimes used for these waves is ‘seismic sea wave,’ suggesting the seismic, or earthquake, origin of most tsunamis.

Warning system

A tsunami warning system for the Pacific has been established by the U.S., with its headquarters in Honolulu, Hawaii. Other countries that border on the Pacific, including Canada, participate in the system.

Canada's direct contribution consists of 2 automatic water level gauges programmed to:

• recognize unusual water level changes that could indicate the passage of a tsunami
• transmit this advice to Honolulu

The gauges are at Tofino on the west coast of Vancouver Island, and at Langara Island off the northwest tip of the Queen Charlotte Islands group.

The tsunami warning centre at Honolulu receives immediate information from seismic recording stations around the Pacific of any earthquake that could possibly generate a tsunami. It then:

• calculates the:
  • epicentre and intensity of the quake
  • arrival time of the as-yet hypothetical tsunami at the water level sensing stations in the network
• initiates a ‘tsunami watch’ at all water level stations in the path, for a generous time interval around the estimated time of arrival of the hypothetical tsunami
• issues tsunami warnings to the appropriate authorities in threatened locations if the water level interpretation indicates that a tsunami has indeed been generated

Melting and freezing

When seawater freezes, only the water forms into ice crystals. The salt becomes trapped between the crystals in a concentrated brine. The brine eventually leaches out, leaving mostly pure ice floating on the surface surrounded by sea water of increased salinity and density.

Freezing

Since the ice displaces its own weight in this denser water, it doesn’t displace as much volume as it occupied before freezing. Because of this, freezing has an effect similar to that of evaporation. It lowers the water level and increases the surface salinity and density.

Surface water must therefore flow toward a region of freezing, while the cold salty water that’s formed must sink and flow away from the region.

In the polar regions, particularly in the Antarctic, freezing produces cold salty water that sinks and flows along the ocean bottom for thousands of kilometres.

Melting

When sea ice melts, mostly fresh water is released. This decreases the salinity and the density of the surrounding water. Melting thus has an effect similar to that of precipitation. It raises the water level and decreases the surface salinity and density.

Surface water must therefore flow away from a region of melting ice. The speed of currents associated with freezing and melting in the ocean are never great.