PHENOMENA

Contents

Tides and tidal streams

Shallow-water effects

Generalities
Tidal Bores
Reversing Falls
Tide rips (Overfalls)

Non-Tidal Influences

Wind-driven currents, and Atmospheric pressure effects (Storm surges)
Seiches
Tsunamis
Melting and freezing

SHALLOW-WATER EFFECTS

Generality

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 is, however, more generally the result of shoaling: as the wave propagates into shallower water, its wave speed decreases and the energy contained between crests is compressed both into a smaller depth and a shorter wavelength. The tide height and the tidal stream strength must increase accordingly. If, in addition, 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. Canadian Tidal Manual

Tidal Bores

Sometimes the front of the rising tide propagates up a river as a bore, a churning and tumbling wall of water advancing up the river not unlike a breaking surf riding up a beach (Fig. 1).

Fig. 1. Generation of a tidal bore

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, and a shallow and gently sloping river bed. Simply stated, the water cannot 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, plus resistance from the last of the ebb flow still leaving the river, causes the top of the advancing front to tumble forward, sometimes giving the bore the appearance of a travelling waterfall.

Fig. 2. Tidal bore on the Petitcodiac River at Moncton, New Brunswick. (Photo by D.G. Mitchell, Canadian Hydrographic Service, 1960)

Fig. 3. Tidal Bore on the Salmon River, near Truro, Nova Scotia. (Photo by F.G. Barber, Ocean Science and Surveys, DFO, 1982)

There are spectacular bores a meter 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, N.B., but there is another in the Shubenacadie River and in the Salmon River near Truro, N.S., all driven by the large Bay of Fundy tides. These are impressive (about a meter) only at the time of the highest monthly tides, and may be no more than a large ripple during the smallest tides. Canadian Tidal Manual

Reversing Falls

The reversing falls near the mouth of the St. John River at Saint John, N.B. is also caused by the large Bay of Fundy tides and the 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 cannot pass quickly enough through the gorge to raise the level of the inner basin at the same rate, so on this stage of the tide the water races in through the gorge, dropping several meters over the length of the gorge. When the outside tide is falling most rapidly, the situation is reversed, and the water races out through the gorge in the opposite direction, again dropping several meters in surface elevation. Twice during each tidal cycle, when the water levels inside and out are the same, the water in the gorge is placid and navigable. The surface of the water in the gorge near the peak flows is violently agitated and the velocity of flow is too rapid and turbulent to permit navigation through the gorge. Canadian Tidal Manual

Fig. 4. The Reversing Falls at Saint John, New Brunswick, at the mouth of the St. John River. (Upper photo by Lockwood Survey, NFB Phototeque, 1966; Lower photos by D.G. Mitchell, Canadian Hydrographic Service, 1963.)

The upper photo is an aerial view at slack water, showing the inner basin, the outer harbour, and the bridge over that separates them. 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). Lower right shows the outflow though the gorge at low water in the outer harbour (0.9 m above hart 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, and at these times the flows would have been correspondingly greater. Canadian Tidal Manual

Tide Rip (Overfall)

A tide rip or 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, by the conjunction of two opposing flows, or by 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, causing 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. Canadian Tidal Manual

Reviewed: 2008-09-15