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PHENOMENAContents
TIDES AND TIDAL STREAMS In a tide wave the horizontal motion, i.e. the particle velocity, is called the tidal stream. The vertical tide is said to rise and fall, and the tidal stream is said to flood and ebb. If the tide is progressive, the flood direction is that of the wave propagation: if the tide is a standing wave, the flood direction is inland or toward the coast, i.e. «upstream.» The flow is the net horizontal motion of the water at a given time from whatever causes. The single word «current» is frequently used synonymously with «flow», but the term residual current is used for the portion of the flow not accounted for by the tidal streams. A tidal stream is rectilinear if it flows back and forth in a straight line, and is rotary if its velocity vector traces out an ellipse. Except in restricted coastal passages, most tidal streams are rotary, although the shape of the ellipse and the direction of rotation may vary. The ellipse traced out by a tidal stream vector is called the tidal ellipse. Slack water refers to zero flow in a tidal regime. The stand of the tide is the interval around high or low water in which there is little change of water level: this need not coincide with slack water. Since the observed tide consists not of a single wave, but of the superposition of many tide waves of different frequency and amplitude, it will never fit exactly any of our simple descriptions. Because of this, we cannot 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 two HWs and two LWs occurring in the same day are designated as higher and lower high water (HHW and LHW), and higher and lower low water (HLW and LLW). It is 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 two days patterns identical. Also, no tide is ever a purely progressive or a purely standing wave, so that slack water should not be expected to occur at the same interval before HW or LW at all locations. Canadian Tidal Manual
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