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Tidal Talk

There is a tide in the affairs of men, Which, taken at the flood, leads on to fortune; Omitted, all the voyage of their life is bound in shallows and in miseries. On such a full sea are we now afloat; And we must take the current when it serves, Or lose our ventures. -From William Shakespeare’s, Julius Caesar

Earth has many natural forces that seize our attention and rock our world. Hurricanes, Tsunamis, floods, tornados, volcanoes, mud slides, sink holes, earth quakes and the like - random, destructive and chaotic in the affairs of mankind - which always garner front page coverage. But in terms of scale, influence and unfathomable power, they all pale in comparison to the predictably calm, steady and unsung movements of the tide. Twice daily (the movement is hindered and altered by land masses in many locations) the almost inconceivable enormity of the earth’s oceans uniformly slosh back and forth across the globe as if a massive pendulum’s 6-hour ticking was slowly arcing its rhythm of the seas back… and forth… and we hardly even take notice other than a few little numbers in a box on the back page of the local weather section. Unlike those earthly, natural, yet inevitably destructive occurrences the tides provide a literally heaven-sent life-giving rhythm without which the depths and coasts would be almost barren of life. Contrary to those calamities which come seething up from the mantle and magma within, the tides are driven by the outside forces of the cosmos, along with the centrifugal forces of the earth’s own daily rotation and seasonal tilts. There you float in your boat in some serene, secluded and sunny cove, but can hardly even notice as the moon and sun - minute by minute, inch by inch - conspire to literally change your world. Although just a minute fraction of the sun’s size, the moon’s much closer proximity to earth more than doubles its tidal effects. The invisible gravitational forces of these two masses reach out across the cold lifeless darkness of space to uncover an oyster bed, hasten your voyage, give you more beach space or make the fish friskier. Countless coastal fauna and flora depend on the tidal rhythm to provide the constant flooding and draining of their habitat without which those creatures would cease to exist. It can’t be argued that the tides have far more of an influence upon life than even the largest jungles and their torrential rains and rivers... combined. The word tide has been one of the most misused of nautical terms, so much so that its misapplication has come to be widely accepted. Properly speaking, tide means only the rise and fall – the vertical movement – of bodies of water resulting from the gravitational pulls of the moon and sun. Commonly but incorrectly, tide is also used to refer to the inflow and outflow of waters as a result of changes in tidal level. Current is the proper term for the horizontal flow of water; tidal current for flows resulting from tidal influences. It is proper to say “a two-knot tidal current” but not “a two-knot tide”. The incoming tidal current running towards the shore or upstream in a river is called the flood; the retreating or downstream current is called the ebb. Slack is the period of time between flood and ebb currents when there is no flow in either direction. Stand is the period when there is no rise or fall of the tidal level and slack and stand usually do not occur simultaneously at any given place. Neither are there rip tides of the sort that carry beach bathers out to sea. The correct term is rip current. And whilst we’re on proper language there is no such thing as a tidal wave as commonly applied to what is correctly called a tsunami, (Japanese for “big wave in harbor”) such as the ones resulting from the massive bottom displacements caused by submerged earthquakes like those that devastated the South Pacific some recent years ago. The only exception is a tidal bore, the initial short wall of water or wave that floods in over the exposed flats at the beginning of an incoming tide in places of extreme tidal swings such as those in Nova Scotia. The highest tides in the world indeed occur at Burntcoat Head near Wolfville in Nova Scotia’s Minas Basin, where the water level at the highest tides can rise an astounding 52.5 feet in just over 6 hours! At mid-tide, the flow in Minas Channel equals the combined flow of all the rivers and streams on Earth and believe it or not, Nova Scotia actually bends when the tide comes in. As 14 billion tons of sea water flow into Minas Basin twice daily, the Nova Scotia countryside actually tilts slightly under the immense load. There is much more to explain as to all the various factors involved with the tides and for a thorough and complete explanation, you should refer to your Chapman’s. But I would like to touch upon a few more of the more commonly used terms associated with the tides. Depending upon the constantly changing positions of the earth, moon and sun relative to one another, the gravitational forces can act in alliance and magnify —or cancel one another out — hence the highs and the lows of the tides constantly change day to day. This is further compounded due to the tilt of the earth’s axis relative to the sun and so on any given day, one tide is always higher and/or lower than the other. We use terms like mean, higher and lower (as in “mean lower low water”) to delineate those differences in tide levels as it quite crucially affects navigation. When passing under a bridge, for instance, you either have sufficient clearance, or you don’t. And a spring tide has nothing to do with a season but instead is the term used to describe the highest tides caused when the combined gravitational influences are strongest, while a neap tide is just the opposite. Due to their immensity and predictability tides are now being tapped as sources of power. The most rudimentary tidal power plants involve a dam called a barrage, placed across an inlet. Gates on the barrage allow the tidal basin to fill on the incoming high tides (sort of like the gates in a lock system) and then be deflected to empty through the turbine system as the tide falls. There are also two-way systems that generate electricity on both the incoming and outgoing tidal currents. Though seemingly quite a “green idea” for generating electricity, tidal barrages can change the tidal level in the basin and cloud up the water with particulates, or negatively affect navigation, recreation and aquatic plants and animals. A tidal fence uses a vertical axis turbine to harness the energy of tides in narrow areas between land masses wherein the water is forced through the turbines, sort of creating an open water version of a damn’s hydroelectric plant. They have less impact on the overall environment than barrages although they will impede the movements of larger marine animals. Then there are horizontally oriented tidal turbines which are basically wind turbines that can be located anywhere there is sufficient tidal flow and depth. Because water is about 800 times denser than air tidal turbines have to be constructed much stronger than wind turbines. As a result, they are heavier and more expensive to build but will be able to capture more energy. It’s mainly the moon driven rise and fall of the tide that generates the tidal currents which nourish aquatic life. Perhaps we can someday harness those same tides to generate much of our electrical power to similarly enrich our lives. I say we ditch those unsightly and fragile solar panels with their little individual solar cells that only work on sunny days anyhow. Let’s start thinking, “Go Lunar!” And although I started this piece with a suitable thought provoking tidal quote, I’d like to close with one as well: Dwell as near as possible to the channel in which your life flows. - Henry David Thoreau

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