![]() ![]() ![]() To check a prediction by hand we first need to know the drying height. Some prediction programs can’t handle that and when they ignore either the first or the second low water the result can be dangerously inaccurate. Tide predictions are generally based on harmonic motions, as if the height of tide is bouncing up and down smoothly between high and low water points, but where a sill creates two low water points the first will be a dead bounce, with no upward turn until the second low water point, when the next flood over-tops the sill. In essence, we have chopped off the bottom of each tidal curve and after each high water we now have two low water points, with a low water stand between them.Ī similar curve can be drawn for a place on a drying estuary, but this is where some computers get confused. To illustrate general principles, I’ll start with a simple example of a man-made tidal sill.ĭiagram 1 (below) shows a typical set of tidal curves (with time and height scales omitted). One obvious way in which a shallow estuary can interfere with the flow is by a natural sill, such as a drying bar. In those cases, using a pencil to sketch adjustments to the almanac’s tidal curve can actually yield more accurate results.įor this it is vital that the skipper should have a good mental picture of what happens when the sea flows into and out of the estuary. The same is true for marinas and harbours protected by locks and sills.Ī full analysis requires intricate maths, usually on a computer, but some computerised output of tidal calculations on websites contains gross errors. On countless rivers around our coast, you wouldn’t want to arrive at the rivermouth bar only to be driven aground because you’re an hour too early, or too late. Tidal calculations: At Granville, the foreshore dries for a long way out but the marina has a sill to retain water. ![]()
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