Arrrr: Tides, Maties....Mon 04 July 2011 by Paul Lescarmure in Blog
Many folks reading this are seasoned SCUBA divers, or recreational mariners & have already been exposed to ideas immediately below; perhaps more accurately & in much greater detail. But, it's become obvious that not everybody in our group has that background--until now. It's my hope this blab can stand as a primer, for anyone in the club who's confused by "Spring tides", "Neap tides", low-water peaks (A.N.s), etc. & wonders why we pay attention to that stuff...
Tides, in general, are "caused" by Sun & Moon. When these 2 celestial influences are conjoined; we get "Spring" tide, with (often, not always) one big low-water anti-node (A.N.) & steadily rising water-level, leading to high-water A.N.-- sometime much later that day. Such tides are "diurnal"--basically once a day.
When the above-mentioned influences are @ right-angles, or are opposite; we get "Neap" tides (often, not always) with two low-water A.N.s & 2 (or almost 2) high-water A.N.s within the day. The changes in water-levels are less extreme. These tides are "Semi-diurnal"--two cycles per day.
Anti-nodes?!? Well, graphs of tidal water-levels are sinusoidal--like those curvy, repetitive math-functions you studied in senior high-school & 1st-year university math courses. But, being natural phenomena, they're not so regular, nor as symmetrical. The "anti-nodes" (A.N.), of those functions (be they Ocean, or "chalk-board" ones) are the high & low points, of those non-circular curves.
We're interested in water-level graphs, because of our (keen!) interest in tidal currents. All divers prefer dive-times with the slowest tidal currents & definitely want to avoid times with fastest currents. The bit-by-bit slopes, of those water-level graphs, are proportional to the (hour by hour) speed of tidal currents. The fastest currents occur @ times when graphs are steepest; the slowest currents occur during the A.N.s, when tide direction slows down, stops & reverses.
@ Victoria Harbor, A.K.A. "The Breakwater", or Ogden Point Marine Park (O.P.M.P.), there is a "Spring" tide pattern July 16-17. Actually occurs from about July 12 to 18th., but we're only interested in weekends. Right?
Looking @ image "Ogden Point Tides: July 16-17" one can see the extreme low-water A.N. actually @ 10:36 hr. After that, O.P.M.P. water-level rises, increasingly faster, then slows down & peaks @ 20:06 hr.
For a 2-hour dive-trip, we'd want to be in water close to 09:30 & dive until 11:30 hr.--for minimal current.
Compare above O.P.M.P. water-level "profile" to that of following weekend, in image "Ogden Point Tides: July 23-24". In that image we can see the tidal current (not fast to begin with) slowing down @ about 09:45 hr & not picking up much until 15:00 hr. The slopes of all sections are much less, hence the tidal currents are much slower--than on 16th.
This "porch", or "shelf" type of water-level "profile" is about the most congenial tidal-current condition to be found.
Compare above to that for Point Atkinson (for all our Howe Sound dive-spots) water-level "profile", in image "Pt. Atkinson Tides: July 23-24". The water-level's moderately low @ 06:35 hr., then rises to a moderately high level @ 13:44. Water-level drops a tiny bit (0.2 m, miniscule for this area) until 17:19 hr., when it rises again, peaking near midnight. This's a very accurate "semi-diurnal" water-level "profile", with 2 low-water A.N.s, each followed by a high-water point.
These patterns, for both O.P.M.P. & Pt. Atkinson, occur alternately on each weekend until @ least the end of August. e.g., O.P.M.P. has a Spring-tide "profile", on July 30-31.
There's a time-lag, between Water-level stages, @ the two areas, with Pt. Atkinson following O.P.M.P. by as much as 2 hours. The time-lag is not constant: there are marked water-level differences in the two areas.
N.B.: Tide-graphs "borrowed from: "WWW Tide and Current Predictor" @ http://tbone.biol.sc.edu/tide/index.html...That site is maintained by University of South Carolina Bio-Sciences Dept.