Thanks for all the informative replies to this question. It seems that a bottleneck will not have slack water coincide with high or low water. So the only way to predict the best time to pass through the bottleneck is either from accumulated experience, published on charts, tide stream tables, sailing directions, or pilot, or maybe a local fisherman. If none of these are available for the location, perhaps it's best to allow plenty of time to hang around and wait. We have this situation nearby at "The Narrows", the eastern entrance to Westernport Bay. At the narrowest, the bottleneck is 600 metres or so wide. The chart marks 5 - 6 knot streams, but unlike the western entrance, used by shipping, there are no time predictions. I gather that the annual swimming event crossing this section is timed for slack water. A boat is placed mid strait. When the boat stops drifting, the starter's gun fires. PT. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Dave Kruger wrote: >Peter Treby wrote: >> Is it generally true that where a tide stream runs through a bottleneck, >> such as a narrow entrance to a widening bay, slack water in the bottleneck >> will not coincide with either high water or low water? >Yes. Depends on size of the bay relative to the narrowness of the neck. Slack >water will occur when the relative levels of the bay and "ocean" outside the >bay are the same height (ignoring the effect of momentum of the tidal stream). >OTBE, this will be somewhere close to mid-tide, and may only occur for a very >short time. Peter, it is always true. By definition, a bottleneck creates a delay so the water won't be the same height on each side of the bottleneck (when the flow becomes "slack") until sometime after high or low tide. What will determine how long that delay will be will depend on many factors. The flow rate through the constriction, the shape of the constriction (deep and narrow vs. high and shallow), the tidal variations due to the orientation of the moon and sun (neap and spring), the type of tide due to the tilt of the earth and other factors (semidiunal, diurnal or mixed), the nearness of the moon and sun (apogee and perigee), the depth of the offshore water (whether there is a continental shelf or not will greatly affect tide height), the shape of the coastal configuration (a bay may be subject to seiches or bores), and the amount of water needed to fill the estuary up to the point when the tide outside has risen or fallen to the same level. With only a very slight constriction in the flow, the delay between high (or low) tide and the "slack current" time will be small. But imagine a situation where only a trickle can get through a tight bottleneck (such as a small vertical gap in lock doors). When the trickle through the lock doors first started (creating a new estuary where one hadn't existed before) the slacks would only be near low tide and most of the time the water would be running in. At some point an equilibrium would be reached where the estuary had finally filled until the water spent about one half the time running in and half running out. Once this balance was reached then the delay would mean that slack current most likely occurred somewhere near mid-tide and the water level in the estuary wouldn't change much at all. However, then variations in the tides (spring, neap, apogee, perigee, etc) would become an important factor shifting slack to nearer the high or low tides (much like when the basin was first filling). With a different type of constriction, say one where the estuary's opening is only reached at the highest of tides, then slack could only occur somewhere near high tide and most of the time water will be running out of the estuary (or it will have drained completely). The variables to knowing when slack will be are potentially so numerous that the best solution for most navigators seeking "slack water" is to use the current tables (if they are available for the area). Gee, I planned a short simple explanation but the complexities and variables kept on rearing their ugly heads and complicating things. I hope this was understandable (and correct). Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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