the northeast-going stream six hours later. This results in a net westward transport of water through the Strait, (Harvey Table 1 .p_523) which in environmental terms explains why the Strait is a lot cleaner than but for the Residual it otherwise might be.
The Wind. As might be expected, the strength of the Residual varies with tidal range. Variations in windstress also reflect directly on the SW Residual, with NW'ly winds augmenting it, while SW'ly winds reduce it or even turn it towards the NE.(Simpson p_251 lines 4-7) This has self-evident and opposite effect upon the strength of both the NE and SW-going streams, both the commencement of 'The Slack' and its duration, and the all important time when 'The Ebb' will begin. Admiralty Pilot : "The streams in the Swellies, and to a less extent in the Strait northeastward and southwestward of the bridges, are affected by winds outside at sea: winds with a southerly component holding the water up, winds with a northerly component causing the stream to turn to the westward earlier. With strong northerly or northwesterly winds both the rate and duration of the southwest-going stream are increased, and the southwest-going stream may begin about a quarter of an hour earlier. Strong south or southwesterly winds have the opposite effect."(Admiralty Pilot Vol 37 . p315)
But in navigational terms the Residual is mostly of little account, seldom exceeding half a knot. However there are times when the Residual is of such a strength that it can even reverse the direction of flow. See for example : Simpson Fig.6 page 251, which records the 59 day duration of his measurements, and notice 23rd /24th March 1968 when there was a strong northeast Residual, and throughout a 36 hour period (three tides) none of the flow was towards the southwest! I myself remember several quite eerie and unsettling periods at the Plas Newydd mooring when the ship lay in a state of 'limbo' right across the Strait for many hours at a time with no flow in either direction. It follows that in transiting the Swellies we must have in mind that there is always the risk that a SW Residual of this sort of strength happening to coincide with 'The Ebb' on a big tide could conceivably produce unusual and very dangerous conditions.
Barometric Pressure : Of course, anything which alters the water levels relative to eachother will alter the time of level parity, and thus also change the time when flow reversal occurs. As early as 1747 Gissler discovered the inverse relationship between sea level and atmospheric pressure (1 cm change in sea level for each 1 mb change in atmospheric pressure). In 1964 Prof J.G. Harvey established that in the Manai Strait atmospheric pressure has a much greater effect on water levels than does the wind, and that : "in the Menai Strait there is a particularly close inverse relationship between water level and barometric pressure twelve hours earlier." (The Effect of Weather on the Water Level in the Menai Strait) This might at first be thought to have uniform effect along the Strait, and therefor not interfere with the normal mechanics of level parity. However, Harvey's paper continues: "A decrease in atmospheric pressure brings about the largest increase in mean water level at Caernarvon and Port Dinorwic (relative to Menai Bridge and Belan). Perhaps surprisingly, as well as having more influence (than the wind) on the overall mean water level, atmospheric pressure also has a greater effect (than the wind) on the differences in water levels between the various positions in the Menai Strait." This makes it plain that changes in atmospheric pressure is yet another highly significant factor affecting both the strength of the stream and the time when flow reversal will occur.
Tidal Constants. It's the practice in the Menai Strait to quote the times of the tides there by reference to the time of High Water at Liverpool (45 miles away !), which assumes a constant time difference, or Tidal Constant on Liverpool. But to quote Reed's Nautical Almanac : "Tidal Constants are not in fact constant, and may vary quite appreciably with changing astronomical conditions." But it isn't only the interval on Liverpool which concerns us. The time interval between HW at Menai Bridge and Port Dinorwic isn't constant, and variations in this interval will also alter the time of level parity and flow reversal. From graphs drawn in association with constructing the bar charts Figs. 1 and 3 it can be shown that a given difference in the duration of that interval alters the time of level parity by about half that difference. It had been my intention to wade through the tide tables to determine to what extent this interval varies, which is no more than to make an academic point. Sufficient to say that in the twenty five tides I did examine in making these bar charts, HW at Port Dinorwic varied between Menai Bridge -20 mins., and Menai Bridge - 52 mins., a difference of over half an hour. This one variable on its own is therefore enough to alter the time when the stream reverses direction by as much as a quarter of an hour.
