Today (Monday 8/4/13) low tide inside Lake Macquarie NSW was at 1:29pm. At Swansea it was 5 minutes earlier at 1:24pm.
I was returning from sea and entered Swansea Channel at around 2:30 pm. I noticed by observing the marker buoys in the first few hundred metres of the channel when returning from sea that the tidal flow in the channel was definitely seaward. I was heading into a strong current.
Based purely on the tide data I expected that the tidal flow would be into the lake at 2:30pm today, not out of the lake. However there must be a lag between the actual flow in the channel and the local tide data because of the volume and area of Lake Macquarie. Is there any data available that predicts the actual tidal flow in the channel?
I was paddling a surf craft and really noticed the current. I was in the north side of the channel near the Blacksmiths breakwater. I had predicted I would be paddling with the current when returning through the channel at 2:30 pm but was proved wrong. I thought the sailors of the area would be the bet resource for this info. Thanks
Very interesting, came over the Swansea bar at 1.35pm yesterday and experienced the same, was expecting the tide to have turned but was still running out. interesting enough, when I picked up the public buoy while waiting for the bridge to open at 2pm, though the current was strong, not once did the yacht pull on the mooring but remain at mid ships to the bouy though the current was very obvious, It was as if there was an incoming current below thee obvious outgoing current, if that is all possible. The lake is also very high at this time after all the rain we have been experiencing and am wondering if this has something to do with the situation. Look forward to anyone else's slant on this...?
I think that you may find that slack water in a situation where you have a relatively large body of water flowing through a small channel will always lag the turn of the tide by up to 3 hours. Port Phillips Heads is a case in point. Any time you cross a bar or enter the Heads you should find out slack water times not tide times. Slack water time is also far more variable based on prevailing weather conditions than is tide time.
Thanks all. Slack water seems to be the term I am after. I wonder where information about slack water in the channel could be obtained?
Good point about the recent rain. Maybe it is affecting things lately.
Slack water at Swansea channel will be approx 3hrs after the advertised time
cheers
And you're probably on the money with the observation about rainfall and surface/bottom currents. In this situation the fresh water from the lake, being less dense, will tend to run out on the surface, while the dense oceanic salt water will be on the bottom. This will probably only happen for a part of the tide - when flow is well established in either direction, the salinity and current flow will be more uniform top to bottom. It's a common situation in rivers or estuaries that have a significant freshwater flow.
Alan Lucas terms it overrun. As the water level in the lake proper hardly changes as the tide at the heads goes up and down, the flow doesn't change direction until about mid tide when the levels balance out and then reverse. The lag depends on the size of the body of water. Forster lags a good 3 hours on a big tide, I expect Lake Macquarie to be similar.
Read Lucas Cruising the NSW Coast or his article in Afloat a few years ago. Critical stuff for sailors navigating bars.
Thanks for all the great information. Hopefully I can use the current to my advantage in future.
The comments above are partly right and partly wrong.
I've been told that the tides in the lake are influenced more by the atmospheric pressure [highs and lows] than the tides out to sea.
On the day you were at the bridge, were we under the influence of a low?
If so there would have been an outflowing of water in addition to what is seen in the channel during a normal tide.
I think I remember seeing special tables for the tide predictions in the lake.
I had a look in my files but couldn't find it.
Just found this link; www.bom.gov.au/fwo/IDN60233/IDN60233.561070.plt.shtml
Atmospheric pressure does have an effect, but a relatively minor one. The tide goes up and down 1-2 metres off the east coast, atmospheric pressure makes a difference in water level measured in single digit centimetres.
Wind can have a big effect as Ramona noted. A strong onshore wind can raise the water level at the heads by lots, conversely a strong offshore wind will lower it. Think storm surges in cyclones.
If you look at those charts attached by NSWSailor you will see the water level is almost constant in the inland reaches of the lake (like at Coranbong or Wyong), and vary more in line with the tide variation nearer the channel i.e. at Belmont a few 10s of cm and at Swansea the water level rises and falls with the tide. In the creeks rain and overflow from weirs would have more affect than anything that we are talking about.
The primary effect on the tidal flows in the channel is the tide at Swansea heads. When the water level there is at top of tide it would be about a metre higher than at the drop over at the lake end of the channel, the same in reverse at low tide.
As the difference in water level between the lake and heads doesn't equal until the tide has got close to mid tide the flow doesn't reverse until some time after the turn of tide at the heads.
The other big effect on tidal flows at Lake Macquarie is heavy prolonged rain. A year or two back the lake water level rose 1 metre after heavy rain, the jetty at the Wangi Workers was under water. As the water level in the lake was higher than even the high tide at the heads the channel had an ebb tide running out for more than 36 hours non stop.
Alan Lucas' explanation, with diagrams, is the best one I have seen. Critical stuff for any yachtie who crosses bars.
Well i cant shed any light on the tidal situation but i once fell off my kiteboard in the mouth of Swansea channel then got circled by a 3 meter shark!..... It was an outgoing tide.
