Can Anyone Please Explain-Tidal Movement

[rickster]

I may not be the sharpest tool in the shed but there's something I still can't quite get my head around. We'll use the georges river as an example as most of us would know it.

Let's just say high tide is at 9.00am at Tom Uglys Bridge. This would put Lugarno at about 10.30 and Milperra Bridge at around 12.00. My problem is this:

At about 10.00, the tide is already running out at Tom Uglys AND GETTING SHALLOWER!!

But at the same time it's still running up at Lugarno AND GETTING DEEPER!! As is Milperra. At around 11.00 it's still getting deeper at Milperra but shallower and shallower at the other two spots. If water is running out at Lugarno how can it still be getting deeper at Milperra???? Where is Milperra getting the water from? As we know, the tide height change is the same at all places.In other words if low tide was 0.8m and high tide 1.8m it will be a difference of 1m at all three places.

This may read like a trick question but I'm serious. I don't have a clue to the answer and neither did the two gentleman I fished with on Saturday. If anyone can shed any light to my mystery please do so, it would be appreciated.

Until then, I'll just continue to lose sleep. Cheers

Rickster

[chrisb]

Ok, this is due to how fast the water is moving and the fact it is trying to play catch up (try to equalise). First things first the further you travel up river the less tidal movement is occurring (i.e. if low tide is 0.8 and high is 1.8 there will not be a 1m difference up river it will be less). If water is running out at Lugarno how can it still be getting deeper at Milperra? This because the water at Milperra is actually still lower than the water at Lugarno (water trying to play catch up). It actually takes quite some time for water to move up river, it is not an instantiations thing. If you have ever put water down a drain and waited for it to come out the other end you will see what I mean. There are a number of factors that influence how fast the water moves such as water depth, size of river mouth even direction of the wind and wind speed.

Hope I haven’t left you more confused; have a look at this link it might help explain how it all works a little better.

link

[kingpig]

I will start off but i bet i dont get it right.

Tides are generated by the gravitational effect of the moon. As we get closer the tides get bigger and reverse when we move away. This happens in cycles just like the seasons happen as we travel around the sun each year. Tides do it every 27 days (i think). So while the gravitational force of the moon is pulling the water of the earth towards it, the tide rises.

The force that is generated buy tidal flow is the same as any other force if you apply it the only thing that will slow it down is if another force acts on it (Thank Mr Newton for that one). The resisting forces are generated by the friction of the things that the tide runs against, (rock, bottom and riverbanks) and the incline in altitude as the water flows upstream. The incomeing water has inertia that is it takes time to stop when it is running fast, just like when you apply the barkes on your car. The negitive acceleration works against the inertia to stop you. Same with the tide, the frictional forces take time to overcome the force of the tidal flow.

There you go bet im wrong all together.

Now while you contemplating that one. If the tide is made by the moon, and the moon goes around the earth once a day, why are there 2 high tides in every day.

Cheers

Kingpig

[rickster]

Firstly kingpin, thanks for your reply (I think) To answer your question, that is very simple. If we had much, much, much longer rivers, we'd only have the one tide

OK I'm only joking and haven't got a clue.

[Day's Fishin]

The moons pull is what makes a high tide. When the moon is on one side of the earth it has a high tide however the opposite side also has a high tide because the moon is still pulling through the earth and makes the water bulge outwards if that makes sense.

Regards Jeff

[WilliamE]

There are a number of factors that influence how fast the water moves such as water depth, size of river mouth even direction of the wind and wind speed.

Good question and excellent answer. Add the those factors already mentioned the actual size of the tide on the day as well as the amount of fresh water (non-tidal) coming down the river due to rain in the catchment.

Basically when you're inside the mouth of an estuary, in the tidal zone, the water will still be running downhill beyond the precise time when sea-level starts to rise at the mouth. The closer you get to the mouth, the shorter the lag will be.

And to complicate things, during the smallest neap tides the far upper reaches of the tidal zone may not even get any incoming flow at all.

Edited August 23, 2010 by WilliamE