Tidal Predictions: can someone explain something for me please...

theMusicMan

theMusicMan

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Hiya All

Though I live adjacent to a river with a huge tidal range - often in excess of 13m - I often use the easytide www site to see what times high and low tides are at certain locations I intend to travel to.

It's easy to see what times high and low tides will occur, and I understand the range of the chart i.e. the difference in water levels between high and low tides. What I don't understand though is what the lower part of the chart means i.e. the bit between low tide and zero;

20090821-cryqdibg45c58a8rtirfkywn9t.jpg


Does anyone know what the '0' represents on the vertical access...?
 
I'm not entirely sure i've grasped what you're asking but i think, i think, i know:shrug:

Anyway looks like minimum amount of water. So on a spring tide, the lowest depth of water will be 80cm.
 
Not sure it's that Merc, as the chart above would imply there's only going to be 0.5M depth of water in the River Severn at low tide this evening! This is what I don't understand, the Severn is still quite deep even at low tide, so I am not sure what the '0' represents.
 
I've edited my post above to say looks like minimum on a spring tide will be 80cm. This will be at the shallowest point. So it looks like worse case scenario for anyone with a boat.

Just because it says 80cm doesn't actually mean that will be the depth of water. Does that make sense? Sorry, heavy night last night.:gag:
 
The figure will be the minimum depth at a given point. Although the river is deep in places, the minimum depth will be for boat traffic using the river. Somewhere, where ever the depth is taken for tide purposes the depth will be in this case 80cm?
Hope this helps.
Mike
 
OK, I understand that 'somewhere' the depth will be 80cm, but 'somewhere' the depth could be anything...!! If at low tide, I wade out 2 steps from the waters edge... the depth might be 10cm, if I wade out 20 steps, it might be 2M!

What I am getting at is what does the '0' point on the chart refer to...? It's meaningless as far as I see it. Boats can come up the Severn even at low tide, it's still incredibly deep in parts, so what does this 'somewhere it's 80cm' thing mean? Nothing?
 
Itis meant to be used in conjunction with Admiralty charts. If you can download one you will be able to see the differences in depth along the river. The charts will state (usually in metres) the shallowest depth. This is designed for boat traffic and not people paddling. The 0 represents no water.

So this is for shipping lanes etc not off the shore.
 
OK Merc, I see what you're saying, but I still don't understand it, sorry. Is that chart then saying, according to your explanation, that in the shipping lanes of the river Severn, there is only 80cm of water at low tide...?

So, 'the shallowest depth'... at what point? it's still meaningless as far as I can tell.
 
A reading of 0 Metres or 0.8 Metres does not really mean 0 or 0.8 Metres. The graph your looking at is not to show the depth of the water, only the the range of the Tide. To work out the depth of the river at a certain point you would need the navigational charts for that area to show the marked depths. The marked depths on these charts will be for mean tide (or something like that, i don't have access to one as im not at work at the moment)

So you would then need to combine the information from the Tide table and the navigational chart for you to work out the depth at the time you want.
 
The zero is what they call 'Chart Datum' which is the lowest tidal point. However the tide can go lower than that if there is high pressure or winds.

Harbours and Estuaries are often dredged so, if it were dredged to say 10m, when the tide is stated as being a 4.3m, the water would actually 14.3m.
 
The zero is what they call 'Chart Datum' which is the lowest tidal point. However the tide can go lower than that if there is high pressure or winds.

Harbours and Estuaries are often dredged so, if it were dredged to say 10m, when the tide is stated as being a 4.3m, the water would actually 14.3m.
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exactly what i was trying to say, but you made the point better:thumbs:
 
Rovers_Andy said:
A reading of 0 Metres or 0.8 Metres does not really mean 0 or 0.8 Metres. The graph your looking at is not to show the depth of the water, only the the range of the Tide. To work out the depth of the river at a certain point you would need the navigational charts for that area to show the marked depths. The marked depths on these charts will be for mean tide (or something like that, i don't have access to one as im not at work at the moment)

So you would then need to combine the information from the Tide table and the navigational chart for you to work out the depth at the time you want.
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Bingo.... thanks Andy, this is exactly what I thought - that the zero on the easytide chart represents nothing but a 'point of reference' to show the range of low to high tide values. I know it has to be there, but in the context of the easytide chart, it is meaningless.

Thanks.
 
It will be the shallowest depth. That may mean it's only the depth in one tiny part of the shipping lane but for safety sake they have to give you the minimum. So in conjunction with a chart you will probably be able to navigate around that obstacle, or it will be marked by a buoy. Anywhere else could have a minimum of say 3m.
 
No problem, glad i could help and the hours on the bridge drinking tea and taking the pee out of the Deckies hasn't been wasted :lol:
 
Rovers_Andy said:
No problem, glad i could help and the hours on the bridge drinking tea and taking the pee out of the Deckies hasn't been wasted :lol:
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Hehe... nice one Andy.

Thanks all for your contributions here... appreciated.
 
Merc said:
Right i'm going back to bed now. Thought i was being helpful, whilst struggling with my hangover. :razz::wave::lol:
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Haha... at least you weren't replying on this thread last night then or we might ALL be confused! :cuckoo::thumbs:
 
0 is Chart datum which is equivalent to the lowest astronomical tide (LAT), ie the lowest tide that could feasibly be expected to occur (not including changes associated with weather conditions). At this point, you might still have a significant depth of water but that depends upon where you are. The actual level of the LAT will vary from place to place.

So basically you'd probably need nautical charts and quite possibly a qualification in navigation to make full sense of a set of tidal charts.
 
The Matt said:
0 is Chart datum which is equivalent to the lowest astronomical tide (LAT), ie the lowest tide that could feasibly be expected to occur (not including changes associated with weather conditions). At this point, you might still have a significant depth of water but that depends upon where you are. The actual level of the LAT will vary from place to place.

So basically you'd probably need nautical charts and quite possibly a qualification in navigation to make full sense of a set of tidal charts.
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Just when I thought I had grasped the whole issue myself :razz:

Tell you what, I will stick to desert trekking, and while here I will be glued to country roads.
 
Wail said:
Just when I thought I had grasped the whole issue myself :razz:

Tell you what, I will stick to desert trekking, and while here I will be glued to country roads.
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As Matt said the 0m on the tide chart is the lowest Astronomical tide, the lowest tide possible (may only occur every hundred years or so). To find the exact water depth at a certain point you need to then use the Admiralty charts. The depths shown on the Admiralty charts (OS maps of the sea essentially) are again connected to chart datum and are the MINIMUM water depths you can get (or 0m on the tide data), you then need to add the tide data to the data on the charts to get the actual depth at the certain time of day.

Here's a practical example that will hopefully help get your head round it. Using your chart (obviously not the right one for the area) and a chart for part of Plymouth sound...

20090821-cryqdibg45c58a8rtirfkywn9t.jpg


http://www.plymouthport.org.uk/images/Cattewater map (too big for the forum unfortunately)

Now look at the chart from Plymouth and find the 2.5 (shown as a big 2 and subscript 5) just to the left of the bouy symbol (semicircle of black, just next to Fl.Y 4s in the middle left just above Mountbatten). That 2.5 is the MINIMUM depth of water in metres that will ever be at that location. Now say you want to know the true depth at 6am and noon.

Look at your tide chart and read off the tide height at 6am (10m) and noon (0.5m), then you simply add it to the depth shown on the map, so 2.5m + 10m for a depth of 12.5m at 6am and 2.5m + 0.5m for a depth of 3m at noon.

The minimum depth is to make sure there is always at least that amount of water beneath your keel, otherwise there may be a few "accidents" as big ships go into ports...:lol:

Hope that clears it up for you (it should all be right, if not I need to go back to school and learn it again...:bonk:)
 
Now that was very clear, thank you so much :thumbs:
 
must be the water where the graph applies...cant have low tide readings at the high tide mark
so lets assume this is the water below low tide for say 5m out
 
The Matt said:
0 is Chart datum which is equivalent to the lowest astronomical tide (LAT), ie the lowest tide that could feasibly be expected to occur (not including changes associated with weather conditions). At this point, you might still have a significant depth of water but that depends upon where you are. The actual level of the LAT will vary from place to place.

So basically you'd probably need nautical charts and quite possibly a qualification in navigation to make full sense of a set of tidal charts.
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YUP!
brilliant matt!:love:
 
The residual amount of water at the bottom of the graph is the height above CHART DATUM.

On charts the depth given is the chart datum and is the theoretical lowest depth. The tidal tables give depths over chart datum.

There are other factors which will affect the actual depth found. Low atmospheric pressure at the location and high atmospheric pressure out to sea will INCREASE the depth of water (it could be quite significant too, if the pressure difference is great) - other way about lowers the depth.

WIND - onshore winds will hold the water back and increase the amount of water at the location (so low water might not actually reach the predicted level due to climatic intervention - ie the wind blows the water in).

Pressure and wind in conjunction can severely affect the height of tide - think of the East Anglia situation this last winter when wind and pressure situations created a tidal surge that broke the flood defences.

Tide tables are worked on sun and moon phases alone and cannot be expected to take climate and weather interventions in to account. You need to understand these effects though - eg, taking a trip across to The Worm and wanting to get back....the tide tables will give you the TIDAL time, but the weather can affect the water movement over an dabove the tidal movement - you could be cut off sooner than the tide tables suggest.
 
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