Thick person asks O-Level geography question

GJ0KYZ · 27 January 2011, 18:22

If a river has a lot of water in it during the winter, will the high tide mark be higher than during the summer (for the same height of tide) when there will be less river water?

bosun · 27 January 2011, 18:32

I would guess it depends how far up river you go,the rising tide will tend to push the rainwater coming down river back up and that's what can cause flooding.Worse on spring tides of course.
I havn't given this a great deal of thought mind,its just that locally we have a river that bursts its banks only when its been raining heavily and the tide is in.

Erin · 27 January 2011, 18:41

Would have thought so. The tide will push in and hold back the natural flow of river water, resulting in it rising to a greater degree.

gotchiguy · 27 January 2011, 18:52

Mock Geography GCSE at 8:30 tomorrow. Really ought to start revising!

chewy · 27 January 2011, 18:55

Quote:

Originally Posted by GJ0KYZ

If a river has a lot of water in it during the winter, will the high tide mark be higher than during the summer (for the same height of tide) when there will be less river water?

Yes.

Channel Ribs · 27 January 2011, 19:01

Quote:

Originally Posted by GJ0KYZ

If a river has a lot of water in it during the winter, will the high tide mark be higher than during the summer (for the same height of tide) when there will be less river water?

Having lived with a tidal river at the bottom of the garden I can say with some certainty yes. Though I have no idea why, but then I struggle to work out why the river goes backwards while falling. Tides eh, what fun!

martini · 27 January 2011, 19:17

I'd say no, any extra water would just flow out, creating a stronger ebb?

Searider · 27 January 2011, 19:32

Yes,

Southampton water tide tables warn that tidal heights may be higher after heavy rain.

Channel Ribs · 27 January 2011, 19:35

Mind you, rain usually ties in with low pressure. Low pressure can be associated with higher tides.

I think, not even sure what day it is now.

jonmitch · 27 January 2011, 20:41

Ask my mate who lives on the edge of the Brisbane river. When he's finished mopping up that is!!

hamster · 28 January 2011, 07:14

As stated already rain is generally associated with low pressure systems. Air pressure can have a huge effect on tidal heights. Tide tables assume a standard air pressure at sea level of 1013mb. As a rough guide variations in pressure either side of this will result in a difference for the predicted height of roughly 1cm per 1mb higher or lower so if pressure was 993 the tide would be approx 20cm higher than predicted and 20cm higher if it was 1033mb.

Combine this with the fact that fresh water is less dense (and there for weighs less) than sea (salt) water and it gets worse. As the fresh water is lighter than the incoming salt water it will be pushed back in a bit but as it's lighter it will flow over the top of the incoming sea water and flow out to sea (downhill). This can be a chunk of water anything up to 18 inches deep depending on geography, how much it's rained etc and can there fore give a big increase in water level and also give the visual effect that the river is ebbing when in fact the level is rising as the fresh water ebbs at the top but the sea water underneath is flooding and therefore increasing in depth.

http://weather.mailasail.com/Franks-...sure-And-Tides

http://en.m.wikipedia.org/wiki/Atmos...edirected=true

GJ0KYZ · 29 January 2011, 09:31

Crikey, thanks Hamster! I had no idea things were that complicated.

tim griffin · 29 January 2011, 10:09

Quote:

Originally Posted by hamster

http://weather.mailasail.com/Franks-...sure-And-Tides

http://en.m.wikipedia.org/wiki/Atmos...edirected=true

Great websites and excellent explanations of weather shall keep them and pass it on to students great resource info for teaching.
Tim

Simon B · 29 January 2011, 14:24

It's also a question of time and space, a larger volume of water either fresh or salt water needs to move through the same space in the same time the level has to get higher, it will also get faster.

I used to live ner the river wear and during heavy rain and higher tides the water level rose a lot, there was also a good deal of mixing so the ideal laminar flow mentioned earlier may not have been happening. It wasnt the sort of river to swim in with your mouth open so a proper test was not conducted.

Also went out of Whitby an a diver cx'n assessment after torential rain and the amount of water and debris coming down ws impressive, 3500rpm just stand still good experience though.