Thread Tools Search this Thread
Old 28 January 2011, 02:14   #11
hamster's Avatar
Country: UK - England
Length: 4m +
Join Date: Aug 2006
Posts: 196
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.

hamster is offline   Reply With Quote
Old 29 January 2011, 04:31   #12
Country: UK - Channel Islands
Town: Jersey
Boat name: Archangel
Make: Ribcraft
Length: 6m +
Engine: ETec 225
MMSI: 235063789
Join Date: Sep 2008
Posts: 1,005
Crikey, thanks Hamster! I had no idea things were that complicated.

GJ0KYZ is offline   Reply With Quote
Old 29 January 2011, 05:09   #13
Trade member
tim griffin's Avatar
Country: UK - England
Town: Newport IoW
Boat name: Amean/Pronto/Rumbo
Make: Solent Rib Princess
Length: 7m +
Engine: 200hp Etec 260x 2
MMSI: lots of them
Join Date: Sep 2003
Posts: 1,857
Great websites and excellent explanations of weather shall keep them and pass it on to students great resource info for teaching.
Tim Griffin
RYA and RLSS UK Training and Rescue Centre
07876 623124 follow us on twitter @GriffMarine
tim griffin is offline   Reply With Quote
Old 29 January 2011, 09:24   #14
Simon B's Avatar
Country: UK - England
Town: Leicester
Boat name: Vixen
Make: Ribcraft
Length: 6m +
Engine: Suzuki OB 175
MMSI: 235071839
Join Date: Jun 2004
Posts: 1,621
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.

New boat is here, very happy!
Simon B is offline   Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Our Communities

Our communities encompass many different hobbies and interests, but each one is built on friendly, intelligent membership.

» More about our Communities

Automotive Communities

Our Automotive communities encompass many different makes and models. From U.S. domestics to European Saloons.

» More about our Automotive Communities

Marine Communities

Our Marine websites focus on Cruising and Sailing Vessels, including forums and the largest cruising Wiki project on the web today.

» More about our Marine Communities

Copyright 2002-2015 Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 18:39.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2016, vBulletin Solutions, Inc.