Ebb rate vs flood?

[rogerroger]

Why does the tide run so much harder on the ebb than the flood ? I'm thinking in particular the entrance to Chichester into which I crept in at about 1.5 knots SOG under full revs and full sails...

Surely as much water goes in as comes out so why is the max ebb at Chi over 6 knots, and the flood only about 1.5 ?



/forums/images/icons/cool.gif Roger Holden /forums/images/icons/cool.gif
<A target="_blank" HREF=http://www.first-magnitude.co.uk>First Magnitude</A>

 

[kingfisher]

Outflow > Inflow

1) More water runs out of a river than into a river: FLOOD = (sea water in)-(river water out) ; EB= (sea water out)+(river water out)

Obi-Wan
http://sirocco31.tripod.com

 

[rogerroger]

Re: Outflow > Inflow

That makes sense but Chichester harbour is really a large creek - it is not a river...

/forums/images/icons/cool.gif Roger Holden /forums/images/icons/cool.gif
<A target="_blank" HREF=http://www.first-magnitude.co.uk>First Magnitude</A>

 

[AndrewB]

Solent Tides

Solent tides are odd in that the flood lasts much longer than the ebb, and the tide drops particularly fast about 3 hours before low water. This is most pronounced in Southampton Water, with its double-high.

Same amount of water in shorter time = greater flow. So the ebb is generally stronger throughout the Solent area.

 

[Gunfleet]

Isn't it Parahandy? 'The tides is just a mystery'

 

[Roberto]

The main determinant (apart from less significant causes) is the shallow water effect.
The speed of a (very) long wave like the tide travels is proportional to the sqrt of depth; when tide amplitude is not too different from depth, like in a bay, the water near the crest (HW) stands over a bigger "thinkess" of water thus travels faster than the water near the trough (LW) water).
In particularly long bays the crest of the tide wave can be as faster as to reach the trough, still amplifying the effect.

 

[oldharry]

Re: Solent Tides

Bit of both happening here. There are quite a number of small rivers and drains emptying into Chi Harbour - including the notorious Lavant (which has worryingly, started running six months early this year), and all the drain and sewage run off from Chichester which enters at Fishbourne, and which betwen them dump quite a lot of water into the harbour. During the height of the Chichester City flooding, the ebb rate out on the bar increased dramatically, increasing the current.

All the land water is held back by the incoming tide, so that not as much seawater comes in and flood tidal currents are reduced in proportion to the amount of landwater being held back. Once the tide turns outside, the level drops quite rapidly from about 2.5 hours after HW, so that a much bigger difference in water level is created, which with the additional 6 hours accumulated land water causes a much stronger run on the ebb.

 

[steve_cronin]

to put it simply m\'dear...

The one runs uphill and the other down. Or at least the water behind the ebb runs downhill and the flood tries to push water uphill as well as having to counteract that wot is still on 'is way down. See?

At least that's how it was explained to me as a child!

Steve Cronin

 

[Rob_Webb]

Rog

I was victim of similar problem over the (spring tide) weekend - see my other post about engine failure..... now dreaming about a shiny new 20hp Vovlo to power me through the ebbs and floods!

Rob

 

[kingfisher]

Imagination ?

I have the same feeling. But I do know that I'm caught more in an adverse eb-stream (returning to late) than an adverse flood-stream ("naah, let's wait a bit more in the pub untill the tide slackens").

When was the last time you left the mooring in a foul tide ?


Obi-Wan
http://sirocco31.tripod.com

 

[rogerroger]

When the rain started lashing down I started dreaming of something big, fast and stinky - with a nice dry canopy !

My worst fear is just crossing the bar fighting against the ebb, the engine failing and getting pushed back onto it!



/forums/images/icons/cool.gif Roger Holden /forums/images/icons/cool.gif
<A target="_blank" HREF=http://www.first-magnitude.co.uk>First Magnitude</A>

 

[Rob_Webb]

I had the anchor on hand but with such a vicious tide and also the swell of much passing traffic didn't fancy that option. Was also ready to give up, sail back out into Poole Bay and wait for the tide to turn but with forecast of NE4-5 blowing over a tide flooding up the Needles Channel, I didn't fancy tackling that with no engine as backup - so took the more immediate option of a two.

By chance, only last week I bought a lovely 20m length of 18mm nylon line - slightly over-rated for my boat but it was the ideal thing for towing - thick enough to do the job but springy enough to save my cleats!

Rob