Tuesday, September 28, 2010

Slack water?

So I'm on my way to watch the Great River Race when I over hear one young man say to another "Yup, slack water, this is as high as the river gets."

I look over at Putney Pier and see the tide swirling by....hmmm.....!

Do I:

a) Approach the lads and say "You're talking total codswollop!"
b) Shake my head and go on my way wondering about the state of the British education system
c) Take a quick snap (above) thinking there's a blog post there somewhere
d) Something else - suggestions please


Tillerman said...

I wish someone would write a blog post explaining how slack water is NOT always at the same time as high tide and low tide. At Hayling Island the organizers provided us with very helpful tide charts showing the direction of the currents every hour before and after high tide. Contrary to what you might expect the direction of flow does NOT reverse there at the time of high tide but at some other time. I don't really understand how this is even possible but I guess the situation is very complicated there with water sloshing into and out of the Channel from the Atlantic and swirling up and down the Solent. It's a mystery to me.

O Docker said...

I don't claim to really understand it either, but I think of it this way.

Slack water doesn't coincide with high tide in many places. It's more likely to in a bay open only to the sea, but a river's flow complicates things.

If there were no tides, a river's height would be stable with a current flowing (over a small time interval). But counter that with an incoming tidal flow until the current is slack and you can see that, at that point, the height must continue to rise (water is converging on the spot of slack tide from both downstream and upstream). Eventually, the incoming tidal flow subsides, the river's flow again exceeds the tidal current, and the height of tide begins to drop.

SF Bay is affected in a similar way. Besides being filled by the ocean's tides, it's also the mouth of a huge river system that drains northern California's central valley.

ChrisP said...

I'm always hesitant to quote Wikipedia on a subject on which I know nothing, but this seems to be plausible, especially in view of what O Docker says:
"In some parts of the world, slack water does not coincide with high water or with low water. This occurs at many river mouths or where a large body of water opens to the ocean through a small outlet, such as The Rip between Point Nepean and Point Lonsdale, Victoria at the entrance to Port Philip Bay, Victoria, Australia or Gibraltar at the entrance to the Mediterranean Sea."
Chichester Harbour, the water round Hayling Island, is connected to the Channel through a very narrow mouth so that would explain the difference between slack water and HT.

Tillerman said...

Chris, we weren't racing in Chichester Harbour but a mile or two off South Hayling towards the Isle of Wight. I assume the tides there are complicated by the incoming tide coming both ways around the Isle of Wight, not to mention that the charts of tidal streams were all timed in reference to high water at Portsmouth which is several miles away and has tides itself influenced by water draining through a narrow harbor entrance.

The entrance to Chichester Harbour was on the route back to the yacht club and tides there were a factor in that we were warned it would be impossible to sail back in at certain states of the tide. One day, I think the day of the practice race, we had to sail in to the beach near the harbour entrance and walk with our boats several hundred yards in the shallow water against the outgoing tide.

ChrisP said...

Ah, I had assumed you would be inside the harbour. The currents in the channel are made very complex by the Isle of Wight sticking out into the tide race in the Channel. The Solent is much narrower at the western end, so the tide flows in faster than it flows out.

JP said...

On the Thames slack water isn't quite aligned with high/low tide but they're quite close in time.

However when that photo was taken the river was pretty much half way between high/low water and could best be described as in flood

However all around the Isle of Wight there are some very odd tidal affects, eddies, multiple highs waters (see Poole) and that's why there's a market for books like "winning tides"

I have a tidal related post I've been waiting to write but alas last 2 weeks have been crazy busy at work.