Chichester tidal curve, Imray app v Harbour guide curves

[FairweatherDave]

I wrote this on the West Wittering thread (re East Head and Chichester harbour)

"Also the tidal curve is not symetrical which means you can end up with a long wait eg you dry out two hours before low water but don't float until four hours after"

Pete, prv wrote "Good reason to use an app like the Imray Tides one, that shows you the curve so you can go off actual depths rather than just times before and after high or low water. "

This prompted me to download the Imray app, (the free bit that gives you the on the day you look info) .......indeed it is a beautiful app, thanks Pete. My normal source for the Chi tidal curve is in the Chichester harbour news and guide (p75 in the 2019 guide). I would say the Imray curve for Chichester harbour entrance shows a much more symetrical shape than the possibilities from the harbour guide. In other words I'm not sure the Imray app would agree with my approximate rule of thumb based on the harbour guide curve. It doesn't seem to show the "young flood stand" as I believe it is called.

 

[prv]

This prompted me to download the Imray app, (the free bit that gives you the on the day you look info) .......indeed it is a beautiful app, thanks Pete. My normal source for the Chi tidal curve is in the Chichester harbour news and guide (p75 in the 2019 guide). I would say the Imray curve for Chichester harbour entrance shows a much more symetrical shape than the possibilities from the harbour guide. In other words I'm not sure the Imray app would agree with my approximate rule of thumb based on the harbour guide curve. It doesn't seem to show the "young flood stand" as I believe it is called.

Thanks, that's an interesting comparison. I believe the Imray app uses Admiralty data (at least, you have to pay for a "UKHO License" to see more than five days ahead) so it'll be as accurate or inaccurate as that. I know the Conservancy commission their own surveys of the bar and entrance channel; apparently they have their own improved local tidal data as well.

(And I certainly wouldn't call the Tides Planner app 'beautiful' . It's a bit ugly and rather fiddly in places, but very functional.)

Pete

 

[lw395]

The curve for the Entrance is only valid for ...... the Entrance.

Trying to apply it in any detail to any other place is of limited value at best.

If you look at sealevelnetworks or whatever it's called, the live tide gauges show reality often diverges from the prediction anyway.

My own observation is that current can still be coming out of the entrance a significant time after predicted LW. I know there is a certain amount of flow through Langstone, across the top of Billy Island, but I'm not sure that's the whole story either.

The big picture of depth in Chi Harbour, there's basically not enough of it, in most places, most of the time....

 

[prv]

The curve for the Entrance is only valid for ...... the Entrance.

True - though I looked at Itchenor in the app and that didn't show Dave's "young flood stand" either.

Pete

 

[lw395]

True - though I looked at Itchenor in the app and that didn't show Dave's "young flood stand" either.

Pete

Sounds like an 'Anchor Bleu' phenomenon?

 

[Sticky Fingers]

I keep my boat at Chi Marina and have run aground in the Harbour at least once, ahem so I'm interested in this.

The "stand" is sort-of visible as a short period of near-nil flow, in the Navionics flood / ebb gauge. Try the Thorney Channel one for example. But none of my tidal Apps show it.

 

[FairweatherDave]

I'm sorry I can't easily post a picture of the harbour guide curve. It might help.
However I also agree about locations relative to the information supplied, I use the guide curve as a clue and enjoy guessing when we might float. You can either call it an approximate science or get deeply scientific factoring in barometric pressure and wind effects etc. I go for the approximate science and enjoy having a bilge keeler safe in the knowledge it should all be okay whatever.
As for the app, well as a non-app sort of person I was impressed. Looking at the App extensions, specifically the Hydrographic license, (£5.99),is that a one off payment and then you just pay annually £3.99 to be able to look as far ahead as you like within that year?

 

[LadyInBed]

I like the BBC Tidal Curve - You get a weeks worth.
https://www.bbc.co.uk/weather/coast_and_sea/tide_tables/8/68#tide-details

 

[prv]

The "stand" is sort-of visible as a short period of near-nil flow, in the Navionics flood / ebb gauge. Try the Thorney Channel one for example. But none of my tidal Apps show it.

The streams atlas in the Imray app does show "slack" in Chichester at 4 hours before HW Portsmouth, with an incoming flow for the hours before and after. I guess that corresponds to the "young flood".

Nothing beyond a very slight kink in the height curve, though, not like the ledge you get part-way up the Southampton one.

Pete

 

[FairweatherDave]

https://www.conservancy.co.uk/page/Weather-and-tides
In case anyone is interested
There is some smart way of converting that to an image but I haven't mastered it yet.....
It may not be a proper stand but if you dry out 2and a bit hours before low you can be in for a long wait, which was the point I was getting accross in the original thread.
But I do like the Imray app, purely for those times when I mess up and don't have the right information to hand in paper form and no access to my computer.

 

[LadyInBed]

 

[lw395]

I would not put too much faith in the fine detail of that graph.
WTF is 'Low water Chichester' anyway?
Chichester is inland, a mile up the canal.
The fall from 3.6 to 2.4m in 15 or 20 minutes at springs seems a little at odds with what we see?

http://www.chimet.co.uk/(S(owzjq545m15oxr45ngjct5yc))/sea.aspx
http://www.cambermet.co.uk/(S(mzye5s550lpqml45ovbgzam2))/sea.aspx

The actual observations seem a far more credible shape, unsurprisingly.
Both at the Bar beacon and mid-harbour.

The published graph could be caused by the averaging process, if the time relative to HW when the level drops through 3.0m varies. There's a classic statistics concept that the 'mean' or 'average' doesn't necessarily exist.

 

[wombat88]

Interesting thread, explains why one always ends up waiting to float at East Head.

Regarding 'stand' or slack water, at the top of the harbour (Fishbourne) I've watched the tide and there appears to be no 'stand' at all. One moment it is coming in and the next it is going out leaving very little time for a beer in the pub.

At Dell Quay it is about two beers worth.

Is it true to say that the further up a creek one gets the less of a stand?

 

[lpdsn]

My own observation is that current can still be coming out of the entrance a significant time after predicted LW. I know there is a certain amount of flow through Langstone, across the top of Billy Island, but I'm not sure that's the whole story either.

I'd expect that. The classic example is Strangford where the flow continues coming out for nearly two hours after LW. Chichester will have less to drain but it'll still flow out for a while until the levels equalise with the rising tide outside.

 

[Roberto]

Is it true to say that the further up a creek one gets the less of a stand?

It cannot be generalised, tide and current behaviour in inlets can oscillate between two extremes, mainly stationary and mainly progressive waves, with all intermediate compositions, google "progressive stationary tide wave" gives a number of explanations and examples.