Bridge Clearance.....help!!!!!

[Daedelus]

I always thought that the air draft clearance under a bridge was measured from MHWS. (Which is also what Reeds says). However, I have now found an RYA reference to Highest Astronomical Tide to measure it.

Anyone got a definitive reference please? /forums/images/graemlins/confused.gif

 

[Vara]

Page three,chapter 8.PDF file;
http://www.reedsalmanac.co.uk/media/Supp0306.pdf

 

[bilbobaggins]

I refer to the Hydrographic Office Chart 5011, which I have always taken as the authoritative reference. My edition, in Cultural Features ID 20, describes 'Vertical clearance above Height Datum', and a note specifies that 'the safe overhead clearance above Height Datum, as defined by the responsible authority, is given in magenta where known; otherwise the physical vertical clearance is shown in black as in ID20'.

I interpret that as requiring me to inspect the Notes printed on the latest, corrected, largest scale official chart published. Then consulting the relevant harbourmaster for any local knowledge that the obstruction was lower than/different to that published.

Does it matter? Yes. One recent MAIB Digest describes the results of a sailing cruiser which attracted an explosive electric arcing to earth from overhead cables. The boat was destroyed, with injuries. I also gather that overhead cables, slung over French tidal rivers, are frequently lower than charted.

....and if it's critical, don't forget to consider the effects of barometric pressure deviation from Standard 1013.25HPa, and other meteorological and tidal effects.

The RYA will take no responsibility for anything they publish, which you or I misinterpret.

FWIW....

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[MarkGrubb]

I have a feeling that they had just redefined it at an international conference and clearance will now be given with respect to Hightest Astronomical Tide, not MHWS. But I can't remember were I read or heard it.

 

[RobBrown]

Yes it is now definitely from HAT (Highest Astronomical Tide)- I was answering some Coastal Skipper/Yachtmaster Course questions last term & used MHWS on basis of previous Day Skipper training and my figures were out. Even my tutor couldn't figure out why until we found a small note at front of the RYA course book highlighting the change. For Air Draft, I think it really makes sense as it adds that little extra safety margin!

 

[boatmike]

It is even more complicated I am afraid because on rivers, the more inland you go, the less important tide is and the more you have to take river height into account due to the amount of rain that has fallen etc. On French canals, especially Canal Du Midi, in a rainy period the water level can be higher than that which is assumed maximum and in drought conditions it can be very low. As the bridges are very low anyway this can concentrate the mind sometimes....

 

[Daedelus]

Re: Bridge Clearance.....thanks.

Colmce has got the definitive bit and I trust I won't get done for copyright if in the interests of safety I copy Reeds update to the 2006 almanac. Just to even things up a bit, if I'd only bought this year's almanac I would have known wouldn't I? I promise I will get one before the boat goes back in the water.

"The clearance height of a bridge or power lines is shown on
charts by the symbol ; see Chapter 1, Fig 1(1). It used to be
measured from the level of MHWS, but HAT (Highest
Astronomical Tide) is now replacing MHWS for this purpose
alone. (Heights of lights, beacons and terrain are still referred
to MHWS). Those Admiralty charts which feature overhead
objects are being changed to HAT when new editions are
published.
It is important to read the text block below the chart’s title. If
the chart is a new edition, under Heights it states:
‘Vertical clearance heights are above Highest
Astronomical Tide. All other heights are above MHWS’.
Older editions do not refer to HAT, in which case continue to
use MHWS. HAT is always a higher level than MHWS."


Thanks to all for helpful comments: this place truly is the home of all knowledge (well at least on things nautical).


(It was all so different before it changed).

 

[nigelhudson]

I've just compared my 2006 Reeds with the new 2007 and it has indeed changed! 2006 states MHWS and 2007 states HAT.

 

[VicS]

[ QUOTE ]
1013.25HPa

[/ QUOTE ]
Bilbo,

having gone to the trouble of quoting the standard barometric pressure to such accuracy you might like to tell us the correction to be applied to tidal heights, to a similar degree of accuracy, for differences from that figure.

The change from MHWS to HAT was either reported in the last RYA magazine or I read it on their website, I can't remember which. I think the effective implementation date is 1 feb.

 

[bilbobaggins]

Ho-kay! Putting on 'pedant' hat...

All of these measures are referred to a Datum. More often than not, such datums are defined precisely by quite august bodies such as the IMO and ICAO. The International Standard Atmosphere is defined - when last I looked - as having a normative datum of 1013.25HPa ( or Hpa, or summat - but HectoPascales - which are the same as millibars, but French, and therefore OK in the EU )

As far as I remember, when traversing the shallows within the Scilly Isles and sneaking all the way up the Beaulieu River to the pool right at the top, one should give some thought to the effects of 'highish' or 'lowish' barometric pressure. Then think about the effects of the recent winds on the timing and heights of the tides....

When push comes to shove, I allow -1cm depression of the sea surface per 1 millibar of corrected atmosheric pressure above the Datum previously given. That's usually a High, and the winds are usually light - and of little consequence.

If the pressure is low, then that usually signifies an active depression. One ought to allow +1cm per millibar of corrected atmospheric pressure above the datum, but I don't bother. I'm far more concerned with the effects of the strong wind on the times and heights of tide, and whether we should be sailing there at all.....

My good friend and mentor Bob Evans RIP argued this with me in an agreeable, clarety way on several trips to, in and around the Scilly Isles. We didn't hold divergent views - we just enjoyed the banter. And we didn't run aground - we 'ran ashore'.

Does that help? Or does it obscure the picture...?

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[AliM]

Agree with all that, but if you look at the actual tidal heights over the last week, you start to worry about the effects of wind, as Bilbo says.

I'll give you an example:
Look at
tidal heights
at, say, Sheerness for the last month. The actual heights are within about 20 cm of the predicted ones for most of that time, when the pressure was generally something like -10mb to + 20mb on average.
Look at 11/12 January, though. The HW height is a good meter higher and the LW about 1.5 m lower than predicted. I was keeping a vague eye on the weather at the time. The pressure was low, but nothing like what it would have to be using bilbo's maths, but the wind was very strong.

I was amazed that the wind can make a difference of 1m or so.

Ali (another pedant)

 

[Lizzie_B]

Like this you mean.

Annapolis after a night of wind blowing down the harbour mouth.

Normal tidal range about 18 inches
This is normal high water springs.

One way to get to work I s'pose

 

[bilbobaggins]

I suppose that's a good example. Sheerness, being at the inner end of the Bristol Channel, is much affected by the 'funneling' effect of a strong westerly wind blowing for many hours - the water gets piled up at the end, around Sheerness. Similarly, heavy rainfall across the Severn's catchment area will also increase the volume of water that is swirling around and stacking up. The tidal stream rates are also affected - expect considerably stronger and rougher around headlands, such as Brean Down and Mumbles, on the ebb.

Such conditions bring the time of actual HW forward a bit, as well increasing as the height. I'm not able to calculate, or give a 'rule-of-thumb' for, such weather-related changes. Your best bet is your local qualified river pilot and/or harbourmaster, who will base his info on local experience.

Around the ports of the southern North Sea - e.g. Essex, Kent - one can get an amplified and destructive 'storm surge' when there is a deep low passing to the north of that area. That is more a consequence of 'hydrographic resonance' , as well as the cumulative effects of wind piling up the water in estuaries, but for a detailed explanation, you need to trap a recently-qualified hydrographer against the bar on a wet Friday night......

Again, if you want to know 'how much', ask your tame harbourmaster - or the guys running the Thames Barrage!



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