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Old 30-06-2020, 09:19   #1
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How to figure Mean High Water on chart?

Going crazy trying to figure out MHW for going under bridges.

The chart shows depth at mean low water, and vertical clearance of bridges at mean high water, but what is the MHW level?

For big, high bridges this is not an issue, but recently we nervously went through a pass with bridge clearance of 75' at MHW with our 65' high boat. It worked out, but I would prefer a more precise method of figuring vertical clearance.

We bumped into a USCG crew and asked them, but they didn't know. Anyone here smarter than the Coast Guard?
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Old 30-06-2020, 09:29   #2
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Re: How to figure Mean High Water on chart?

It should be in your tide book/program for your location, but, if not and you have internet access, for the US you can go to the NOAA datum website and find all the information for the nearest tide station.
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Old 30-06-2020, 10:09   #3
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Re: How to figure Mean High Water on chart?

Tide table show the levels of low and high tide, but not MHW. I haven't see that page on the NOAA website and it looks like exactly what's needed. Why it's such a mysterious secret I have no idea. Thanks!

In the attached screenshot for Brownsville, which is relatively close to Agate Pass and the 75' bridge in question, arrow #2 shows 10.96 feet at MHW, and arrow #1 says the figures are relative to MLW, which is what's on the chart.

So, if I add 10.96 to the 24 feet on the chart I get 35 feet of depth total at MHW.

So I can expect 75 feet of clearance at MHW of 35 feet, and if my depth sounder were to say 45 feet (possible if the depth data is old and the area is subject to shoaling) it would be a no-go.

Does my reasoning and math seem correct?
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Old 30-06-2020, 10:43   #4
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Re: How to figure Mean High Water on chart?

You're on track, but you are depending on elements with very few significant units to make a calculation with bunches of significant digits. Your depth finder isn't that well calibrated, the bottom is not likely to be correct to the nearest foot, have you actually measured your mast height as loaded, and so on.

My experience has been that there's a bit more air under bridges than they admit, except in the case of that notorious example in Lake Okeechobee.
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Old 30-06-2020, 10:44   #5
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Re: How to figure Mean High Water on chart?

In most cases, the listed bridge height is to the absolute lowest point anywhere over the channel. Depending on the bridge design, that may mean that a large portion of the channel has more clearance. One local bridge mentions on a sign that the center point of the span is 6 feet higher than listed clearance.
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Old 30-06-2020, 10:56   #6
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pirate Re: How to figure Mean High Water on chart?

Mean High Water (MHW) is a Tidal Datum representing the average of all the daily tidal high water heights observed over a period of several years. In the United States this period spans 19 years and is referred to as the National Tidal Datum Epoch
So SV Grace I would take it that the average depth under that bridge is around 10.96 at HW.
The second mark is the average depth at MLW.
But I am not that bright so I could be wrong..
So you have 75ft + 10.96 to play with.. If your draft is 6ft and your mast 75ft go under when its down to 8ft.. at least 3hrs after HW roughly.. depending on if the range is big enough between HW and LW.
This will vary at Springs with more or less than the average and also prevailing local conditions where wind can affect tidal flow such as Pamlico Sound etc.
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Old 30-06-2020, 11:14   #7
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Re: How to figure Mean High Water on chart?

I don't think that anyone with a mast height within 5 feet of a listed bridge height goes under an unfamiliar bridge without some degree of anxiety...
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Old 30-06-2020, 11:19   #8
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Re: How to figure Mean High Water on chart?

You can use air gap information if available, see e.g. https://co-ops.nos.noaa.gov/stationh...4&units=metric (scroll down)

Or, here's the old-fashioned way:
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Old 30-06-2020, 11:19   #9
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pirate Re: How to figure Mean High Water on chart?

Quote:
Originally Posted by tkeithlu View Post
I don't think that anyone with a mast height within 5 feet of a listed bridge height goes under an unfamiliar bridge without some degree of anxiety...
Do what climbers advise one never to do..
Look down Not up..
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Old 30-06-2020, 12:07   #10
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Re: How to figure Mean High Water on chart?

Quote:
Originally Posted by SV__Grace View Post
In the attached screenshot for Brownsville, which is relatively close to Agate Pass and the 75' bridge in question, arrow #2 shows 10.96 feet at MHW, and arrow #1 says the figures are relative to MLW, which is what's on the chart.
Relative to MLLW, to be correct.

Quote:
Originally Posted by SV__Grace View Post
So, if I add 10.96 to the 24 feet on the chart I get 35 feet of depth total at MHW.

So I can expect 75 feet of clearance at MHW of 35 feet, and if my depth sounder were to say 45 feet (possible if the depth data is old and the area is subject to shoaling) it would be a no-go.
Shoaling would of course reduce the number your depth sounder displays. Thus, if there had been 10 feet of shoaling since the survey, and your sounder read 35 feet, it could similarly be a no-go. Fortunately it looks like it was surveyed around 2010 with a good quality scan; allowing for a few extra feet should be sufficient.

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I would take it that the average depth under that bridge is around 10.96 at HW
I think I get what you're getting at, but using depth here is just confusing!
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Old 30-06-2020, 12:28   #11
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pirate Re: How to figure Mean High Water on chart?

Quote:
Originally Posted by requiem View Post
I think I get what you're getting at, but using depth here is just confusing!
I'm a Brit.. It comes naturally..
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Old 30-06-2020, 12:45   #12
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Re: How to figure Mean High Water on chart?

Quote:
Originally Posted by rslifkin View Post
In most cases, the listed bridge height is to the absolute lowest point anywhere over the channel.
Sigh... no, that is a potentially dangerous bit of misinformation, but I suppose it depends upon the chart or reference being used or the area of the world you are in.

Most of us in the U.S. use charts created and updated by NOAA and the digital charts like Navionics (which I use) use the NOAA data.

According to NOAA, as stated on Page 1 in the Coast Pilot -

Vertical clearances of bridges and overhead cables
are in feet above mean high water unless otherwise stated
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Old 30-06-2020, 12:57   #13
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Re: How to figure Mean High Water on chart?

Quote:
Originally Posted by requiem View Post
You can use air gap information if available, see e.g. https://co-ops.nos.noaa.gov/stationh...4&units=metric (scroll down)

Or, here's the old-fashioned way:
Air Gap info is fantastic but not available everywhere, darn.

Most of us don't use paper charts anymore and the electronic charts don't have that info- double darn!
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Old 30-06-2020, 13:03   #14
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pirate Re: How to figure Mean High Water on chart?

Quote:
Originally Posted by boatman61 View Post
I'm a Brit.. It comes naturally..
What I am saying is it matters not what the depth is you have 75ft air draught at MHW of 10.96.
If you need more, go through before or after HW but your air draught will depend on the tidal range not how deep the channel is.. unless the chart states your boats draught will run you aground at LW.
If the difference is 9ft between H and L thats all you have to work with.. 75ft to 84ft. Simple.
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Old 30-06-2020, 13:31   #15
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Re: How to figure Mean High Water on chart?

Doing the math doesn’t seem to be the problem for the OP, finding the relationship between MLLW and MHHW is. It’s a problem I’ve consistently had too.

Apparently SanFrancisco Bay Area charts have the desired data on them. I haven’t found the data on any other charts and I have looked. I’ll look again.
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