I've been trying unsuccessfully to work out what the absolute max clearance would be under the Hawkesbury Rail Bridge at the lowest tide i.e. willy weather tide chart showing 0.1m tides.willyweather.com.au/nsw/sydney/hawkesbury-river--railway-bridge.html

The clearance of the bridge is 11.8m MHWS from what I can find online. Would any of our resident experts know how to calculate what the clearance would be when the tide chart is showing 0.1m?

Thanks

Mean height so find max springs High tide and low Springs high tide then find the middle. then you'll have the figure to work from. So if high tide at springs is 1.9, the next high is 1.6, the mean is 1.75 It's what I've been doing for years but if I'm wrong I'm happy to be corrected.

MHWS is the average highest tides over a period of time ( years ) what you need to know is what that tide is at the bridge, then calculate the difference between the MHWS and the tide you are going to use and add that to the clearance. You will also have to take into account tide time differences being upstream and any fresh in the river

I don't know why HAT isn't used instead.

Thanks for the responses.

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woko said..
what you need to know is what that tide is at the bridge

This is one of the pieces of the puzzle I was trying to find but no luck. Any idea where you would get this data from? The other option is I just go there with a laser distance measurer

It is not an obvious calculation, but it is not difficult if you know what you're looking for. I've done it many times in my profession, because sea levels affect flooding.

You need to consider the semi-diurnal tidal planes, which are charts relating the various sea-levels due to astronomical tides. You also need to know how the various planes relate to each other. For example, Chart Datum (CD) is usually Lowest astronomical tide (LAT), which is below Australian Height Datum (AHD). Bridge clearances are usually expressed in terms of a high tide, e.g. MHWS. So, the charts usually tell you the least clearance from submerged rocks (LAT) and the least clearance to overhead hazards (MHWS). The actual vertical dimension between a submerged rock and an overhead bridge will be greater than the difference of the two numbers, because they are measured from two different datums*.

The weather will also affect sea levels, which is why these are called astronomical tides.

Page 67 of the OEH document in the following link provides a detailed analysis of semi-diurnal tidal planes of the Hawkesbury River at Patonga relative to Australian Height Datum (AHD). AHD approximates to, but is not equal to mean sea level (MSL). In this case Mean Sea Level (MSL) is 0.047m above AHD.
s3-ap-southeast-2.amazonaws.com/www-data.manly.hydraulics.works/www/publications/oeh/tidalplanes/mhl2053%20OEH%20tidal%20planes%20analysis%20final%20report.pdf

I find it useful to tabulate the planes to AHD and LAT, but you're interested in what 11.8 MHWS means at low tide.

Using the averages in the chart, MHWS is 1.02m above AHD.

So, the underside of the bridge is 11.8 + 1.02 = 12.82 m above AHD

The problem with the Willy Weather data is that it does not seem to tell you what datum it refers to. I'm going to assume it is CD/LAT, but it is something you'll need to check. Also, the OEH data does not provide an LAT number, so I'll have to assume the LAT is the ISLW number, which is 0.83m below AHD. (The tables equate LAT to ISLW at Norfolk Island, don't ask me what ISLW means because I'd have to guess)

So, the underside of the bridge is 12.82 + 0.83 m = 13.65m above Chart Datum/LAT/ISLW?

At a low tide of 0.1m above Chart Datum, the clearance to the underside of the bridge is 13.65 - 0.1 = 13.55m

(*The plural of datum is actually data, but I'm being a pedant)

Very interesting indeed, you would assume that information such as MHWS etc would be readily available, like inside the cover of every annual tide chart.

This is getting on a bit but given it's a 19 year average we could be due for an update.

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woko said..
Very interesting indeed, you would assume that information such as MHWS etc would be readily available, like inside the cover of every annual tide chart.

This is getting on a bit but given it's a 19 year average we could be due for an update.

That highest tide in Sydney [may 74] would have to relate to that huge east coast low storm we had that year?

Yes must of been some phenomena. I find it difficult to fathom why information like MHWS is so hidden I found that in the same place that Achernar gave a link to, I just went there a different way, by the way that was an excellent deduction Achernar. And the much simpler calculation of adding the tide difference to the clearance gives a result a few centimetres different. But why the secrecy ? Or is it just an old school term being forgotten and to be replaced by a more digitally savvy term like above datum then we just need to subtract the tide and we have our clearance.

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woko said..
Yes must of been some phenomena. I find it difficult to fathom why information like MHWS is so hidden I found that in the same place that Achernar gave a link to, I just went there a different way, by the way that was an excellent deduction Achernar. And the much simpler calculation of adding the tide difference to the clearance gives a result a few centimetres different. But why the secrecy ? Or is it just an old school term being forgotten and to be replaced by a more digitally savvy term like above datum then we just need to subtract the tide and we have our clearance.

Woko, thanks for your comments.

I don't think it is a matter of secrecy. Its probably a legacy thing - there are land surveyors (who work on land) and hydrographers (who work on water) and never (or rarely) the twain shall meet. Its only when someone like me comes along and wants to know how a tide level will affect a river level that we need to put the numbers together, which can only be done by establishing the different datums.

But, it also needs some knowledge. You've got to know the difference between an astronomical tide and weather effects. The May 74 "tide" in Sydney that nswsailer refers to might actually have been heavily influenced by river flooding, storm surge, inverse barometric effects, wave runup, which are independent of astronomical tide levels. For example, tidal ranges on the east coast of Australia are generally in the 2 to 3m range, but a storm surge can add 8m to that, so its not an insignificant factor. Add to that the question of climate change and you soon need to learn a specialist language and start to think in terms of probabilities rather than events. Its not impossible to do, but the internet generally likes to dumb it down, so it does not like to give you a "straightforward" answer.

Wow - Thanks for the amazing response Achernar :)