On Sun, 5 Nov 2023 16:54:16 +0000, Roland Perry <roland@perry.uk> wrote:

>In message <ui155l$2difu$2@dont-email.me>, at 21:43:17 on Thu, 2 Nov
>2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:

>The official headroom under the railway bridge just north of the station
>is 3.06m, and it's a truss design (no pork markets involved) where the
>tracks are not very far above the lowest bit of the truss. [Roughly
>speaking visualise two parallel RSJs with the sleepers on the bottom
>flange]
>
>https://www.geograph.org.uk/photo/5765703
>https://www.geograph.org.uk/photo/1622566
>
>Like the nearby underpass, I have surveyed both in great detail.
>
>The river at Ely is usually around 0.55m - this morning it's 0.44m
>because they are letting lots of water out at Denver, in order to be
>able to accommodate runoff from the recent heavy rains. The highest I've
>seen it myself in the last ten years is 0.65m, at which point there's a
>handful of boats which can't make it under the slightly downstream truss
>bridge (headroom 3.1m). {Those boats - Dutch barges mainly - would not
>expect to be able to reliably get under the upstream railway bridge}

Bear in mind that the river level at Ely is measured Above Stage Datum
(ASD), not Above Ordnance Datum (AOD). To get the height above Average Mean
Sea Level (AMSL) you need to add the AOD value for the Ely gauge, which,
conveniently, is 1m. So a typical level of 0.55m ASD makes 1.55m AOD, which,
when added to the 3.06m headroom on the bridge, takes you to 4.61m AOD.

Given that Google Earth's elevation data seems to be rounded down to the
nearest integer, that's consistent with it reporting that the track at the
station is 4m above sea level. But the elevation data isn't particularly
granular anyway, it doesn't report a difference in elevation between the
river itself and the land either side of it when in reality it's a couple of
metres in most places. So that 4m may not be entirely reliable either.

Mark

Roland Perry <roland@perry.uk> wrote:
> In message <ui155j$2difu$1@dont-email.me>, at 21:43:15 on Thu, 2 Nov
> 2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:
>> Roland Perry <roland@perry.uk> wrote:
>>> In message <uhuca1$1p7ld$2@dont-email.me>, at 20:26:41 on Wed, 1 Nov
>>> 2023, Coffee <martin.coffee@round-midnight.org.uk> remarked:
>>>
>>>>>>>> Meanwhile, my new SUV can wade through <drumroll> 50cm, so that's OK.
>>>>>>>
>>>>>>> I hope it can handle waves and tides!
>>>>>>
>>>>>> I'm not sure how many of either are going to be presenting themselves 40
>>>>>> miles inland.
>>>
>>>>> Do you think you’ll still be 40 miles inland?
>>>>
>>>> Depending on where he lives Roland might find his plot will be under
>>>> the North Sea. So will Ely railway station. Rail Replacement Boats
>>>> anyone?
>>>
>>> My house is significantly higher than Ely Railway station, hence my
>>> numerous references to the risk to the Fen Line which are clearly
>>> falling on deaf ears.
>>
>> What makes you think your references to the Fen Line are being ignored?
>
> Some scepticism being expressed about exactly how low bits of it are.

OK, not from me though - I’m aware how the line(s) leapfrog over the water
courses and how some of the water courses are raised above the land.

Sam

--
The entity formerly known as Sam.Wilson@ed.ac.uk
Spit the dummy to reply

On 05/11/2023 16:54, Roland Perry wrote:
> In message <ui155l$2difu$2@dont-email.me>, at 21:43:17 on Thu, 2 Nov
> 2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:
>> Roland Perry <roland@perry.uk> wrote:
>>> In message <uhuhtp$1q8ac$1@dont-email.me>, at 22:02:33 on Wed, 1 Nov
>>> 2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:
>>>
>>>>> Depending on where he lives Roland might find his plot will be
>>>>> under the
>>>>> North Sea.  So will Ely railway station.  Rail Replacement Boats
>>>>> anyone?
>>>>
>>>> I know someone who lives not far from Roland and I think his house is
>>>> techically at 0’ AMSL.  Its definitely below the level of the
>>>> rivers.  I
>>>> think Roland lives on the Isle of Ely, though, so he’ll probably be OK.
>>>
>>> According to Google Earth my land is 8m ASL. The tracks at Ely Station
>>> are 4m ASL.
>>
>> I’m surprised it’s only that.  The tracks cross the river immediately to
>> the north of the station and, on the eastern line, pretty close the
>> station
>> on the south.  In both cases there is clearance for navigation and I
>> think
>> that river is one that is above sea level.
>
> The official headroom under the railway bridge just north of the station
> is 3.06m, and it's a truss design (no pork markets involved) where the
> tracks are not very far above the lowest bit of the truss. [Roughly
> speaking visualise two parallel RSJs with the sleepers on the bottom
> flange]
>
> https://www.geograph.org.uk/photo/5765703
> https://www.geograph.org.uk/photo/1622566
>
> Like the nearby underpass, I have surveyed both in great detail.
>
> The river at Ely is usually around 0.55m - this morning it's 0.44m
> because they are letting lots of water out at Denver, in order to be
> able to accommodate runoff from the recent heavy rains. The highest I've
> seen it myself in the last ten years is 0.65m, at which point there's a
> handful of boats which can't make it under the slightly downstream truss
> bridge (headroom 3.1m). {Those boats - Dutch barges mainly - would not
> expect to be able to reliably get under the upstream railway bridge}
>
> https://www.geograph.org.uk/photo/4879196
>
> Remember also that "Sea Level" is the average, and the tidal range at
> Kings Lynn is typically 8m, so you can only actually let water out under
> gravity for a few hours of the day - at low tide. Most of the time the
> barrier at Denver is preventing the tide from flooding upstream, which
> is of course why they built it.

Roland

Are you sure about the range of 8m? The next full moon is 27/28 November
(i.e. spring tide) has a max height on 28th 6.3m and a LW of 0.4m (on
27th) giving a range of 5.9m. I appreciate that some spring tides are
higher (but cannot find an on-line resource) and atmospheric effects can
raise or lower tides, I would not agree with your comment of "Typically" 8m.
https://www.tide-forecast.com/tide/Kings-Lynn/tide-times

--
Colin

In message <37nhkip7jgkqiscvdigm11qu2lrq557f6i@4ax.com>, at 12:48:10 on
Mon, 6 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
remarked:
>On Sun, 5 Nov 2023 16:54:16 +0000, Roland Perry <roland@perry.uk> wrote:
>
>>In message <ui155l$2difu$2@dont-email.me>, at 21:43:17 on Thu, 2 Nov
>>2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:
>
>>The official headroom under the railway bridge just north of the station
>>is 3.06m, and it's a truss design (no pork markets involved) where the
>>tracks are not very far above the lowest bit of the truss. [Roughly
>>speaking visualise two parallel RSJs with the sleepers on the bottom
>>flange]
>>
>>https://www.geograph.org.uk/photo/5765703
>>https://www.geograph.org.uk/photo/1622566
>>
>>Like the nearby underpass, I have surveyed both in great detail.
>>
>>The river at Ely is usually around 0.55m - this morning it's 0.44m
>>because they are letting lots of water out at Denver, in order to be
>>able to accommodate runoff from the recent heavy rains. The highest I've
>>seen it myself in the last ten years is 0.65m, at which point there's a
>>handful of boats which can't make it under the slightly downstream truss
>>bridge (headroom 3.1m). {Those boats - Dutch barges mainly - would not
>>expect to be able to reliably get under the upstream railway bridge}
>
>Bear in mind that the river level at Ely is measured Above Stage Datum
>(ASD), not Above Ordnance Datum (AOD).

I forgot that (but we are in danger of losing the plot, which is the
railway line being ~4m above AOD, not near or below it).

Your site, which is the one I usually look at, rather glosses over the
Datum thing, whereas the EA's site has it a click away from the
headline, and explains what it really means.

It's the AOD height of the river bed, thus the number we see is the
depth of the water, not the height of the surface. Looking at other
measuring stations, they appear to round it to the nearest 10cm.

>To get the height above Average Mean Sea Level (AMSL) you need to add
>the AOD value for the Ely gauge, which, conveniently, is 1m.

Or 1.0m, rather than 0.9m or 1.1m

>So a typical level of 0.55m ASD makes 1.55m AOD, which,
>when added to the 3.06m headroom on the bridge, takes you to 4.61m AOD.

Hurrah! That's within scope. Although I have yet to find a source which
says what river level (10%|50%|90%) that rather precise 3.06 is
calibrated from. Next time I meet an appropriate person I'll ask them!

>Given that Google Earth's elevation data seems to be rounded down to the
>nearest integer, that's consistent with it reporting that the track at the
>station is 4m above sea level.

No, Google Earth data I looked at yesterday wasn't as severely rounded
as that. Maybe I'm using a different version of the app? For example one
reading I got just now was 8.53m

>But the elevation data isn't particularly
>granular anyway, it doesn't report a difference in elevation between the
>river itself and the land either side of it when in reality it's a couple of
>metres in most places. So that 4m may not be entirely reliable either.

Again, I don't see that. A few feet away from that 8.53m I get 8.25m

>>According to Google Earth my land is 8m ASL.

Those are both on different bits of my land, and I've now remembered
that the 25ft OS contour runs through a different bit of it (I was
considering putting a meridian-style copper band on the ground), and
=7.62m so it all seems very consistent.

I chose Google Earth as the authority because people will have heard of
it, and could find it and verify results for themselves. Earlier today I
looked at a much more obscure topological site (but with much better
mapping overlays), which said my land was "9m", which I presume is
rounded *up* to the next integer.

One can use such topological mapping to look at things like the wider
circumstances of the Fen Line, and plenty of it's lower than the bit at
Ely station.

Meanwhile the station forecourt (which is much the same level as the
platform) is reported as 6m, which is consistent with the tracks being
at around 4.6m

ps A recently launched local tourist app has scraped various
descriptions of 'places of interest' and reports that Ely Station is
70 miles 30 chains from Liverpool St* I seriously doubt more than 1%
of the public will find that precision helpful. On the other hand, I
have a vintage surveyors' chain in my shed! And confess my back
garden is almost exactly one chain. (A Test Match Cricket Pitch
length or 0.40 of an Olympic Swimming Pool).

* Because historically the Fen Line is a branch of the Liverpool St to
Norwich line, not a section of Kings Cross to Kings Lynn.
--
Roland Perry

In message <uib97q$igvo$1@dont-email.me>, at 17:54:02 on Mon, 6 Nov
2023, ColinR <rail@greystane.shetland.co.uk> remarked:
>On 05/11/2023 16:54, Roland Perry wrote:
>> In message <ui155l$2difu$2@dont-email.me>, at 21:43:17 on Thu, 2 Nov
>>2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:
>>> Roland Perry <roland@perry.uk> wrote:
>>>> In message <uhuhtp$1q8ac$1@dont-email.me>, at 22:02:33 on Wed, 1 Nov
>>>> 2023, Sam Wilson <ukr@dummy.wislons.fastmail.co.uk> remarked:
>>>>
>>>>>> Depending on where he lives Roland might find his plot will be
>>>>>>under the
>>>>>> North Sea.  So will Ely railway station.  Rail Replacement
>>>>>>Boats anyone?
>>>>>
>>>>> I know someone who lives not far from Roland and I think his house is
>>>>> techically at 0’ AMSL.  Its definitely below the level of the
>>>>>rivers.  I
>>>>> think Roland lives on the Isle of Ely, though, so he’ll probably be OK.
>>>>
>>>> According to Google Earth my land is 8m ASL. The tracks at Ely Station
>>>> are 4m ASL.
>>>
>>> I’m surprised it’s only that.  The tracks cross the river
>>>immediately to the north of the station and, on the eastern line,
>>>pretty close the station on the south.  In both cases there is
>>>clearance for navigation and I think that river is one that is
>>>above sea level.

>> The official headroom under the railway bridge just north of the
>>station is 3.06m, and it's a truss design (no pork markets involved)
>>where the tracks are not very far above the lowest bit of the truss.

>>[Roughly speaking visualise two parallel RSJs with the sleepers on
>>the bottom flange]

>> https://www.geograph.org.uk/photo/5765703
>> https://www.geograph.org.uk/photo/1622566

>> Like the nearby underpass, I have surveyed both in great detail.
>> The river at Ely is usually around 0.55m - this morning it's 0.44m
>>because they are letting lots of water out at Denver, in order to be
>>able to accommodate runoff from the recent heavy rains. The highest
>>I've seen it myself in the last ten years is 0.65m, at which point
>>there's a handful of boats which can't make it under the slightly
>>downstream truss bridge (headroom 3.1m). {Those boats - Dutch barges
>>mainly - would not expect to be able to reliably get under the
>>upstream railway bridge}

>> https://www.geograph.org.uk/photo/4879196

>> Remember also that "Sea Level" is the average, and the tidal range
>>at Kings Lynn is typically 8m, so you can only actually let water out
>>under gravity for a few hours of the day - at low tide. Most of the
>>time the barrier at Denver is preventing the tide from flooding
>>upstream, which is of course why they built it.
>
>Roland
>
>Are you sure about the range of 8m? The next full moon is 27/28
>November (i.e. spring tide) has a max height on 28th 6.3m and a LW of
>0.4m (on 27th) giving a range of 5.9m. I appreciate that some spring
>tides are higher (but cannot find an on-line resource) and atmospheric
>effects can raise or lower tides, I would not agree with your comment
>of "Typically" 8m.

>https://www.tide-forecast.com/tide/Kings-Lynn/tide-times

I'm happy to review the data (using MarkG's site). And it seems whatever
tool I was using is reporting the very lowest displayed to very highest,
both of which could be anomalies. So yes, the normal fluctuation is 4m.
--
Roland Perry

On Fri, 10 Nov 2023 13:27:55 +0000, Roland Perry <roland@perry.uk> wrote:

>In message <37nhkip7jgkqiscvdigm11qu2lrq557f6i@4ax.com>, at 12:48:10 on
>Mon, 6 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
>remarked:

>>Bear in mind that the river level at Ely is measured Above Stage Datum
>>(ASD), not Above Ordnance Datum (AOD).
>
>I forgot that (but we are in danger of losing the plot, which is the
>railway line being ~4m above AOD, not near or below it).
>
>Your site, which is the one I usually look at, rather glosses over the
>Datum thing, whereas the EA's site has it a click away from the
>headline, and explains what it really means.

Well, on my site it's a short scroll away from the headline. I'm not sure
that's objectively worse.

>It's the AOD height of the river bed, thus the number we see is the
>depth of the water, not the height of the surface. Looking at other
>measuring stations, they appear to round it to the nearest 10cm.

Bear in mind also that the river bed isn't flat. The stage datum is an
approximation, based on an average depth.

Mark

In message <5ldtkilg9kfm8cuom98m4qh2fbsnlgjlmt@4ax.com>, at 23:06:24 on
Fri, 10 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
remarked:
>On Fri, 10 Nov 2023 13:27:55 +0000, Roland Perry <roland@perry.uk> wrote:
>
>>In message <37nhkip7jgkqiscvdigm11qu2lrq557f6i@4ax.com>, at 12:48:10 on
>>Mon, 6 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
>>remarked:
>
>>>Bear in mind that the river level at Ely is measured Above Stage Datum
>>>(ASD), not Above Ordnance Datum (AOD).
>>
>>I forgot that (but we are in danger of losing the plot, which is the
>>railway line being ~4m above AOD, not near or below it).
>>
>>Your site, which is the one I usually look at, rather glosses over the
>>Datum thing, whereas the EA's site has it a click away from the
>>headline, and explains what it really means.
>
>Well, on my site it's a short scroll away from the headline. I'm not sure
>that's objectively worse.

I could only find it at the bottom of the page, hiding amongst a
multitude of other geeky stuff, and it didn't explain the bit
about being derived from the river bed.

>>It's the AOD height of the river bed, thus the number we see is the
>>depth of the water, not the height of the surface. Looking at other
>>measuring stations, they appear to round it to the nearest 10cm.
>
>Bear in mind also that the river bed isn't flat. The stage datum is an
>approximation, based on an average depth.

Is that an average perpendicular to the river flow? After all, rivers
are somewhat u-shaped when it comes to their bed.

Which probably explains why boats which look like they have a draft of
more than say 0.5m are still able to navigate up and down the Great Ouse
at Ely, by sticking to the middle of the river.

Can you answer my question about the datum for the headroom under the
railway bridges, or is that above your salary level?
--
Roland Perry

On Sat, 11 Nov 2023 14:18:01 +0000, Roland Perry <roland@perry.uk> wrote:

>In message <5ldtkilg9kfm8cuom98m4qh2fbsnlgjlmt@4ax.com>, at 23:06:24 on
>Fri, 10 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
>remarked:
>>On Fri, 10 Nov 2023 13:27:55 +0000, Roland Perry <roland@perry.uk> wrote:
>>
>>>In message <37nhkip7jgkqiscvdigm11qu2lrq557f6i@4ax.com>, at 12:48:10 on
>>>Mon, 6 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
>>>remarked:
>>
>>>>Bear in mind that the river level at Ely is measured Above Stage Datum
>>>>(ASD), not Above Ordnance Datum (AOD).
>>>
>>>I forgot that (but we are in danger of losing the plot, which is the
>>>railway line being ~4m above AOD, not near or below it).
>>>
>>>Your site, which is the one I usually look at, rather glosses over the
>>>Datum thing, whereas the EA's site has it a click away from the
>>>headline, and explains what it really means.
>>
>>Well, on my site it's a short scroll away from the headline. I'm not sure
>>that's objectively worse.
>
>I could only find it at the bottom of the page, hiding amongst a
>multitude of other geeky stuff, and it didn't explain the bit
>about being derived from the river bed.

The point about stage datum being derived from the river bed is on a generic
info page. I think the EA site dumbs it down a little. What it actually says
is

This station measures height from a fixed point on or close to the
riverbed. This point is 1.00m above sea level.

That's accurate, but it doesn't tell you anything more about what that fixed
point means. In particular, it doesn't mention the fact that it's a spot
reading and doesn't necessarily reflect the actual navigable depth.

>>>It's the AOD height of the river bed, thus the number we see is the
>>>depth of the water, not the height of the surface. Looking at other
>>>measuring stations, they appear to round it to the nearest 10cm.
>>
>>Bear in mind also that the river bed isn't flat. The stage datum is an
>>approximation, based on an average depth.
>
>Is that an average perpendicular to the river flow? After all, rivers
>are somewhat u-shaped when it comes to their bed.

It varies from location to location. And I don't have detailed information
about each location, because that data isn't published by the EA. As I
understand it, where a gauge sits on the river itself, then the stage datum
is usually the actual river bed at the location of the gauge. But in some
cases, the physical gauge isn't in the river itself. For example, at Ely,
it's in the entrance to the EA depot near Cuckoo Bridge. In cases like that,
using the river bed at that point would probably result in a misleading
reading, so the datum is taken from a point nearby on the main line of the
river. But, as you say, rivers are typically somewhat U-shaped, so parts of
it will be shallower than implied by the published levels and part of it
will be deeper.

>Which probably explains why boats which look like they have a draft of
>more than say 0.5m are still able to navigate up and down the Great Ouse
>at Ely, by sticking to the middle of the river.

Yes, indeed.

>Can you answer my question about the datum for the headroom under the
>railway bridges, or is that above your salary level?

It's not data that I have access to.

Mark

Mark Goodge wrote:

> On Sat, 11 Nov 2023 14:18:01 +0000, Roland Perry <roland@perry.uk>
> wrote:
>
> > In message <5ldtkilg9kfm8cuom98m4qh2fbsnlgjlmt@4ax.com>, at
> > 23:06:24 on
> >Fri, 10 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
> > remarked:
> >>On Fri, 10 Nov 2023 13:27:55 +0000, Roland Perry <roland@perry.uk>
> wrote:
> > >
> >
> > I could only find it at the bottom of the page, hiding amongst a
> > multitude of other geeky stuff, and it didn't explain the bit
> > about being derived from the river bed.
>
> The point about stage datum being derived from the river bed is on a
> generic info page. I think the EA site dumbs it down a little. What
> it actually says is
>
> This station measures height from a fixed point on or close to the
> riverbed. This point is 1.00m above sea level.
>
> That's accurate, but it doesn't tell you anything more about what
> that fixed point means. In particular, it doesn't mention the fact
> that it's a spot reading and doesn't necessarily reflect the actual
> navigable depth.
>
> > > > It's the AOD height of the river bed, thus the number we see is
> > > > the depth of the water, not the height of the surface. Looking
> > > > at other measuring stations, they appear to round it to the
> > > > nearest 10cm.
> > >
> > > Bear in mind also that the river bed isn't flat. The stage datum
> > > is an approximation, based on an average depth.
> >
> > Is that an average perpendicular to the river flow? After all,
> > rivers are somewhat u-shaped when it comes to their bed.
>
> It varies from location to location. And I don't have detailed
> information about each location, because that data isn't published by
> the EA. As I understand it, where a gauge sits on the river itself,
> then the stage datum is usually the actual river bed at the location
> of the gauge. But in some cases, the physical gauge isn't in the
> river itself. For example, at Ely, it's in the entrance to the EA
> depot near Cuckoo Bridge. In cases like that, using the river bed at
> that point would probably result in a misleading reading, so the
> datum is taken from a point nearby on the main line of the river.
> But, as you say, rivers are typically somewhat U-shaped, so parts of
> it will be shallower than implied by the published levels and part of
> it will be deeper.
>
Two points:
(1) If the gauge is at a weir, two water levels are recorded and the
datum for the upstream level will be the weir crest.
(2) Gauge datums are typically set once-and-for-all and do not change
if the river-bed changes through deposition or erosion of sediment.

On 11/11/2023 14:18, Roland Perry wrote:
> In message <5ldtkilg9kfm8cuom98m4qh2fbsnlgjlmt@4ax.com>, at 23:06:24 on
> Fri, 10 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk> remarked:
>> On Fri, 10 Nov 2023 13:27:55 +0000, Roland Perry <roland@perry.uk> wrote:
>>
>>> In message <37nhkip7jgkqiscvdigm11qu2lrq557f6i@4ax.com>, at 12:48:10 on
>>> Mon, 6 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
>>> remarked:
>>
>>>> Bear in mind that the river level at Ely is measured Above Stage Datum
>>>> (ASD), not Above Ordnance Datum (AOD).
>>>
>>> I forgot that (but we are in danger of losing the plot, which is the
>>> railway line being ~4m above AOD, not near or below it).
>>>
>>> Your site, which is the one I usually look at, rather glosses over the
>>> Datum thing, whereas the EA's site has it a click away from the
>>> headline, and explains what it really means.
>>
>> Well, on my site it's a short scroll away from the headline. I'm not sure
>> that's objectively worse.
>
> I could only find it at the bottom of the page, hiding amongst a
> multitude of other geeky stuff, and it didn't explain the bit
> about being derived from the river bed.
>
>>> It's the AOD height of the river bed, thus the number we see is the
>>> depth of the water, not the height of the surface. Looking at other
>>> measuring stations, they appear to round it to the nearest 10cm.
>>
>> Bear in mind also that the river bed isn't flat. The stage datum is an
>> approximation, based on an average depth.
>
> Is that an average perpendicular to the river flow? After all, rivers
> are somewhat u-shaped when it comes to their bed.
>
> Which probably explains why boats which look like they have a draft of
> more than say 0.5m are still able to navigate up and down the Great Ouse
> at Ely, by sticking to the middle of the river.
>
> Can you answer my question about the datum for the headroom under the
> railway bridges, or is that above your salary level?

It's different for tidal waters and the inland waterways.

For tidal waters the information is available from charts and almanacs
and the Navigator has to undertake tidal calculations.

The inland waterways are a nightmare with only conflicting information
available from multiple sources or even the same source. Sometimes the
only way you can be sure of fitting under the bridge is when you're there.

On Sun, 12 Nov 2023 21:52:03 +0000, Coffee
<martin.coffee@round-midnight.org.uk> wrote:

>On 11/11/2023 14:18, Roland Perry wrote:
>>
>> Which probably explains why boats which look like they have a draft of
>> more than say 0.5m are still able to navigate up and down the Great Ouse
>> at Ely, by sticking to the middle of the river.
>>
>> Can you answer my question about the datum for the headroom under the
>> railway bridges, or is that above your salary level?
>
>It's different for tidal waters and the inland waterways.
>
>For tidal waters the information is available from charts and almanacs
>and the Navigator has to undertake tidal calculations.
>
>The inland waterways are a nightmare with only conflicting information
>available from multiple sources or even the same source. Sometimes the
>only way you can be sure of fitting under the bridge is when you're there.

It's also worth bearing in mind that the river level data published by the
Environment Agency (and their equivalents in the Celtic fringes) is for the
purpose of assessing flooding risk and is explicitly *not* intended for
navigational use. The navigation authority is responsible for supplying that
information, but unlike the flood risk data there is no obligation to keep
it up to date or to make it available free of charge.

As it happens, the Environment Agency also happens to be the navigation
authorityfor the Great Ouse, and does publish bridge headway values. But
these will, of course, be affected by the level.

https://www.gov.uk/guidance/river-great-ouse-bridge-heights-locks-and-facilities

Mark

Am 12.11.2023 um 15:14 schrieb David Jones:
> Mark Goodge wrote:
>
>> On Sat, 11 Nov 2023 14:18:01 +0000, Roland Perry <roland@perry.uk>
>> wrote:
>>
>>> In message <5ldtkilg9kfm8cuom98m4qh2fbsnlgjlmt@4ax.com>, at
>>> 23:06:24 on
>>> Fri, 10 Nov 2023, Mark Goodge <usenet@listmail.good-stuff.co.uk>
>>> remarked:
>>>> On Fri, 10 Nov 2023 13:27:55 +0000, Roland Perry <roland@perry.uk>
>> wrote:
>>>>
>>>
>>> I could only find it at the bottom of the page, hiding amongst a
>>> multitude of other geeky stuff, and it didn't explain the bit
>>> about being derived from the river bed.
>>
>> The point about stage datum being derived from the river bed is on a
>> generic info page. I think the EA site dumbs it down a little. What
>> it actually says is
>>
>> This station measures height from a fixed point on or close to the
>> riverbed. This point is 1.00m above sea level.
>>
>> That's accurate, but it doesn't tell you anything more about what
>> that fixed point means. In particular, it doesn't mention the fact
>> that it's a spot reading and doesn't necessarily reflect the actual
>> navigable depth.
>>
>>>>> It's the AOD height of the river bed, thus the number we see is
>>>>> the depth of the water, not the height of the surface. Looking
>>>>> at other measuring stations, they appear to round it to the
>>>>> nearest 10cm.
>>>>
>>>> Bear in mind also that the river bed isn't flat. The stage datum
>>>> is an approximation, based on an average depth.
>>>
>>> Is that an average perpendicular to the river flow? After all,
>>> rivers are somewhat u-shaped when it comes to their bed.
>>
>> It varies from location to location. And I don't have detailed
>> information about each location, because that data isn't published by
>> the EA. As I understand it, where a gauge sits on the river itself,
>> then the stage datum is usually the actual river bed at the location
>> of the gauge. But in some cases, the physical gauge isn't in the
>> river itself. For example, at Ely, it's in the entrance to the EA
>> depot near Cuckoo Bridge. In cases like that, using the river bed at
>> that point would probably result in a misleading reading, so the
>> datum is taken from a point nearby on the main line of the river.
>> But, as you say, rivers are typically somewhat U-shaped, so parts of
>> it will be shallower than implied by the published levels and part of
>> it will be deeper.
>>
> Two points:
> (1) If the gauge is at a weir, two water levels are recorded and the
> datum for the upstream level will be the weir crest.
> (2) Gauge datums are typically set once-and-for-all and do not change
> if the river-bed changes through deposition or erosion of sediment.

The historical variation of Gauge datums leads to some anomalies:

The Rhine Gague at Worms read at IIRC -0.75m in October 2018, with the
river still being pasable for small leasure boats.

In German 'federal navigable rivers', there is a definition of an
'equivalent gague' (GlW), which is that gague level at which you just
reach the nominal navigable depth; this is re-calculated every 10 years
to promise meaningful ship loading abilities.

<https://www.elwis.de/DE/dynamisch/gewaesserkunde/wasserstaende/index.php?target=2&gw=RHEIN>

The upper Rhine has a guaranteed depth of 190cm, so gague Worms with a
GlW of 61, the gague differs from the naviagable depth by 119cm.

The lower Rhine down of Dusseldorf has a guaranteed depth of 250cm,
gague Dusseldorf has a GlW of 91cm, the gague differs from the
naviagable depth by 159cm.

<https://www.bafg.de/DE/07_Nachrichten/Archiv/2015/20151111_nw_download.pdf?__blob=publicationFile#page=3>

Rolf