Definition of "Mean High Water" on Ordnance Survey Maps
Richard Chambers
stages, 3 or 4 miles at a time, from its source in the Yorkshire Dales to
its outflow into the River Ouse some 10 miles south of the city of York. The
section we did today was close to Tadcaster, in the lower reaches of the
river.
The Ordnance Survey 2.5-inch map marks a feature of the river with the
legend "Mean High Water". I have tried to look up the definition of this
term using Google, but the only definition that I have been able to find is
(paraphrased) "the difference in height between mean sea level and mean high
tide". This definition is fine for an oceanologist, but is inappropriate and
meaningless when applied to the marking of a river-feature on a map. Could
somebody please supply the cartographical definition of the term, when
applied to a feature on a river.
I do not believe (although I could be wrong) that the river water would ever
become saline at high tide, at the position marked. This position is still
30 miles or so upstream from the sea. The map contour on the land adjacent
to the marked feature is 5m above mean sea level, although the river itself
obviously will lie somewhat below this level.
Richard Chambers Leeds UK.
Raymond O'Hara
"Richard Chambers" <richard.cham...@ntlworld.net> wrote in message
news:AqidnaOuyPiSEvfU...@posted.plusnet...
It's the average high water level as determined by averaging several years
worth of observations.
Don Aitken
>It's the average high water level as determined by averaging several years
>worth of observations.
>
And the river is certainly tidal at this point. That has nothing to do
with whether the water becomes saline, which it probably doesn't.
--
Don Aitken
Mail to the From: address is not read.
To email me, substitute "clara.co.uk" for "freeuk.com"
Peter Bennett
"Mean High Water" is the mean (a form of average) of all the high
tides averaged over 19 years (I think that's the full tidal cycle).
Mean High Water is likely the level above which heights of things
ashore, and clearance under bridges, etc. are measured.
You may want to consult a nautical chart for further information.
--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
Chuck Riggs
Unfortunately for the boater, a chart does not spell out the dynamic
changes to river levels, day by day. Other means are available to him,
one being local newspaper reports, another being books and magazines
that specialize in white water canoeing. They give the boater a good
idea of seasonal variation for various rivers, streams and bays. A
third source of information is local knowledge and a fourth is your
own eyewitness readings of river levels at the boat launching point,
if nowhere else.
"Mean high water" means the average high water line you can expect on
a particular date, for whatever the reason, not simply waterline
changes due to tidal movement. I'm not familiar with the River Wharfe,
but for most rivers, rainfall into contributory streams is the major
factor, not tidal motion.
--
Regards,
Chuck Riggs
Near Dublin, Ireland
Richard Chambers
Chuck Riggs wrote
> Peter Bennett wrote:
>
>>Richard Chambers originally asked:-
Not just you Charles, but all the others who have responded to this
question, seem (if I have interpreted them correctly) to have mis-read or
misunderstood the question. If everybody has misunderstood, it must be my
fault, not theirs. So I shall try again.
The map simply marks a position on the river, some 30 miles inland, with the
legend "Mean High Water". It does not say, for example, "Mean High Water =
1.5 metres". So all the definitions based on the difference of vertical
height between "Mean Sea Level[1]" and "High Tide", averaged over 19 years,
are irrelevant or inappropriate to the information the map is displaying.
Vertical height differences are of interest to the oceanologist, but
evidently are not of interest to the cartographer who prepared this map.
[1] The mean of the height of the sea at low tide and at high tide. This
definition of "Mean Sea Level", according to the glossary on my map, is the
base from which all subsequent contour heights are measued.
The replies that I have received have at least stimulated me to think more
deeply about the problem. Here is the idea I have come up with:-
In the larger estuary, the dominant flow will be caused by the tides. When
the adjacent sea is rising due to the tide, the major flow of water into the
estuary will be from the sea into the estuary. This flow will overwhelm the
flow of water from the river into the estuary. Hence, as far as the river is
concerned, there will be a back-flow of water into the estuary when this
happens.
If we go to a point well upstream in the river, well away from the coast,
there will never be a back-flow of water at this point in the river,
whatever the state of the tide. The river at this point is too high up to be
affected by the tides.
Would it be correct for me to assume that the point marked "Mean High Water"
on the map is the most downstream position in the river where there is no
tidal back-flow, under average tidal conditions?
(i.e. neither neap nor spring tide, but the 19-year average).
Because of the flow of water in the river, I would (intuitively) not expect
the water to become saline, even for part of the day, at the "Mean High
Water" point (assuming I have correctly guessed what that is). Is there a
name for most downstream point of the river where the water never becomes
saline, even at a Spring Tide?
Richard Chambers Leeds UK.
Peter Duncanson (BrE)
Googling >define: Mean High Water< finds a number of definitions including:
http://www.sf.adfg.state.ak.us/SARR/AWC/index.cfm/FA/defs.definitions
"Mean high water" means a tidal datum used in referring to tidelands or
the tidally affected portion of the stream, that is equal to the average
of all high tides over a 19-year Metonic cycle, as established by the
National Ocean Service of the National Oceanic and Atmospheric
Administration.
This says something similar:
http://amsglossary.allenpress.com/glossary/browse?s=m&p=17
mean high water裕he average level of all high waters at a place over a
19-year period.
Perhaps the point on the map is at (approximately) Mean High Water level.
There may well be a physical marker there showing the level.
--
Peter Duncanson, UK
(in alt.usage.english)
Paul Wolff
>>>Richard Chambers originally asked:-
>>>>
>>>>legend "Mean High Water".
>If everybody has misunderstood, it must be my
>fault, not theirs. So I shall try again.
>
>The map simply marks a position on the river, some 30 miles inland, with the
>legend "Mean High Water".
What sort of mark is it? Does the map show the river having breadth
there, or is it just a thin blue line?
If the river has breadth, I'd want to know if the mark specifically
indicates a point on one or both sides of the riverbank, or in the
river.
Generally, the OS 1:25 000 maps (at least, the Explorer series) show MHW
as a line, not a point, along the boundary between dry land and mud or
sand -- foreshore, if you like -- on either side of the blue ribbon that
shows the river at Mean Low Water.
But when I read of a "feature" and a "position" marked MHW, then I don't
visualise the feature as a continuous shoreline.
Without further information, I'd reason that river height in Tadcaster,
thirty miles from the estuary, is variable from season to season and
according to the past week's precipitation, so the mark is unlikely to
be related to the water height in the river. More likely, it points out
where the land height equals mean high water at sea. I doubt if the OS
measures river bed heights above sea level. But river banks go up and
down.
Can it mean a point at which the river would no longer burst its banks
if the sea were held at high tide long enough for the back flow upriver
to cease? But why would the OS bother with that? They already mark the
Normal Tidal Limit as NTL. Can you check how far away your MHW feature
is from the NTL?
>The replies that I have received have at least stimulated me to think more
>deeply about the problem. Here is the idea I have come up with:-
>In the larger estuary, the dominant flow will be caused by the tides. When
>the adjacent sea is rising due to the tide, the major flow of water into the
>estuary will be from the sea into the estuary. This flow will overwhelm the
>flow of water from the river into the estuary. Hence, as far as the river is
>concerned, there will be a back-flow of water into the estuary when this
>happens.
>
>If we go to a point well upstream in the river, well away from the coast,
>there will never be a back-flow of water at this point in the river,
>whatever the state of the tide. The river at this point is too high up to be
>affected by the tides.
>
>Would it be correct for me to assume that the point marked "Mean High Water"
>on the map is the most downstream position in the river where there is no
>tidal back-flow, under average tidal conditions?
> (i.e. neither neap nor spring tide, but the 19-year average).
That is the point that should be marked NTL. It should be lower than
MHW if the river is long, narrow and slow, because of the tide's turning
and ebbing before the flood tide flow has travelled right up to its
potential farthest point. Or so I reason, from the idea that the last
few inches of tidal rise are going to take a very long time indeed to
drive water 30 miles up a narrow channel. But the ebb may take nearly
as long to catch up, I suppose.
>
>Because of the flow of water in the river, I would (intuitively) not expect
>the water to become saline, even for part of the day, at the "Mean High
>Water" point (assuming I have correctly guessed what that is). Is there a
>name for most downstream point of the river where the water never becomes
>saline, even at a Spring Tide?
Pass.
--
Paul
Mike Lyle
[...]
>>
> And the river is certainly tidal at this point. That has nothing to do
> with whether the water becomes saline, which it probably doesn't.
I've never been tempted to taste the Severn Bore near Gloucester. Even
before I was passed by the bloated body of a sheep which had been in the
water so long it had lost all its wool, and for a moment I feared it
might be human.
Oh, and, by the way, is it true that surfers and kayakers in sufficient
numbers actually /break down/ a bore?
"I don't mean to be boring," he said eagrely.
--
Mike.
Pat Durkin
news:gkg5h5$jc8$1...@news.motzarella.org
Oh, this doesn't augur well, he said boringly
Don Aitken
<boun...@two.wolff.co.uk> wrote:
>Richard Chambers <richard.cham...@ntlworld.net> wrote
>>Would it be correct for me to assume that the point marked "Mean High Water"
>>on the map is the most downstream position in the river where there is no
>>tidal back-flow, under average tidal conditions?
>> (i.e. neither neap nor spring tide, but the 19-year average).
>
>That is the point that should be marked NTL. It should be lower than
>MHW if the river is long, narrow and slow, because of the tide's turning
>and ebbing before the flood tide flow has travelled right up to its
>potential farthest point. Or so I reason, from the idea that the last
>few inches of tidal rise are going to take a very long time indeed to
>drive water 30 miles up a narrow channel. But the ebb may take nearly
>as long to catch up, I suppose.
>>
Nope. That water moves upstream *fast*. I don't have figures for other
Yorkshire rivers, but the Trent, which is tidal for more than 50 miles
from the sea, floods for about two and a quarter hours and ebbs for
the rest of the 12 hours. The Wharfe is probably a bit less extreme.
Paul Wolff
channel narrows from a wide estuary, but we shouldn't take it for
granted that the peak height of the sea tide reaches the same level
miles up the river.
--
Paul
Hatunen
<boun...@two.wolff.co.uk> wrote:
>Richard Chambers <richard.cham...@ntlworld.net> wrote
>>>>Richard Chambers originally asked:-
>>>>>
>>>>>The Ordnance Survey 2.5-inch map marks a feature of the river with the
>>>>>legend "Mean High Water".
>
>>If everybody has misunderstood, it must be my
>>fault, not theirs. So I shall try again.
>>
>>The map simply marks a position on the river, some 30 miles inland, with the
>>legend "Mean High Water".
>
>What sort of mark is it? Does the map show the river having breadth
>there, or is it just a thin blue line?
>
>If the river has breadth, I'd want to know if the mark specifically
>indicates a point on one or both sides of the riverbank, or in the
>river.
>
>Generally, the OS 1:25 000 maps (at least, the Explorer series) show MHW
>as a line, not a point, along the boundary between dry land and mud or
>sand -- foreshore, if you like -- on either side of the blue ribbon that
>shows the river at Mean Low Water.
>
>But when I read of a "feature" and a "position" marked MHW, then I don't
>visualise the feature as a continuous shoreline.
Could it be that it simply means the map, as drawn, is drawn as
if the water were at MHW?
--
************* DAVE HATUNEN (hat...@cox.net) *************
* Tucson Arizona, out where the cacti grow *
* My typos & mispellings are intentional copyright traps *
Hatunen
Surely you mean it doesn't auger well.
Robin Bignall
wrote:
>"Mike Lyle" <mike_l...@REMOVETHISyahoo.co.uk> wrote in message
"I bring glad tidings", he said, with gravity.
--
Robin
(BrE)
Herts, England
Adam Funk
> On Mon, 12 Jan 2009 13:34:20 -0600, "Pat Durkin"
><dur...@sbc.com> wrote:
>>Oh, this doesn't augur well, he said boringly
>
> Surely you mean it doesn't auger well.
Is this a real alarm, or just a drill?
--
Nam Sibbyllam quidem Cumis ego ipse oculis meis vidi in ampulla
pendere, et cum illi pueri dicerent: beable beable beable; respondebat
illa: doidy doidy doidy. [plorkwort]
Paul Wolff
>On Mon, 12 Jan 2009 19:06:47 +0000, Paul Wolff
><boun...@two.wolff.co.uk> wrote:
>
>>Richard Chambers <richard.cham...@ntlworld.net> wrote
>>>>>Richard Chambers originally asked:-
>>>>>>
>>>>>>The Ordnance Survey 2.5-inch map marks a feature of the river with the
>>>>>>legend "Mean High Water".
>>
>>>If everybody has misunderstood, it must be my
>>>fault, not theirs. So I shall try again.
>>>
>>>The map simply marks a position on the river, some 30 miles inland, with the
>>>legend "Mean High Water".
>>
>>What sort of mark is it? Does the map show the river having breadth
>>there, or is it just a thin blue line?
>>
>>If the river has breadth, I'd want to know if the mark specifically
>>indicates a point on one or both sides of the riverbank, or in the
>>river.
>>
>>Generally, the OS 1:25 000 maps (at least, the Explorer series) show MHW
>>as a line, not a point, along the boundary between dry land and mud or
>>sand -- foreshore, if you like -- on either side of the blue ribbon that
>>shows the river at Mean Low Water.
>>
>>But when I read of a "feature" and a "position" marked MHW, then I don't
>>visualise the feature as a continuous shoreline.
>
>Could it be that it simply means the map, as drawn, is drawn as
>if the water were at MHW?
>
for Tadcaster. The Isle of Wight it was, actually.
The Wight map was drawn with rivers at MLW. Then there was a tasteful
mud-coloured band on either side up to the line marked MHW. Said
mud-coloured band tended to end at NTL.
--
Paul
Roland Hutchinson
> On 2009-01-12, Hatunen wrote:
>
>> On Mon, 12 Jan 2009 13:34:20 -0600, "Pat Durkin"
>><dur...@sbc.com> wrote:
>
>>>Oh, this doesn't augur well, he said boringly
>>
>> Surely you mean it doesn't auger well.
>
> Is this a real alarm, or just a drill?
<bravo!>
--
Roland Hutchinson Will play viola da gamba for food.
NB mail to my.spamtrap [at] verizon.net is heavily filtered to
remove spam. If your message looks like spam I may not see it.
R H Draney
>
>On 2009-01-12, Hatunen wrote:
>
>> On Mon, 12 Jan 2009 13:34:20 -0600, "Pat Durkin"
>><dur...@sbc.com> wrote:
>
>>>Oh, this doesn't augur well, he said boringly
>>
>> Surely you mean it doesn't auger well.
>
>Is this a real alarm, or just a drill?
Merely a caution of the possible presence of a tocsin....r
--
"You got Schadenfreude on my Weltanschauung!"
"You got Weltanschauung in my Schadenfreude!"
Mike Lyle
> Adam Funk wrote:
>
>> On 2009-01-12, Hatunen wrote:
>>
>>> On Mon, 12 Jan 2009 13:34:20 -0600, "Pat Durkin"
>>> <dur...@sbc.com> wrote:
>>
>>>> Oh, this doesn't augur well, he said boringly
>>>
>>> Surely you mean it doesn't auger well.
>>
>> Is this a real alarm, or just a drill?
>
> <bravo!>
"Another clap!" he laughed infectiously.
--
Mike.
Wood Avens
wrote:
>Adam Funk filted:
>>
>>On 2009-01-12, Hatunen wrote:
>>
>>> On Mon, 12 Jan 2009 13:34:20 -0600, "Pat Durkin"
>>><dur...@sbc.com> wrote:
>>
>>>>Oh, this doesn't augur well, he said boringly
>>>
>>> Surely you mean it doesn't auger well.
>>
>>Is this a real alarm, or just a drill?
>
>Merely a caution of the possible presence of a tocsin....r
That sounds a bit off, she muttered venomousy.
--
Katy Jennison
spamtrap: remove the first two letters after the @
Richard Chambers
Paul Wolff wrote
> < ... >
> Can it mean a point at which the river would no longer burst its banks if
> the sea were held at high tide long enough for the back flow upriver to
> cease? But why would the OS bother with that? They already mark the
> Normal Tidal Limit as NTL. Can you check how far away your MHW feature is
> from the NTL?
-----------------------------
On the River Ouse (the major river, into which the Wharfe flows) the "MHW"
is marked at a point approximately 5 km upstream from the confluence of the
two rivers. The NTL is marked a further 5km *upstream*, at Naburn Lock. This
is near the village of Naburn, and is about 7 km south of the southern
outskirts of York. There is a weir at this point in the river, so the tidal
limit is man-made, not natural, at this point.
The interesting thing about this is that the NTL is upstream from the MHW,
contrary to your conjecture later in your posting.
On the River Wharfe, there is a "MHW" marked at approximately 1km (as the
crow flies) or 2 km (as the river meanders) upstream from its confluence
with the Ouse. I cannot find a "NTL" associated with this marked point. I
suspect that there might be a cartographical error, because there is a
second "MHW" marked a further 5 km upstream on the same river, near the
village of Ulleskelf. The associated "NTL" is a further 1 km upstream.
The reason why I think there may be a cartographical error is simply a
matter of logic. By definition, there can be only one MHW per river.
To get back on topic (but only briefly and in passing, I hope) should I
write < a "MHW" > or < an "MHW" >?
---------------------------------------
< ... >
>>Would it be correct for me to assume that the point marked "Mean High
>>Water"
>>on the map is the most downstream position in the river where there is no
>>tidal back-flow, under average tidal conditions?
>> (i.e. neither neap nor spring tide, but the 19-year average).
>
> That is the point that should be marked NTL. It should be lower than MHW
> if the river is long, narrow and slow, because of the tide's turning and
> ebbing before the flood tide flow has travelled right up to its potential
> farthest point. Or so I reason, from the idea that the last few inches of
> tidal rise are going to take a very long time indeed to drive water 30
> miles up a narrow channel. But the ebb may take nearly as long to catch
> up, I suppose.
----------------------------------------
It is a little known fact that the Humber Estuary (into which the Ouse
flows) has a tidal bore, very much the little brother of the Severn Bore,
but nevertheless of appreciable height. This might affect your conjecture,
and place the NTL upstream of the MHW.
----------------------------------------
Richard Chambers Leeds UK.
Jonathan Morton
> On the River Ouse (the major river, into which the Wharfe flows) the "MHW"
> is marked at a point approximately 5 km upstream from the confluence of
> the two rivers. The NTL is marked a further 5km *upstream*, at Naburn
> Lock. This is near the village of Naburn, and is about 7 km south of the
> southern outskirts of York. There is a weir at this point in the river, so
> the tidal limit is man-made, not natural, at this point.
>
> The interesting thing about this is that the NTL is upstream from the MHW,
> contrary to your conjecture later in your posting.
No, that is quite logical and correct. The MHW is the *mean* of high tides,
but the NTL (Normal Tidal Limit) is the point to which the highest *spring*
tides flow - so it will be higher than the MHW.
You should also find on your OS (both 1:25,000 and 1:50,000) that the lines
at the edge of the river are darker and thicker on the tidal sections. The
distinction isn't immediately obvious but it will be if you look either side
of the NTL.
Regards
Jonathan
Paul Wolff
measure. I had assumed that it was fixed at the coast. But of course
local is what matters in a map.
--
Paul
Irwell
Is Teddington a 'Tide ending town' or is it called after a man named
'Tuda'?
Peter Duncanson (BrE)
http://en.wikipedia.org/wiki/Teddington
The name 'Teddington' derives from an Old English tribal leader, and it
was known in Saxon and Norman times as Todyngton and Tutington.[2] The
name does not derive from 'Tide's End Town', as claimed by Rudyard Kipling
among others.
[2] John Sheaf, Ken Howe: Hampton and Teddington Past, Historical
Publications, October 1995 ISBN 0-948667-25-7 page 9
That's Teddington, Greater London. There is also Teddington, Gloucestershire:
http://en.wikipedia.org/wiki/Teddington,_Gloucestershire
Teddington was rumoured to have been founded by a traveller who was fond
of making drinks. He wanted to open a pub in an area of flourishing
population so that he would get lots of business. Finally he sought out a
town but with very little people, but still he decided to stay and ask if
the people there would help him build his tavern. The inhabitants agreed
and they built the bar. After the structure was completed the people
thought that it was such a magnificent building that they named the
area, Teddington – as was the mans last name- and the name of the
pub, Teddington Hands as it had been made from scratch with the makers’
bare hands. Of course there are thousands of rumours and stories of how it
was founded but this is one of the most believeable. However the
Teddington Hands Inn was originally known as The Cross hands Inn and the
name change only took place in the late 1980s after construction of the
Teddington Hands Roundabout which realigned the Stow Road away from the
side of the premises due to a high volume of serious road traffic
accidents. The area gets its name from the historic fingerpost which
formerly stood at the crossroads but now stands adjacent to the entrance
to the pub. The finger post is a listed structure.
The article also says:
The village has a population of less than 300, of which the majority are
professional commuters and elderly pensioners.
What, I wonder, is the difference between a professional commuter and an
amateur commuter?
Mike Lyle
[...]
>
> Is Teddington a 'Tide ending town' or is it called after a man named
> 'Tuda'?
"If you've seen one Teddington, you've seen the lot," said Tuda weirily.
--
Mike.
Paul Wolff
>
>Paul Wolff wrote
>
>> < ... >
>> Can it mean a point at which the river would no longer burst its banks if
>> the sea were held at high tide long enough for the back flow upriver to
>> cease? But why would the OS bother with that? They already mark the
>> Normal Tidal Limit as NTL. Can you check how far away your MHW feature is
>> from the NTL?
>-----------------------------
>On the River Ouse (the major river, into which the Wharfe flows) the "MHW"
>is marked at a point approximately 5 km upstream from the confluence of the
>two rivers. The NTL is marked a further 5km *upstream*, at Naburn Lock. This
>is near the village of Naburn, and is about 7 km south of the southern
>outskirts of York. There is a weir at this point in the river, so the tidal
>limit is man-made, not natural, at this point.
>
>The interesting thing about this is that the NTL is upstream from the MHW,
>contrary to your conjecture later in your posting.
I don't mind at all. How else to learn?
>
>On the River Wharfe, there is a "MHW" marked at approximately 1km (as the
>crow flies) or 2 km (as the river meanders) upstream from its confluence
>with the Ouse. I cannot find a "NTL" associated with this marked point. I
>suspect that there might be a cartographical error, because there is a
>second "MHW" marked a further 5 km upstream on the same river, near the
>village of Ulleskelf. The associated "NTL" is a further 1 km upstream.
>
>The reason why I think there may be a cartographical error is simply a
>matter of logic. By definition, there can be only one MHW per river.
That seems to make sense, but I am uneasy still about MHW being a point
in the river. I want it to be extensive, and marked along the banks.
Are you sure that it isn't just a repeated label to a continuous line?
The way the height designation numerals are repeated at intervals along
contour lines?
>
>To get back on topic (but only briefly and in passing, I hope) should I
>write < a "MHW" > or < an "MHW" >?
So long as you put it in inverted commas, it takes an 'an'. That's my
vote, anyway.
>
>>>Would it be correct for me to assume that the point marked "Mean High
>>>Water"
>>>on the map is the most downstream position in the river where there is no
>>>tidal back-flow, under average tidal conditions?
>>> (i.e. neither neap nor spring tide, but the 19-year average).
>>
>> That is the point that should be marked NTL. It should be lower than MHW
Lower than coastal MHW is what I was thinking at the time.
>> if the river is long, narrow and slow, because of the tide's turning and
>> ebbing before the flood tide flow has travelled right up to its potential
>> farthest point. Or so I reason, from the idea that the last few inches of
>> tidal rise are going to take a very long time indeed to drive water 30
>> miles up a narrow channel. But the ebb may take nearly as long to catch
>> up, I suppose.
>----------------------------------------
>It is a little known fact that the Humber Estuary (into which the Ouse
>flows) has a tidal bore, very much the little brother of the Severn Bore,
>but nevertheless of appreciable height. This might affect your conjecture,
>and place the NTL upstream of the MHW.
>----------------------------------------
Bores are an added complication, because I can envisage the slug of
water overshooting the elevation (altitude) corresponding to MHW at the
coast. Presumably a long, tapering estuary has the effect of
accelerating the water on a rising tide. I imagine stepping on to a
tube of toothpaste with the cap off, or more accurately, dispensing
mastic through a tapering delivery nozzle. Considering the water, a
large mass entering the mouth of a wide channel at slow speed is
translated into the same mass (perforce) shooting up the narrowing
channel at a range of speeds, all higher. Where the inherently greater
momentum comes from, I can't say. Shouldn't it be conserved? Maybe the
greater momentum of the faster water up the river is offset by the lost
momentum of the water in the estuary that doesn't actually go anywhere
else, or, as we say in the trade, "stops".
So this bolus of water thrown up a suitably narrowing river may well
travel beyond coastal MHW.
--
Paul
Paul Wolff
>'Tuda'?
Yes, Henry Tuda. A man with an ego the size of nearby Hampton Wick,
with which he went a-courting.
--
Paul
Richard Chambers
Paul Wolff wrote
>
> Bores are an added complication, because I can envisage the slug of water
> overshooting the elevation (altitude) corresponding to MHW at the coast.
> Presumably a long, tapering estuary has the effect of accelerating the
> water on a rising tide. I imagine stepping on to a tube of toothpaste
> with the cap off, or more accurately, dispensing mastic through a tapering
> delivery nozzle. Considering the water, a large mass entering the mouth of
> a wide channel at slow speed is translated into the same mass (perforce)
> shooting up the narrowing channel at a range of speeds, all higher. Where
> the inherently greater momentum comes from, I can't say. Shouldn't it be
> conserved? Maybe the greater momentum of the faster water up the river is
> offset by the lost momentum of the water in the estuary that doesn't
> actually go anywhere else, or, as we say in the trade, "stops".
The tide is more akin to a sound wave than to a surface wave on the sea, and
travels at 700 to 1000 mph around the earth. Conventional sound waves travel
at a similar speed through sea water.
The moon, which causes the tide, performs a rotation around the earth once
every 28 days. Underneath this moon, the earth rotates at one revolution per
24-hour day. To a reasonable first-degree approximation, we can therefore
neglect the slow 28-day motion of the moon, and think only of the spin of
the earth under a nearly-stationary moon.
The circumference of the earth at the equator is 24,000 miles. If the peak
of the tide is to remain under the moon, the tide must travel at
approximately 1000 mph around the equator. Approximately 700 mph at latitude
45 degrees.
It is this speed that provides the momentum you are asking about. As the
tide enters the estuary, the gently shelving sea-bed and the shallower water
converts the tide-wave into a surface wave. Hence, a bore is formed. The
recent tsunami in Thailand/Indonesia/Sri Lanka etc was formed by a similar
process.
Do not, however, confuse the speed of the wave with the speed of any part of
the water. No part of the ocean moves at 1000 mph, but the tidal wave does
move at that speed. In a similar way, when you speak the sound waves you
produce go forward at 650 mph, but the air leaving your lips might well move
at only 1 mph.
Richard Chambers Leeds UK.
Paul Wolff
A well-placed reminder. But, there's a but. I have this idea that the
water level behind a bore is higher than the level before it.
>No part of the ocean moves at 1000 mph, but the tidal wave does
>move at that speed. In a similar way, when you speak the sound waves you
>produce go forward at 650 mph, but the air leaving your lips might well move
>at only 1 mph.
>
Pffui.
(That's an experiment, not a judgement.)
--
Paul
Garrett Wollman
Paul Wolff <pa...@two.wolff.co.uk> wrote:
>Presumably a long, tapering estuary has the effect of accelerating
>the water on a rising tide.
Hydraulic damming, innit? Same process as made the Columbia River
Gorge and the Grand Coulee....
>So this bolus of water thrown up a suitably narrowing river may well
>travel beyond coastal MHW.
[giggle]
-GAWollman
--
Garrett A. Wollman | The real tragedy of human existence is not that we are
wol...@csail.mit.edu| nasty by nature, but that a cruel structural asymmetry
Opinions not those | grants to rare events of meanness such power to shape
of MIT or CSAIL. | our history. - S.J. Gould, Ten Thousand Acts of Kindness
Nick Spalding
on Tue, 13 Jan 2009 15:10:24 -0800:
The tide didn't end there until the lock and weir were built.
--
Nick Spalding
BrE/IrE
jgharston
the level of the high tides averaged over the tidal cycle reaches.
On OS maps it is shown as a thick line.
The Normal Tidal Limit is the point on a river below which there
is a tidal range and above which there is no tidal range. This is
shown on OS maps by the thick line showing Mean High Water ending
change changing to a thin line.
Paul Wolff
The 1:25 000 sheet that I was looking at doesn't show a solid black line
for MHW unless it's the edge of a solid structure like a concrete jetty,
a quay or a fortification.
And MHW does seem to extend up rivers, so the line is not strictly
limited to the (sea) coast.
--
Paul
Nick
> It is a little known fact that the Humber Estuary (into which the Ouse
> flows) has a tidal bore, very much the little brother of the Severn Bore,
> but nevertheless of appreciable height. This might affect your conjecture,
> and place the NTL upstream of the MHW.
Not that little known - it continues up the Trent in particular and was
the subject of a particularly painful pun by - I think - Mike Lyle a few
days ago.
Another "interesting" fact about the Ouse is that it has neither a
source nor a mouth. It starts where the River Ure merges with the Ouse
Gill Beck, and ends where the Ouse and the Trent merge to form the Humber.
--
Online waterways route planner: http://canalplan.org.uk
development version: http://canalplan.eu
Adam Funk
> That's Teddington, Greater London. There is also Teddington, Gloucestershire:
> http://en.wikipedia.org/wiki/Teddington,_Gloucestershire
...
> The article also says:
>
> The village has a population of less than 300, of which the majority are
> professional commuters and elderly pensioners.
>
> What, I wonder, is the difference between a professional commuter and an
> amateur commuter?
Amateur commuters do it for sport; once they get paid, they change
status.
--
In Karhide king and kyorremy have a good deal of control over what
people do, but very little over what they hear, and none over what
they say. Here, the government can check not only act but thought.
Surely no men should have such power over others. (LeGuin 1969)
Paul Wolff
>"Richard Chambers" <richard.cham...@ntlworld.net> writes:
>
>> It is a little known fact that the Humber Estuary (into which the Ouse
>> flows) has a tidal bore, very much the little brother of the Severn Bore,
>> but nevertheless of appreciable height. This might affect your conjecture,
>> and place the NTL upstream of the MHW.
>
>Not that little known - it continues up the Trent in particular and was
>the subject of a particularly painful pun by - I think - Mike Lyle a few
>days ago.
>
>Another "interesting" fact about the Ouse is that it has neither a
>source nor a mouth. It starts where the River Ure merges with the Ouse
>Gill Beck, and ends where the Ouse and the Trent merge to form the Humber.
But it does have another incarnation in Sussex. It is to be commended
for finding a way from the High Weald to the sea through the South Downs
without a map.
I write as one who has been thrown into the Sussex Ouse, thrown others
in, forded it, fallen in it, raced along it, bridged it, been flooded by
it, and generally had an intimate relationship with it. Such are one's
schooldays. Character-building, y'know.
--
Paul
HVS
> Nick <3-no...@temporary-address.org.uk> wrote
>> "Richard Chambers" <richard.cham...@ntlworld.net>
>> writes:
>>
>>> It is a little known fact that the Humber Estuary (into which
>>> the Ouse flows) has a tidal bore, very much the little brother
>>> of the Severn Bore, but nevertheless of appreciable height.
>>> This might affect your conjecture, and place the NTL upstream
>>> of the MHW.
>>
>> Not that little known - it continues up the Trent in particular
>> and was the subject of a particularly painful pun by - I think
>> - Mike Lyle a few days ago.
>>
>> Another "interesting" fact about the Ouse is that it has
>> neither a source nor a mouth. It starts where the River Ure
>> merges with the Ouse Gill Beck, and ends where the Ouse and the
>> Trent merge to form the Humber.
>
> But it does have another incarnation in Sussex. It is to be
> commended for finding a way from the High Weald to the sea
> through the South Downs without a map.
It has to detour through East Anglia to get there, though.
--
Cheers, Harvey
CanEng and BrEng, indiscriminately mixed
Paul Wolff
--
Paul
Jonathan Morton
news:HTv1djoN...@fpwolff.demon.co.uk...
>>
>>You should also find on your OS (both 1:25,000 and 1:50,000) that the
>>lines
>>at the edge of the river are darker and thicker on the tidal sections. The
>>distinction isn't immediately obvious but it will be if you look either
>>side
>>of the NTL.
>>
> It had not occurred to me until I read this that MHW might be a local
> measure. I had assumed that it was fixed at the coast. But of course
> local is what matters in a map.
MHW is, in effect, a line which runs all the way round the coast. When it
reaches a river estuary it turns up the river until it reaches the mean high
water mark of tides, at which point it crosses to the other bank and runs
back down to the coast.
Regards
Jonathan
Irwell
Or the difference between an elderly pensioner and a young pensioner.
Irwell
Sung to the tune of 'The old ash grove'?
Paul Wolff
Streamlets meandered and I pensively roved; but otherwise I'm whooshed.
--
Paul
Robert Bannister
> The name 'Teddington' derives from an Old English tribal leader, and it
> was known in Saxon and Norman times as Todyngton and Tutington.
So, that old King Tut got around a bit.
--
Rob Bannister
jgharston
> That's quite handy. But perhaps it only applies to the 1:50 000 maps.
> The 1:25 000 sheet that I was looking at doesn't show a solid black line
> for MHW unless it's the edge of a solid structure like a concrete jetty,
The 1:25,000 uses a solid blue line for MHW and a change from coastal
feature (eg mud, sand, etc) to water for MLW.
> And MHW does seem to extend up rivers, so the line is not strictly
> limited to the (sea) coast.
"Coast" is that part of an island's geography that has a tidal range.
London is on the coast, as is Ruswarp in the above link.
--
JGH
Paul Wolff
so, but general English usage would not accept that Richmond-upon-Thames
was a coastal town. Nor would I give that honour to Newport IoW:
<http://www.multimap.com/maps/?qs=newport+isle+of+wight&countryCode=GB#ma
p=50.6995,-1.29323|11|4&bd=useful_information&loc=GB:50.6995:-1.29323:14|
newport%20isle%20of%20wight|Newport,%20Newport%20and%20Ryde,%20Isle%20Of%
20Wight,%20England,%20PO30%201>
or possibly more conveniently: <http://tinyurl.com/9gcpbv>
--
Paul