Tidal Water Volume Slop

Tidal Water Volume Slop

Post by Alistai » Sat, 23 Mar 2013 21:19:15


On a tidal piece of water the consensus is that after a lot of rain
rowing on the flood tide will be slower than normal because the flood is
fighting the volume of rain coming downstream.

It sort of makes sense, but I'm not convinced - or at least it's
probably not that simple. It could be that the greater volume of water
simply gets picked up and sloshes back up the way it came, negating the
greater volume coming down, and in fact the net result is pretty much zero.

Or perhaps there is a period where the net effect is an increase in the
speed of the flood because there's more volume in the tidal reaches.

Hmmmmm.

 
 
 

Tidal Water Volume Slop

Post by Richard du » Sat, 23 Mar 2013 23:54:19

Alistair - consider two extreme cases?

1] No water at all coming down the river, ie the Tideway is just a little bit of ocean that happens to snake inland. I'd expect flood and ebb tides to be roughly equal in duration, roughly equal and opposite in velocity. The floating log drifts up and down over the same range until it disintegrates.

2] More water than you could ever imagine, at least without some - er - climate change, coming down - say - Old Father Thames, from the swamps of Gloucestershire, Oxfordshire &c. Tide does its best, but is overwhemelmed, and in fact the outward flow of the stream surface never reverses, just increases and decreases a bit. This is what I've seen on London's Tideway more than once in the last couple of decades; indeed there was a UBR in recent memory, the latter part of which may have been rowed against an outward stream, tide tables notwithstanding? The floating log drifts purposefully [sorry!] out into the ocean.

If this spectrum is basically monotonic, then we normally find ourselves at some intermediate point, maybe? - with ebb tides longer than flood tides [when the metaphorical bath is filling from both ends]. Do we perhaps see a highly variable outward river flow, adding to and subtracting from simplish harmonic tidal flow, varying regularly only as the moon and the sun line up, or not?

- or have I missed your point?

Richard du P

 
 
 

Tidal Water Volume Slop

Post by Alistai » Sun, 24 Mar 2013 00:47:03

Quote:

> Alistair - consider two extreme cases?

> 1] No water at all coming down the river, ie the Tideway is just a little bit of ocean that happens to snake inland. I'd expect flood and ebb tides to be roughly equal in duration, roughly equal and opposite in velocity. The floating log drifts up and down over the same range until it disintegrates.

> 2] More water than you could ever imagine, at least without some - er - climate change, coming down - say - Old Father Thames, from the swamps of Gloucestershire, Oxfordshire &c. Tide does its best, but is overwhemelmed, and in fact the outward flow of the stream surface never reverses, just increases and decreases a bit. This is what I've seen on London's Tideway more than once in the last couple of decades; indeed there was a UBR in recent memory, the latter part of which may have been rowed against an outward stream, tide tables notwithstanding? The floating log drifts purposefully [sorry!] out into the ocean.

> If this spectrum is basically monotonic, then we normally find ourselves at some intermediate point, maybe? - with ebb tides longer than flood tides [when the metaphorical bath is filling from both ends]. Do we perhaps see a highly variable outward river flow, adding to and subtracting from simplish harmonic tidal flow, varying regularly only as the moon and the sun line up, or not?

> - or have I missed your point?

> Richard du P

 >

All good points.

And I don't doubt that the volume on the ebb must be greater than the
volume on the flood, otherwise Oxfordshire would be underwater.

And there is the complicating factor that tides vary greatly in strength.

But with a strong tide, could the peak flood actually be increased by
this greater volume as the moon grabs all this water and slops it back
up the way it came?

I don't actually know, but I suspect the monolithic idea that more rain
= slower flood tide is not as simple as all that.

 
 
 

Tidal Water Volume Slop

Post by James H » Tue, 26 Mar 2013 16:11:36

Quote:

> On a tidal piece of water the consensus is that after a lot of rain

> rowing on the flood tide will be slower than normal because the flood is

> fighting the volume of rain coming downstream.

> It sort of makes sense, but I'm not convinced - or at least it's

> probably not that simple. It could be that the greater volume of water

> simply gets picked up and sloshes back up the way it came, negating the

> greater volume coming down, and in fact the net result is pretty much zero.

> Or perhaps there is a period where the net effect is an increase in the

> speed of the flood because there's more volume in the tidal reaches.

> Hmmmmm.

I think that the incoming tide is an effect of the moon on the ocean - a much larger body of water, that is simply overspilling up the thames, rather than the water itself being dragged up (I argue this because non tidal rivers do not exhibit tidal nature - despite being water and under the moon.

However, what I have seen more and more recently is that the tide seems to come up under the flow of rain water on top. On the scheduled turn of the tide I have often seen the river bank in one tidal flow direction and the stream in the other!

Makes for a fun decision as to which direction to launch, but only fools look at their tide timetables and then decide which way to row! (For those that do not know the Tideway has 3 lanes - 2 controlled by COLREGS and one local bylaw which allows rowing against the tide .... depending on which way the tide is flowing. So this one way street changes direction with the tide, and knowing the direction of the tide determines which way you use this lane .....needless to say, there are always several views as to which way the tide is actually flowing! (With some potentially disastrous consequences).

James

 
 
 

Tidal Water Volume Slop

Post by Chris » Sat, 06 Apr 2013 07:29:10

Quote:

> On a tidal piece of water the consensus is that after a lot of rain

> rowing on the flood tide will be slower than normal because the flood is

> fighting the volume of rain coming downstream.

> It sort of makes sense, but I'm not convinced - or at least it's

> probably not that simple. It could be that the greater volume of water

> simply gets picked up and sloshes back up the way it came, negating the

> greater volume coming down, and in fact the net result is pretty much zero.

> Or perhaps there is a period where the net effect is an increase in the

> speed of the flood because there's more volume in the tidal reaches.

> Hmmmmm.

my tidal stream calculator suggests that the speed of the flood is decreased by lots of land water

Here are the figures (for a position around about Chiswick Steps) for the Boat race with an average amount of land water for March (101 cubic metres per second). Time (UTC), height (m), stream velocity (m/s)

31/3/2013 13:20 0.54    -0.26
31/3/2013 13:30 0.64    0.17
31/3/2013 13:40 0.85    0.73
31/3/2013 13:50 1.13    0.98
31/3/2013 14:0  1.45    1.06
31/3/2013 14:10 1.79    1.07
31/3/2013 14:20 2.14    1.06
31/3/2013 14:30 2.48    1.05
31/3/2013 14:40 2.78    1.02
31/3/2013 14:50 3.03    0.99
31/3/2013 15:0  3.26    0.98
31/3/2013 15:10 3.49    0.99
31/3/2013 15:20 3.72    1.01
31/3/2013 15:30 3.97    1.02
31/3/2013 15:40 4.22    1.03
31/3/2013 15:50 4.48    1.02
31/3/2013 16:0  4.74    1
31/3/2013 16:10 4.99    0.97
31/3/2013 16:20 5.24    0.92
31/3/2013 16:30 5.48    0.86
31/3/2013 16:40 5.7     0.77
31/3/2013 16:50 5.88    0.65
31/3/2013 17:0  6.03    0.5
31/3/2013 17:10 6.12    0.31
31/3/2013 17:20 6.14    0.06
31/3/2013 17:30 6.06    -0.27
31/3/2013 17:40 5.88    -0.65
31/3/2013 17:50 5.63    -0.94
31/3/2013 18:0  5.36    -1.1
31/3/2013 18:10 5.09    -1.16
31/3/2013 18:20 4.84    -1.16
31/3/2013 18:30 4.59    -1.15
31/3/2013 18:40 4.36    -1.14
31/3/2013 18:50 4.13    -1.13

And here are the same figures for 3  times average (300 cubic metres per second)

31/3/2013 13:20 1.43    -0.71
31/3/2013 13:30 1.51    -0.41
31/3/2013 13:40 1.68    -0.06
31/3/2013 13:50 1.95    0.34
31/3/2013 14:0  2.3     0.61
31/3/2013 14:10 2.67    0.69
31/3/2013 14:20 3.03    0.65
31/3/2013 14:30 3.36    0.58
31/3/2013 14:40 3.63    0.51
31/3/2013 14:50 3.86    0.45
31/3/2013 15:0  4.04    0.44
31/3/2013 15:10 4.2     0.46
31/3/2013 15:20 4.37    0.52
31/3/2013 15:30 4.57    0.59
31/3/2013 15:40 4.82    0.64
31/3/2013 15:50 5.09    0.65
31/3/2013 16:0  5.37    0.61
31/3/2013 16:10 5.62    0.54
31/3/2013 16:20 5.83    0.45
31/3/2013 16:30 5.99    0.36
31/3/2013 16:40 6.13    0.28
31/3/2013 16:50 6.26    0.19
31/3/2013 17:0  6.35    0.04
31/3/2013 17:10 6.37    -0.18
31/3/2013 17:20 6.28    -0.46
31/3/2013 17:30 6.12    -0.72
31/3/2013 17:40 5.9     -0.91
31/3/2013 17:50 5.67    -1.05
31/3/2013 18:0  5.44    -1.14
31/3/2013 18:10 5.21    -1.19
31/3/2013 18:20 4.98    -1.21
31/3/2013 18:30 4.76    -1.22
31/3/2013 18:40 4.55    -1.23
31/3/2013 18:50 4.35    -1.23

 
 
 

Tidal Water Volume Slop

Post by Alistai » Tue, 09 Apr 2013 01:27:01

Quote:

>> Hmmmmm.

> my tidal stream calculator suggests that the speed of the flood is decreased by lots of land water

> Here are the figures (for a position around about Chiswick Steps) for the Boat race with an average amount of land water for March (101 cubic metres per second). Time (UTC), height (m), stream velocity (m/s)

That's good stats, thanks. But how is the average amount of land water
calculated? Is that from the non-tidal bit?

And are you saying that this year there was THREE TIMES the amount of
water coming downstream as normal?

 
 
 

Tidal Water Volume Slop

Post by Chris » Tue, 09 Apr 2013 02:59:03

Quote:


> >> Hmmmmm.

> > my tidal stream calculator suggests that the speed of the flood is decreased by lots of land water

> > Here are the figures (for a position around about Chiswick Steps) for the Boat race with an average amount of land water for March (101 cubic metres per second). Time (UTC), height (m), stream velocity (m/s)

> That's good stats, thanks. But how is the average amount of land water

> calculated? Is that from the non-tidal bit?

> And are you saying that this year there was THREE TIMES the amount of

> water coming downstream as normal?

The average is calculated for 130 years' worth of records for Kingston from the National River Flow Archive. This is about 101 cumecs for March, 75 for April. This year on 31st March there was about 180 cumecs on the day - it is difficult to get reliable figures until they're archived, but this was countered by a reasonably good tide and a following wind, hence a reasonable quick time, 12th fastest if I've counted correctly. I'll convert my spreadsheet for calculating boat race winning times and stick it on Google docs.
 
 
 

Tidal Water Volume Slop

Post by Chris » Tue, 09 Apr 2013 03:00:45

Quote:



> > >> Hmmmmm.

> > > my tidal stream calculator suggests that the speed of the flood is decreased by lots of land water

> > > Here are the figures (for a position around about Chiswick Steps) for the Boat race with an average amount of land water for March (101 cubic metres per second). Time (UTC), height (m), stream velocity (m/s)

> > That's good stats, thanks. But how is the average amount of land water

> > calculated? Is that from the non-tidal bit?

> > And are you saying that this year there was THREE TIMES the amount of

> > water coming downstream as normal?

> The average is calculated for 130 years' worth of records for Kingston from the National River Flow Archive. This is about 101 cumecs for March, 75 for April. This year on 31st March there was about 180 cumecs on the day - it is difficult to get reliable figures until they're archived, but this was countered by a reasonably good tide and a following wind, hence a reasonable quick time, 12th fastest if I've counted correctly. I'll convert my spreadsheet for calculating boat race winning times and stick it on Google docs.

You'll also notice as James has posted that with lots of land water it is possible for the tide to have turned but with the level continuing to rise.
 
 
 

Tidal Water Volume Slop

Post by Alistai » Tue, 09 Apr 2013 04:02:23

Quote:

>> The average is calculated for 130 years' worth of records for Kingston from the National River Flow Archive. This is about 101 cumecs for March, 75 for April. This year on 31st March there was about 180 cumecs on the day - it is difficult to get reliable figures until they're archived, but this was countered by a reasonably good tide and a following wind, hence a reasonable quick time, 12th fastest if I've counted correctly. I'll convert my spreadsheet for calculating boat race winning times and stick it on Google docs.

Ok, so we know how much water is coming down on a day-by-day basis
thanks to the NRF archives, but for the flow on the tidal part of the
river you speak of a "tidal stream calculator"....? So is this an
estimate or an actual device?
 
 
 

Tidal Water Volume Slop

Post by Alistai » Tue, 09 Apr 2013 04:56:40

Here are the past twenty years of Boat Race times and water flow:

27/03/1993      29.5    17
26/03/1994      71.4    18.09
01/04/1995      70.5    18.04
06/04/1996      34.5    16.58
29/03/1997      7.09    17.38
28/03/1998      48.4    16.19
03/04/1999      45.9    16.41
25/03/2000      48      18.04
24/03/2001      346     19.36   (Goldie)
30/03/2002      68.2    16.54
06/04/2003      52.4    18.06
28/03/2004      49.9    18.47
27/03/2005      21.9    16.42
02/04/2006      114     18.26
07/04/2007      62.1    17.49
29/03/2008      153     20.53
29/03/2009      49.9    17
03/04/2010      159     17.35
26/03/2011      40.3    17.32
07/04/2012      15.8    16.41   (Isis)

I looked at this and thought: that looks like bollocks, but popped it
into Excel anyway and did a scatter graph with the flow represented
logarithmically and I was surprised to see a definite trend there.

I am forced to admit that scatter graphs can reveal non-obvious stuff.

CAVEAT - relying on flow volume to PREDICT a (record) time is still idiotic.

 
 
 

Tidal Water Volume Slop

Post by stewie.. » Tue, 09 Apr 2013 05:52:45

Quote:

> Here are the past twenty years of Boat Race times and water flow:

> 27/03/1993 29.5    17

> 26/03/1994 71.4    18.09

> 01/04/1995 70.5    18.04

> 06/04/1996 34.5    16.58

> 29/03/1997 7.09    17.38

> 28/03/1998 48.4    16.19

> 03/04/1999 45.9    16.41

> 25/03/2000 48      18.04

> 24/03/2001 346     19.36   (Goldie)

> 30/03/2002 68.2    16.54

> 06/04/2003 52.4    18.06

> 28/03/2004 49.9    18.47

> 27/03/2005 21.9    16.42

> 02/04/2006 114     18.26

> 07/04/2007 62.1    17.49

> 29/03/2008 153     20.53

> 29/03/2009 49.9    17

> 03/04/2010 159     17.35

> 26/03/2011 40.3    17.32

> 07/04/2012 15.8    16.41   (Isis)

> I looked at this and thought: that looks like bollocks, but popped it

> into Excel anyway and did a scatter graph with the flow represented

> logarithmically and I was surprised to see a definite trend there.

> I am forced to admit that scatter graphs can reveal non-obvious stuff.

> CAVEAT - relying on flow volume to PREDICT a (record) time is still idiotic.

Where is that water flow figure from? Groundwater cumecs at Kingston from the NRF?
 
 
 

Tidal Water Volume Slop

Post by Carl » Tue, 09 Apr 2013 06:25:14


Quote:
> Here are the past twenty years of Boat Race times and water flow:

> 27/03/1993    29.5    17
> 26/03/1994    71.4    18.09
> 01/04/1995    70.5    18.04
> 06/04/1996    34.5    16.58
> 29/03/1997    7.09    17.38
> 28/03/1998    48.4    16.19
> 03/04/1999    45.9    16.41
> 25/03/2000    48    18.04
> 24/03/2001    346    19.36    (Goldie)
> 30/03/2002    68.2    16.54
> 06/04/2003    52.4    18.06
> 28/03/2004    49.9    18.47
> 27/03/2005    21.9    16.42
> 02/04/2006    114    18.26
> 07/04/2007    62.1    17.49
> 29/03/2008    153    20.53
> 29/03/2009    49.9    17
> 03/04/2010    159    17.35
> 26/03/2011    40.3    17.32
> 07/04/2012    15.8    16.41    (Isis)

> I looked at this and thought: that looks like bollocks, but popped it
> into Excel anyway and did a scatter graph with the flow represented
> logarithmically and I was surprised to see a definite trend there.

> I am forced to admit that scatter graphs can reveal non-obvious stuff.

> CAVEAT - relying on flow volume to PREDICT a (record) time is still
> idiotic.

The snag with logarithmic scaling on any axis is that it so deforms the
plot as to make even random data look meaningful.

I recall a fascinating paper on data correlation which showed this with
great clarity.  It gave various plausible graphical correlations
generated from a given data set, each showing a few supposedly ignorable
outliers.  Having gone through these, the author then showed the
original data set, plotted on linear-linear paper - it was just a
meaningless scatter of points.  His (expert) advice was that if you
can't draw a reasonable curve through the data plotted to linear axes,
then any supposed correlation you might get by applying various
functions to any of the data sets was likely to be simple hogwash.

Working on a linear-only basis, one might generate a simple straight
line passing through those data points, but it requires attaching undue
significance to the real outlier (at maximum stream) and going slap
through the middle of the ball of noise you see at the slower flows.

One factor which, at high stream flows, might be nearly as relevant as
stream, & is certainly more relevant at low stream flows, is the
weather.  The wrong sort of wind & waves can dramatically change a race
time, not least (but not only) when boats are racing with lots of water
on board.

We could contemplate a multivariate analysis, but how many variables
should we consider & what might be the available accuracy of those data
items?  Critical, e.g., would be not just the size of the waves but
where the water turned rough in the race, for how long, how much water
the crews then collected, which wind direction, etc.

Maybe that's a great thing about the Boat Race - the course, the
weather, equipment choices and human foibles are so hard to factor into
the final result?  At least there were no errant swimmers this year,
just half of the SAS, SBS, MI5, MI6, Uncle Tom Cobley & all lurking in
the background.  And that's a real shame - the degree of irreversible
over-reaction which ratchets itself up these days whenever some fool
does something daft.  Had no one noticed last year's swimmer, maybe he'd
been chopped unseen into tiny pieces by the following fleet's
propellers.  That'd have saved a whole lot of daft security over-kill
for what remains a private affair between consenting ***s in public.

Cheers -
Carl

--
Carl Douglas Racing Shells        -
     Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write:   Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find:    tinyurl.com/2tqujf


 
 
 

Tidal Water Volume Slop

Post by Alistai » Tue, 09 Apr 2013 06:42:52

Quote:

> The snag with logarithmic scaling on any axis is that it so deforms the
> plot as to make even random data look meaningful.

> [Carl demonstrates that Excel can be made to show anything]

> Cheers -
> Carl

For the reasons you mention, I was surprised to find even a suggestion
of correlation. But even without logarithmising the flow axis there's
still a trend to my eye. Pressing the Log button just made it neater. I
could press some other buttons too and make it irrefutable, but I can't
find them right now.

Someone cleverer than me could  probably quantify its statistical
significance, but if you're going to do that it would be better to get
at least double the data.

But whatever you think, it is often generalised that Oxford is the more
arty-farty place and Cambridge more scientific. And when you save an
"Excel" file, it's in Cambridge blue, more or less, yet its degenerate
rage-inducing cousin "Word" is dark blue.  I bet you'd say that's
"co-incidence" too.

 
 
 

Tidal Water Volume Slop

Post by Alistai » Tue, 09 Apr 2013 17:51:00

Quote:

> The snag with logarithmic scaling on any axis is that it so deforms the
> plot as to make even random data look meaningful.

Here is 40 years of boat race times / flow on a normal scale:

http://goo.gl/9Rwxl

... and here with a log scale:

http://goo.gl/lJYF6

I don't think the logarithmic scale actually adds anything. But I would
suggest there's a definite (although very unpredictable) correlation
between land water flow and time, which is what you'd expect.

The unexpected thing was that with all the other factors that you've
mentioned that this is noticeable at all.

 
 
 

Tidal Water Volume Slop

Post by Carl » Tue, 09 Apr 2013 18:20:41


Quote:

>> The snag with logarithmic scaling on any axis is that it so deforms the
>> plot as to make even random data look meaningful.

> Here is 40 years of boat race times / flow on a normal scale:

> http://goo.gl/9Rwxl

> ... and here with a log scale:

> http://goo.gl/lJYF6

> I don't think the logarithmic scale actually adds anything. But I would
> suggest there's a definite (although very unpredictable) correlation
> between land water flow and time, which is what you'd expect.

> The unexpected thing was that with all the other factors that you've
> mentioned that this is noticeable at all.

Someone's been hard at work!

I think we agree that introducing the log scale adds no insights beyond
those visible on the linear scales.

That said, I'm not surprised by the trend we both see - that scorching
streams do leave their mark on the results.  I don't even find this
unexpected, but what is always surprising is how large is the apparent
influence of the other relevant but undefined variables.  If rowers were
automata and the only variable the stream speed, I'm sure we'd have a
compelling correlation.  Fortunately (I think) there are many variables
which may be equally influential - the Boat Race is no mere drag race.

The beauty of the Boat Race is that the course is pretty fair but highly
susceptible to tactics, hence the arguments for & against the station
choices.  And it's a cox's course, if ever there was one, which is great
for the smaller participants in our sport.  Contrast that with the
weather-dictated lottery with which 2k multi-lane racing is cursed, as
particularly at Nottingham & Dorney in the UK.  It always bemuses me
that crews & nations will cheerfully spend fortunes to train for races
held on such courses in the clear knowledge that racing will likely be
entirely unfair & that the blazerati will then cook the lane allocations
to favour those they think are the fastest.  Does that make any sense?

Cheers -
Carl

--
Carl Douglas Racing Shells        -
     Fine Small-Boats/AeRoWing Low-drag Riggers/Advanced Accessories
Write:   Harris Boatyard, Laleham Reach, Chertsey KT16 8RP, UK
Find:    tinyurl.com/2tqujf