Why does non Tidal Thames go up and down daily?

[Chopdog]

We are moored at Penton Hook. I am curious (and a fierce Sunday morning debate here) - why does this non tidal part of the Thames go up and down daily by about 5cm?

 

[Old Crusty]

We are moored at Penton Hook. I am curious (and a fierce Sunday morning debate here) - why does this non tidal part of the Thames go up and down daily by about 5cm?

Minor fluctuations in water level will be caused by changes in flow rates from acquifers upstream, the water flowing into the Thames from tributaries, run-off from surface water drains and abstraction and outflows at water treatment plants. Lock movements can have a small impact on levels too.

Greater fluctuations are usually down to heavy rain and the subsequent moving of weir gates to alter the flow and maintain the levels within the normal range.

The tidal effect can influence levels upstream as far as Penton Hook especially around spring tides when the high tide at Teddington obstructs the flow over the weir causing the river to back up. If you look at the graphic data available on the gauge map web site you can see the influence.

 

[STILL AFLOAT]

Well, The Locks, have their influence, between opening & closing & water running downstream.
On top of that, perhaps river boaters, are not examined as much as Coastal Skippers & Yachtmasters are.
Any Coastal Skipper, or Yacht master, will just shrug & point to the Barometer !
High Pressure, prevents the river water to rise.
Low pressure, allows the river water, to rise.
Add to that, local winds, and you have the river flow, pushed back, by high pressure & wind !
Or Vice Versa
I did 6 Transatlantics, prior to retiring to the river malarkey

 

[Old Crusty]

Well, The Locks, have their influence, between opening & closing & water running downstream.
On top of that, perhaps river boaters, are not examined as much as Coastal Skippers & Yachtmasters are.
Any Coastal Skipper, or Yacht master, will just shrug & point to the Barometer !
High Pressure, prevents the river water to rise.
Low pressure, allows the river water, to rise.
Add to that, local winds, and you have the river flow, pushed back, by high pressure & wind !
Or Vice Versa
I did 6 Transatlantics, prior to retiring to the river malarkey

That's a coincidence, I've done six Transatlantics too, not to mention the Med, Biscay, Channel and the North Sea but I never blamed the barometer for going aground.

I recall from my meteorology studies that a cubic metre of air at sea level weighs a kilo and a change in atmospheric pressure of one millibar (old money, now known as hectopascals I believe) can influence sea level by one centimetre. Therefore, a change in barometric pressure from say 990mb to 1020mb could lower the level by 30cm which can be critical if navigating over sandbanks and similar.

Atmospherics and moon pull will have a very minor effect on water levels but not as much as fluvial flow which is the key factor on the non tidal Thames. I'll take off the anorak now.

 

[Scholar Gypsy]

Not an answer to the question, but ...

I moor at Ely on the River Great Ouse. It is maintained at a level that is below high water at Kings Lynn, by the sluice at Denver. This operates as a huge one way valve, which lets fresh water out around low tide and stops the sea coming in around high tide. The net effect is that, when there is a decent amount of fresh water flow in the river, the level at Ely fluctuates daily, by 10cm or so, with "high water" about 5 hours after Kings Lynn.

 

[rotrax]

That's a coincidence, I've done six Transatlantics too, not to mention the Med, Biscay, Channel and the North Sea but I never blamed the barometer for going aground.

I recall from my meteorology studies that a cubic metre of air at sea level weighs a kilo and a change in atmospheric pressure of one millibar (old money, now known as hectopascals I believe) can influence sea level by one centimetre. Therefore, a change in barometric pressure from say 990mb to 1020mb could lower the level by 30cm which can be critical if navigating over sandbanks and similar.

Atmospherics and moon pull will have a very minor effect on water levels but not as much as fluvial flow which is the key factor on the non tidal Thames. I'll take off the anorak now.

I am reliably informed that high pressure in the Pacific and low pressure over the Tasman can delay or stop the tide flowing in the Cook Strait from time to time.

Thats special!

 

[Outinthedinghy]

Might be the effect of the Laleham intake which is a major water abstraction between PH lock and Dennets.

The tide catagorically does NOT have any effect on anywhere above Molesey.

The River was once tidal as far as Staines but with the building of pound locks this ended.

 

[boatone]

Note, also, that the EA target is to maintain the levels between -3" and +6" of the nominal level at each lock by weir adjustments as and when required.

 

[Outinthedinghy]

As I understand it it's not just a target it is a statutory requirement to maintain a certain level of water.

This obviously causes some problems in drought conditions when water abstraction for reservoirs conflicts with the needs of river users.

I was chatting with the keeper at Penton recently about this and apparently the Laleham intake is indeed quite a major influence on the River as is the one down at Datchet.

 

[Outinthedinghy]

The simple fact is that with a fully shut in Weir and no rain and water abstraction for drinking water you have a problem.

Weir adjustments can only work if it's not all shut in already...

 

[boatone]

The specific measurement limits were removed some time ago and the current Customer Charter statement is as follows:
[FONT=&quot]"We manage water levels to balance the many uses of the river including boating, retaining enough for water companies to abstract, managing flood risk and ensuring conservation sites, fisheries and water quality are protected. We measure water levels at lock sites and monitor them 24 hours a day.
We operate the weirs and other structures to provide the best conditions possible for all uses of the river. Our ability to control water levels may change in flood or drought conditions."[/FONT]

 

[Old Crusty]

Just had an additional thought. Romney weir has an automatic gate linked to the hydro power screws which will influence levels downstream by small amounts as it adjusts Romney headwater to keep the screws spinning at optimum generating power.

 

[Lower Limit 1909]

There are statutory (Government Act) requirements to maintain a certain level (1.72M above ODN to be precise) between Richmond Half Tide Lock and Teddington Lock (except during the annual draw off) but above Teddington it is 'targets' with no statutory backing.

Probably sensible that it is this way but it does leave the EA as 'Judge and Jury' having to decide between competing claims on the water. I'm no fan of how the EA run many things on the Thames but they do seem to manage this pretty well but by definition won't always keep everybody happy.

If the EA were remotely like the PLA they would provide lots of open information and even run events to explain how it all works and the criteria used but .....