Sea Level and River Flow

Alan B · #1 by **Alan B** » Feb 19, 2020 11:59 am

The recent severe flooding in the UK attributed to unusual heavy rainfall (which in turn may be due to Climate Change) has made me ask the question: Has an increase of just a few cms or so in the sea level caused a tendency for the rivers to flow more slowly into the sea and 'back-up' thereby contributing to the excessive flooding with rivers bursting their banks and other anti-flood defences?

Has anyone carried out research into sea level and river flow and how much change in sea level can cause a particular change in river flow?

Cito di Pense · #2 by **Cito di Pense** » Feb 19, 2020 12:33 pm

Alan B wrote:The recent severe flooding in the UK attributed to unusual heavy rainfall (which in turn may be due to Climate Change) has made me ask the question: Has an increase of just a few cms or so in the sea level caused a tendency for the rivers to flow more slowly into the sea and 'back-up' thereby contributing to the excessive flooding with rivers bursting their banks and other anti-flood defences?

Has anyone carried out research into sea level and river flow and how much change in sea level can cause a particular change in river flow?

Water is a fluid, Alan. You can't make dams out of water.

felltoearth · #3 by **felltoearth** » Feb 19, 2020 12:36 pm

Depends. In most cases it doesn’t slow the flow as long as the elevation of the majority of the river is substantially above sea level. Think of a pipe above a pond. The level of the pond doesn’t affect the rate of flow. The gradient does (as well as friction within the pipe.)
Surges are an issue. The Thames barrier exists for that reason. The biggest issue is at the mouth of the river where erosion puts coastline at risk and the change in fresh and salt water mix affects ecosystems.

newolder · #4 by **newolder** » Feb 19, 2020 12:58 pm

A river breaching a sand berm can give what? Minutes of fun. "Git yer shovel!"

Alan B · #5 by **Alan B** » Feb 19, 2020 2:22 pm

Cito di Pense wrote:
Alan B wrote:The recent severe flooding in the UK attributed to unusual heavy rainfall (which in turn may be due to Climate Change) has made me ask the question: Has an increase of just a few cms or so in the sea level caused a tendency for the rivers to flow more slowly into the sea and 'back-up' thereby contributing to the excessive flooding with rivers bursting their banks and other anti-flood defences?

Has anyone carried out research into sea level and river flow and how much change in sea level can cause a particular change in river flow?

Water is a fluid, Alan. You can't make dams out of water.

If the sea level rises a river's rate of flow into the sea could decrease. This could cause 'overflowing' upstream. (I think. :ask:

)

felltoearth · #6 by **felltoearth** » Feb 19, 2020 2:25 pm

Alan B wrote:
Cito di Pense wrote:
Alan B wrote:The recent severe flooding in the UK attributed to unusual heavy rainfall (which in turn may be due to Climate Change) has made me ask the question: Has an increase of just a few cms or so in the sea level caused a tendency for the rivers to flow more slowly into the sea and 'back-up' thereby contributing to the excessive flooding with rivers bursting their banks and other anti-flood defences?

Has anyone carried out research into sea level and river flow and how much change in sea level can cause a particular change in river flow?

Water is a fluid, Alan. You can't make dams out of water.

If the sea level rises a river's rate of flow into the sea could decrease. This could cause 'overflowing' upstream. (I think. )

A rising sea can’t cause any river higher than sea level to flow more slowly. See my comment about the pipe above.

Cito di Pense · #7 by **Cito di Pense** » Feb 19, 2020 4:05 pm

Alan B wrote:
Cito di Pense wrote:
Alan B wrote:The recent severe flooding in the UK attributed to unusual heavy rainfall (which in turn may be due to Climate Change) has made me ask the question: Has an increase of just a few cms or so in the sea level caused a tendency for the rivers to flow more slowly into the sea and 'back-up' thereby contributing to the excessive flooding with rivers bursting their banks and other anti-flood defences?

Has anyone carried out research into sea level and river flow and how much change in sea level can cause a particular change in river flow?

Water is a fluid, Alan. You can't make dams out of water.

If the sea level rises a river's rate of flow into the sea could decrease. This could cause 'overflowing' upstream.

Flooding happens because there's too much water in the channel -- because there's too much runoff in the drainage basin that feeds into that channel.

If you're thinking about the UK flooding, it's because there was a lot of rain. Some channels could not contain the flow.

The volume rate of the flow is related to the gradient (gravity) and then to the cross section area of the channel. The average gradient of a stream that runs for tens of kilometers and descends only tens or hundreds of meters is not changed appreciably by the fact that the terminus of the stream hits water that is centimeter higher than without sea-level rise. Flooding is happening far from this terminus in many cases.

Macdoc · #8 by **Macdoc** » Feb 19, 2020 9:51 pm

A glacier can be slowed by your "visualization" water cannot. :coffee:

TopCat · #9 by **TopCat** » Feb 21, 2020 1:27 pm

All rivers, as they discharge into the sea, are at sea level. If the sea level rises, the point at which the river level is at sea level will move upstream.

This happens twice a day, of course, as the tide comes in. Take the Thames for instance. The river flows upstream while the tide is coming in, until the most downstream permanent lock*. Hence the term tideway, in relation to the Thames. Teddington (a couple of miles from where I live) is in West London, a long way from the estuary. Yet downstream from Teddington, the river flows in the upstream direction while the tide is coming in.

If the downstream flow is sufficient, then of course it will overflow the banks. This happens frequently at Richmond, in past winters making an ice rink out of Old Deer Park.

So as sea level rises due to global warming (and there's no 'may be' about it, OP) then such floods will become more likely.

However, by far the largest contributory factor, as several have said, is simply the volume of water which cannot be contained by the channel. All the serious flooding on the Thames (Chertsey and Staines, for instance, which are upstream of the tideway) in recent years has been caused by this.

Teddington has seen very high water levels recently, but it has (just) avoided being actually flooded, despite the fact that the weir can allow a lot of water to bypass the lock.

_____
* there is also a lock at Richmond. However, it isn't permanently raised, so it doesn't stop the Thames being tidal as far upstream as Teddington.

Sgt Kelly · **#10** by **Sgt Kelly** » Feb 21, 2020 3:18 pm

TopCat wrote:Old Deer Park.

Sorry for the slight - and I promise short - derail but is that the park of Fenton fame ?

TopCat · **#11** by **TopCat** » Feb 21, 2020 4:34 pm

Sgt Kelly wrote:
TopCat wrote:Old Deer Park.

Sorry for the slight - and I promise short - derail but is that the park of Fenton fame ?

If it was this Fenton, then no... that was Richmond Park. :grin:

They're not far apart.

Funnily enough, there are no deer in Old Deer Park.

Alan B · **#12** by **Alan B** » Feb 23, 2020 3:35 pm

TopCat wrote:All rivers, as they discharge into the sea, are at sea level. If the sea level rises, the point at which the river level is at sea level will move upstream.

This happens twice a day, of course, as the tide comes in. Take the Thames for instance. The river flows upstream while the tide is coming in, until the most downstream permanent lock*. Hence the term tideway, in relation to the Thames. Teddington (a couple of miles from where I live) is in West London, a long way from the estuary. Yet downstream from Teddington, the river flows in the upstream direction while the tide is coming in.

If the downstream flow is sufficient, then of course it will overflow the banks. This happens frequently at Richmond, in past winters making an ice rink out of Old Deer Park.

So as sea level rises due to global warming (and there's no 'may be' about it, OP) then such floods will become more likely.

However, by far the largest contributory factor, as several have said, is simply the volume of water which cannot be contained by the channel. All the serious flooding on the Thames (Chertsey and Staines, for instance, which are upstream of the tideway) in recent years has been caused by this.

Teddington has seen very high water levels recently, but it has (just) avoided being actually flooded, despite the fact that the weir can allow a lot of water to bypass the lock.

_____
* there is also a lock at Richmond. However, it isn't permanently raised, so it doesn't stop the Thames being tidal as far upstream as Teddington.

This is what I remember, being an ex-Londoner. The Thames flowing backwards or at least stationary at Woolwich.

felltoearth · **#13** by **felltoearth** » Feb 23, 2020 4:43 pm

Alan B wrote:
TopCat wrote:All rivers, as they discharge into the sea, are at sea level. If the sea level rises, the point at which the river level is at sea level will move upstream.

This happens twice a day, of course, as the tide comes in. Take the Thames for instance. The river flows upstream while the tide is coming in, until the most downstream permanent lock*. Hence the term tideway, in relation to the Thames. Teddington (a couple of miles from where I live) is in West London, a long way from the estuary. Yet downstream from Teddington, the river flows in the upstream direction while the tide is coming in.

If the downstream flow is sufficient, then of course it will overflow the banks. This happens frequently at Richmond, in past winters making an ice rink out of Old Deer Park.

So as sea level rises due to global warming (and there's no 'may be' about it, OP) then such floods will become more likely.

However, by far the largest contributory factor, as several have said, is simply the volume of water which cannot be contained by the channel. All the serious flooding on the Thames (Chertsey and Staines, for instance, which are upstream of the tideway) in recent years has been caused by this.

Teddington has seen very high water levels recently, but it has (just) avoided being actually flooded, despite the fact that the weir can allow a lot of water to bypass the lock.

_____
* there is also a lock at Richmond. However, it isn't permanently raised, so it doesn't stop the Thames being tidal as far upstream as Teddington.

This is what I remember, being an ex-Londoner. The Thames flowing backwards or at least stationary at Woolwich.

Because the Thames is lower than sea level when the sea swells during tides and storms. Any river higher than sea level will not “slow down”

Ironclad · **#14** by **Ironclad** » Feb 23, 2020 9:39 pm

https://en.m.wikipedia.org/wiki/Severn_bore?wprov=sfla1
This creature passes my way. Worth a skim.

Cito di Pense · **#15** by **Cito di Pense** » Feb 24, 2020 9:08 am

Ironclad wrote:https://en.m.wikipedia.org/wiki/Severn_bore?wprov=sfla1
This creature passes my way. Worth a skim.

I especially liked this:

Bores are present on about 130 days in the year

Here, they are present every day. If the wave breaks, would that be called a crashing bore?

Ba-dumm-tsssssh.

Ironclad · **#16** by **Ironclad** » Feb 24, 2020 10:07 am

Go to your room!!

felltoearth · **#17** by **felltoearth** » Feb 24, 2020 1:50 pm

Cito di Pense wrote:
Here, they are present every day. If the wave breaks, would that be called a crashing bore?

Ba-dumm-tsssssh.

felltoearth · **#18** by **felltoearth** » Feb 24, 2020 1:50 pm