River Nene conundrum

[blackrose]

I'm currently moored at Billing Aquadrome for winter where at the peak of the recent flooding the water level rose about 5 and a half ft. At my usual mooring at Cogenhoe a few miles downstream, the river level came about 1" from breaching the top of the thick wooden plank that holds the the steel piling to the river bank. That's a rise of about 3 and a half ft. There is open river with no locks or weirs between Billing and Cogenhoe. In the 4 years I've been living here I've never figured out how that's possible and none of the locals or other boaters that I've talked to seem to know (to be honest none seem very interested ?)

 

It must be running off into fields, etc, but is there really a 2ft water level gradient between Billing and Cogenhoe during flood conditions? It seems difficult to believe.

Edited January 1, 2021 by blackrose

[Phoenix_V]

I am sure the Thames has similar gradients between weirs, that is why mooring above the weir rather than below is always advised?

[Sea Dog]

37 minutes ago, blackrose said:

It must be running off into fields, etc, but is there really a 2ft water level gradient between Billing and Cogenhoe during flood conditions? It seems difficult to believe.

It seems difficult to believe to me too!

[TheBiscuits]

43 minutes ago, blackrose said:

It must be running off into fields, etc, but is there really a 2ft water level gradient between Billing and Cogenhoe during flood conditions? It seems difficult to believe.

 

I don't know the Nene, but on Google maps it appears to split into two between Billing and Cogenhoe and rejoin further downstream.  Is it simply that half the water goes the other way?

[Keeping Up]

I can certainly imagine such a gradient. There could be several inches difference through the bridge below Billing for example. As referred to in post #2 the level above a weir changes by much less than that below it; the proximity of the weir crest prevents it rising too far but it requires a significant gradient to push the large quantity of water along the intervening miles to the next one.

[blackrose]

14 minutes ago, TheBiscuits said:

 

I don't know the Nene, but on Google maps it appears to split into two between Billing and Cogenhoe and rejoin further downstream.  Is it simply that half the water goes the other way?

 

Yes the river divides shortly downstream of Billing and then again at Cogenhoe where the mill stream splits off before rejoining the river. I moor at the head of the mill stream on the south bank.  That could well be the answer, but then it still seems counter-intuitive to me that a river can be at very different levels along the same reach creating a gradient.

 

Edited January 1, 2021 by blackrose

[Graham Davis]

Not just weirs, but any constriction against the flow, be that a bridge or even the narrowing of the valley the river runs through.

Good example of the first can be seen at many bridges at a time of strong flow. I've seen nearly a foot difference either side of Worcester Bridge, or Eckington Bridge on the Avon. 

And an example of the second can be seen in the Wye going South from Ross on Wye.

[blackrose]

5 minutes ago, Keeping Up said:

I can certainly imagine such a gradient. There could be several inches difference through the bridge below Billing for example. As referred to in post #2 the level above a weir changes by much less than that below it; the proximity of the weir crest prevents it rising too far but it requires a significant gradient to push the large quantity of water along the intervening miles to the next one.

Yes, you must be right.

[TheBiscuits]

30 minutes ago, blackrose said:

it still seems counter-intuitive to me that a river can be at very different levels along the same reach creating a gradient.

 

If it didn't it would be a lake, not a river ...

[stagedamager]

The river narrows significantly at Billing, and the bridge creates a large bottleneck, probably forcing the water into the marina lake as the path of least resistance. If the washland sluices are opened upstream, the volume of water will tend to fill the lake before levelling off.

Having tried, and failed, to take a full length breasted pair out of the marina and turning upstream, there is quite a significant flow even in non flood conditions.

[Mike Todd]

The rise will depend on width. In the extreme, imagine at one point the river is infinitely wide (well, something like Tixall anyway) and at another point it is just over a boat's width. With a constant flow, the impact of an increase at the narrow point will be significant whilst at the wide it will be negligible. 

 

The height of flood water at one point is no guide at all to what it will be elsewhere.

[magpie patrick]

In extreme flood conditions the gradient can be very significant - locks and weirs disappear, which, if you think about it, means that the river immediately below the weir is much more flooded than the river immediately above - if each lock were six feet lift then the river is now sloping six feet from lock to lock where it is normally (more or less) level and each weir is level when it should have a fall of six feet

[Tam & Di]

Wind can have an astonishing effect too. The 13½ mile Paddington Arm of the Grand Union can be as much as 9" higher at one end than the other after a couple of days steady wind in one direction.

 

Tam

[matty40s]

You also have the fact that the Nene Flood Alleviation Plan is designed to protect Northampton and Northampton alone from flooding, Billing has water dumped on it, not only from the Nene, but also from Billing Brook which drains a large area north of the city in the Overstone/Sywell direction. Once Overstone Park lake is full, sluices are full open to the Nene.

Below to Cogenhoe, the water has a chance to spread out over the flood plain.

Edited January 1, 2021 by matty40s