Understanding the Trent's tidal flow pattern.

[nicknorman]

Or not.

 

Anyway, here's what I think I know. Any corrections gratefully received!

 

The incoming tide surges up the river quite fast, but there is still a substantial lag between HW at hull, and HW further up the river, say 3.5hrs to Torksey and 5 hrs to Cromwell. By the time it reaches Cromwell it is pretty much spent so there is only a few feet level difference between HW and LW.

 

Whilst at hull the tidal cycle is roughly sinusoidal, the further up the river you go, the shorter the flood time and the longer the ebb, so that at Torksey it is perhaps 3 hrs flood and 10 hrs ebb (varies according to tide size and "fresh"). So the level rises rapidly from LW to HW, and then falls off very slowly back to LW.

 

Thus, if depth of water over the cill is an issue, the remarks in the Trent guide about the lock being available 3 hrs either side of HW seems quite wrong.

 

Finally, the bit I definitely don't understand is that according to the Lockie, when there are spring tides the depth of water over the cill is not likely to be an issue at pretty much any time, whereas on neap tides it will be. Tides are reasonably symmetrical in that whilst a spring tide has a high high, it also has a low low. Therefore I don't see why a spring tide should result in a generally higher level of water upstream, rather than merely a larger variation in level upstream.

[Naughty Cal]

Your lockie was talking shite.

 

It isn't as much an issue on neaps as it is on springs.

 

The tide runs in for two (ish) hours, has a brief period of slack then ebbs for the rest of the tide.

[Tony Dunkley]

Or not.

 

Anyway, here's what I think I know. Any corrections gratefully received!

 

The incoming tide surges up the river quite fast, but there is still a substantial lag between HW at hull, and HW further up the river, say 3.5hrs to Torksey and 5 hrs to Cromwell. By the time it reaches Cromwell it is pretty much spent so there is only a few feet level difference between HW and LW.

 

Whilst at hull the tidal cycle is roughly sinusoidal, the further up the river you go, the shorter the flood time and the longer the ebb, so that at Torksey it is perhaps 3 hrs flood and 10 hrs ebb (varies according to tide size and "fresh"). So the level rises rapidly from LW to HW, and then falls off very slowly back to LW.

 

Thus, if depth of water over the cill is an issue, the remarks in the Trent guide about the lock being available 3 hrs either side of HW seems quite wrong.

 

Finally, the bit I definitely don't understand is that according to the Lockie, when there are spring tides the depth of water over the cill is not likely to be an issue at pretty much any time, whereas on neap tides it will be. Tides are reasonably symmetrical in that whilst a spring tide has a high high, it also has a low low. Therefore I don't see why a spring tide should result in a generally higher level of water upstream, rather than merely a larger variation in level upstream.

 

I did make a brief reference to your query about the depth over Torksey outer (bottom) cill 3 hours ether side of (local) HW in an answer in your 'Boston and back' topic, but perhaps more info. should have been included.

Firstly, you need to be clear that Torksey HW minus 3 hours is in fact Torksey LW minus approximately 1 hour. With sufficient fresh coming downriver there could be several feet over the bottom cill at this time, but after a longish dry spell with no fresh coming down you could stand on the cill at LW Neaps, in a good pair of boots, without even getting your feet wet.

The depth on the cill 3 hours after (local) HW can also vary by big amounts, down to 2', or less. Poor tides (below prediction Neaps) during a Summer dry spell can struggle to make 3' at HW, and the rate of fall in the water level is fastest in the first half hour after HW.

Big Springs can make upwards of 7' to 8' on the cill at HW, even without the assistance of any fresh. As I said before, because so many factors have a marked effect, including atmospheric pressure over the North sea, wind direction and strength both at sea and in the Humber and Trent, it's not possible to say, with any reasonable accuracy, what depths can be expected at any particular time for more than a few hours ahead, and broad generalizations such as the suggestion that Torksey lock will always be available 3 hours either side of HW are clearly just nonsense.

The lock keeper you were talking to is quite right and Springs do result in considerably more depth in the upper reaches of a tidal river at LW than do Neaps, but I'd rather wait awhile before explaining why in order to give the Forum's self appointed expert on all matters tidal, another opportunity to make herself look and sound even more foolish and ignorant than she already has. In fact, the explanation is so simple that you will probably have figured it out for yourself . . . . just think about why it takes longer to empty a sink that's been filled to higher level than the same sink when containing much less water.

Edited July 7, 2015 by Tony Dunkley

[Tony Dunkley]

Your lockie was talking shite.

 

It isn't as much an issue on neaps as it is on springs.

 

 

No, he wasn't, you're the one doing that . . . . yet again.

[GoodGurl]

Your lockie was talking shite.

 

It isn't as much an issue on neaps as it is on springs.

 

The tide runs in for two (ish) hours, has a brief period of slack then ebbs for the rest of the tide.

 

 

 

No, he wasn't, you're the one doing that . . . . yet again.

Can we please not start another thread of bickering and stay on topic.

GG

[Scholar Gypsy]

 

 

The lock keeper you were talking to is quite right and Springs do result in considerably more depth in the upper reaches of a tidal river at LW than do Neaps, but I'd rather wait awhile before explaining why in order to give the Forum's self appointed expert on all matters tidal, another opportunity to make herself look and sound even more foolish and ignorant than she already has. In fact, the explanation is so simple that you will probably have figured it out for yourself . . . . just think about why it takes longer to empty a sink that's been filled to higher level than an identical one containing much less water.

 

Thanks - this is interesting & I can see why it could be the case, depending on the shape of the river etc.

 

So there is a point on the river (which will move I guess according to the amount of fresh water at Cromwell?), such that below that point LW is lower at Springs than at Neaps (that is certainly the case at Hull, for instance); and above that point LW is higher at Springs than at Neaps.

 

Where this point is (I wonder if there is a proper technical name for it) could be modelled by some big computers, or just observed in practice. I think you are saying it is below Torksey?

[Scholar Gypsy]

 

Thanks - this is interesting & I can see why it could be the case, depending on the shape of the river etc.

 

So there is a point on the river (which will move I guess according to the amount of fresh water at Cromwell?), such that below that point LW is lower at Springs than at Neaps (that is certainly the case at Hull, for instance); and above that point LW is higher at Springs than at Neaps.

 

Where this point is (I wonder if there is a proper technical name for it) could be modelled by some big computers, or just observed in practice. I think you are saying it is below Torksey?

 

Just checked this out with the PLA's tide tables. So at Tower Bridge LWS is 0.9m below LWN, and this difference gradually reduces to zero by Brentford (where low water level is between 1.9m and 1.0m above LW at Tower - at Springs and Neaps respectively).

 

Unfortunately the next data point upstream (where one might expect this number to go negative) is at Richmond, which is the wrong side of the half tide weir, which rather spoils the experiment.

[Tony Dunkley]

 

Thanks - this is interesting & I can see why it could be the case, depending on the shape of the river etc.

 

So there is a point on the river (which will move I guess according to the amount of fresh water at Cromwell?), such that below that point LW is lower at Springs than at Neaps (that is certainly the case at Hull, for instance); and above that point LW is higher at Springs than at Neaps.

 

Where this point is (I wonder if there is a proper technical name for it) could be modelled by some big computers, or just observed in practice. I think you are saying it is below Torksey?

 

Yes, it's a long way below Torksey, and particularly noticeable at Gainsborough where you can count the stones showing on the piers of Gainsborough Bridge on the last of the ebb.There can be up to a bit over 2' more depth at LW Springs than at LW Neaps.

The shallowest bar that's closest to Trent End is at Mere Dyke, so I would say that's likely to be where the LW depths start to conform with the normal pattern of LW Springs running down to less than LW Neaps.

Edited July 7, 2015 by Tony Dunkley

[nicknorman]

 

Thanks Tony. Just out of interest, with the river around its "summer average" fresh (according to Cromwell Lockie) and with a tide of 7.6m @Hull (ie modest spring), last night at local LW the gauge was showing 0.9m over the cill, enough for most narrowboats. Thus the lock was available throughout the tidal cycle (opening hours permitting)

 

The lock keeper you were talking to is quite right and Springs do result in considerably more depth in the upper reaches of a tidal river at LW than do Neaps, but I'd rather wait awhile before explaining why in order to give the Forum's self appointed expert on all matters tidal, another opportunity to make herself look and sound even more foolish and ignorant than she already has. In fact, the explanation is so simple that you will probably have figured it out for yourself . . . . just think about why it takes longer to empty a sink that's been filled to higher level than the same sink when containing much less water.

No I still don't really get this. Yes of course a higher high will push more water up the river, but since flow rate is related to slope, I would expect the water to flow out into a lower sea (spring low) level faster than it would into a higher sea (neap low). But there is some mechanism making it a non-symmetrical effect.

 

Just thinking around that then, could it be related to the fact that mass flow is in fact gradient plus depth? With a spring flood, a large volume of water comes up both in terms of flow rate and depth, whereas the ebb is primarily limited by reduced level during the course of the river, rather than an increased gradient near the end?

 

Still a bit puzzled! Although as you say fresh, tides, atmospheric pressure all play a part, I still think it is a deterministic system ie can ultimately be mathematically modelled. Only wind would be difficult to fully model since it is an unknown at local level and obviously very variable in its effect throughout the course of the river.

 

I can't help feeling that study and modelling of these effects must have been done by academia, it would make a great PhD for a hydrologist. I know a hydrology lecturer at Dundee university so I'll ask him if there are any papers on the subject.

[peterboat]

It must be the variables that decide if there is going to be a hagar or not. I have moored at gainsborough 4 times twice hagar twice not

 

Peter

[nicknorman]

It must be the variables that decide if there is going to be a hagar or not. I have moored at gainsborough 4 times twice hagar twice not

 

Peter

Presuming hagar = aegir it was explained to me by the Cromwell Lockie that shallow water promotes the aegir in the same way that waves and tsunamis build up as the water shallows approaching a beach. So low levels of fresh (shallower at LW) and a high spring tide seems to be the main factors? Edited July 7, 2015 by nicknorman

[Tony Dunkley]

Thanks Tony. Just out of interest, with the river around its "summer average" fresh (according to Cromwell Lockie) and with a tide of 7.6m @Hull (ie modest spring), last night at local LW the gauge was showing 0.9m over the cill, enough for most narrowboats. Thus the lock was available throughout the tidal cycle (opening hours permitting)

 

 

No I still don't really get this. Yes of course a higher high will push more water up the river, but since flow rate is related to slope, I would expect the water to flow out into a lower sea (spring low) level faster than it would into a higher sea (neap low). But there is some mechanism making it a non-symmetrical effect.

 

Just thinking around that then, could it be related to the fact that mass flow is in fact gradient plus depth? With a spring flood, a large volume of water comes up both in terms of flow rate and depth, whereas the ebb is primarily limited by reduced level during the course of the river, rather than an increased gradient near the end?

 

Still a bit puzzled! Although as you say fresh, tides, atmospheric pressure all play a part, I still think it is a deterministic system ie can ultimately be mathematically modelled. Only wind would be difficult to fully model since it is an unknown at local level and obviously very variable in its effect throughout the course of the river.

 

I can't help feeling that study and modelling of these effects must have been done by academia, it would make a great PhD for a hydrologist. I know a hydrology lecturer at Dundee university so I'll ask him if there are any papers on the subject.

 

I think it's the influence of the bars in the lower end of the river limiting the rate that water can run back out that causes the upriver increase in difference between the periods of flood and ebb, and the inverted Spring / Neap LW effect.

As you say, the ebb is affected by the dropping (reducing) water level, but that reducing water level, is also having to flow through a reducing transverse X-sectional area and then in the last few miles it has to pass over the bars, which are fixed height obstacles to water flow, rather like a weir.

[nicknorman]

Thanks Tony, it makes sense. One thing is clear, from the lack of input to this thread from more than a handful, I am not the only one who doesn't understand the issues! I guess everyone else is happy just to take the lock keepers' advice.

[cuthound]

Presuming hagar = aegir it was explained to me by the Cromwell Lockie that shallow water promotes the aegir in the same way that waves and tsunamis build up as the water shallows approaching a beach. So low levels of fresh (shallower at LW) and a high spring tide seems to be the main factors?

He might mean there are Vikings about