9 Autumn 2005: Water levels in Lincoln

"Water Levels in Lincoln" - Rob Wheeler

 

I

n 1743 John Grundy senior and John Grundy junior ran a chain of levels from Boston up the Witham to Lincoln.  They were rather proud of their work and the son was still using it 19 years later.  Nevertheless, it is clear that, by the time they reached Lincoln, a significant height error had accumulated.  I had hoped to use their survey to determine land levels at Fiskerton and hence the extent of subsequent shrinkage there but the accumulated error was too great for this.  A potential solution was to fix the Lincoln end with respect to a known height there.  That raised the question of whether mid-C18 water levels at Lincoln were known independently of the Grundys' survey.  The general history of the improvement of the river through Lincoln has been recorded by Hill[1] but his account contains few figures.

 

Fig 1 Profile of River Witham through Lincoln

Actually, the Grundys' survey provides a starting point: even though their results may require correction before they can be used absolutely, they ought to provide sufficient accuracy to show local profiles.  The profile of the river through Lincoln has been extracted from their map[2] and is shown at Figure 1, though it must be understood that only the ringed points come from the map.  More explanation can be found in the 1762 Report of John Grundy and John Smeaton on the practicability of improving the Fossdyke Navigation.[3]  That makes it clear that the water level in the Fossdyke was constrained on the one hand by the need to maintain an adequate depth for boats and the instability of the bottom, which made dredging difficult; and on the other hand by the existence of land draining into the Fossdyke that was a mere 5 inches higher than the Fossdyke's summer level.  This is encouraging in that one should be able to use the summer level of the Fossdyke (and hence of Brayford Pool) as a Lincoln datum.

The mechanism by which the height of water was maintained was the 'natural staunch' or shoal where the water flowed out of Brayford Pool towards High Bridge. The depth of water over this shoal in summer was only some 4 inches and carriages apparently used it as a short cut from one side of Brayford Pool to the other.

 

From High Bridge eastwards the bottom dropped fairly rapidly, and the water level likewise.  The 1762 Report quotes a fall of 14 inches to where the old Sincil Dyke came out[4], a further 5½ inches to Stamp End, and a further 10 inches to Bailey's Sluice, below Lincoln.  These figures are consistent with Fig 1.

These are of course summer levels.  In winter the Brayford Pool rose by as much as 4 feet (a 5ft 9in rise was quoted in 1782) and the depth of water through Lincoln increased correspondingly.  Even so, because of the narrowness of the channel, the rate at which water passed through Lincoln would be far less than occurs now when the river is in spate.  Consequently levels remained high for many weeks at a time.

 A major change occurred downstream in 1771 with the building of Lincoln Lock.  Under the 1762 Act[5], any locks constructed by the Navigation Commissioners were not to hold up water within less than 2ft of the adjoining low lands: making the river navigable was not to be done to the detriment of drainage.  In practice, this rule was stretched in summer by placing 'waste boards' so as to raise the height of the staunch temporarily.  At Lincoln, this was done on the authority of the Mayor[6].  Smeaton in 1782 stated that the water was held by this means a full 2 ft above the maximum permitted level.

 This practice may explain the variability in water depths reported at High Bridge.  In 1743 there was a mere 4 inches.  Jessop in 1792 reported that the depth under normal conditions was 1 ft, whereas a survey by Green in the same year records 2ft 4½ in.  Perhaps some form of periodic scouring also took place.

 The next major change followed the Act of 1792[7].  The river was to be dug out through Lincoln to allow the passage of boats drawing 3ft 6in; this would involve lowering the floor of High Bridge.  The Act leant over backwards to satisfy drainage interests downstream by laying down the principle that the river should not pass more water than would have flowed hitherto – but then added a rider allowing more water to be passed in time of flood so long as lands adjoining the lower Witham were not endangered thereby.  Detailed estimates for the work were submitted 6 Sep 1794 by John Tompson[8] and Michael Pilley[9] for deepening the river, underpinning the south wall and moving back the other wall to widen the waterway.  Lincoln Lock was rebuilt on a new site, the old lock being filled with spoil from the excavation[10].  The Sincil Dyke was extended to join the Witham below the new lock (Figure 2), thereby giving Lincoln a lower level for drainage purposes.  The old Thorn Bridge was replaced by a swivel bridge on the line of Broadgate Street.  Work was completed by 3 Aug 1797, when the Navigation Commissioners agreed to pay their share of the final expenses.

The new arrangements included a tunnel under the upper Witham so that lands in Boultham could drain via Great Gowt into the Sincil Dyke.  There was a desire to extend this to what became known as the Lincoln West Drainage.  John Rennie inspected the river through Lincoln on 6 Oct 1802[11] and produced a Report (the first of many) on 1 Dec.  He proposed that Swan Pool and Cuckoo Pool should be drained, the upper Witham and Fossdyke embanked, and their levels raised by a foot.  This proposal had already been made by Smeaton as early as 1762, something Rennie took care to acknowledge.  Lincoln Lock was to be rebuilt a second time and Sincil Dyke extended to a lower outfall at Horsley Deeps near Bardney.  The old outfall in due course became Clayton's dock.

 The rebuilding (on a new site) of Lincoln Lock took place only in 1826-7.  The navigation was temporarily diverted to the Sincil Dyke and all the improvements planned for the river through Lincoln were completed by 1828.

 Thereafter, concerns focused on High Bridge and the lack of headroom there.  Rennie had wanted to replace it; subsequent proposals were made to run a canal around it, but came to nothing.  In 1848, the Witham Navigation was taken over by the Great Northern Railway[12]; it was to be maintained but all impetus for improvement had now vanished.

 

 

Fig 2 Skeleton Plan of the Witham through Lincoln, showing alterations

 

This short account suggests that the only change to the level of Brayford Pool was the increase of 1 ft in 1826.  Its present height is 15.3ft above the OS Liverpool datum[13] (4.45m above Newlyn datum).  Prior to 1826, it was therefore about 14.3ft above Liverpool (4.15m above Newlyn).  This has therefore been used as a datum for tieing down the Grundys' work in Figure 1.

 Now Grundy had noted in 1745[14] that deepening the river through Lincoln would be "all Pick-Axe Work"; in other words, the bottom was hard.  Although the river may to some extent have cut into the hard material (whatever that was) in the post-glacial era, essentially Figure 1 gives us a west-east geological section to the east of High Bridge – we know that to the west of Brayford Head the floor was covered in recent sediment.  This section thus shows a scarp of harder material and it would seem that this scarp is holding back Brayford Pool and causes its eastern shore to lie where it does.  The scarp would also provide a firm route across the Witham – which is perhaps why Ermine Street/ Fosse Way crosses here.  In winter, it would not be surprising if the water overflowed this scarp in more than one channel: indeed, the present arrangements at Great and Little Gowts may represent a controlled version of this. 

 Is it legitimate to project back the mid-C18 arrangements as far as the Iron Age?  If so, perhaps the waterlogged ground north and south of the 'sand island' at 181-183 High Street[15] should be interpreted as a firm length of the scarp between points where the water overflowed in winter.  It may make the difference between this spot being "in the centre of a complex of pools and meres" and it being part of an established trackway, with open water to its west and summer grounds to its east.

 I end with some open questions.

1.      How can this view of the geology be reconciled with Michael Drury's north-south section of 1888?  If Drury is correct, what does the north abutment of High Bridge rest on?  His section implies mud.

2.      Where exactly was the Brayford shoal?  Documentary evidence seems to suggest it was across the lower Witham and offered a short cut from the east shore of Brayford Pool to the north shore.  Some writers have associated it with deposition of sediment brought down by the upper Witham in winter and deposited when the fast-flowing waters entered the relatively still Brayford Pool.  Theory suggests that siltation would occur by the exit from the upper Witham and in due course extend across to the north shore and might form a barrier from where the University Library stands now to the Square Sail.  This would still provide the impediment to navigation that is described but does not fit other aspects (such as stepping stones) so well.  So perhaps the real impediment to navigation came from a shelf of hard material where the water flowed out into the lower Witham.

 3.      If this shelf were at Brayford Head, it would form the obvious crossing point, so why was High Bridge not built there?  Perhaps the strata form a double scarp, of which the higher formed the best route way and determined the siting of High Bridge but had subsequently been cut away, whereas the lower remained as the mechanism by which summer levels of Brayford Pool were maintained.  Could the deposition of sand on or against this scarp, and between winter overflow channels, account for the 'sand islands' at the 181-183 High Street excavation?

4.      The old floor of High Bridge – a wooden structure forming part of the foundation is implied – was taken up in 1796.  Its upper surface must have been at about 11.7ft above Liverpool (3.35m above Newlyn) and the new bottom needed to be at least a foot lower.  How old was this old floor?


[1] Sir Francis Hill, Georgian Lincoln, 1966, p131-135

[2] John Grundy, A Map of the Antient River Witham as Reduced from the Original, 1743. LCL Map 661.

[3] The Fossdyke reports seem not to survive at Lincoln but can be found in Reports of the late John Smeaton, FRS, Vol I (London, 1837)

[4] The line of the old Sincil Dyke is effectively shown on Padley's 1868 plan by the broken line labelled 'Line of Old City Wall'.

[5] 2 Geo III c32

[6] LAO Monson 7/17/65

[7] 32 Geo III c107

[8] LCL UP 2164.  He may be the 'Mr Thompson' who was surveyor to the River Don Navigation.

[9] He was jointly leasing the Witham tolls – an early Public/Private Partnership – and, according to Smeaton (1782) also acted as agent in navigation matters for Ellison.

[10] PRO RAIL 885/1, folder WTN 1/1, minute of 25 Aug 1796

[11] NLS MS 19870

[12] 9&10 Vic c71

[13] Because the OS determined levels with respect to Liverpool once only, they are effectively fixed with regard to the underlying geology.  This is more useful for historical work than a datum like Newlyn, which changes (with respect to the underlying geology) every time eastern England sinks a little lower.

[14] A Further Illustration of Messrs. Grundy's Scheme for Restoring and making Perfect, the navigation of the River Witham, from Boston to Lincoln (Stamford, 1745)

[15] D Stocker City by the Pool, 2003, p26