Page updated 20 December 1999
Drainage
The text on this page is taken from a 10 page A5 format booklet
which also includes a hand drawn map (36k).
The Middle Level
The Area, The Problems and The Solutions
Introduction
The Middle Level Commissioners are responsible for land drainage in that part of the Great Level Cambridgeshire Fens reclaimed during the l7th Century which lies between the River Nene to the north west and the Great Ouse ( Old Bedford River) to the east, and which is bounded by low clay hills to the south and west and by the marine silts of Marshland to the north.
The Middle Level has a total catchment area of 70,000 hectares of which approximately 48,500 hectares are rateable fenland. The latter area, most of which lies below mean sea level, is divided into 39 Internal Drainage Districts from which run-off is pumped to the main Middle Level arterial drainage system by 78 pumping stations.
Formation of the Fens
In the distant past the British Isles were part of the continental land mass and the rivers of eastern England were tributaries of the River Rhine which flowed northwards aver a flat plain and out into the sea just west of present day Norway.
During the Ice Ages the Arctic ice caps spread southwards as far as the Thames Valley on several occasions moving clay from higher ground and depositing it to form the thick layers of clay which today lie beneath the fen deposits.
Melting of the ice when it retreated caused a gradual rise in sea level and the sea advanced across the north sea plain, between the Lincolnshire Wolds and the heathlands of Norfolk and into the low lying area of present day fen-land. When the sea subsequently receded the latter area became a freshwater swamp cut off from the sea by silt banks (estuarine deposits) which the rivers formed in the area now known as Marshland.
Inside the swamp the continuous growth and decay of trees and other vegetation formed thick layers of peat on top of the earlier glacial deposits. For a time this process was interrupted when the sea broke in again so that over much of the fens there are two layers of peat divided by a thin layer of buttery clay which was deposited by the sea.
A maximum depth of approximately 9 metres of peat was deposited in the south west part of what is now the Middle Level Area.
Silt brought from inland by the rivers, combined with the peat deposits, slowly raised the ground levels but large expanses of water remained (Whittlesey Mere etc.) and the fenland rivers which followed slow meandering courses frequently overflowed and flooded the surrounding land.
In places the underlying clay projected above the inundated area forming the islands upon which towns and villages have grown up.
Reclamation of the Fens
John Morton, Bishop of Ely, who in 1490 constructed a straight cut over 19 kilometres long from Stanground to Guyhirne, was aware that large straight drains were an answer to the problem of overflowing rivers. In this way large quantities of water from the hills inland could be conveyed straight to the sea without flooding the fens but the theory was not carried further until the l7th Century.
In 1630 the Earl of Bedford and thirteen other "Gentlemen Adventurers" employed the Dutch Engineer Cornelius Vermuyden to undertake the drainage of the fens. The object was not to drain the land so that it was permanently dry, but to provide lands fit for grazing in the summer ( "Summer Lands" ) and, in an attempt to achieve this, the Old Bedford River was cut 34 kilometres long between Earith and Salter's Lode to relieve the meandering course of the River Ouse.
In,1650 Vermuyden was again employed to execute a scheme which would make the fens dry all year round, and the New Bedford River was cut to the east of the Old Bedford (the latter had been found inadequate) with washlands in between to store flood water. The outer banks were built up to prevent overtopping into the adjacent fen areas.
The works finally segregated the Middle Level Area completely from the adjacent fens and the Forty Foot, 'Twenty Foot and Sixteen Foot Rivers were constructed to direct drainage of the Middle Level to Salter's Lode and Welches Dam.
Drainage of the Middle Level since the Reclamation
Following reclamation it soon became obvious that the work of drainage would never finish and that constant maintenance and improvement work would be necessary, due to a continuous lowering of land levels caused by shrinkage of the peat soils.
In 1844, to provide an improved gravity outfall, the Middle Level Main Drain was excavated through the silts of Marshland to connect the Middle Level Area with a point some 14 kilometres down the River Ouse at St. Germans, where tidal levels were up to 2.5 metres lower than those at the existing upstream outfall at Salter's Lode. Since construction of the Main Drain, run-off from the entire Middle Level Area has been discharged into the River Ouse at St. Germans by one means or another.
In 1862 the Middle Level was separated from the Bedford Level Corporation by Act of Parliament, and since that time the Middle Level Commissioners have executed a continuous Programme of improvement works, gradually lowering water levels to counteract the effects of peat shrinkage and increasing run-off.
By the 1920's gravity drainage alone was no longer sufficient to protect the shrinking fens from flooding and construction of a pumping station on the Main Drain at St. Germans was imperative.
St. Germans Pumping Station
St. Germans Pumping Station was constructed between 1930 and 1934 by direct labour and with the aid of a special grant conditional upon 75% of the workforce coming from the distressed areas.
The station, which consists of two identical pump houses separated by two 35 feet wide gravity sluice pens, has concrete foundations supported on 1315 reinforced concrete bearing piles.
Three similar 1000 H.P. diesel driven pumping units were initially installed and gravity drainage was still used when water levels in the River Ouse allowed.
By 1951 however, gravity drainage was no longer effective and further lowering of water levels in the Middle Level System and increases in discharge made it necessary to install a fourth diesel driven unit of 1500 H.P. The need for this fourth pump had been anticipated when the pumping station was constructed, and it was installed in the south pump house where the necessary space had been provided.
Run-off had again exceeded pumping station capacity under worst conditions by 1966, and the capacity of the two original pumping sets in the north engine house was therefore increased in 1969/70 by installing new impellers of modified design and increased rotational speed driven by 1550 H.P. electric motors.
The total capacity of St. Germans Pumping Station when pumping against the worst tidal period was thus increased from 2500 tonnes per minute in 1934 to 3990 tonnes per minute in 1970.
This increase in capacity was necessary to keep pace with the large increases in run-off would have occurred due to substantial improvements in land drainage standards over the years, including the extensive execution of tile drainage schemes and improvement of highland brooks to relieve local valley flooding in the west and south of the catchment area.
Since the construction of St. Germans Pumping Station over 100 new pumping stations have been constructed by the Minor Boards in the Middle Level Area and the total pumping capacity of the Boards has been increased by around 50% since 1940 to cater for the considerable improvements in field drainage.
The Middle Level Major Improvement Scheme (1977 to 1983)
By the early 1970's lowering of water levels, particularly in the south west of the drainage system, was again necessary to counter the effects of increasing run-off and land shrinkage, but the degree to which further channel improvements could re used to lower water levels was by now severely limited by lack of gradient.
The problem in the south west of the drainage system was of major concern. This part of the system is remote from St. Germans Pumping Station, and land levels in the area, which are already the lowest in the catchment, coincide with the maximum depths of remaining surface peat.
Clearly a situation which was already critical would continue to deteriorate over the years and the time had arrived when a new approach was necessary which would provide a satisfactory long term solution to the problems of the area.
Investigations were therefore carried out to establish a scheme most ideally suited to deal with the future drainage requirements of the area and with the aid of proposals drain up by the Commissioners' former Engineer Mr. L.F. Fillenham, and the assistance of Binnie & Partners, a combination of works was agreed with the Ministry of Agriculture, Fisheries and Food in 1976 under the heading of "Middle Level Major Improvement Scheme".
An economic appraisal by Mr. D.B. Wallace of Cambridge University showed the scheme to have an unusually large benefit/cost ratio of 9.37:1, and when submitted to the Commission of the European Communities for grant from the E.E.C. Agricultural Fund, a maximum subsidy of £824,151 was awarded in addition to the 50% U.K. Government grant which had already been secured.
The works which have been designed and supervised by the Commissioners' own Engineering Staff, commenced in January 1977.
To resolve the problems of the South west area a booster pumping station has been constructed at Tebbits Bridge on Bevills Leam, and the capacity of the station has been geared to that of the downstream drainage system by regulating flows from the highland brooks which enter this part of the system from the low clay hills in the west of the catchment.
Water levels can now be lowered over the years as necessary within the boosted area without disturbance of the downstream system. The problem of increasing run-off has been eliminated, and a long term solution to the problem of peat shrinkage in the south west area provided.
The pumping station contains six similar axial flow pumps, of which three are driven automatically by electric motors and three by manually operated diesel engines. This arrangement provides great flexibility of pumping and the plant is small enough to be handled comparatively easily through removable roof hatches, although the total capacity of 1080 tonnes per minute is equivalent to , of that, available at St. Germans.
Regulation of the highland run-off is being achieved by diverting the highland brooks to Great Raveley Drain by means of a new catchwater channel and by utilising Woodwalton Fen National Nature Reserve as a flood storage area.
The Catchwater Drain, which has best constructed along the edge of the highland fringe, has an average top width of around 30 metres and is approximately 12 kilometres long. Construction of the new charnel involved the excavation of over 450, 000 cu.m. of material and the installation of 12 vehicular bridges, including reinforced concrete box culverts under the A1 and B660 carriageways. Great Raveley Drain was widened and deepened to accommodate the increased run-off from the Catchwater Drain.
To utilise Woodwalton Nature Reserve for flood storage purposes, a control sluice has been constructed on Great Raveley Drain and impounding embankments have been built to contain the water which flows into the reserve when the control sluice is brought into operation.
The control sluice has two 3.5 m wide electrically operated weir type penstocks which can be raised or lowered to regulate flow to Bevills Leam Pumping Station as the situation demands.
The impounding works facilitate the storage of 1.8 million cubic metres of water on the Nature Reserve (200 ha.) when flooded to a maximum depth of 0.9 metre. This will allow reduction of flow at the control sluice to 400 tonnes per minute under maximum design conditions, which is less than 1/5th of the maximum designed flow from the highland catchment.
The Scheme will benefit the Nature Reserve by maintaining a high water table there in the future, and clay for the impounding embankments was obtained by excavating two meres in the Reserve. The Scheme has therefore been of mutual benefit to nature conservation and land drainage, and has been carried cut with the approval and cooperation of English Nature formerly the Nature Conservancy Council.
The channels upstream of Bevills Leam Pumping Station have been improved as necessary, including reconstruction of three bridges. Construction of an isolating lock on the Old River Nene at Lodes End near Ramsey brought the scheme into full operation by the end of 1983.
In the remainder of the system, low banks on the Old River Nene were raised and clay-puddled where necessary to reduce seepage in the early stages of the works, and major improvements to the Middle Level Main Drain, Sixteen Foot and Forty Foot Rivers have been carried out to maintain acceptable, water levels.
These channel improvements involved the excavation of over 700,000 cu.m. of material, and nine old bridges/culverts on the Sixteen Foot River, which were in poor condition and severely restricted flow, were demolished and replaced by eight new reinforced concrete structures.
Finally, the No.3 unit at St. Germans Pumping Station has been uprated by installation of a new impeller of modified design, together with a modern diesel engine and gearbox designed to increase the rotational speed of the pump from 84 to 98 r.p.m. This has furnished No.3 pump with a capacity equal to that.of one of the electrical units, and provides a considerable increase in the diesel driven capacity available at the station. An increase in pump capacity of 200 tonnes per minute required by the Major Scheme has been more than met.
Bevill's Leam Pumping Station
Details
| Catchment Area | Low lying fenland drained into Middle Level system by I.D.B. Pumps.......... | 9990 hectares |
| Highland.......... | 8280 hectares | |
| Total... | 18270 hectares | |
| Designed Capacity | I.D.B. pumps + regulated highland flow | 1080 tonnes per minute. |
| Pumping Plant | Contractor: W.H. Allen | |
| Pumps: W.H.Allen 1000mm diameter vertical spindle axial flow pumps, each of 180 tonnes per minute capacity at 367 r.p.m. | 6 No. | |
| Motors:Laurence Scott 168kw squirrel cage induction motors | 3 No. | |
| Engines: Dorman 8 cylinder, 274 b.h.p. normally aspirated diesel engines running at 1200 r.p.m. | 3 No. | |
| Gearboxes: David Brown 90° reduction gearboxes with Twiflex clutches. | 3 No. | |
| Main Control Panel: Cutler Hammer Europa. | ||
| Engine Starter Panels: Eledyne Ltd. | ||
| Mains Failure Generator: C. & G. Couzens Electrical Engineers Ltd. | ||
| Telemetry: K.D.G. Bestal-Dean. | ||
| Civil Engineering | Bovis Civil Engineering. |
| Sluice Gates | Ham Baker & Co Ltd. |
| Superstructure | Steelwork: D.A. Green & Sons Ltd. Brickworks: T. Hussey (March) Site Works: Direct Labour |
| Weedscreen Cleaning | Richard Smalley (Engineering) Ltd. |
| Associated Drainworks | P. Dalton Contracts (Earthmoving) Ltd. |
St. Germans Pumping Station
| General Description of Pumps | ||
| Makers: Allen Gwynnes Pumps Ltd | ||
| Type: Double Entry Centrifugal | Double Inlets 84" dia | |
| Single Outlet 102" dia | ||
| Impeller 95" dia | ||
| Arrangement of Plant | ||
| (A) | North Engine House (Units 1 and 2) | |
| 2 No original units modified 1969/70 | ||
| Pumps: As above | Rotational Speed 98rpm | |
| Electrical Motors: Laurence Scott Squirrel Cage | 1500 HP @ 98rpm | |
| (B) | South Engine House (Units 3 and 4) | |
| 1 No original unit modified in 1982/83 | ||
| Pumps: As Above | Rotational Speed 98rpm | |
| Engine: W.H. Allen | 3 cylinder turbo charged | |
| 1550 HP @ 1000rpm | ||
| 1 diesel driven unit (4th Unit) installed 1951 | ||
| Pumps: As Above | Rotational Speed 95rpm | |
| Engine: Crossley Premier | 8 cylinder V-A-V | |
| 1550 HP @ 215rpm | ||
Pumping Capacity of Individual Units
| Static Head (ft) | Capacity of Each Unit (tons per minute) | ||
| Fourth Unit 1951 |
Original Units modified 1969/70 |
Original Units modified 1982/83 |
|
| 5 | 1110 | 1190 | 1190 |
| 10 | 995 | 1080 | 1080 |
| 18 | 610 | 870 | 870 |
The table indicates the considerably greater capacities of the fourth unit installed in 1951 and the original units after the modifications in 1969/70 (2No.) and 1982/83 (1No.).
Total Capacity of Plant when Pumping Against Worst Tidal Period
| 1) | 3 Original units in 1934 | ..2500 tons/minute |
| 2) | After installation of fourth unit in 1951 | ..3500 tons/minute |
| 3) | After modification of two original units in 1970 | ..3900 tons/minute |
| 4) | After modification of third original unit in 1982/83 | ..4260 tons/minute |