The Middle Level - the area, the problems and the solutions

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 at Tebbits Bridge 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 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 been 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 Natural England, 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.

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