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Loch Thom and Greenock Cut
Greenock, Inverclyde, Scotland, UK
associated engineer
Robert Thom
date  1825 - 1827
era  Georgian  |  category  Water Supply/Pipes  |  reference  NS251733
ICE reference number  HEW 616
Loch Thom and the open cut aqueduct known as Greenock Cut that runs north from the loch system down to Greenock on the River Clyde, once supplied water to the town and power to industry. Its engineer Robert Thom designed the network and its sluices, and constructed waterfalls to provide the water pressure for power generation. The cut can still be seen, though the water now runs in pipes.
In an earlier scheme, James Watt (1736-1819) had planned and supervised the provision of a water supply for Greenock following a 1773 Act of Parliament. Water from two small reservoirs constructed on the lower slopes of Whin Hill flowed through wooden pipes to a cistern at Well Park. Watt and other engineers of the day, including John Rennie (1761-1821), deemed impracticable the suggestions that a body of water known as Shaws Water could be used to boost the supply. However, by the 1820s the existing supply was inadequate and often disappeared altogether in dry weather.
In 1824, hydraulic engineer Robert Thom (1774-1847) was asked to survey the hinterland of Greenock by Sir Michael Shaw Stewart (1788-1836). He reported that enough water could be supplied for domestic and industrial uses using an aqueduct from a new reservoir that could be created by damming Shaws Water.
Thom had previous experience. At Rothesay Cotton Mills (NS086646) on the Isle of Bute, which he purchased with William Kelly in 1813 (he became sole owner in 1826), there was insufficient water to provide power. An attempt at powering the mills using coal proved too expensive, so Thom built a series of ‘cuts’ to convey water to the site. He doubled the capacity of the service reservoir that fed the cuts by raising its dam, and installed self-regulating sluices of his own design to control water levels. By 1821 he had increased the combined power of the two mills from about 22kW to around 52kW.
With this impressive track record, Thom was engaged to design and direct the new scheme for Greenock. The Shaws Water Joint Stock Company was incorporated in June 1825 with a capital of £31,000, and the work was carried out piecemeal by contractors responding to local advertisements.
An earth embankment dam 14.6m high was built around Shaws Water and land south of it, impounding a reservoir 119.3ha in area and containing over 8 million cu m of water. It was called the Great Reservoir originally, and later renamed Loch Thom after its engineer.
The nearby Compensation Reservoir was built to the west of the loch, and lived up to its name. Its embankment is 7m high and it contains 409,400 cu m of water, with a surface area of 16.2ha. A spillway cut feeds water from Loch Thom, which is higher, into the northern end of Compensation Reservoir. From there it flows through an outlet (NS247720) originally fitted with six timber sluices, and discharges into the 8.9km open aqueduct of Greenock Cut.
Greenock Cut follows the hillside at a gradient of 1 in 600 to Long Dam at the Town Head service reservoir (NS269748) at Overton, about 156m above the Firth of Clyde. Here water was diverted into a watertight covered stone channel 380mm square, and through three filters each 15.2m long, 3.7m wide and 2.4m deep before joining the town’s supply.
Water first flowed along Greenock Cut on 16th April 1827. The cut falls some 15.2m over its length and is about 1.5m wide, lined with rubble masonry. It has a downside bank with an inner clay puddle wall and when operational contained up to 600mm of water.
Flow was controlled by the sluices at Compensation Reservoir but water from various streams also contributed. To prevent overflow when the streams were full, stone sluice houses (with vaulted roofs) were built on the downslope side of the cut. Their walls are 600mm thick, the doors square with the sluice exit vertically below, and they ranged in size from 2.5-3.8m wide to 1.7-3.6m long. Each building contained an automated overflow mechanism or 'waster' — a device invented by Thom.
When maximum water level in the cut was reached, water passed via a pipe into a suspended cylindrical bucket with small holes in its base. As the bucket weight increased, it descended and operated a mechanism that opened the waster valve. The water in the bucket escaped through the holes and the bucket rose, closing the valve.
Only five waster houses survive. The best example is near Dunrod Hill (NS233726) where the iron sluice mechanism is intact. Timber sluice gates mounted in iron frames are also located on the cut itself, south (NS238738) and east (NS246746) of Spango. The water overflows from these into ashlar culverts beneath the aqueduct.
Many of the 22 stone bridges that crossed the cut are still in place. They are generally hump-backed single spans, around 3.5m wide with 1m high parapets — Overton Bridge (NS266748) is wider because it carries road traffic. Some have cast iron sluice runners on their upstream sides. Stone bothies with fireplaces were also built at intervals along the cut and two of these remain. They once provided shelter for the workmen who kept the cut free of snow and ice.
The flowing water was harnessed for power generation on its way to Greenock. Thom believed it could deliver the water power of 50 horses to '"public works". Waterfalls were constructed at 19 levels along the cut and rented out to individual companies. They provided power for manufacturing by turning waterwheels at various mills, refineries, foundries and other industrial concerns. On completion of the first phase of the scheme in 1827, the Shaws Water Joint Stock Company guaranteed to its subscribers a supply of up to 34 cu m of water per minute for 12 hours a day, 310 days a year.
One subscriber, the Shaws Water Cotton Spinning Co. used its 19.5m waterfall to drive what was then one of the world's largest and most powerful iron waterwheels at its mill in Greenock. The wheel was 21.4m in diameter, 3.65m wide and weighed 119 tonnes. It was designed and fabricated c.1830 by engineer James Smith (1789-1850) at his Deanston works in Stirling. The wheel produced about 143kW net from the mill seats, assuming 75% efficiency, and it operated 12,216 mule spindles and 13,544 throstle spindles. It was replaced by a turbine in 1881, and dismantled in 1918.
As late as 1900 there were still 25 mill seats let to industries on the cut. They varied in the power produced, from 15.7kW at Scott’s Sugar Refinery to 431kW from the six falls at the worsted mills of Fleming Reid & Co.
The estimated cost of the whole scheme, including numerous smaller reservoirs and channels, was £16,000 in 1824. Another five auxiliary reservoirs were completed alongside the cut after 1827, each with its own regulating sluice, and by the time the whole project was completed the cost had risen to some £90,000.
By 1845 the demand for domestic supply and industrial process water was increasing rapidly, and from 1852 the cut emptied in dry weather. In the 1870s the embankment around Loch Thom was raised by 2.4m to create 308,000 cu m more storage, and two large new reservoirs were constructed on Gryfe Water (NS286716). The total cost was £207,000.
The Lower Clyde Water Board maintained the cut until the 1970s. It was rendered obsolete in October 1971 when a tunnel opened between Loch Thom and Greenock — designed by consulting engineer Babtie Shaw & Morton — to deliver water to the Overton Water Treatment Works (NS269748) south of Overton Reservoir.
Loch Thom and Greenock Cut were together listed as a Scheduled Ancient Monument in 1972. An exhibition on them is housed in Greenock Cut Visitor Centre (NS246721) at Cornalees Bridge. In 1995, engineer Scott Wilson Kirkpatrick was commissioned to report on the feasibility of restoring the cut. Work began in 2005 and was completed around 2010. Greenock Cut is a popular walking route in Clyde Muirshiel Regional Park.
Research: ECPK
"A Brief Account of the Shaws Water Scheme” by Shaws Water Joint Stock Company, Columbian Press, Greenock, 1829
"Obituary: Robert Thom, Born 1774” in Minutes of ICE Proceedings, Vol.7, pp.7-9, January 1848
reference sources   CEH SLBBDCE1

Loch Thom and Greenock Cut