Loch Thom is a reservoir located in the hills above the town of Greenock on the west coast of Scotland. If you go for a walk or even a drive in the area, you could be forgiven for assuming it was a natural loch. It is a substantial body of water set in an area of stunning natural beauty.
In reality, the Loch is manmade and was constructed to provide water power for Greenock during its industrial revolution. The project was built between 1825-1827 by Scottish engineer Robert Thom.
Preface
It is important to note that there are no known engineering plans for the construction of Loch Thom. It is believed that Robert Thom did create engineering documents for the project but they were lost forever when they were destroyed by fire. The following information is based on hypothesis, history and clues.
The Plan
The concept behind the Loch Thom project was not new. It had been proposed by Sir Michael Shaw-Stewart (of Ardgowan) and George Robertson (Greenock Magistrate) in the past. The concept was put to James Watt who declined to carry out the work because he thought it was “impracticable”.
The outline of the project was to build a dam over an existing water course called the Shaws Water. By doing so, this would result in a large body of water being formed. Initially, this was referred to as the Great Reservoir. There was also a requirement to construct a second, smaller reservoir to act as a compensation dam.
The Great Reservoir would be the primary storage reservoir. Water could then flow down into the compensation dam when the level was high enough for it to be able to pass over a spillway. When the level in the Great Reservoir was below the spillway level, a sluice could be opened to allow water to travel from the great reservoir to the compensation dam.
There was then an aqueduct that carried the water from the compensation dam to the town of Greenock. The Aqueduct was named the “Greenock Cut”.
In this map view, we have added some coloured sections to point out the different areas of the project. The largest area (green) is the Great Reservoir. The smaller body of water (blue) is the compensation dam. The structure located between them (purple) is the main dam complete with its spillway. The dot (orange) is the location of the sluice. There are also two watercourses included in the map. The Greenock cut (yellow) and the original watercourse known as the Shaws Water (red) Currently the lower section of the former Shaws Water is the Kip Water. The upper section (that enters the Great Reservoir) is the North Rotten Burn.
Here we see a very old map, Roys’ military map (lowlands) showing the layout of the Shaws Water prior to the construction of Loch Thom.
Robert Thom
Robert Thom was already an expert when it came to water power. He owned a cotton Mill located on the Isle of Bute. He had personally overseen the successful replacement of coal power with water power at his own mill. He designed and built a water system capable of providing all the power needed to drive his own industry. In Scotland, rain is plentiful and coal was expensive.
Sir Michael Shaw-Steward asked Robert Thom to build the water system at Greenock. Robert Thom accepted and they formed the Shaws Water Joint Stock Company.
Note: It is believed the first engineer to be approached was James Watt. He declined the proposal. Sometime later, Robert Thom was asked to oversee the project but was unable to invest the time due to commitments with his existing ventures. It is thought George Robertson asked another engineer to survey the land and carry out the construction but he did not believe the plan was possible. It is not known who this person was or if they undertook any similar engineering projects. In around 1823 Robert Thom was once again approached and this time agreed to be the project’s engineer.
Construction
When you are building a dam across an existing waterway you need to divert the water away from the proposed dam site. It became pretty apparent that this would have been the case during the Loch Thom construction. We really didn’t know where the diversion was so we started using Google Maps to see if there was any evidence still present. At this point, all we were looking for was a mark on the ground that could have gone from The Great Reservoir into the compensation dam.
The above image is a screenshot from Google Earth and this looks promising. What we are looking at is the area of ground between the two bodies of water. To the right is Loch Thom and to the left is the compensation dam. We can see what looks like a river running down into the compensation dam. There is also scarring on the landscape from the top leading over to Loch Thom. Was this used to bypass the proposed dam site?
Upon visiting this location it quickly became clear that this was not the bypass site. the land was very elevated in the centre and any channel that was dug to transport water would have been extremely deep. What we did find was a natural spring. Water was flowing in both directions down to Loch Thom and also into the compensation dam so this would explain the land scars.
An old map was able to confirm the water source (Issues) and the stream also had an arrow symbol on it indicating the direction of its flow.
One thing that did interest us was how much more eroded the lower section of this land scar was. When you visit the location it’s a fairly slow-moving low-volume stream yet the channel it flows in is quite substantial. That really didn’t make a lot of sense at the time.
This was the next clue. It is the exact same location but this time we are viewing a LIDAR map (Light Detection and Ranging). LIDAR can be a great tool for looking at the terrain of an environment and not just the surface. LIDAR can show terrain features that may be hidden under vegetation.
I have added a sequence of dots to the image to highlight the presence of an extension to the existing stream channel. We are now beginning to wonder if Robert Thom had been able to extend a natural watercourse to act as his bypass. If this was the case it certainly shows smart thinking. Why dig an entire channel when you can make use of what nature has provided and use that as a starting point?
We looked through various maps, lidar images and were beginning to convince ourselves that we had found the bypass. For now, it was no more than a hypothesis, but that was soon to change…
The French Connection
It turns out that a French engineering publication was also very interested in the work of Robert Thom, in fact, they published a lengthy article on the engineering behind the Loch Thom project. The specific edition appears to have been written around the subject of water engineering projects because it mainly featured Loch Thom and the Manchester ship canal.
When reading through the pages you see various diagrams, illustrations and maps. The original plans and designs used for the loch construction were destroyed in WWII during the Greenock Blitz. These copies, reproduced in a French publication are probably the closest we will ever get to seeing the genuine plans.
Then it happened… Standing out like a sore thumb on a map of the project was a line leading through the land between the great reservoir and the compensation dam.
The bypass was precisely where we had predicted it would be. Robert Thom had indeed used the natural feature of a watercourse as part of his bypass cut.
Now we know that the water had been bypassed, we need to think about how the entire project would have been accomplished. As has already been mentioned there are no copies of the original plans that we have any knowledge of.
Workers would have manually dug out the channel for the bypass cut. It would have made sense for them to start at the existing stream and work towards the Shaw’s Water. It would make no sense to start digging from the other side and be battling against the water flooding the area you are digging.
Once they reached the Shaw’s water, the bypass cut would breach its banks and the water would begin to flow. It is possible they would have built some form of a temporary dam across the Shaw’s water to ensure all water was redirected along the bypass.
The Main Dam
Once the watercourse had been diverted work could commence on the main dam. The main dam also consisted of a spillway that would dictate the maximum level the water could reach within the Great Reservoir. If it exceeded that level it would overflow into the compensation dam. The dam construction also contained a sluice to allow manual control over the flow between the two bodies of water.
The main dam would have required deep foundations to prevent water being able to penetrate below it. Contained water will always follow the path of least resistance, and Robert Thom had his own specialised method of constructing Dams.
He used a method of building up different types of clay into layers. “puddled peat, alluvial earth and gravel beaten together with wooden dumpers until completely mixed”. Robert Thom also used layers of ash near the surface of the dam to prevent animals from burrowing into the structure and weakening it.
once the entire dam was completed the sluice could be opened. They could then breach the temporary dam and water would start flowing towards the main dam. It would then Travel throng the open sluice and use its original path once again as it would have before the dam was put in place.
The bypass will no longer be required and can be filled in once again. They may have dug the channel out and left the spoil at the top ready to be shovelled in during the refill stage.
Cornalees Bridge
Cornalees Bridge is the starting point for the main aqueduct known as the Greenock Cut. The “Cut” carried water from the south side of the compensation dam all the way to the Overton area of Greenock. The Grenock cut is just over 6.5 KM long and today is classed as a scheduled monument by Historic Environment Scotland (HES).
The current bridge dates back to the construction of the compensation dam and passes over the dam’s spillway between the compensation dam and Kip Water.
The southern end of the compensation reservoir is also dammed with an embankment. This dam also contains a sluice and the previously mentioned spillway. This dam may have been built at the same time as the previous dam or before/after the main Loch Thom dam was constructed.
The sluice built into the compensation dam (Green dot) allows water to pass through a tunnel into a “pond” located directly below the dam. From there the water can be controlled using a further two sluices. One leads into the Greenock Cut (red dot) and the other allows water to flow into the Kip Water (orange dot). By opening the sluice between the pond and the Kip Water and closing the sluice leading into the Greenock Cut (whilst the tunnel sluice is also open) the water would rejoin its original route.
Dry Pit Construction
This smaller dam was almost certainly built using a different technique compared to the main dam. it is very probable that it was built in two stages. It is likely that the original flow of water ran almost where the current spillway is located.
The above diagram shows how the dam may have been constructed. the pink line shows the original path of the Shaws Water. Without altering this, a pit could have been dug (grey) to allow the first stage of the dam to be built (orange). It is important to note that the first stage also comprised the sluice and tunnel. When stage one was finished the sluice could be opened and the space between the Shaws Water and the pit could be breached. This would force the Shaws Water to now flow into the excavated pit and through the sluice and tunnel. This would cause the original watercourse to run dry and allow the second stage (yellow) to be constructed. The 2nd stage contained the spillway.
During the first stage of construction, the water course would be unaffected. During the second stage, the water would flow through the sluice and tunnel into the pond before being reintroduced to its natural course through the sluice into the Kip Water.
This method of construction would ensure the Kip Water was kept flowing during the dam construction. This was an important factor because there were at least two mills located downhill that used the Kip Water for power.
The Greenock Cut
The Greenock cut starts at its sluice with the pond and travels for 6.5 km downhill to Greenock. The cut follows a pre-determined path hugging the hills and maintaining a downhill gradient along its route. Next to nothing is known about its construction technique. What we do know is it was manually dug out and block lined along its entire distance. The blocks were also sealed with Puddling clay to ensure the aqueduct remained as watertight as possible.
We do not know how many people were involved in the construction or whether they worked in groups at different stages along the way. Did they start at one end and progress along or did one group start at the top and one at the bottom and they met halfway? These are questions that will probably never be answered.
Flooding The Land
In theory, you could simply close the sluice on both dams and the area of the Great Reservoir would flood and fill. It would then pour over the overspill on the main dam and fill the compensation dam. This method is very unlikely because it doesn’t allow for safe testing of the water system, nor does it maintain the flow of water for the mills on the Kip Water. It is much more likely that a more methodical method was used.
Let’s call the main dam “Dam 1” and the dam at the end of the compensation dam “Dam 2”. Let’s also take into account the working hours of the mills on the Kip Water. For the purpose of this example let’s imagine they work from 9 am to 6 pm 7 days a week.
At this point, the sluice on dam 1 is still open and the Shaws Water is able to flow through the planned area of the Great Reservoir, through this sluice and across the area that will become the compensation dam. It then passes through the tunnel into the pond and down into the Kip Water.
On day one of the flooding someone will close the sluice on dam 2 at 5.30 PM. This means the water will be greatly limited to the area of the existing mills by 6 PM. The following morning the sluice on dam 2 will be re-opened at 8.30 AM. This will ensure the water is fully available by 9 AM when the mill starts operating. The same process will be repeated every day to ensure water is only being sent down the Kip Water when needed for Mill operations.
When water is not being sent down the Kip Water it will be accumulated in the compensation dam. If we imagine the flow of water used to be 24 hours per day. It is now limited to only 9 hours per day. This will lead to a net gain in water volume within the compensation dam.
At this point in time, no water is being sent down the Greenock Cut. Currently, the entire water system is being tested and all water being expelled is being sent into the Kip Water.
Through time as the water level continues to rise within the compensation dam, it will reach its spillway. When this happens water will be able to flow over the spillway and into the Kip Water without the need of the sluice. The compensation dam has now reached its designed maximum level. The spillway should regulate this by not allowing the water level to get any higher.
With the compensation dam now full the sluice can be closed on Dam 1. This will cause the water to back up and begin to fill the area of the Great Reservoir. It will then be a case of monitoring the level of the water in the compensation dam. If it falls below a pre-determined level the sluice on Dam 1 can be opened to bring the water level back up. When the water in the compensation dam is within its operating range the sluice in Dam 1 can remain closed. it will still be necessary to open and close the sluice on Dam 2 when the water level is not passing over the spillway.
Over time (probably months) the level in the great reservoir will increase until it too reaches its spillway. When that occurs the water can flow directly from the great reservoir into the compensation dam, if the compensation dam is also full it can waste into the Kip Water. The pair of Spllways act like overflows to ensure the entire system is not able to exceed its design levels.
Using this method to raise the water level not only ensures a continued flow to the Mills on the Kip Water, it also allowed both bodies of water to be filled in a safe and controlled manner. If there had been a problem with the compensation dam it was vital it was known about and addressed whilst the main reservoir was still empty. They could then use the sluice to lower the water as required. The same was true of the main reservoir. Before filling that they had to know 100% that the compensation dam worked exactly as required. If there was to be a problem with the main reservoir they would need to drain it through the compensation dam.
It is possible that the entire water system may have been put through several cycles of flooding and draining to ensure everything worked as required. This could also help flush out debris that would have no doubt been present in the freshly flooded land.
One little detail that we did spot was the height of the sluices leading from the pond to the Greenock cut and the Kip Water. The sluice that feeds the Greenock cut is set into a small raised dam. The sluice that leads to the Kip Water is set into a lower weir. If the sluice on Dam 2 was ever to jam or fail the excess water would flow over the weir into the Kip Water as opposed to flooding the Greenock Cut.
Supplying The Cut
Only when the Greenock cut aqueduct was fully completed could water to sent from the pond into the cut. There is a report that on the opening day of the Greenock Cut, the then magistrate opened the sluice before getting onboard a small boat and travelling down the Cut to Overton. Whilst this may have happened, it is unlikely to have been the first time the water flowed. There would have had to be testing to ensure that the cut was able to supply the correct amount of water and that all the regulation devices along the way worked as they were supposed to.
One such device was the self-regulating automatic sluice that was invented by Robert Thom. This worked by having a pipe set horizontally into the side of the aqueduct, at the maximum height the water was designed to reach.
If the water got to this weight, it flowed through the pipe and filled a bucket. The bucket would gain weight and pull a lever to open a sluice allowing water to flow out of the aqueduct and into a smaller river, that went back to the Kip Water.
The bucket has several small holes drilled into it so it could drain slowly. If the flow of water stopped the bucket would drain and lose weight. As a result of this, the sluice would close.
Whats in a name
The entire project was originally known as the “Shaw’s Waterworks”, It was later named the “Greenock Waterworks”. The Great Reservoir was also renamed Loch Thom in honour of Robert Thom (Its engineer).
Today the Greenock Cut is no longer in use, although it has been preserved and given the status of a scheduled monument. Loch Thom currently supplies the local area with drinking water using a tunnel that was blasted through the hillside to a new water treatment plant in the early 1970s.
In 2021 and 2022 there was extensive engineering work carried out at Loch Thom. This was to ensure the longevity of the water scheme and future-proof it. This included repairs all around the banks of the loch and strengthening work carried out on Dam 1.
This was not the first time changes had been made to the waterworks. In the 1840s a new aqueduct was built to bring additional water from the Kelly Dam to the Compensation dam. This new aqueduct was called the Kelly Cut.
In the 1870s the main loch (Loch Thom) was enlarged by raising the spillway. This also involved building a brand new dam to the east shore of the Loch.
In this image, we can see Dam 3. This was taken during a period of maintenance and as a result, the water level was extremely low. Directly in front of the dam, we can see a causeway of land that projects out into the loch. This is what remains of the pre-1870s shoreline.
The old shoreline remains largely intact because it was lined with rock and gravel (almost like railway ballast) We believe this was laid to prevent the shoreline from being eroded away due to water action. It certainly did work because the shoreline is intact but the land behind it has been eroded away.
The Lost Road
Another interesting aspect of the loch being enlarged is the Old Largs Rd. It used to travel across an area that is now usually submerged under Loch Thom. As part of the 1870s expansion, a new road had to be built next to the present-day bank.
Summary
Loch Thom will continue to supply the local area with drinking water well into the future. No doubt future work will be required to keep the facilities relevant and capable. It is incredible to think that just under 200 years ago this was the brainchild of one engineer who was presented with a challenge.
