A Canal through Limestone?

Limestone is a permeable rock, since it has cracks through which water can pass.

The Sapperton Tunnel, on the Thames and Severn Canal, tunnels through the limestone rock so that the canal can pass through the Cotswold Hills.

The 3½ km tunnel took 5½ years to build. Along the line of the route, 25 shafts were dug down from the ground surface and the tunnel was excavated from the bottom of these, until they were all connected together. Once the alignment had been checked, the tunnel was enlarged to the correct size.

Before water could be allowed in, the channel had to be made watertight. This was done by lining it with clay, an impermeable rock that doesn’t let water through. The channel was then finished with brick and the sections that were not through solid limestone were also given a brick roof.

Water could then be pumped in to this section of canal, enabling it to be used by boats.

The problem was that the rainwater soaking down into the permeable Cotswold Hills was leaking into the tunnel. This provided an additional water supply in winter, but in summer the springs dried up. Where the spring had pushed a hole through the clay canal lining, the canal water could then leak out through the same hole.

More water was needed to make up for leakage than to transport boats through the locks. The wind pump, by the Thames source, was replaced by a steam engine pump, which was used in the drier summer months, typically from June to October, and could deliver 3 million gallons a day into the canal.

There was a constant need for repairs to deal with the holes. Sections were eventually lined with concrete, but that was over 100 years after the canal first opened. Meanwhile loads of water had been taken from the ground near the source of the Thames. It had leaked back into the ground but not at the same place.

The Thames and Severn canal was closed in 1933, due to the need for repairs, and the Sapperton Tunnel is now impassable as the roof has collapsed in a number of places. You can find lots more information, including diagrams and pictures, on this page of the Cotswold Canals website.

Water shortages caused problems for boats both in the canal and in the river. Join me next week to find out more about travel on the Thames.

Thames and Severn Canal

In 1789 the Thames and Severn Canal was completed, allowing cargo to be transported from Gloucester, through the countryside, to London, by boat. Via a series of locks, it climbed the western side of the Cotswolds to Sapperton, where it entered what was then the longest tunnel in Britain, at 3½ km.

This emerged from the hillside near the source of the Thames.

The canal then descended, via more locks to meet the Thames at Inglesham, near Lechlade…

…from where the river was navigable to London.

To pass through the Cotswolds in either direction, boats had to use a series of locks, to reach the highest level of the canal.

This lock is in Shropshire but illustrates the point. Coming from the lower level, the boat enters the lock, the gates are closed and then water is allowed in from the higher level to fill the lock and raise the boat. Each time the lock is used, water moves down to the lower level. With this happening at both ends of the tunnel section, this top part of the canal needed a water supply.

Water was taken from the River Churn at Cirencester to feed the canal but this alone wasn’t enough. The solution was found in the area near the source of the Thames, where the meadow was described as having “numerous little fountains” – presumably lots of springs. A borehole was dug and a wind pump was used to transfer several tons of water per minute to the canal.

It’s a controversial point, but this is likely to have lowered the level of the water in the limestone and contributed to the drying up of the spring at the Thames source.

But water wasn’t just leaving the canal via the locks. Next week we will be looking more closely at Sapperton tunnel and what was happening to the water inside.

Water from the Cotswolds

Much of the rainwater falling in the Cotswolds disappears into the ground, soaking into the permeable limestone rock. It may eventually emerge lower down the hillside at a spring…

…or maybe not, as in here at what was once the source of the Thames.

Water that has trickled through limestone usually comes out pretty clean and carrying various dissolved minerals, particularly calcium. This usually results in a pleasant taste and gives the water health benefits, calcium being needed for strong bones. Thus it is excellent as drinking water.

But you don’t have to live in the Cotswolds to taste it for yourself. Several companies sell spring water that is bottled at its Cotswold source. These enterprises vary in size, with one company having a licence to remove 73 million litres of water per year from its borehole drilled into the limestone rock.

And then there are the water supply companies. They need to source enough water to keep it flowing down the pipes direct into our homes. Thames Water supplies 15 million customers. It gets around 40% of its water from groundwater and some of that comes from the Cotswolds.

So if the water is taken directly out of the rock and at a faster rate than rainwater puts it back, the spring may dry up.

The river may eventually appear from a different spring at a lower level. Compare this…

…with this.

But in the area of the source of the Thames, water was not just needed for drinking. A major project needed a water supply and it turned out to need much more than was anticipated. Find out all about it next week.

Springs in the Cotswolds

The River Thames once began as a spring, in this field, on the lower slopes of the Cotswold Hills.

The Cotswolds are made of limestone, the same type of rock that we encountered in Wharfedale, where the river Wharfe disappears into the ground. On the Cotswolds you won’t find much in the way of rivers for the same reason. Limestone is full of cracks, enabling rainwater to sink into the ground. It is a permeable rock.

The water flows down through the permeable limestone until it can’t go any further, either because the rock has changed and no longer has holes, or because the holes are already full. The water will then flow sideways, out of the hillside, forming a spring, the source of a surface stream.

So if there hasn’t been rain in a while, water stops flowing out and the spring dries up.

But where the Thames is concerned, that is only part of the story. Find out more next week.

Uses of Limestone

Last week we looked at some of the issues involved with extracting limestone from the landscape.  But why are we so keen to get hold of this stuff anyway?

Limestone has been quarried in Britain since at least Roman times.  Stone walls make use of odd shaped pieces, while more carefully shaped blocks are used for buildings.

Portland limestone, from Dorset, has been used for many notable buildings, including St Paul’s Cathedral.

In limestone areas you will often find old lime kilns, because it was discovered that burning limestone produced another useful substance.  Lime could be used as a mortar to fix blocks together, while sprinkling it on farmland results in better crop growth.

In industry, limestone is used in the production of cement and concrete and to help remove impurities from iron, in the making of steel.

It is often under your feet, in road construction, paving slabs and carpet backing.  It is used as a filler in paint, sealant and glue.  It is consumed in animal feed and also by us, in food such as bread and pills.

Use this link to find lots more information.

Limestone then has a multitude of uses, so it seems that quarries are necessary scars on our landscape.