Caving Phil Hendy

Finding the source of water

ITis generally accepted that when Mendip rain sinks underground, it will eventually emerge at the base of the hill. Swallets engulf streams flowing from the impervious sandstone and see the light of day again at various springs and resurgences lower down.

Most of the villages on the lower slopes were founded because of these water sources and they were used to power mills as well as providing water for drinking and other domestic uses.

Over the years, water tracing in various forms has established which swallets feed which resurgences. In many cases, the combined input is far less than the output, but this is because the rain which falls on the limestone seeps underground, to join eventually with the main subterranean conduit.

There are, however, anomalies, where the source of water at a well or spring is not immediately evident. One such case is the Monk’s Well on Steep Holm. This lies near Cliff Cottage at the north east corner of the island, and is 64 metres below the highest point.

It is the main source of potable water on the island, although there are other small seepages. It is likely that the spring is fed by percolating rainwater from the higher parts of the island, although there is a theory that it is fed by stream sinks in Wales.

This idea may be strengthened by the upwelling of fresh water from the seabed near Weston-super-Mare. In the late 1940s, the theory was propounded that water from Gough’s Cave, first seen in the Skeleton Pit before emerging just below the cave entrance, also had its source in Wales.

Water tracing soon firmly established the real connection between the swallets south of Black Down and Gough’s Cave.

The strangest unexplained appearance (at the time) of fresh water occurred many miles north, under the bed of the River Severn. In March 1873 work began on what is still the longest underwater rail tunnel in the world – the Severn Tunnel. It is 7.1 kilometres long.

Excavation began on the Welsh side and soon water started leaking from the roof. This was brackish, so obviously came from the river above. It was easily dealt with. However, in October 1879 a great rush of water was encountered, which flooded the Welsh side of the tunnel.

Pumping and closure of a watertight door by a diver eventually led to the spring being capped and work continued. The Great Spring flows at around 20 million gallons (90.1 million litres) per day. This is almost enough to fill four Olympic swimming pools. A channel

With PHILIP HENDY

runs under the rail bed, and the water is removed at the Sudbrook Pumping Station in the Forest of Dean.

Until the 1960s, Cornish pumps were used, until being replaced with electric pumps. The first train ran through the tunnel on December 1st, 1886, 14 years after work started.

The water from the Great Spring is fresh and so cannot be leakage from the Severn. The most likely source is from stream sinks in the Forest of Dean area, and it was found that there is around a three week delay, after heavy rain, before the spring responds.

The water is of high quality and has been used by local industries. In the tunnel, the water emerges from what would be a fairly roomy cave passage, but an attempt to dive there was thwarted as the pressure of water was too great for divers to enter and make progress.

There are other swallets and springs on Mendip where the outflow or source has yet to be determined. Usually, the flow rate is so slow that water tracing would be impossible. Yet cavers and scientists are forever pushing the boundaries of our knowledge of caves and water tracing, so maybe in time these puzzles will be solved.

The Barracks, Steep Holm Steep Holm

Phil has been caving for more than 50 years and is a member of the Wessex Cave Club. He has been involved in producing several caving publications and until his retirement was a caving instructor at Cheddar. His main interest is digging for new caves