Cave Hydrology

A group descended James Hall (JH) to visit the White River Series on Saturday 25th February 2012. The water levels in JH were low with a trickle of water flowing through the Plumtree Connection, and Speedwell was found to be very low, with The Bung only just flowing. A routine trip to and from the White River Series via Block Hall was had. On reaching the Speedwell Stream on the exit from Block Hall, nothing out of ordinary was noticed, it was on reaching the The Bung, the visible ladder they climbed down on the way in, was under a thundering waterfall now coming down it. The whole party found it difficult to climb the ladder, the streamway was roaring, the party was very concerned about drowning so were very relieved to get to the Boulder Piles and back into base of James Hall. The Plumtree Connection and JH was the same as it was on the way in and when back at the surface, they found it was dry. Thinking there had been a thunderstorm on the surface while they were underground, they reported it to some Technical Speleological Group members who infomed them that there had been no rain all day.

There is a rainfall guage at Sparrowpit that is as representative as it gets for the Peak system catchment, this recorded 5.1 mm of rain over the previous three days while 1 mm of rain was recorded the same day. A gauge located in Castleton recorded 9 mm of rain over the previous three days while no rainfall was recorded on the same day. The Buxton weather station clocked 13 mm on the preceding Thursday. The waster levels in the area were very dry for this time of year and with little rain so if a pulse strong enough to cause cavers difficulty happened, there was no meteorological reason for this.

Why did this happen:

1. the catchment for the Peak system is bigger than believed, with an unsuspected extension southwards towards Buxton which causes an increase in discharge in Speedwell a few days after rain in the Buxton area but this is unlikely, because 13 mm of rain in what has been a very dry February, 10 miles away isn't going to come through to Castleton as a pulse big enough to make the Bung nearly impassable.
2. A natural event within the system, likley to be an inlet-switch between the two inlets of Main Rising and Whirlpool Rising. After a big flood, the main inlet to Speedwell can switch suddenly from Main Rising to Whirlpool Rising, or vice versa. It can also happen in a long spell of low water. Why this happens is not clear, though various mechanisms have been suggested. It is likely that there is a period of no flow while water quietly fills a siphon before it starts to operate vigorously and creates a large pulse. How does it happen? Think of the great underwater shafts found by divers in Main Rising. Imagine a gradual build up of sediment with too large a particle size to be raised up the shaft by normal flow, slowly building up in the bottom at a steeper and steeper angle. Eventually this slope will collapse and effectively block the bottom of the shaft. Main Rising goes dry as water then backs up and flows by a different route, and it can stay so for a long period, sometimes years. Whirlpool Rising is known to pulse strongly so there is another siphon operating in the Whirlpool route. Somehow this also becomes blocked, and the pressure of the resulting backup then bursts explosively through the blockage in the other route.

On the day in question, the water levels were reported to be low but nobody was aware if the Whirlpool was flowing or not. This trip occured during a dry February so you could say it was a period of unusually low water, there hadn't been a meteorological event so it must have been a flow switch between Whirlpool Rising and Main Rising. The cavers only felt relieved once they got to Boulder Piles, so the main flow was coming from Main Rising. There is no way to predict when these will occur so cavers should beware of Speedwell’s unpredictable behaviour, as Speedwell Streamway can be a fearsome beast.

Due to the above phernonama, research is currently being gathered on flow patterns and water levels at various places in the Speedwell system. This will hopefully be published at some point in the future.