Saturday 25 July 2015

Cwm Dwr II - The Wrong Dig

Regular rain showers had kept the Brickworks Dig topped up with water, and with only enough people available for one days digging we had sought out an alternative site to work on, as most of our time would be spent dealing with the water at the Brickworks. We chose an old site in Cwm Dwr II, and after arranging permissions with the OFD Management Committee and Natural Resources Wales we were able to give this location some attention. There had once been a short sump in the entrance series to this cave - Helen's Horror which had to be syphoned to gain access, but this was removed some time ago and now is no more than a short wet crawl.

We entered the cave and rigged the 10m pitch down into Default Chamber. It is here that a slope leads up to a 20m long sandy passage that leads to the Sand Sump. The sand had previously been dug and only a small access crawl led through to what was once a body sized sump which had now silted up somewhat.

Keith in the sandy passage that approaches the Sand Sump

A view of the sump at the start of the dig with bucket for scale

A close-up view of the sump at the start of the dig

To work the dig efficiently we removed the sand fill on the approach to the sump, clearing it from the sides and lowering the floor by 30-50cm. After a couple of hours and 40 buckets of spoil removed we had good access to the sump and we could now start draining it. Only a small part was visible of this perched reservoir, we expected to drain it fairly quickly, bailing it with buckets. After 100 buckets of water had been removed and the water level only dropping a few inches we decided on a Plan B. Brendan went back up the ladder and to Helen's Horror where a long firehose that was once used to syphon off the water could be found. This 40 metre heavy duty hose was brought back to the pitch and then lowered so it could be used to syphon the sump. At first we attempted to start the syphon by sucking on the pipe, but this failed, so Mark headed back up the ladder, the bottom end of the hose hauled up and the 10m head of water was used to fill the hose. Once the hose was full of water, it was dropped to the floor of the chamber where it began to syphon off the water immediately. The flow of water was directed to join the stream and then we returned to the sump. At this point most people went off to explore the cave while Mark and Brendan returned to the sump to monitor progress. It became clear that there was a large volume of water in the passage beyond as the water level very slowly fell. Rather than wait we continued to drop the floor of the approach to the sump. With everyone back at the dig over the next three hours we were able to clear about 40cm depth of sediment from the bottom of the passage over a 4 metre length, following the water as it slowly receded, until after three hours an airspace appeared. A short while later the hose broke the surface of the water and the syphon stopped. The passage is still too silted up to get through at the lowest point. At this point we had removed a further 60 buckets of spoil and were all exhausted and cold and so after all having a look through the airspace to the passage beyond we headed out of the cave knowing that on our next visit we would be in some interesting territory.

Keith at the sump at the end of the days dig

On our next visit we plan to set up the syphon again and drain as much water from the sump as possible. We will continue to remove the infill from the bottom of the sump to give easy access to the passage beyond, where we will be able to assess the future possibilities.

More information on Cwm Dwr II on this link.

Present: Mark Burkey, Keith Edwards, Chloe Burney, Mark Burgess, Ian Millward and Brendan Marris

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