With the weather still holding we headed to South Wales and the Dan Yr Ogof cave system. Our aim was to give the Battle of Britain series a better looking at and maybe findout what has caused the change in water flow in DYO. This time Andrew avoided the snuffles and after a quick carry in we both dived through on the line placed last time. In the Hard Cheese Airbell the dive gear was stowed and we climbed up into the passages beyond, quickly reaching the place I turned at last time. Andrew’s memory of the passage up to this point was as poor as mine had been last week.
At the point where it looked like the roof comes down to the water we followed the in-situ dive line along the right hand wall (North wall) around a slight curve. At this point Andrews memory of the place started to return. The line was originally put in place to mark the route through the passage using the available airspace. After several meters of swimming we entered Gwyn Saunders Hall where a waterfall lands from a rift high above in the roof.
We skirted the pool in the middle of the Hall and scrambled up the bank to the South West to look at Lake Zero; this is the route via which the water from the upstream cave enters Battle of Britain series. The water in Lake Zero was clear, unlike the water in the pool we had just climbed out of which was very cloudy. Further up slope from Lake Zero was a tall sand bar which came close enough to the roof to reduce us to crawling. Dropping down the other side of this we found the other entrance into the sump that connects Lake Zero with Lake One; the water was clear.
Having looked at the sump we climbed another sandbank that this time did hit the roof, leaving only a small opening to crawl through to reach the end of the Hall. At the most Westerly point of the chamber, next to some formations there was a notable draft.
Returning down the sandbanks we had another look at the water colour and levels. We then looked at the sumps in the canal passages to the South of Gwyn Saunders Hall and the passage out.
Before heading out we had a look at a bedding crawl below the climb up at the shelved passage and followed it to two sumps. This passage is not marked on any of the surveys we have of the area but we think it is very close to the Hard Cheese Airbell. The dive back through to the resurgence cave was uneventful.
We think that the water levels in the main cave area are responding much more quickly to rainfall owing to sand deposits in Gwyn Saunders Hall. The sandbank between Lake Zero and the flood overflow exit from the sump is restricting the flow rate in times of high flow. We also think that there may be a partial blockage in the sump passage that leads between Lake Zero and the pool in Gwyn Saunders Hall. When the flow rate increases these two restrictions are causing the water level in Lakes One up to Four to rise as the water backs up and fails to flow into Gwyn Saunders Hall at the normal level.
Series - DYO Battle of Britain
- Dan Yr Ogof, The Battle of Britain Series
- DYO Battle of Britain rematch
- A sunny day for BoB
- BoB(ing) along