The survey brings back memories. Back in the day when a belt mounted carbide generator was cutting edge technology we did a through trip from Trou de Glaz to Guiers Mort.
We spent a few days rigging / exploring from each end. The night before the first group did the through trip (of which I was one) it was a bit rainy to say the least (even worse than a Mendip summer). Still the next morning the water seemed to have settled and no sign of flood pulses.
So off we went. All was fine, if a little tiring, as we were really bombing it, until soooo close the exit. I was in front and I put my foot in some of the clearest water you've ever seen (well, I didn't see it, was that clear). What was the walking way out was now under lots of water. (I'm guessing it's the oval black blob on the survey on the right at the back the Guiers Mort part.) Previously this was open passage, and an easy stroll out. No way was it in any way free diveable - too long, deep, and cold and in furry suits. A bit too drowny for our liking. So we decided to wait for a while and get the bivvy bags out and sit and see if the water would drop, especially as everyone was a bit knackered and a bit chilly. By that time nature indicated that a pee was needed. Of course, there's no way you'd go anywhere near the underwater way out as that's just filling the sump up even more. A pebble was put at water level to check for any changes and after a long while (hour or two...) there was no change.
So the other options considered were: a) wait and get hauled out by the French (lovely as they are) - Non. b) Go all the way back up to the Trou de Glaz - Non. c) One of the group recalled a bypass - Oui.
So we scrabbled about a bit, found a draught, and abseiled down the side of the Guiers Mort (I guess it's the 8m pitch marked on the survey). Ran down the mountain to the waiting minibus (adrenalin is a good thing), ate Mars bars or similar, and found a large bottle of whiskey, which was nice.
Moral of the story, take the safety gear just in case.