Definitely. It crossed my mind in one drain in manchester that while I knew the outfall was not blocked, there were only two of us, noone really knew where we were, and we got a whiff of hydrogen sulphide and stupidly ploughed through it with no measuring or protective equipment. All was ok as there was a good air supply in the drain, and we were nearing the outfall, but I was always more cautious after that and you are dead right having a plan b and c and is always better, especially in these tidal systems, or ones designed to handle huge amounts of stormwater.