If they aren't for the general public's consumption, become part of the un-general public. Claim to be an civil engineering student, or in construction or something. If all that fails, and I don't think it would, do some library research Storm drains often are built to follow the original terrain, which may have nothing to do with what you see today. See if you can't find topographical maps, or a hydrological survey from the ninetenth century. Personally, I've had a lot of luck with just driving or walking up and down riverbanks and keeping an eye out for big slabs of concrete.
Many explorers like to bring along markers, stickers or spraypaint to mark their progress.
A hat is also a good idea when looking at drains with icky stuff hanging from the ceiling. More specialized supplies, like climbing aids, are cumbersome, so it makes sense to only bring these when you know they'll be needed. Check weather reports before draining. If there's a chance of rain, put the expedition off. In Canada and similar climes, melting snow can be just as dangerous.