I am not an engineer. Nor do I play one on TV. My knowledge of the subject is (extremely!) limited to how it relates to the fantasy genre, particularly PC and tabletop gaming, but the Obligatory Sewer Adventure is also something that comes up in written fiction all the time. (And since I'm adapting a campaign to a series of novels, I gotta pay attention to this stuff.) Personally speaking, I don't think this is any more cliche than any other common fantasy trapping; it's all in the execution, and sadly, the execution seems to be where most of us get it wrong.
In this blog post, an engineer Dungeon Master gives several easy to understand examples of just how badly game designers do this. Even the Big Boys like Wizards of the Coast get this wrong, and they do it as recently as Dungeons and Dragons Online.
As I said, I am not an engineer, so you'll want to listen to this guy more closely than you listen to me. Below is a snippet from his post, which contains some of the fundamental principles of real sewer design:
That last point in particular presented a great worldbuilding opportunity for my husband. He actually wanted the culture in which Obligatory Sewer Adventure takes place to be more advanced than typical medieval Europe. And of course, since it's fantasy, he could think about how magic could shape the way sewers were built and maintained. So yes, these people know enough to separate the poop from the run-off, but that doesn't make walking around their sewers any less dangerous. Let's just say that the characters were extremely thankful both for the existence of antitoxins and that they could accomplish their task without going to the sanitary section of the magic district.1) Water flows downhill. Almost every sewer has some slope to it to make sure it drains.
2) Digging underground tunnels (of any size) is expensive and potentially dangerous. So designers design for as little excavation as possible.
3) Bigger pipes are more expensive than small pipes. So designers always use the smallest pipe possible to convey the storm.
4) Wet climates need bigger pipes than dry climates.
5) Big cities with lots of impervious surfaces (rooftops and roads) produce more runoff and effluent than small towns.
6) Sewer systems tend to follow the natural contours
7) In the modern era, sanitary sewers and storm sewers are kept separate so that surface waters and ground waters are not polluted by effluent. In the fantasy realm, this is not typically so (this is called “combined sewer” by the way), which can have excellent narrative consequences.
I welcome any corrections or additions a real expert can make. I'm just the wife of a veteran Dungeon Master who scrapped hours of map preparation in favor of a more true to life sewer experience. I'm sure that we still got a few things wrong, but I think we're on the right track, and the story has been improved tenfold by his research.
And really, that's the whole point, right? A better story?
(Note: please excuse any typos or other weird errors. I wanted to type this up before I went to work. I'll be back later tonight!)