As I am a professional engineer, I thought I would share some knowledge with you in a series of engineering related articles. If I can help to add some authenticity, or at least help you to avoid some of the engineering fallacies that I hear in popular culture, then I will have achieved my aim. How's that for a bit of bloviating?!
The first topic on the agenda is SEWERAGE.
Don't you love it when I talk dirty?
First let's get some terms sorted.
A drain serves a single source, such as a house, or a factory or a highway. A sewer serves more than one property, or source. The network of sewers is called sewerage - not to be confused with sewage which is the smelly stuff that flows in a sewer.
A sewer can either carry just rainwater, in which case it is called a surface water or stormwater sewer or it can carry dirty water in which case it is called a foul or sanitary sewer. Older sewers, particularly those in cities, carry both rainwater and dirty water and are called combined sewers. It is these combined sewers which can be very large - sufficiently large for a person to walk in.
On the subject of size. 1000 houses would produce, on average, a FOUL WATER design flow of 45 litres per sec, and would need a pipe 225mm or 9 inches in diameter. Not something your hero would fit into. On the other hand 1000 houses would produce 1400 l/s in STORMWATER runoff in the sort of storm that you might see once a year. (This depends on where in the world you are - the figure quoted is relevant to the UK). This would need a pipe 900mm or 3 feet in diameter. Large enough to crawl in. However, your hero should beware - it takes 4 minutes for rainwater to work it's way into the sewer, with the flows building to a peak in 10 to 30 minutes depending on the network. I mention all of this just in case your hero should ever get trapped in a sewer during a rainstorm! Sadly the truth is that every year people are drowned in sewers, all too often children who think it is cool to hang out in their "den" which might have been cosy and dry for weeks.
Assuming you haven't zoned out by now here is a MYTH-BUSTER. The idea of a fish being flushed down a toilet finding its way to the sea is, alas, nonsense. (Even for Nemo!). Even if you live in a seaside town where flows can sometimes discharge to the sea, sewage has to pass through a screen with only 6mm gaps (1/4"). That is enough to stop the solids, sanitary towels and piscine escape artists. Inland, the sewers run to Treatment Works, where the sewage is screened, solids are removed in flocculation tanks, sedimentation tanks, and the resulting liquid is sprayed onto gravel filtration beds. Not a good place for a fish.
One last note. The large sewers have only been around since the 19th century. Not strictly accurate to have your medieval protagonist escaping through the sewers. In medieval times, householders disposed of their waste into a cesspool which was emptied daily by the night soil men. (A real lowest of the low job). Other waste ran in open channels in the road.
I hope this article has given you something to think on, and you never know, perhaps one day it might come in handy!