Life under the city can be made interesting, if of course there is a Sewer System there to take advantage of. A quick way to and from points of interest concealed from the city guard, a den of intrigue and network of corridors used by thieves and beasts alike. The sewage system leading from the ground drains to the system tend to be about 2' in diameter and almost always vertical accessed via drain covers, garderobes or cesspits, nice.
Stormwater Drains: Tunnels draining heavy rainwater, 3' in diameter running vertical or at a 40 degree angle. The entrance to these drains are usually heavy grilled at the surface.
Access Tunnels: Built for access to the drains and exclusively used by barges and thus 65% of them will lead to a Barge Wharf. 6' in diameter with a 45 degree+ entrance via manholes found in back streets or cellars leading to wharves usually with a locked and barred oaken door with a bell rope to ring, barges will usually arrive 1-6 hours later, not the quickest of services.
Channels: Sewers and Storm Water drains after 50 - 100' will feed into channels. 30'-40' in diameter, 5'-15' in depth with a mix of rainwater and sewage.
Cisterns: Large chambers holding stormwater to avoid the streets flooding, square 300' in width and 50' high, but usually only filled 3-4' in depth. The largest of these are usually used as a harbor for barges. Often there are larger ones up to twice the size that only rainwater will feed into.
Locks: Built to avoid waterfalls or to climb to a higher level, the barge will enter a lock close the gate behind and open the nest to flood the lock raising the barge and so on.
Barges: Long narrow boats, with no deck or cabin, though usually with a canvas cover with up to 4 barges pulled behind a narrow boat. (2-3 mph)
Cargo: Usual cargo transported through a large system would include Liquor, Spices, Tobacco, Ale, Cloth, Thieves, Escapees etc...
Events: What might you find down in the sewers? Though these tend to be kept fairly well under control by the Sewer Guard... Rats, giant rats, scum creepers, shambling mounds, spiders, giant toads, troll, green hag, muckdweller, snakes, crocodiles, black pudding, brown mold, yellow mold, algoid, bloodworm, giant weed eels, green slime, subterranean lizard, lizard men, rot grubs and gelatinous cubes.
Other Events: Flash Flood: Water level will rise in channels and cisterns by 2 - 20 feet.
Waterfall: Cannel drops to another channel
Whirlpool: Where several channels meet into a cistern which itself drains into a vertical shaft speed will increase up 100 yards away.
Corpses: 40% chance of a sever, chronic disease of the blood, but also a small chance of treasure, usually the only corpses found in the sewers are those of victims of crime and thus not usually carrying treasure anymore