The sewers are well designed and sturdily built, with carefully cut stones and arched supports to keep the tunnels from collapsing.
The Nature of the Sewers
The largest sewer tunnels measure about sixteen feet wide. A channel ten feet wide flows through the middle of the tunnel, with a three-foot ledge on either side that stays dry—in theory (in reality, even these walkways are under a foot or two of water after a heavy rain). The channel is ten feet deep. The ceilings in these main tunnels are about eight feet high, except where a sewer grate offers access to the surface, at which point the ceiling is about fifteen feet high. At some intersections, stone bridges about two feet wide arch a couple feet above the level of walkways, enabling one to get from one side to the other while still allowing the water and sewage to flow beneath.
A middle-sized sewer tunnel measures about ten feet wide. Here the central channel is six feet wide and six feet deep, with only two-foot-wide walkways. The current of the flowing water is sometimes stronger in these tunnels than in larger ones.
The smallest sewer tunnels are merely drainage conduits about two and a half feet in diameter. They typically hold only a trickle of water, except during a heavy rain. These are large enough for a human to crawl through very uncomfortably with some risk of getting stuck. A smaller creature (a halfling, a dire rat, or a ratling) risks no chance of getting stuck.
More than anything, the sewers are used to drain rainwater from the streets, alleys, and buildings. The folk dump old cooking water, wash water, and garbage of all sorts into the sewers, including rotten food, spoiled ale, waste from manufactories, dyes from textile mills, alchemical waste, and dead animals. And, of course, the waste from privies and chamber pots all over the city goes into the sewers as well.
The fact that the sewers dump this unholy admixture into the river makes it abundantly clear why no one uses the river as a source of drinking water.
Sewer entrances in the streets and alleys are covered with iron grates. Prying up one of these grates requires a Athletics + Might (Strength) check. Damaging the grates is a crime, however, so if the Watch catches anyone prying them up, they will arrest the would-be delvers.
During dry periods
, the water in the sewers moves very slowly. It is almost entirely waste water and raw sewage. The water in the central channel of a large or medium tunnel is two to four feet deep, and the drainage conduits are merely damp, punctuated by piles of garbage and organic waste. The sewers stink horribly during these dry times, both in the tunnels and on the surface near the grates.
During or after a rain
, the sewers flow heavily with runoff and rainwater. Water rises to the top of the central channels, and the drainage conduits flow with six to twelve inches of water. After a particularly heavy rain (or after many days of rain), the sewer channels overflow, filling the entire tunnel with water—sometimes up to three feet over the top of the walkways. Conduits remain more than half full of rapidly rushing water.
Characters wading through running water in a sewer must make Athletics + Dexterity (Acrobatics/Dexterity) checks or slip and fall in, at which point Athletics + Might (Athletics) checks are required. Characters crawling through a drainage conduit with rushing water must make a Athletics + Might (Athletics) check each round or be swept along with the water. Once a character is so swept, a Athletics + Dexterity (Acrobatics/Dexterity/Reflex Save) is required to stop his or her movement.
Clogs in the sewers are common, and one of the duties of the System Monitors is finding and clearing such blockages to prevent waste water from flowing back into the city, a dire occurrence which unfortunately happens from time to time.