Minecraft wiki entry for Trapdoor_uses, associated text: Do the same with the drawbridge, then in the pit (make the pit only 1 block tall) add water, the amount of water you put is the amount of trapdoors wide. Only add water at the beginning of the sewer,if the sewer runs out of water (after 7-8 blocks) make the pit one block lower where the last water block is. Now, you can decorate it. Add mossy cobble at the sides and for added effect, connect it to a house, or make a whole water system in a town.