A recent Washington Post article written by a plumbing expert explained it this way, “Imagine a giant ready-mix concrete truck filled with a slurry of sand, small rocks, and water. Drop the chute and discharge this mix into a 12-inch-diameter city sewer. If you don’t add a significant volume of clear water immediately after dumping the sand and gravel into the sewer pipe, the pipe will start to choke off. The clear water is needed to transport the sand and gravel to the end of the pipe.”