When San Diego sewer lines break and spew their contents into the bay or ocean, how do they always know how much got discharged? Are there meters on all these pipes just in case? One big holding tank with numbers on the side, so they can see how much is lost?
-- Phyllis, the net
Heck, no, Phyllis. They race from house to house recruiting neighbors to break out the measuring cups, gas cans, empty Sparkletts bottles, all vessels of known volume, and head over to the spill site. Everybody fills 'er up, then they run a total. If they can't scare up volunteers, they fall back on their wits. How witty they need to be depends on where the line break is. If it happens at a pump station, with a flow-measuring device, they can get a very accurate picture. If it's just oozing out of some random pipe, they can eyeball the flow rate (gallons per minute) and multiply that times the duration of the break.
So how good are these estimates? Well, they go to school to learn the skill. Somewhere in San Diego is a mock-up of a manhole with pipes, gauges, and valves to adjust and measure the flow rate. The student watches water gurgling up through the manhole at various rates and commits the pictures to memory. As backup, they carry with them in the field an album of snapshots of same. They can scope out the manhole, then find the matching picture. Even less interesting than pictures of vacations and grandkids. Anyway, in addition to being America's Finest City, we're America's Flow-Rate-Estimate Capital; other cities come to us to learn how we do it. I guess one gets very good at something one must do very often.