Engineering Rainwater

by Don Friedman on June 29, 2016

New York City has combined sewers mostly because when our sewer system was started in the mid-1800s, no one had yet come up with a reason to separate clean water and “black water.” This occasionally causes trouble during heavy rains, when a literal flood of water can overwhelm the sewers. New buildings are designed to temporarily retain rain water on their roofs to help reduce the immediate surge of water during rain, but old buildings don’t and water on the streets goes straight to the sewers.

The combined sewer overflow at the north end of the Gowanus Canal. Too much rain, and sewage comes out here.

 

Gowanus in Brooklyn has several related problems, including inadequate sewers, an elevation barely above sea level, and the presence of its eponymous canal. In heavy rains, storage tanks in the sewers can be overwhelmed, allowing sewage to flow into the canal. A developer has a solution: using dumpsters to create miniature gardens that can absorb water. An interesting idea, but can it work?

The dumpsters are described as being 11 feet long and 4 feet 6 inches high. No width is given but the standard dumpster width is 8 feet so that they can fit in a single traffic lane. That means the interior volume of one of the dumpsters is 396 cubic feet.* There are 7.48 gallons per cubic foot, so an empty dumpster can hold 2962 gallons of water. But the claim is that the garden-filled dumpsters can hold 2000 gallons, which is possible as long as the soil isn’t filled too high: soil can absorb water, so if the soil were something like 18 inches deep, the water in the soil and the water on top of the soil would probably be about 2000 gallons.** I’m no plant expert, but it seems to me that having so much water above the soil would likely kill the plants…but that’s not my topic.

How much rain does that represent? Our monthly rain/snow stats are fairly consistent: we get 3.5 to 4.5 inches of rain per month in 8 to 12 days of precipitation. Pure average, we get 0.38 inches of rain per day of precipitation. A typical large New York rowhouse is 20 feet wide and 50 feet deep, so it gets 31.7 cubic feet of rain on that average day. In other words, each dumpster can hold rain equivalent to the roof runoff of 63 rowhouses. And the rain that will be captured is rain that would otherwise fall in the street and go straight to the sewer. So the capacity is good, but will the water get there?

Let’s look at it another way. Bond Street is the north-south street west of the canal and Nevins Street is to the east. They’re about 600 feet apart. The plan includes three dumpsters in the rectangle bounded by Bond, Nevins, Sackett, and Union Streets, which is about 150,000 square feet of area. The three dumpsters in that rectangle have an open top area of 264 square feet or 0.18% of the area, which doesn’t seem like enough to do much good. In that typical day of rain, how much water falls into the 264 square feet of dumpster? Only 8.4 cubic feet, and that’s in three 2000-gallon capacity dumpsters. Or in other words, it would take almost 95 times the average day of rain to get 2000 gallons of water into a dumpster. Or in other words, it’s never going to happen.

This is really common sense: the dumpsters are too small in ground area to capture any appreciable amount of rain. The numbers are nice because analysis, rather than common sense, is a good way to support or attack an argument. The idea of capturing water before it causes sewer problems is a good one, enshrined in the code for new buildings. Maybe if rainwater were shunted to the dumpsters from some of the big flat roofs nearby…


* Ordinarily when I’m doing calculations, I’d round 396 to 400 and 7.48 to 7.5. For the sake of clarity, I’m using unrounded numbers.

**The rendering seems to show soil right up to the top of the dumpster, which would reduce the water-holding capacity well under 2000 gallons, but I’ll grant that as artistic license.

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