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  • Marie McKinsey

How to Calculate Storm Water Run-off from Your Property

Updated: May 25

The EPA estimates that over 770 cities in the US combine waste water and stormwater in the same pipe and send it to sewage treatment plants. The treated water is then released into a nearby body of water. The problem with this practice is that when there are heavy rains or snow melts, that extra volume of water overwhelms the treatment plants. To prevent flooding and sewage backups into homes and businesses, overflows of combined stormwater and raw sewage are released. Where I live, in Seattle, those overflows go directly into Puget Sound. This is a serious source of pollution and a threat to fish, wildlife and public health.

Sewage outflow warning at Alki Beach West Seattle

It is better for storm water to be absorbed on site - in water gardens, rain barrels, or straight into the ground - than to carry it off to waste water facilities.

I wrote the post below in 2013, when my neighbors and I were trying to point out the negative environmental impact of a proposed development in the Alki Beach neighborhood. You can use this method of calculation to determine how much water is accumulated on your property, and then decide how best to manage it yourself.

Originally published - 5/27/13

First, take the square footage of the roof area. In this case, we're going to use the square footage of the combined building footprints for the Alki 11 since we don't have actual roofs to measure. According to the developer's Environmental (SEPA) Checklist, this is 5,511 sf.

Now multiply the square footage by 144 to get the number of square inches.

5,511 X 144 = 793,584 sq. in.

Multiply the number of square inches by the amount of rainfall to get the number of cubic inches. In this case, we'll use one inch.

793,584 X 1 = 793,584 cubic inches

There are 231 cubic inches in one gallon of water. So to find the number of gallons of runoff that will come off these buildings after one inch of rain falls, divide the number of cubic inches by 231.

793,582 cubic inches divided by 231 = 3,435 gallons of runoff from rowhouse buildings for every inch of rainfall


Using the same process, here's the amount of runoff that will come off the Alki 11 autocourt.

The square footage, again according to the developer, is 3,244 sf

3,244 X 144 = 467,136 square inches

467,135 sq. in. X 1 inch of rainfall = 467,135 cubic inches

467,135 cubic inches divided by 231 = 2,022 gallons of runoff from rowhouse autocourt

Buildings + autocourt combined total = 5,457 gallons of runoff for every inch of rainfall

For comparison, this rowhouse project would replace housing that has zero runoff - all stormwater from roofs and parking is currently absorbed on site.

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