Monday, May 27, 2013

How To Calculate the Amount of Stormwater Runoff the Proposed "Alki 11" Rowhouses Will Produce

Here are the steps you can use to evaluate any project you are concerned about.

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 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

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.