Analysis: soil and stormwater run-off

Good design starts with good site analysis…

What sorts of soils do I have?  Completing a soil percolation test helped us determine the size and composition of the rain garden…

  1. Dig test holes for percolation test
  2. Fill the hole and completely drain at least twice before measuring infiltration
  3. Once rate of draining stabilizes, measure rate of water surface drop until hole is completely empty (this is infiltration rate)
  4. You’ll want at least 1″ / hour  infiltration rate to create an effective rain garden

The City of Seattle Rainwise program has information regarding soil perc tests and the amount of site run-off from each and every property in the City!  Check it out…

How much run-off am I dealing with?

  1. First, map out your downspouts.  The size of your rain garden will be dependent on the amount of roof area you want to drain.
  2. Determine the volume of water you will be dealing with.

This is easy.  Seattle avg. rainfall is 38.6 in/yr or 3.2 ft/ yr.  This needs to be multiplied by the roof area your dealing mitigating.  In our case it was 763 sf.  So, 3.2 ft/ yr x 763 sf = 2442 cf/ yr.  That’s a lot.  But, to put into into gals, you divide by .14.  There are .14 gals in 1 cf.  So, 2442 cf/ .14 = 17,443 gals / year!   So, in our case, we’re mitigating 35% of this in the rain garden.

.35 x 17,443 gals/ year = 6105 gals/ year of untreated storm water kept out of Piper’s Creek!

Now, you need to size garden to accommodation this volume of water.

  1. Size the rain garden accordingly and your off to the races.  Typically, the bottom area of rain gardens need to be about 15% of total runoff area for effective infiltration.  Otherwise you will have more of a bio-swale function during heavy rains, etc. The City of Seattle Rainwise program has information regarding rain garden sizing as well.  The link below is a good start.  If this feels too daunting, it might be a good idea to talk to a professional whom can offer tips and or put together a design plan.


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