The Minneapolis stormwater credit: volume credit is nigh impossible

Rainwater harvesting, thanks to Hennepin County

I was awake extremely early this morning monitoring contractions (not my own).  As of now, it seems like a false alarm, so I started reading the news, which got me wanting to post, which brings me here.

Since we’ve moved to the new house, I have wanted to try to put some of the stormwater practices we preach as professionals in to practice at the home stead.  Eventually there will be some sort of food garden in the back, and reusing graywater or stormwater instead of potable for watering seemed like a natural first step towards easing our demands on “the grid” and providing myself with a little DIY-time outdoors.  One of the easiest ways to do this is a rain barrel. I eventually got things figured out, and have some notes on acquisition, installation and pursuing the Minneapolis stormwater credit after the break.

After exploring some commercial and build-it-yourself options and deeming them too expensive (the later because of the need for new tools I probably wouldn’t use again), I found out that Hennepin County is selling rain barrels to residents for what I now know is a discounted rate of $62.  After some sawing of downspout pipe and two trips to the hardware store, everything was in place.

I didn’t expect to save any money on this, but out of curiosity I looked into the Minneapolis stormwater credit.  In Minneapolis, most property is required to pay a stormwater fee to maintain the stormwater system (our monthly fee is around $7).  You can get a credit two ways, through a quantity reduction or a quality improvement.  Rain barrels would fall into the quantity category, as they reduce the amount of stormwater flowing off your property into the stormsewer.  To get a 50% reduction in your fee, you need to retain a “10-year, 24-hour type II SCS storm event to pre-developed conditions”.  In our area, a 10-year event corresponds to a 4.2″ rainfall over 24 hours (special thanks to Emily Resseger at Bonestroo for providing rain event information).  To get a 100% reduction in your fee, you need to retain a 100-year event.  This equals a 6″ rainfall over 24 hours.

Now for some math.  Given that a 1″ rainfall on a 1,000 square foot roof yields 625 gallons of water, 10-year and 100-year events would yield 2,625 and 3,750 gallons respectively (discounting any evaporation).  Our roof is roughly 1,000 square feet, possibly a little larger.  The rain barrel holds about 50 gallons.  If we were to attempt to get the quantity credit using rain barrels, we’d need 53 or 75 for the 10- and 100-year events respectively.  Basically our yard and driveway would be filled with green barrels.  Plus I’d be out $3,000 to $5,000 for barrels or for a giant cistern. I’d see payback in roughly 70 years.

The message: unless you want your yard to look like a treatment plant like this guy, or you’re a big commercial users with higher fees, you’re better off pursuing the quality credits.  They’re much more accesible to the average homeowner, and your rain garden would make all your neighbors jealous.

2 thoughts on “The Minneapolis stormwater credit: volume credit is nigh impossible

  1. You’d be surprised how little water 50 gallons actually is. Certainly not enough to sustain any amount of landscaping through our recent drought. We have four small evergreen trees (arborvitae?) that started to turn brown during that month and a half. That barrel was only enough to water each for about 15 minutes on a slow trickle. The trees are still mottled.

    I hope the County has this deal next year, I’ll probably get two more and link them all together.

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