Urban Slobber

So what’s the big deal about stormwater? It’s a BIG deal because it’s a BIG volume of water! And it can carry a lot of pollutants into our streams and rivers. That’s why stormwater is sometimes called “urban slobber”! You may laugh…until you do the math on stormwater’s wrath!

Learn more!

Let’s begin with a question that will help account for stormwater’s amount: “How much stormwater runoff is caused by one inch of rain falling over 1 acre of paved surface?” To picture an acre…think of a high school football field (a football field is a little bigger than an acre; it equals about 1.32 acres). We can use simple math and known conversion factors to calculate stormwater volume.

Stormwater: Visualize the Volume

How much stormwater does 1 inch of rain on 1 acre make?

Volume = Area x Height
= 1 acre x 1 inch
= 1 acre x 43,560 ft2 / 1 acre (known fact) x 1 inch x 1 foot / 12 inches
→ Volume of 1 inch of rain on 1 acre = 3,630 ft3

Even though large volumes of water use are expressed in cubic feet, let’s convert cubic feet to familiar gallons.

7.481 gallons / 1 ft3 (known fact) x 3,630 ft3

= 27,156 gallons!

Hold up a 1-gallon milk jug. If you could capture the first inch of rain during a downpour…

  • Imagine 27,000 of these jugs filled with stormwater coming off every paved acre in our watershed!
  • Imagine 27,000 gallons of stormwater rushing fast and furiously downhill, picking up pollutants all along the way. Then, delivering them untreated into our streams and rivers!
  • Imagine the force of this stormwater as it powerfully erodes streambanks, causing sedimentation and ecological havoc in our waterways!

No wonder it is important to reduce impervious surfaces, a term used to describe areas where rain can not sink in to the ground. When we allow water to sink into the ground, it can recharge groundwater and replenish streams!

Be a Stormwater Sleuth! Calculate the Runoff Pouring Off Your Roof!

To calculate how much stormwater comes off your school’s rooftops…

  1. Ask your Principal what the square footage is of all school buildings
    OR
  2. Measure (using your pace or a trundle wheel) the width & length of each building. Width times length equals the number of square feet of each building. Total the square footage for each building.
  3. Divide this number by the area of an acre: 43,560 square feet
  4. Your school will either be smaller (a smaller % of an acre), or perhaps it’s larger!
    = _ of an acre x 27,154 gallons per acre
    = ___ gallons coming off the impervious rooftops of your school buildings!

In addition, you could add the amount of stormwater coming off your school’s impervious playground, sidewalks, tennis courts, greenhouses or other outbuildings, and parking lots!

Question: Can you name the three kinds of impervious surfaces in this photo?

school

Answers: 1. The entire roof of the school building. 2. The cement sidewalks. 3. The asphalt street.

Compare the amount of surface runoff “before” and “after” construction.

Question: What made the difference?

school

Answers: The amount of natural vegetation that allows rainwater to soak into the ground.

Take Action! Clobber the Urban Slobber!

What can you and your classmates recommend to:

  • Slo-oo-ooow the stormwater on your campus?
  • Treat the stormwater on your campus?
  • Reduce the volume of stormwater on your campus?
  • Educate others about stormwater on your campus?

What Can I Do?

Additional Resources

NC DENR’s “Grade Your Schoolyard” asks “What is your school doing to conserve and protect water?”

Interact with the “Virtual Runoff Model” from the Living Classrooms Foundation in Baltimore, MD and Washington, DC.