Calculating Stormwater Runoff

RainTankGoal: Estimate the amount of impervious surface on your school campus and calculate stormwater runoff from that area.

Objectives: Students will…

  • Measure shape dimensions
  • Calculate area from shape dimensions
  • Use a scale to convert units
  • Read a map to determine surface types
  • Calculate stormwater runoff using a formula with unit conversions
  • Identify stormwater outlets
  • Map water flow based on actual topography

Materials: tape measure (the long reel kind if you have them), clipboards, rulers, calculators, site maps

Advance Preparation: obtain overhead site maps of your campus

Timing Estimate
10 min Measure width and length of impervious areas and calculate area
10 min Calculate runoff in 1” storm and for whole year
25 min Campus tour
5 min Back to class and pack up

Instructional Plan

Part 1: Impervious Area Calculations

Give each small group (approx 4 students) a copy of the school site map and ask them to measure dimensions and calculate area of each hard surface (roof and pavement). For irregularly shaped areas, ask them to estimate, showing on the map the measurements they used. Have them record on the map the square footage of each area as they estimate it.

formularUsing a one inch storm as a typical significant rain event, have them calculate the number of gallons of stormwater running off each impervious surface using the following calculation:

Ask students to tally the total runoff from all of the areas they measured.

Repeat the runoff calculations for the total amount of rain that fell on your campus last year (Cincinnati’s average precipitation is 42 inches/year).

(If you are concerned about this process taking too long, you could assign each group a specific area and then have each group share their calculations with the rest of the class, but within each group, different students could be working on different areas at the same time.)

Part 2: Campus Tour

Take a walk around your campus (bring the maps on clipboards and pencils) and make observations about where water is going on your site (this is one outdoor activity that is great to do in the rain!).

    Have students search for downspouts, manholes, storm drains and detention basins and mark them on their maps.

  • Once they identify an outlet for stormwater, have them figure out what areas are draining to it using the principal that water always flows down hill. You can bring containers of water out to test the flow of any areas that are confusing. Draw flow arrows on maps.
  • Have each group measure and record the

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