Get to Know Your Rain Garden Site

plant_01Goal: Calculate the amount of runoff flowing into the stormwater control and the slope of the site.

Objectives: Students will …

  • Measure length
  • Calculate area
  • Calculate runoff
  • Practice unit conversion
  • Measure rise and run of slope
  • Calculate slope

Materials: Rain Garden Handout, clipboards, pencils, campus maps, topographic map of campus, for each small group: 2 stakes or sticks, thin string, level, tape measure and/or meter stick, calculator

Advance Preparation: : request a site map of your school that includes roof pitch so you can see how much area drains to each downspout (if relevant)

Extension: Have students analyze site using topographic maps

Timing Estimate: 1 classes
5 min get ready and go outside
15 min Assess contours of land and measure areas
5 min Calculate runoff#
10 min Measure slope#
5 min Calculate slope
5 min Site observations and return to class
# You could switch order of these two to make best use of outdoor time.

The size of a stormwater control (rain garden, bioswale or tank) depends on a number of factors:

  • Area feeding into it
  • Volume of runoff coming off area (typically calculated from a 1” rain)
  • Slope of rain garden site
  • Depth of rain garden (which will depend on slope and soil drainage rate)

If you plop a rain garden into the ground without considering these factors, you may create a space that consistently floods and is a nuisance for your school.

In this lesson your students will work through several steps which are all leading towards calculating the size of the rain garden.

Calculate Area Feeding Rain Garden

Have students bring their campus map outside to the spot you plan to put the rain garden. Study the contours of the land. Where will the garden be collecting water from? Will you be running a downspout from the roof*? Will it collect water from sidewalks, driveways or parking lots? Only surfaces that are uphill from the rain garden and not blocked by a curb will send water to it. If necessary, bring a source of water or a ball outside to test the flow direction of areas students are debating.

Calculate the area of the different surfaces draining to the rain garden. Use the measurements on your map and actual measurements students collect in the field.** Calculate area separately for sections of lawn, paved surfaces and roof top. You may want to be prepared with area formula sheets as reminders.

* Keep in mind that a roof has many downspouts. How will you calculate the area of roof draining to the downspout you will be using?

** If students are calculating the area of a hill or sloped roof, they should look at the area from a bird’s eye view perspective, not the actual surface area. You can imagine rain falling from and onto a horizontal plane: that is the area you want to capture with a measurement.

Calculate Runoff from Area^

Ask students to use the area measurements to calculate the number of gallons of runoff coming off each type of area in a typical 1” rain (1” = 1/12 foot of rain). They have a table in the Rain Garden Handout, which walks them through the process. The basic formula is:

area (ft2) x depth of rain (ft) x 7.48 gal/ft3 x runoff coefficient = gallons of runoff

  • The runoff coefficient takes into account that there is an average amount of water that is absorbed or evaporates off of each surface type and therefore does not run off the surface.
  • As a rough check, 1” of rain falling on 1000 ft2 generates 623 gallons of water.

^ You could wait to do this part until returning to the classroom, moving directly on to slope measurement in the field.

Calculate Slope of Site

The depth and area of the rain garden will be influenced by the slope of the surface where it is located and how quickly the soil drains. The steeper the slope, the deeper you will make the rain garden. A slope greater than 12% is not suitable for a rain garden. This activity works best in small groups of four. Have each group measure slope in a different section of the rain garden area.

  1. Set one stake at a high point of the rain garden site. Tie one end of the string at the base of the stake so it is sitting on the ground.
  2. Measure 10 feet along the string, holding the string so it is horizontal as you move downhill. Put the other stake in the ground once the string is extended 10 feet. Tie the string to the top of the second stake so the string stays level (use a level to check). If your stick isn’t tall enough, have someone hold the string at the bottom so it is level.
  3. Measure the distance between the ground and the downhill end of the string. This is your rise.
  4. Double check the length of the string. This is your run.
  5. Calculate the percentage slope of the surface (remind students to use consistent units):

Ask students to share their results and calculate an average slope for the site.
(Slope 1 + slope 2 + slope 3…)/number of slopes = average slope

Check out Other Plantings

While you are outdoors, have students look at the existing landscaping at your school, especially near the rain garden site. What colors and types of plants are already planted? If you can find the names, list the plants near the rain garden.

Click here to download PDF file.