Title: Runoff Experimet
Essential Question: How does ground cover affect the amount of silt in runoff?
Materials: “Runoff Board”, sod squares to fit, water containers for pouring and collecting
(plastic pitchers, beakers), protractors, rulers or balances, support for the runoff board when in
use
“Runoff” Board
Construct “runoff board” – plywood, pressed board, etc. with two bumper strips (quarter round
works well) glued on in a “V” to hold a square of sod.
Size – about legal clipboard size works well – 11” by 14” or so
Engage
Begin with a discussion of runoff and what it carries to streams. The most abundant pollutant in
streams is the silt carried by runoff. Ask students how could this silt be controlled – what
methods are used to ensure that silt and soil don’t get into streams? They might have seen silt
fences in construction site for example, but the most common way is to have vegetation on the
land.
Proceed to the experimental design – hand out student paper.
Explore / Explain
Experimental Design:
Question: Does the amount of “cover” on soil actually reduce the amount of sediment reaching
streams?
Discuss the dependent and independent variables as given in the question (DV – amount of
sediment; IV – amount of “cover”)
Hypothesis – have students write a hypothesis using an If…independent variable…., then …
dependent variable…. format.
As a class, design a simple set of procedures to follow. Discuss constants to be used (amount of
water used, angle of board, size of sod square, height water is poured from, rate at which water is
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poured) and how the amount of sediment will be measured. Fill in the appropriate places on the
student handout.
Procedures 2 through 5 below are what the students should do.
Procedures:
1) Cut sod square to the same size to fit “runoff” board. This should be done by the teacher prior
to the experiment.
2) Place square in “V” – Sod side up for one test, sod down for the next
3) Angle the “runoff board” - outside up against the school wall works well, supported high
enough to have a collection container under the “V”. Try to maintain the same angle for all setups.
4) Pour a measured amount of water (500 to 1000 mL) slowly over the square of sod, letting the
water collect in the container below.
5) Allow the sediment to settle in the container and measure the amount to sediment collected.
Measurement might be height of sediment (be sure all collection containers are the same) or
weight of dry sediment (takes a long time to dry out samples) or some other method.
Conclusion – compare the amounts collected – class averages for each variable, percent
difference, graphs etc.
How does the cover affect the amount of sediment in the samples? How does this relate to
construction areas? Usually the first thing to go in a new construction zone is the “cover” – top
layers of soil are removed leaving bare soil. Plastic fences are set up to help contain the runoff.
Often storm drains are blocked with straw or some other material to contain sediment.
Sometimes grass strips are left on the edges of the construction site to absorb runoff.
How does this relate to streams? Wide areas of diverse vegetation along stream banks (riparian
zones) are essential in absorbing silt and other pollutants before they reach the stream.
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Does the amount of “cover” on soil actually reduce the amount
of sediment reaching streams?
Question:
Title: Runoff Experiment
Hypothesis:
If soil is covered with sod, then there will be less sediment
collected in a “rain” event.
Procedure:
IV:
Bare soil (sod down)
Covered soil(sod up)
Cover
DV:
Amount of
sediment
Constant variables: amount of water used, angle of board, size of sod square,
height water is poured from, rate at which water is poured
Conclusion:
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Does the amount of “cover” on soil actually reduce the amount
of sediment reaching streams?
Question:
Title: Runoff Experiment
Hypothesis:
Procedure:
IV:
DV:
Constant variables:
Conclusion:
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