Why Rivers Carry Sediment As Well As Water
Erosion - an exercise over 4 days
|
Concepts:
|
Rivers carry more than water; they carry sediment from erosion and runoff.
Certain types of soil are more easily eroded.
The steeper the gradient, the more erodible each soil type.
A single heavy downpour is more apt to cause erosion than many days of steady light rain.
|
Objectives:
|
To determine how much water runs off and/or is retained by various soils with and without vegetation.
To determine how much sediment, of what type, will erode most quickly in the watershed at varying gradients and differing intensities of "rainfall."
|
Materials:
|
8 plastic-lined shallow boxes of the same size
wedges of two different angles (10 and 30 degrees) to tilt boxes
9 wide-mouth quart jars
2 jar lids (one punctured with small holes & the other with large holes)
and screw bands
various soils (clay, sand, loam, fine gravel, gravelly clay, etc. - see soil types)
pail
data sheets & graph paper, or computer spread sheet
2 time watches.
|
Subjects:
|
Science and Mathematics
| | | |
Preparation and Procedure: Cut a "V" shape in box ends and line boxes and "V's" with plastic. Adhere plastic with duct tape to exterior sides of the boxes. Select students to bring 4 different soil types. Tamp these soils tightly into lined boxes of the same size. Label boxes according to soil type.
-
Fill a quart jar with water. Screw on lid with small holes. Place a box of soil at a 10 degree tilt using wedges, making sure "V" groove is at the bottom and that it hangs over the edge of the table. Pour water from lidded jar over the high end of the box onto the soil, using a watch to time the pour (all boxes should be watered within the same amount of time). Collect water runoff from box into a pail. A second student times the amount of minutes and seconds from the first "runoff" through the box "V" until it stops dripping.
- Record the type of soil, timed pour, minutes of "runoff," and condition of soil in the box after the exercise. Pour collected "runoff" into labeled quart jar and allow to settle overnight. Measure amount of water in the jar, and sediment on the bottom of the jar.
- Continue same procedures with all other soil boxes, using the large holed lid. Record all measurements.
- Re-do exercise at a 30 degree tilt, still using the lid with small holes. Record all measurements.
- Re-do exercise, at both the 10 and 30 degree tilts, using the large hole lid. Record the same data as before.
- Graph or chart results, and arrive at written conclusions.
You can modify this experiment to study the effect of other factors. For example, what would be the effect of having the soil in the trays loose instead of being tamped down? Or, what would be the effect of having more organic matter in the soil (add peat moss to the soil in half of the trays)? Or, what would be the effect of rooted vegetation (in one tray fit a piece of sod cut from the ground; in the other put soil from the same site but where the vegetation and roots have been removed)?
This experimental design is very powerful because it allows you to separate out and compare the effect of various factors. As you include more and more factors the number of tests increases and may become overwhelming. To prevent that from happening you can remove other factors if they are not interesting or relevant.