Riprap: It's Not Hip Hop But Erosion Stop
AbstractHave you ever been to the Grand Canyon and seen what water can do over millions of years? When you turn on the faucet, do you see water come out, or mud? In this experiment you'll find out how engineers help prevent erosion, which keeps dirt out of our water.
Kristin Strong, Science Buddies
In this science project, you will build a model of a water channel and determine how best to prevent erosion in that channel.
Water has tremendous force, meaning it can carve into soil and even solid rock. As water carves, it picks up little bits of soil and rock and carries them away. This process is called erosion.
Practically any place where water and land meet-river banks, river bottoms, irrigation ditches, canals, shorelines, beaches, or farmland-you'll find civil and environmental engineers working hard to slow erosion. One way they slow erosion is by placing pieces of jagged rock, called riprap, on the land in or around waterways. This riprap blunts or dulls the force of the water so that it cannot break down the soil or carry it away as readily.
In this project you'll build a model of a waterway and see what kinds of riprap (fine or coarse, one layer or two) work best at keeping sand in the waterway and out of the water downstream.
Terms and Concepts
To do this type of project you should know what the following terms mean. Have an adult help you search the Internet or take you to your local library to find out more.
- Civil engineer
- Environmental engineer
- Displacement of water
- Archimedes' principle
- What size of riprap works best at slowing erosion (fine or coarse)?
- What shape of riprap works best at slowing erosion (smooth or angular)?
- Are multiple layers of riprap better than a singer layer?
- This source shows images of riprap and discusses its historical and modern uses:
Robinson, L. (2002, June 30). Mats, Concrete, Blocks, and Rocks: The Lowdown on Riprap. Erosion Control. Retrieved May 1, 2014.
Materials and Equipment
To do this experiment, you will need the following materials and equipment:
- Adjust-A-SpoutTM downspout extender, available at hardware stores or online. An alternative would be approximately 4-6 feet of a rain gutter.
- Sturdy box or stool
- Dry measuring cups
- Liquid measuring cups (4-cup size or larger), need 2. The read-from-above types are easiest to use.
- Play sand, 50 pounds
- Plastic baggies, gallon-sized (4)
- Fine gravel approximately 1/16th to 3/8 inch (shown in Figure 1), need approximately 2 gallon-sized plastic bags full
Coarse gravel, approximately ½ x ½ x 1 inch (shown in Figure 1), need approximately 2 gallon-sized plastic bags full
Figure 1. Shown here are examples of fine and coarse gravel (riprap).
- Lab notebook
- Garden hose
- Place to dump water, sand, and rocks
- Graph paper
Set-up your materials, as shown in Figure 2, first by placing the swivel-end of the extender on the box or stool and the other end on one of the liquid measuring cups. The swivel-end of the extender should be slightly higher than the end resting on the liquid measuring cup.
Figure 2. Experimental setup.
You will be testing four types of riprap, three times each. Make a data table to record your observations, as shown below:
Riprap Type Trial 1
Sum of Trials Average of Trials Control:
No Riprap, Only Sand
Sand + Fine Gravel Sand + Coarse Gravel Sand + Fine Gravel + Coarse Gravel
Sprinkle sand evenly over the gutter part of the extender, as shown below in Figure 3. The amount of sand needed to evenly cover the bottom of the gutter will depend on the length of your extender. For example, a 4-foot extender will need approximately 1 cup of sand to evenly cover the bottom, while a 6-foot extender will need almost twice that. Record in your lab notebook how much sand you used so you will remember for each trial. It is important that you use the same amount of sand each time you run a trial in the experiment.
Figure 3. Sprinkle sand evenly along the extender.
- Measure out 3 cups of water into a liquid measuring cup.
- Pour the water all at once into the swivel-end of the extender.
- Wait 3-5 minutes to allow the water to completely drain out of the gutter. The exact time you wait is not critical. Just make sure you wait the same amount of time for each trial in the experiment. Record in your lab notebook about how much time you waited so you will remember for each trial.
- Record the water level in the liquid measuring cup at the end of the extender.
- Dump and rinse out the measuring cup.
- Thoroughly rinse off the gutter.
- Repeat steps 3-9 two more times using only sand on the gutter.
Perform steps 3-9, this time using sand topped by 1 cup (or more, as needed) of evenly distributed fine gravel, as shown in Figures 4 and 5. Record in your lab notebook how much fine gravel you needed to evenly cover the sand. Be sure to use this same amount of sand and fine gravel each time you do a trial.
Figure 4. Sand topped with a layer of fine gravel in the extender.
Figure 5. Water poured all at once over the sand and fine gravel layers.
- Perform steps 3-9 three times, using sand topped by a layer of coarse gravel (evenly distributed). Record in your lab notebook how much coarse gravel you needed to evenly cover the sand. Again, make sure to use the same amount of sand and coarse gravel each time you do a trial.
- Finally, perform steps 3-9 three times with the sand topped by a layer of fine gravel, and then topped by a layer of coarse gravel (all evenly distributed). Use the same amount of sand and fine and coarse gravel for each trial, recording the amount of each in your lab notebook.
- Plot your results for each type of riprap on the graph paper. Which riprap type displaced the most water (had the highest water level reading)? This is the riprap type that did the poorest job at preventing erosion. Which riprap type displaced the least water (had the lowest water level reading)? This is the riprap that did the best job at preventing erosion in the channel. Did two layers work better than one?
Ask an Expert
- Compare smooth, round riprap vs. angular riprap.
- Compare which sequence works best in layering-fine riprap first and coarse second, or coarse riprap first and fine second.
- Add a layer of grass seed to any of the riprap types, sprinkle lightly with water, and let it grow until blades can be seen. After the grass has grown, do not water for 2-3 days and then repeat the trial to see if the addition of a plant mat does a better job at preventing erosion than riprap alone.
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