Dear Washington Rock is a new blog series in which people ask our representative, The Rock, questions about which materials to use for their projects. If you’re interested in having your question answered, send us an email.
Dear Washington Rock: My friend and I are working together on an earth science project for our 8th grade class. We have to solve an erosion problem on the hillside behind our school. Our task is to stop major erosion on the hillside due to large amounts of water flowing down it.
We are competing with other teams to find the best solution. Each team will present its findings to the class and make a model of their research. Then our class will determine the best way to save the hillside.
We don’t need to place rocks on 100% of the hillside—only the part where the majority of the erosion is taking place. The area most affected measures approximately 10 feet by 10 feet.
One of our ideas was to cover the hillside in river rocks to slow the erosion, because the rocks will not be eroded like the soil would be.
Which type of rocks would be best for our project? How many yards do we need? —Earth Science Students
Dear Earth Science Students: River rock would naturally help siphon off water. The drawback to this solution is that without adequate anchoring, round rocks may roll down the hillside.
An Optimal Solution
A more foolproof solution would be to use 4”x 8” quarry spalls or light loose riprap, which is approximately 8”x 18” in size. Most projects of this type use our 4” x 8” quarry spalls.
Both products are angular, which would help them stay in place. The material could be spread evenly over the hillside. Landscape rocks could also be placed throughout the hillside to enhance its visual appeal.
The great thing about our quarry rock is that it is a beautiful blue color. That blue color really comes through after it rains—which happens often here in the Pacific Northwest.
Points of Reference
Riprap and quarry spalls are often used to cover shorelines to prevent erosion. The Point Defiance Breakwater Peninsula, which will be finished sometime this year, was lined with our quarry spalls and riprap to provide a defense against wave erosion.
If you cover the entire hillside, you will need 22 yards of material. But if you cover just the affected area, 3-4 yards will do. —The Rock