Costs and Installation Tips When Building a Riprap

Lake Michigan Shireline in Evanston with distant Chicago
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Riprap is commonly used to protect soil from erosion in areas of concentrated runoff. It consists of a layer of very large stones interlocked together and acts as a barrier on slopes that are unstable because of seepage problems or in areas that are receiving a large concentrated flow.

Riprap can be used to stabilize slopes to prevent erosion and provide support. Areas where it sometimes is used to stabilize slopes and prevent erosion include bridges, drainage structures, grade stabilization structures, storm drains, streambanks, and the sides and slopes of channels and culvert inlets and outlets.

Constructing Your Riprap

When building a riprap, it's important to use well-graded rock of different sizes. The variation in size allows the rocks to create a better interlocking system than rocks of the same size would. Rocks ranging in size from 2 to 24 inches are recommended, and they should be durable enough to withstand freeze and thaw cycles. Using blocky and angular shaped rocks with similar dimensions on all sides and sharp, clean edges will make it easier to build the riprap.

Before installing the first layer of rocks, apply a synthetic geotextile membrane to prevent the soil from moving through the riprap. To ensure that the riprap is anchored to the soil, be sure to make the riprap at least two times as thick as the diameter of the largest rocks.

Extend your riprap all the way to the top of the bank so it can control erosion more efficiently. Consider using chain link fencing or wire mesh to secure riprap installations, especially on steep slopes or in high flow areas. You can use galvanized wire mesh for better efficiency and durability.

Things to Consider During Installation

It's important to consider the steepness of the slope before installing riprap. A 2:1 ratio is generally the steepest a slope should be when using riprap. This means the slope should extend horizontally at least 2 feet for every 1 foot of drop. In other words, if the horizontal distance from the top edge of the slope to the bottom of the slope is 100 feet, the height from the bottom of the slope to the top should be no more than 50 feet. Slopes greater than 2:1 can cause the riprap to fall down, which actually can increase the amount of erosion.

Smaller rocks can be used if you choose to go with grouted riprap. This means the rocks will be interlocked with grout and cement, but maintaining this type of riprap sometimes can be more challenging. Grouted riprap must be securely protected against toe scour or undermining, and it cannot be self-repair like the ordinary installation. After every storm or major runoff event, the system needs to be inspected and maintained. If the wire mesh or the stone has been damaged or altered, be sure to repair it immediately to prevent a cascade effect with the whole system.

Erosion Control Costs

The cost of riprap varies depending on location and the type of material selected. A cost of $35 to $50 per square yard of ungrouted riprap can be used as a basic quote, while grouted riprap ranges from $45 to $60 per square yard. These costs will depend on the availability of resources, accessibility, and total area to be covered.