The Various Sizes of Riprap Rock and How They Can Be Used

outfall protection

The Various Sizes of Riprap Rock and How They Can Be Used

Outfall protection is the act of ensuring that drainage systems are maintained well enough so that the natural body of water they meet is preserved. Outfall protection is often completed through the use of riprap. Depending on the project, the sizes of the crushed stone may vary to better suit the job.

Four to Five Inches

This type of stone riprap is the smallest available and isn’t well suited for projects that involve heavy water flow such as a rushing river. However, this type of riprap can be a preferred choice for preventing erosion from water runoff on low-grade slopes and drainage ditches. Four- to five-inch riprap is also great for preventing the sprouting of weeds and is often used in the base layer of driveways and gravel parking lots.

Six to Nine Inches

Riprap stone that is between six and nine inches is the most common type of riprap used in civil construction. These larger stones are typically preferred for preventing and/or controlling possible erosion along lakes, streams, rivers, and even ponds because it is able to withstand strong currents. This sized rock is large and quite heavy, making it ideal to reinforce the banks and can even be a deterrent to foot traffic from pedestrians. This sized riprap is also used as railroad ballast or to create retaining walls.


The bulkiest and largest of riprap sizes, this stone is an effective measure of shoreline protection. In areas where heavy erosion is bound to happen, nine-inch riprap is commonly used to help prevent major movement. It may also be used on banks that have already had erosion damage to help restore their integrity. Nine-inch riprap can further be used to fill ditches or large holes as well.

When an outfall is compromised, it’s essential to restabilize it as soon as possible.  The drain water gains speed as it moves throughout the system due to slick surfaces. As the water leaves the outfall and touches the natural ground it can become violent, which can create sediment pollution and erosion. Land-based sources are to blame for roughly 80% of marine pollution each year. Outfall protection aims to reduce sediment pollution in water and allow for sustainable, efficient water drainage.

Looking for a civil construction company to stabilize an outfall or build a retaining wall? Give us a call today.

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