Erosion Control

Rip Rap Boulders – What is it?

If you’re not already familiar with the effective, erosion-resisting construction aggregate, you might think “Rip Rap” is the latest music sub-genre your sons, daughters or younger relatives can’t seem to get enough of and that you won’t be able to escape having to listen to.

The origin of the name is actually rooted in the nautical term “riprap”, which was used in the early 19th century to describe the rippling or ‘tearing’ of water surfaces caused by underwater currents along with the ‘rapping’ or striking of waves crashing into the shore.

The history of the name is fitting as the product is most often used in building seawalls to protect land and property from the impact of waves. The material is also sometimes called shot rock, rubble, rock armor or armor stone.

What is it exactly?

Rip Rap is a loose, angular stone that comes in a range of sizes from 4 inches to 2 feet and can be made out of a variety of materials like fieldstone or concrete rubble from building and paving demolition – but most commonly it is made of limestone or granite.

Rip Rap size classifications and designations like ‘Type 1, 2, A or B’ will vary by state, which is why most gravel pits simply sell the rock by measurements in inches.

Rip Rap is often a landowners’ first choice when their bank starts eroding. However, Rip Rap carries many negative consequences for the river and for other landowners. These consequences include:

• Rip Rap is very expensive ($100 to $500 per foot), and permitting can result in hundreds of thousands of dollars in mitigation fees
• Increased flow velocity, putting your downstream neighbors at increased risk of erosion and flooding
• Interfering with stream dynamics, causing more erosion on adjoining banks
• Destroying riparian vegetation necessary for healthy ecosystems and fisheries
• Improperly constructed Rip Rap will eventually fail and is very expensive to fix

Rip Rap should be considered a last resort for protecting banks. If you select a construction site taking floodplains and channel migration zones into account, Rip Rap should not be necessary. Some of the problems with Rip Rap can be mitigated by adding woody vegetation to the engineered structure.

Rip Rap requires permits from the local conservation district and the US Army Corps of Engineers. Learn more about what permits you need.

Only use Rip Rap when:

• Long-term durability is needed
• Design discharge and shear stress is high
• There is substantial threat to high-value property
• Impacts to channel stability and fisheries would be minimal
• Effective alternative practices are unavailable

Some guidelines for Rip Rap engineering and construction:

• Use bioengineering and vegetative plantings to stabilize the upper bank.
• The keyed in rock must be placed below scour depth.
• The toe is the most important part of a rip-rap project. This is the zone of highest erosion.
• Rock is unnecessary above high-water mark.
• 2:1 is the recommended slope. 1.5:1 is the steepest slop on which Rip Rap will stabilize.
• Rock must be angular, not rounded, for greatest strength.
• Rock is sized according to shear stress criteria for engineered designs.
• Rip Rap is flexible and not impaired by slight movement from settlement.

Takeaways

• Rip Rap should be a last resort as it causes serious damage to river health and landowners.

Let’s look at some examples of less-than-ideal bank protection in Montana

This landowner probably removed the native riparian shrubs and trees and replaced them with grass to “improve access”. Unfortunately, this caused erosion that will need to be addressed using rip-rap or (preferably) alternative stabilization methods.

Consider vegetation your first line of defense against erosion and flooding.  Scientific research clearly documents that Riparian Buffers, particularly forested buffers and those along headwater streams, deliver tremendous economic, ecological and other benefits.  Among these benefits, riparian buffers:

• protect the quality of the water we drink.
• intercept non-point source pollutants carried by surface water runoff and remove the excess nitrogen, phosphorus and other     substances that can pollute water bodies.
• stabilize stream banks and minimize erosion.
• decrease the frequency and intensity of flooding and low stream flows.
• prevent sedimentation of waterways.
• through shading, reduce swings in stream temperatures and prevent elevated temperatures harmful to aquatic life.
• provide food and habitat for wildlife of the land, water and air and allow for wildlife movement within natural corridors; and
• replenish groundwater and protect associated wetlands.

Takeaways

• For the safety of your investment, maintain a robust buffer zone of native vegetation between any structure and the river.
• Removing riverbank vegetation to improve the view and access requires a 310 permit and greatly increases the risk of property damage from erosion and flooding. Is it worth it?
• Lawns provide almost no protection. Avoid building your lawn to the river’s edge.

This landowner applied for a 310 permit to remove the brush on the upper bank. However, with a house located this close to the river, the thick shrubs are essential to protect the property from erosion. Removing this brush would very likely require the installation of Rip Rap at great cost to the landowner and to the detriment of river health.