- Rip-rap should be a last resort
- Consider alternative practices wherever possible
- Rip-rap causes serious damage to river health and landowners
Rip-rap is often 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.
- 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 key 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.