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Erosion Control

It is important to control erosion and sediment when doing construction near streambanks to prevent sediment from entering the stream and harming aquatic life.

Erosion Control involves protecting the soil surface against erosion with the goal of keeping sediment in place. Some strategies for erosion control are surface mulch, seeding to establish vegetation cover before construction, and erosion control blankets and mats.

Sediment Control seeks to prevent sediments that have been mobilized from entering the waterway. Methods include silt fences, fiber rolls, and sandbag barriers. Healthy riparian vegetation also catches sediment before it enters the stream.

A stilt fence and erosion control matting are used to reduce erosion at this site. Note that plastic mesh such as this is not ideal as it takes a long time to degrade.

 

Failing Bank Protection

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

Vertical retaining walls such as these often fail, provide little to no fish habitat, and increase flow velocity.

Grass does not have the root structure to hold banks in place. 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.

Cars placed in the Missouri River in Great Falls in the 1950s. Using auto bodies for rip-rap is now against the law in Montana (Reference:  MT Law 75-7-106)

Broken concrete pieces are no longer permitted for bank protection.
Concrete bank armor is ugly, harmful, and subject to failure. It will eventually become undercut and start to break up.

Alternatives to Bank Armor

Fortunately, there are other options for addressing a failing bank other than rip-rap that work with, instead of fighting against, natural stream processes. These typically involve using woody debris, native plants, and other natural materials to reestablish natural features that stabilize the bank while improving river habitat.

Watch this video about a successful soil lift project on the Beaverhead River in Montana.

Revegetation

Healthy riparian vegetation is the best defense against erosion. Establishing vegetation on previously bare banks will start a trajectory towards a healthy, stable riverbank. This usually involves installing cuttings of native willows and cottonwoods, transplanting native shrubs and trees, and spreading native riparian seed. On eroding banks, revegetation needs to be combined with soil bioengineering to allow the plants to become established before they are eroded away. Vegetation provides important functions for fish and aquatic organisms by providing complex habitat, providing an energy source for macroinvertebrates by shedding leaves and woody debris, and by cooling the river by creating shade.

Soil bioengineering

Soil bioengineering seeks to utilize plants and organic materials in an engineered design, with the goal of reestablishing a self-sustaining natural riverbank. Natural materials such as trees, fiber mats and blankets, bundles of cuttings, and rocks of a similar size to those in the stream are used to stabilize banks while providing ecological benefits for the river.

A key feature of bioengineering is roughness, the frictional resistance experienced by flowing water. Increasing roughness slows water, reducing its ability to erode. Creating roughness using organic materials also creates complex habitats that help native fish thrive.

Some examples of bioengineering methods (USDA):

Resources

USDA Practical Streambank Bioengineering Guide

 

Bank Armor

Takeaways

  1. Rip-rap should be a last resort
  2. Consider alternative practices wherever possible
  3. 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.

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 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.

Vegetation and Landscaping

Consider vegetation your first line of defense against erosion and flooding. Robust riparian (river-adjacent) vegetation absorbs and slows flood flows and stabilizes banks by holding the soil in place. A healthy buffer of native plants between the river and your house will offer protection from erosion far cheaper than conventional bank armor, and provides essential functions needed to maintain a healthy river ecosystem. A vegetation buffer also provides water quality benefits by intercepting and degrading pollutants in run-off, such as fertilizer from gardens and lawns.

      Takeaways

  1. For the safety of your investment, maintain a robust buffer zone of native vegetation between any structure and the river.
  2. 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?
  3. 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. Notice that the brushy area is higher (less eroded) than the bare area to the left.

 

 

 

The vegetation along this riverbank will provide stability, reduce flooding, and maintain river health

Flood Insurance

Most homeowner’s policies do not cover flood insurance! To protect your investment from damaging floods you will need to specifically purchase flood insurance. Flood insurance is available for most places in Montana through FEMA’s National Flood Insurance Program or through private insurance. Any insurance agent can sell you flood insurance.

Flood insurance can be obtained within floodplain, but it will be MUCH more expensive than outside the floodplain. This is why it is important to talk to your insurance agent before building.

“Building on a floodplain is like pitching a tent on a highway with no car coming”

Flood insurance resources

FEMA FloodSmart – Evaluate your risk and learn about the National Flood Insurance Program

Montana Commissioner of Securities and Insurance has FAQs about flood insurance and a list of private options for buying flood insurance

Montana DNRC has more resources and tips for obtaining flood insurance

Protecting My Investment

This section contains several things you need to know about protecting your investment from floods and bank erosion.

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