Using Natural Materials to Control Erosion on a Riverbank

Gabion system
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Water erosion on the banks of a river, creek, or lake can be a troublesome problem on constructions sites. Not only can erosion complicate construction, but federal regulations and other applicable laws require you to have an erosion control plan to prevent soil from entering water bodies or affecting other areas.

Fortunately, there are a number of ways to control erosion on a construction site if the right techniques are used at the right time. And while some methods call for synthetic materials, there are also several methods that use natural materials to help diminish soil erosion problems along river embankments while construction projects are underway. Some methods use biodegradable materials, while others can serve as permanent solutions that can actually enhance the appearance of a stream and benefit wildlife.

Coir Geotextile

Coir geotextile fabric consists of woven fibers of coconut. The fabric is available in rolls that can be rolled out over the surface of an embankment, preventing soil erosion on newly graded slopes. Sometimes vegetation can be established over the geotextile, so that the roots of the vegetation growing up through the fabric interlock with the fibers. Unlike non-woven geotextile fabric made from synthetic polypropylene or other materials, coir geotextile is a biodegradable material that will not harm the environment.

This method is usually used on the banks of small streams that have a consistent surface water level.

Caution: Don’t use geotextile fabric along channels that transport large quantities of sediments, because if stream sediment is deposited on the geotextile, it will destroy any vegetation that has grown up through it.

The advantages of geotextile:

  • Simplicity of installation
  • A fully biodegradable product
  • Relatively low maintenance

Brush Mattress

A brush mattress, also known as live brush mat, is a means protecting stream banks by combining live brush cuttings, facines (bundles of sticks or other materials) and stakes to cover and stabilize stream banks. Construction involves placing a thick mat of dormant brush cuttings on the bank and anchoring them with stakes. The cuttings eventually root into soil and stabilize the bank. The technique is often used with other methods.

This method is ideal for capturing sediment carried down banks by rain. The drawback is that the facine bundles can be washed away if the rains are very heavy.

Advantages of this technique:

  • Ideal for use along streams that receive plenty of sunlight, which will help the cuttings quickly root
  • Quick installation
  • Fully biodegradable system
  • Low maintenance

Rootwad Composites

A rootwall composite system consists of a combination of interlocking tree materials and other vegetation. A mass of tree roots, known as a rootwad, is combined with other vegetative materials, and is often combined with other techniques to stabilize stream banks. The rootwad is placed within the stream bed or at the base of the slope to direct the stream flow away from the bank, thus reducing erosion.

In addition to stabilizing the stream bank by lowering the water-flow velocity, the roots provide complex habitats for the establishment of aquatic wildlife. Although this is an inexpensive system, the installation is highly complex, which puts some limitations on its use.

Advantages of a rootwad composite system includes:

  • Inexpensive to install
  • Eventually decomposes completely
  • Provides habitat for marine life

Tree Revetment

In a tree revetment system, small fallen trees are laid horizontally along the stream bank and anchored in place to prevent erosion. The trees slow down the flow of water along the back, greatly diminishing the rate of erosion. The trees also block silt and sand from flowing down banks into the stream, instead causing them to be deposited along the bank and within the tree branches. (Better choices are evergreens or other trees with many branches.) The deposited sediment also forms an excellent soil bed into which seeds of additional trees and other vegetation can take root.

The resulting network of roots spreading through the revetment can serve as a permanent deterrent to erosion, as well as providing great fish and wildlife cover.

This system is not recommended for areas where high erosion has already taken place. It is also a somewhat difficult system to maintain.

Advantages of a tree revetment system include:

  • Inexpensive
  • Fully biodegradable
  • Can enhance wildlife habitat

Gabions

The last system is somewhat less natural than the others. Gabions combine the use of natural stone with manufactured metal wires. The system consists of pouches, baskets, or boxes made of high-strength, double-twisted hexagonal mesh filled with stone (or sometimes concrete rubble). The word gabion comes from an Italian word meaning "big box." Gabions are used for bank stabilization in areas where high soil erosion is likely to occur and where the bank is composed of small rocks that help resist the water forces.

Gabion's mattresses or boxes are not particularly attractive, and there is always the possibility of damage to the mesh.

A gabion can be used almost everywhere, but special conditions and precautions must be taken on areas susceptible to rapid erosion and unstable water flow. Special attention must also be considered when designing its footing. There are several cases where the footing has failed, leaving the gabion without its proper support.

Despite some drawbacks, there are some advantages to gabions:

  • Can be installed underwater to "train" streams to a desired path
  • Very strong system that resists heavy water flow
  • Porous construction slows the velocity of water flow
  • Invisible when submerged along a stream bank