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Gabion

General

Gabions are rock-filled, multi-celled, PVC coated wire baskets that are placed in ponds (as an outlet structure), swales, and vegetated channels to dissipate the water’s energy. Gabions absorb a great deal of the water’s energy by forcing water to pass through the voids in the structure, which reduces its velocity, promoting sedimentation and reducing channel erosion.

Gabions may be used in swales and vegetated channels that outlet to sediment traps and basins. They are very versatile structures that may conform to a wide variety of situations and sites and may be constructed on site or purchased commercially. As a result, materials should be selected carefully to ensure proper function and stability.

Advantages

  • Relatively inexpensive
  • Requires little maintenance
  • Easy to construct
  • Can be aesthetically pleasing if designed properly, which may increase adjacent property values
  • Supports plant life
  • Excellent for retrofit applications

Disadvantages

  • Adjoining materials may require additional stabilization to prevent erosion

Design

DESIGN The size of the structure will depend upon the site, but should have a height of at least 1 foot; have a minimum bottom width of 3 feet; and should extend across the entire conveyance structure with slopes no steeper than 2:1. In addition, gabions must be underlain with geotextile filter fabric to protect the structure from undercutting, which may cause the failure of the device.

The stone selected for use in gabions will vary depending upon the individual needs of the site, but should be 1 to 8 inches in diameter and be clear of fines and other sediment. Gabions may be filled by mechanical methods, but it is generally recommended that they be filled by hand. Hand filling ensures that the entire volume of the gabion is occupied, increasing the strength and durability of the practice.

Baskets are constructed of PVC coated wire mesh that is resistant to corrosion. After they have been filled, the gate should be closed tightly and securely wired shut. Additional structures, if necessary, may be used and should be securely wired to existing gabions.

Construction

  • To maintain its shape, the basket shall be braced with wire supports in both directions
  • Gabions are anchored into the walls of the channel laterally and to the ground vertically by weight - buried portions must be wrapped with 12 ounce, non-woven filter fabric
  • To prevent the erosion of downstream materials, stone should be placed at the toe of the structure
  • Gabions should be underlain by geotextile filter fabric
  • Gabions should be constructed immediately after grading is completed on the conveyance structure

Maintenance

  • Gabions should be inspected periodically and after all storm events for evidence of undercutting and the erosion of adjacent materials
  • Gabions may require additional stone to offset settlement and loss

Method to Determine Practice Efficiency

Gabions reduce the amount of suspended sediment in stormwater by reducing the flow velocity of water. The efficiency for this practice is determined by calculating the settling efficiency for the device from the equation below.