By John McCullah
Agencies that govern critical fisheries habitat are becoming increasingly concerned about the widespread use of gabions, riprap, and other inert streambank structures. According to Department of Fish and Game officials, “The continuing use of gabions and riprap for bank stabilization is resulting in ongoing cumulative habitat degradation. Riprap and gabions result in chronic long-term habitat problems. Alternative bioengineering solutions should be considered.”
Vegetated gabions are a viable alternative to conventional gabion installations. This practice is applicable where flow velocities exceed 6 ft./sec. and where vegetative streambank protection alone is not sufficient. Whenever possible, gabions should be installed above the mean high-water mark. Vegetated gabions are also useful to retain earth at culvert outlets or inlets or to provide a flexible toe wall to reduce steepness of slopes.
Live branch cuttings of a rootable species such as willow or cottonwood should be integrated into construction of gabions, not as an afterthought. The branch cuttings, or live “poles,” can be installed between the gabions, but for optimum geotechnical benefit they should be inserted through the baskets, before the baskets are completely filled with rock. The basal ends of the poles should extend downward and into the native soil behind the gabions. After the poles are inserted through the basket and into the slope, the basket should be backfilled and closed. Up to 50% fines can be used to fill the baskets, which is most beneficial to plant establishment. The use of filter fabrics can inhibit root penetration; therefore, a graded sand filter should be considered.
Vegetated gabionsprovide an important component to a biotechnical soil stabilization solution and allow for the growth and establishment of natural vegetation while providing strong protection from the onset. The rooted cuttings provide tremendous pullout resistance, which greatly reduces failure of gabion structures. The vegetation serves to provide habitat diversity and aesthetic screening of the inert structure. The branches protruding from the gabion baskets will provide overhead cover for fish and will also protect the wire from abrasion.
– From the CD “Bio Draw 1.0: Compendium of Biotechnical Soil Stabilization Solutions.” This technique and other biotechnical BMPs will be offered in a training course titled “Biotechnical Soil Stabilization for Slopes and Streambanks” at the 2001 IECA conference in Las Vegas.
