VEGETATED GABION BASKET

1. CATEGORY

1.0 – River Training

2. DESIGN STATUS

Level I

3. ALSO KNOWN AS

Vegetated gabions, Gabion walls

4. DESCRIPTION

Gabions are rectangular baskets made of triple-twisted hexagonal or welded-wire mesh of heavily galvanized, and sometimes poly-vinyl chloride coated, steel wire. Gabions are delivered as flat wire panels that are folded into the basket form, filled with rock, and laced shut. Vegetation can be incorporated by placing cuttings through the mesh of the basket during filling, or between the baskets after the baskets have been laced shut. These pervious structures can be used singly or stacked like building blocks.

5. PURPOSE

Vegetated gabions are used as pervious retaining and armoring structures, culvert outlet or inlet stabilization, and as flexible toe-walls that reduce the steepness of slopes or streambanks. This technique is typically used where large rock is unavailable, as gabion baskets filled with small rocks can resist higher tractive forces than the rocks would normally be able to withstand without wire reinforcement.

Vegetated gabions are mitigation for unvegetated gabions. Besides providing habitat enhancements, vegetating gabions with woody plants during construction will increase longevity of the structures.

Vegetated gabions are used where streambanks are subject to excessive scour. Gabions are often recommended for sites where either there is insufficient supply of large angular stone or where the bank must be constructed at an angle steeper than the angle of repose (greater than 1V:1.5H) therefore reinforcement is necessary. The gabion baskets can be stacked to construct relatively high banks. Gabions can also be used to armor the bed and/or banks of channels, or as deflectors or groins that divert flow away from eroding streambank sections. Gabions are useful where flow velocities exceed 2 m/sec (6 ft/sec), and where vegetative streambank protection alone is not sufficient. This is a technique that can be used in place of riprap where required rock size exceeds that available, or where a steep slope (greater than 1V:1.5H) requires structural support (USDA, 1996). These structures can also be used to stabilize culvert outlets or inlets, or to provide a flexible toe-wall and reduce steepness of slopes or streambanks.

Complexity:

Moderate.

Design Guidelines / Typical Drawings:

Filter Material: An important consideration when installing gabion baskets is choice of filter material. Filter material prevents migration of soil through the installation, which could lead to undermining, settlement or flanking, and subsequent failure. The only time that filter material may be omitted is when the foundation material clearly does not need it (Freeman & Fischenich, 2000).

There are several choices for filter material; one can either use filter fabric, a gravel or sand filter, or a combination of these two. Filter fabric is commonly used; however, vegetative roots may not penetrate filter fabric. Therefore, a sand or gravel filter is the best material to use when installing vegetated gabion baskets or mattresses.

If using a filter fabric, it is important to use a non-woven fabric (170 g (6 oz) is recommended), as woven fabrics have poor flow-through properties, and can actually lead to increased erosion from bank, due to fabric movement (Maccaferri, 2001).

The following example (see Table 1) composition can be used as a granular filter below gabion baskets and mattresses. The composition of all filter blankets should be based on site-specific conditions. Other filter designs can be found in Brown et al, 1998.

TABLE 1: Sieve Size and Passing

Sieve Size	Percentage Passing
25 mm (1 in)	100
19 mm (3/4 in)	90-100
10 mm (3/8 in)	40-100
No. 4	25-40
No. 8	18-33
No. 30	5-15
No. 50	0-7
No. 200	0-3

Stability: Gabion basket walls should be installed with a batter of at least 6° from the vertical into the slope. Two options are available for achieving this batter, as shown in the illustration below.

Another important consideration when designing a gabion wall is how to protect it against scour. The bottom-most rank of baskets should ideally be placed below the expected maximum scour depth. If this is not feasible, gabion mattresses (scour apron) can be placed at the toe to fall (self-launch) into any scour holes that occur. It is critical that factors such as scour depth be determined for each particular project and be incorporated into project design. Overall height of gabion structures should not exceed 3 m (9 ft), including the toe (Maccaferri, 2001).

Rock sizing: Table 2 provides a guide for sizing rock fill based on expected velocity (Freeman & Fischenich, 2000; Maccaferri, 2001):

TABLE 2: Rock Sizing Based on Expected Velocity

Thickness m (ft)	Filling Stone Range cm (in)	D50 cm(in)	Critical Velocity (m/s (ft/s))	Limit Velocity (m/s (ft/s))
0.5–1 (1.5-3)	10-20 cm (4-8 in)	15.25 cm (6 in)	5.8 (19)	7.6 (24.9)
0.5–1 (1.5-3)	13-25 cm (5-10 in)	19 cm (7.5 in)	6.4 (21)	8.0 (26.2)

The following equation (from Freeman & Fischenich, 2000) provides more accurate rock sizing guidelines:

The variables in the above equation are defined as:
	Cs = stability coefficient
	Cv = velocity distribution coefficient, where
	and, Cv = 1.25 at the end of dikes and concrete channels.
	dm = average rock diameter in gabions
	d = local flow depth at V
	g = acceleration due to gravity
	K1 = side slope correction factor (see Table 1)
	R = centerline bend radius or main channel flow
	Sf = safety factor (1.1 minimum)
	V = depth-averaged velocity
	W = water surface width of main channel
	Υs = unit weight of stone
	Υw = unit weight of water

Other rock sizing guidelines from Racin and Hoover (2001, Appendix A) are:

• For PVC-coated gabions, click on the following hyperlink: http://www.dot.ca.gov/hq/esc/oe/specifications/SSPs/99-SSPs/Sec_10/61-75/72-300_A02-10-00.doc. For your own copy of the file, choose the “save as” option from the Microsoft ® (MS) Word menu.

• For zinc-coated gabions, click on the following hyperlink: http://www.dot.ca.gov/hq/esc/oe/specifications/SSPs/99-SSPs/Sec_10/61-75/72-305_A02-10-00.doc

Wire type: There are several different types of wire mesh available for gabions. The first choice to make is welded-wire vs. twisted wire. Racin & Hoover (2001) found that there was no significant difference between the two types in terms of load bearing ability, flexibility, or deflection. Another choice that must be made is whether to purchase PVC-coated wire, or zinc-coated (uncoated) wire. Table 4, based on 16 years of observation, indicates expected life-span of the two choices in a variety of conditions. The life-span was determined when the wires lost half of their strength, wires disintegrated, or other loss of function occurred. The designer should match the desired lifespan of the structure with site conditions to determine whether or not coating (and its accompanying higher costs) is necessary.

TABLE 4. Life-span Comparison Between PVC-coated Wire and Zinc-coated Wire.
(from Racin & Hoover, 2001)

Site Conditions	PVC coating	Zinc
Vandalism (wires cut, rocks emptied) in urban/suburban sites	1 year	1 year
Beach where ocean waves break	1.5-4 years	1.5-4 years
Slough connected to ocean – cyclic rising and falling of tide waters, wind-driven surface ripples	16+ years	3-6 years
Creeks, streams and arroyos with storm runoff that conveys abrasive soil particles and debris, corrasion (erosion of corroded compounds), or cyclic rising and falling water levels	9-15 years	9-15 years
Fresh water, stagnant pools with low dissolved oxygen	16+ years	10 years
Saturated soils	16+ years	10 years
Wave splash and spray, near-shore fog	16+ years	16+ years
Freshwater sites with intermittent, channelized storm runoff and/or streams that transport little or no suspended particles greater than 0.074 mm and/or low velocity water with low conductivity and high dissolved oxygen	16+ years	16+ years
Well-drained and/or dry soil	16+ years	16+ years
Atmosphere	16+ years	16+ years
Rock fall impact that could break wire	16+ years	16+ years

Soil bioengineers believe that woody vegetation used in conjunction with gabions can increase the baskets' longevity by protecting the wire from debris that could cause wear or abrasion. In addition, if and when the wire fails, the vegetation will protect the slope or streambank.

Vegetation: There are two methods used to vegetate gabions; pole planting and brushlayering. Pole planting, where branches are inserted through the basket mesh during filling, helps to anchor the baskets to the slope, while simultaneously enhancing aesthetic appeal. At least 3 poles should be installed per m (1 pole/ft), with a minimum of 3 branches per basket. Brushlayering, where branches are placed between the basket layers, is a very quick technique that enhances aesthetics and habitat. The gabion basket provides all geotechnical stability necessary for the slope, so 6 branches per m (2 per ft) should be sufficient for aesthetic and habitat benefits.

Vegetated Gabion Baskets Typical Drawing
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Vegetated Gabion Baskets Installation Typical Drawing
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7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Some environmental agencies, such as the California Department of Fish and Game, have reported concern about the use of rock-filled gabions in streams with sensitive aquatic resources. See the San Vacinte Case Study for more information on the CA Fish and Game concerns. The Canadian Department of Fisheries and Oceans states that gabion baskets provide marginal fish habitat and their use is not encouraged. As mentioned previously in this report, unvegetated riprap and rock-filled gabions, especially when used as resistive structures along an outer bend, provide very limited habitat. Vegetated riprap is often required as mitigation for unvegetated riprap (Washington, 2003). This consideration is probably apropos for gabions also. Since gabions have a finite lifespan, the ultimate long-term solution for stable streambanks is vegetation, and mimicking the naturally-occurring riparian plant community is a valuable consideration.

There are several environmental benefits offered by vegetated gabion baskets, most of which are derived from the planting of willow (Salix spp) or other woody species capable of rooting in the installation. The vegetation provides canopy cover to the stream, which gives fish and other aquatic inhabitants cool places to hide. The willow also supplies the river with vegetative debris, which is a major portion of the base of the aquatic food chain. Birds that catch fish or aquatic insects will also be attracted by the increased perching space next to the stream (Gray & Sotir, 1996). Rock contained within the gabion provides substrate for aquatic invertebrates, and the small rock size means that there is a significant amount of surface area available for colonization (Freeman & Fischenich, 2000). The small spaces between the rocks also provide hiding places for small fish and fry.

8. HYDRAULIC LOADING

Fischenich (2001) reported a maximum allowable velocity of 4.3 m/s (14.1 ft/s) for gabion baskets.

9. COMBINATION OPPORTUNITIES

Vegetated gabions for toe protection and other techniques, such as brushlayering and/or VMSE with organic geotextiles can be used to reinforce the rest of the bank.

10. ADVANTAGES

Vegetated gabions have a more natural appearance and are less visually intrusive than structural treatments alone (Gray & Sotir, 1996).

Branches planted within and between the baskets anchors them to the bank, and once established, can help protect against failure due to wire damage.

Wetland Research Program (1998) provides the following advantages of gabion baskets:

•  The smaller stone used in a gabion can provide equivalent protection as the larger stone used in a riprap revetment.

•  They can support vegetation.

•  Gabions can be cost-effective when using local, commercially available stone fill.

•  Gabions require less tonnage of stone than riprap – as gabion thickness is approximately 1/3 that of comparable riprap.

•  They are flexible and durable if properly maintained.

•  Gabions can be stacked to obtain near-vertical side slopes where right-of-way is limited.

•  If necessary, gabions can be built without heavy-equipment.

•  Gabions are flexible and can adjust to minor substrate settling.

•  They can be repaired easily by mending or replacing damaged baskets and refilling them as needed.

11. LIMITATIONS

•  Wire mesh is subject to damage from strong waves, floating debris, corrosion, high-velocity sediments, and vandalism. A study conducted by Caltrans (Racine and Hoover, 2001) found that gabion wire in a stream or coastal environment had an average longevity of approximately 15 years. Vinyl coated wire increased the longevity by 5-7 years. However, it is theorized that vegetation, especially woody vegetation will increase longevity by protecting the wire from abrasion. Secondarily, the vegetative stems, branches and roots, if well established, will continue to stabilize the bank long after the wire has deteriorated.

•  Baskets require significant monitoring and maintenance to identify wear before failure occurs.

•  Installation is labor-intensive.

12. MATERIALS AND EQUIPMENT

Gabion baskets, hog-ring staples and stapler, stiffeners or corner ties, rock for gabion fill, filter fabric or gravel, woody cuttings.

13. CONSTRUCTION / INSTALLATION

The following guidelines are derived from Gray & Sotir (1996); Freeman & Fischenich (2000); E. Richards - Maccaferri (personal communication, 2003); McCullah (2004) and USDA (1996).

1. Remove loose material from the foundation area, and excavate a footing area as deep as specified in plans (typically 0.6 to 1 m (2-3 ft)). The key trench should be uniform, and slope down into the bank at least 6° from the horizontal.

2. Install filter material (fabric or gravel) as specified by manufacturer. Filter gravel should be installed to a uniform thickness; compaction typically is not necessary (Freeman & Fischenich, 2000).

3. Assemble the gabions as specified by the supplier, and position them according to plans.

4. Once the gabions are in place, fill them halfway with rock and insert poles at the density specified. They should be inserted through the basket, with the growing tip higher than the butt end; the butt end should extend far enough into the substrate behind the gabions to reach the vadose zone. Fill the basket the remainder of the way with rock, close lid, and lace it shut.

5. Rocks should be placed carefully, so as not to damage gabion wires. If necessary, stiffeners or corner ties should be installed during filling. Typically, 0.3 m (1 ft) of rock is placed, stiffeners are installed, the next layer of rocks is placed, and so on.

6. Once the first rank of baskets is installed, brushlayers should be placed at the designed density, with the butt ends extending far enough into the substrate behind the gabions to reach the vadose zone. The 6° minimum batter into slope will ensure that growing tips are elevated above butt ends.

7. Backfill behind the gabions and compact to ensure optimum stem to soil contact prior to installing the next rank of baskets. Baskets should be placed such that vertical joints are staggered (as shown in design guidelines).

8. Repeat this sequence until the wall is the desired height.

For other standard highway specifications for gabions, please see the following website: http://www.dot.ca.gov/hq/esc/oe/project_plans/HTM/stdplns-met-new99.htm

14. COST

Costs for gabion baskets are very high compared to other bank protection methods. Freeman & Fischenich (2000) report costs for the baskets between $16.14 and $34.43/m² ($1.50 and $3.20/ft²). Total project cost (including baskets, assembling and filling, stone fill, and basket closure) is approximately $195 to $585/m³ ($150 to $450/yd³). This does not include vegetating the baskets, which runs about $1 per cutting (including all harvesting, handling, transport, installation, etc) (McCullah, 2004). Using both pole planting and brushlayering, vegetation will increase costs by about $11/m³ ($9/yd³), a comparatively small sum.

15. MAINTENANCE / MONITORING

Gabions should be checked regularly for broken wires, and repaired immediately if necessary, to prevent loss of rock from the structure. The structure should also be monitored for signs of undermining or flanking; should any be found, corrective measures should be taken. The area should also be monitored for any signs of geotechnical failure, such as shifting or bulging away from the bank.

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

• Baskets not adequately filled, allowing stones to shift and cause abrasion and fatigue failures of wire.

• Baskets damaged by floating debris, wear, corrosion, or vandalism.

• Flanking or undermining of the structure due to insufficient keying, or unstable banks on upstream or downstream end.

17. CASE STUDIES AND EXAMPLES

Tecalote Canyon

Substantial erosion from gullying caused by storm water discharge from the mesas into Tecolote Creek, sheet flow and rill erosion on hillsides from the increased runoff from developed areas, and streambank and streambed deterioration from increased flow volumes was resulting in sediment deposition at the mouth of Tecolote Creek and in Mission Bay . Tecolote Creek had incised through the alluvium in the canyon floor to the bedrock. The creek was undercutting, causing widening and dewatering of the channel banks, which subsequently caused all riparian vegetation to die from dehydration. This project involved constructing a series of gabion weirs across Tecolote Creek for grade control, and installing vegetated riprap along the banks of the creek to halt further bank erosion.

Branciforte Creek

High flows during large storms events resulted in erosion of a streambank located on the downstream end of a low radius bend with highly erodible soil. The erosion threatened a residential structure located on the high right-descending bank. Between January and March of 2000, vegetated riprap and vegetated gabions were installed to halt erosion. In June of 2000, vegetated gabions and brush layering with soil wrap were installed above the previous installation to stabilize the bank and encourage revegetation.

Miscellaneous

Photos courtesy Maccaferri Gabions
These gabions were very difficult to vegetate after construction. Coir rolls were installed in an attempt to aide willow stake establishment. Soquel Creek is an important Coho salmon and steelhead stream. Santa Cruz, CA 1997	The rock spur provided an additional benefit of increasing sedimentation at the toe, which then provided an excellent substrate for willow establishment. 1999. J. McCullah
Soquel Creek gabions are now well vegetated after 7 years. 2004. Photo by J. McCullah	These gabions and streambank will probably be long-lived as the riparian vegetation is providing mutual protection.
Unvegetated gabions in Morro Creek, endangered steelhead habitat, required mitigation work before approval by NOA NMFS. Getting willow cutting to establish after construction is extremely difficult. 1999. Morro Creek, San Luis Obispo County, CA Photo by John McCullah.	After several efforts, a few willows became established. 2000. Photo by J. McCullah.

Please visit the Photo Gallery for more pictures.

18. RESEARCH OPPORTUNITIES

Long term (more than 15 years) performance of vegetated gabions.

19. REFERENCES

Brown, Scott A. & Clyde, Eric S. (1989) Design of Riprap Revetment, Hydraulic Engineering Circular No. 11, HEC-11, US Department of Transportation, Federal Highway Administration, Office of Implementation HRT-10, March, 1989.

Canadian Department of Fisheries and Oceans (unk) Fish Habitat and Shoreline Stabilization, Fact Sheet C-4.

Fischenich, J. C. (2001).  Stability thresholds for stream restoration materials.  EMRRP Technical Notes Collection (ERDC-TN-EMRRP-SR-29), U.S. Army Engineering Research and Development Center, Vicksburg, MS. (pdf)

Freeman, G. E. & Fischenich J. C.  (2000).  Gabions for Streambank Erosion Control.  EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-22), U.S. Army Engineer Research and Development Center, Vicksburg, MS. (pdf)

Gray, D. H. & Sotir, R.  (1996).  Biotechnical and Soil Bioengineering Slope Stabilization. John Wiley and Sons, New York, N. Y.

Maccaferri, Inc. (2001). Soil Bioengineering and Ecological Systems Techniques. Maccaferri, Inc.

McCullah, J. A. (2003).  Bio Draw 2.0.  Salix Applied Earthcare, Redding, CA

Racin, J. A. & Hoover, T. P. (2001).  Gabion Mesh Corrosion: Field Study of Test Panels and Full-scale Facilities. (Final Report No. FHWA-CA-TL-99-23) State of California Department of Transportation, Division of New Technology and Research, Sacramento, CA. (pdf)

USDA Soil Conservation Service. (1996). Chapter 16: Streambank and Shoreline Protection. Part 650, 210-EFH, Engineering Field Handbook, 88 pp. (pdf)

Washington Dept of Fish & Wildlife (2003). Integrated Streambank Protection Guidelines, published in co-operation with Washington Dept. of Transportation and Washington Dept. of Ecology, June 2003. (Chapter 6 pdf) (Appendix L pdf) (Appendix H pdf) http://www.wa.gov/wdfw/hab/ahg/ispgdoc.htm (April 2003)

Wetland Research Program, (1998). Shoreline and Channel Erosion Protection: Overview of Alternatives (WRP Technical Note HS-RS-4.1).

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