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VEGETATED GABION MATTRESS

 

1. CATEGORY

Revetments

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Vegetated Reno Mattress

4. DESCRIPTION

Gabion mattresses are designed to protect the bed or lower banks of a stream against erosion. In this they differ from gabion baskets, which are meant to protect the mid to upper bank. The difference between gabion mattresses and baskets is the thickness and aerial extent of the basket. Gabion mattresses are shallower (0.5-1.5 m (1.5-5 ft) deep) rectangular containers made of welded wire mesh, and filled with rock.

5. PURPOSE

A gabion mattress is a gabion basket used as a revetment to stabilize a streambank, or when used in a channel, to decrease the effects of scour.

6. PLANNING

Useful for Erosion Processes:

Toe erosion with upper bank failure Scour of middle and upper banks by currents
Local scour Erosion of local lenses or layers of noncohesive sediment

Erosion by overbank runoff   General bed degradation

Headcutting

Piping
Erosion by navigation waves
Erosion by wind waves
Erosion by ice and debris gouging   General bank instability or susceptibility to mass slope failure
Spatial Application:
  Instream Toe Midbank   Top of Bank
Hydrologic / Geomorphic Setting Resistive   Redirective Continuous   Discontinuous Outer Bend   Inner Bend   Incision   Lateral Migration   Aggradation

Conditions Where Practice Applies:

Gabion mattresses are useful when available rock is too small to act as riprap, and withstand the erosive and tractive forces of the stream, but when stabilized with wire, the composite can buffer the banks (Freeman & Fischenich, 2000). Gabion mattresses can often be 1/3 as thick as riprap, and still provide an equivalent amount of protection. This lower profile is often preferred in areas with high visibility. Riprap is, however, much more cost effective when either method will suffice (Freeman & Fischenich, 2000).

Complexity:

Moderate.

Design Guidelines / Typical Drawings:

There are three major factors to consider when designing a gabion mattress ( Freeman & Fischenich, 2000):

1. Is the foundation sufficiently stable?
2. Is the mattress appropriately protected at the toe and flanks, such that it will not be undermined?
3. Will the installation be able to withstand the shear stress and velocity of the river?

An important consideration when installing a gabion mattress 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 is clearly not subject to erosion ( 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, the roots of the vegetation that is planted cannot penetrate filter fabric. Therefore, a sand or gravel filter is the best material to use when installing vegetated gabion baskets or mattresses.

TABLE 1: Example Filter Material Composition

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

Another important consideration when designing a gabion mattress is how to protect it against scour. The bottom-most layer of the mattress should ideally be placed below the expected maximum scour depth. If this is not feasible, mattresses can be placed at the toe to fall into any scour holes that occur.

Guides exist for sizing rock fill for the velocity encountered in a stream (see Table 2).

TABLE 2: Rock Fill Sizing Suited for Stream Velocity.

Thickness
(cm / ft)

Filling Stone Range

D50

Critical Velocity
(m/s (ft/s))

Limit Velocity
(m/s (ft/s))

15 / 0.5

7-10 cm (3-4”)

8.6 cm (3.4”)

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3.5 (11.5)

4.2 (13.8)

15 / 0.5

7-15 cm (3-6”)

10.9 cm (4.3”)

4.2 (13.8)

4.5 (14.8)

23 / 0.75

7-10 cm (3-4”)

8.6 cm (3.4”)

4.5 (14.8)

4.9 (16.0)

23 / 0.75

7-15 cm (3-6”)

11.9 cm (4.7”)

4.5 (14.8)

6.1 (20.0)

30 / 1.0

7-12 cm (3-5”)

10.2 cm (4.0”)

4.1 (13.6)

5.5 (18.0)

30 / 1.0

10-15 cm (4-6”)

12.7 cm (5.0”)

5.0 (16.4)

6.4 (21.0)

Vegetated Gabion Mattress Typical Drawing

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7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

There are several environmental benefits offered by Vegetated Gabion Mattresses, most of which are derived from the planting of willows or other species in the installation. The willow 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). An additional environmental benefit is the rock contained within the gabion; many aquatic invertebrates make their home on rocks, and the small rock size means that there is a significant amount of substrate available for colonization (Freeman & Fischenich, 2000). The small spaces between the rocks also provide hiding places for fish including fry.

8. HYDRAULIC LOADING

Limit velocities for vegetated gabion mattresses range from 4.2 m/sec (13.8 ft/sec) for a 15 cm (6 in) thick mattress to 6.4 m/sec (21 ft/sec) for a 30 cm (12 in) thick mattress (Freeman & Fischenich, 2000).

9. COMBINATION OPPORTUNITIES

Gabion Baskets are frequently used to secure the toe of the installation.

10. ADVANTAGES

Gabion mattresses provide similar bank protection as riprap, but have a lower profile. They also slow water velocities close to the bank, and encourage sedimentation. Vegetation has many environmental benefits, which are described above. Establishing vegetation in a gabion mattress secures the mattress to the bank with a network of roots.

11. LIMITATIONS

This technique is relatively expensive and labor intensive when compared to other bank protection methods.

12. MATERIALS AND EQUIPMENT

Gabion baskets, appropriately sized rock (see design guidelines), live willow stakes (or stakes of other adventitiously-rooting species).

13. CONSTRUCTION / INSTALLATION

• Grade the bank or slope back to no steeper than 1V:1½H.

• If the soil is very fine or sandy, a filter medium will be required to prevent loss of this material during high flows. An accepted gravel filter mixture is described above in the design guidelines. A less expensive option is to use filter fabric; however, this method is not recommended, as it may inhibit root penetration.

• Lay a smooth and uniform layer of whatever filter material has been chosen.

• Construct the gabion mattress according to the manufacturer's installation instructions.

• Fill the baskets with rock. The rock must be somewhat more uniform than that used for riprap, as the smallest rocks must be larger than the mesh openings, while the rocks must not be so large that they do not lock together well (Biedenharn, Elliott & Watson, 1997). Maccaferri suggests rock with a diameter between 85 and 140 mm (about 3-6 in) (Maccaferri, 2001).

• Once the baskets have been filled, insert live stakes (see Technique: Live Staking) through the rock, any filter material, and down deep enough to penetrate the vadose zone.

• The vegetation should be placed at a density of 2 to 4 cuttings per m2 (2-4 cuttings/yd2). Placement will depend on where there are holes between the rocks. In many cases, it may be necessary to use an iron bar to make pilot holes for stake placement.

• After all vegetation has been installed, pull the top of the mattress over the rock, and secure it as directed by manufacturer's specifications.

14. COST

Gabion mattresses are one of the most expensive types of slope protection. Just the baskets themselves run from $11.80 to $18.80 per square m ($1.10 to $1.75 per square ft) for a 30 cm (12 in) deep mattress.The cost of rock will vary depending on availability in your area, but typically runs between $22 to $67 per metric ton ($20 to $60 per ton), delivered. Vegetating the mattress costs very little, and can be estimated at 2-5 m2/work hour (22-54 ft2/work hour).

15. MAINTENANCE / MONITORING

Mattresses 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.

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

• Mattresses 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.

• Omission of a filter layer below the installation.

• Attempts to vegetate the mattress after construction is complete.

17. CASE STUDIES AND EXAMPLES

Information Unavailable

18. RESEARCH OPPORTUNITIES

None identified.

19. REFERENCES

Biedenharn, D. S., Elliott, C. M., & Watson, C. C.  (1997).  The WES Stream Investigation and Streambank Stabilization Handbook.  US Army Engineer Waterways Experiment Station, Vicksburg, Mississippi. (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.

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