COCONUT FIBER ROLLS

1. CATEGORY

1.0 – River Training

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Coir rolls, fiberschines, reed rolls, fiber logs, coir geotextile rolls, coir fascines.

4. DESCRIPTION

Fiber rolls are manufactured, elongated cylindrical structures that are placed at the toe of a streambank. They are typically made of coconut husk fibers bound together with a geotextile netting. The external binding or netting forms a type of open weave "sock" that contains and provides some shape to the internal mass of fiber husks (see Figure 1). When natural, coconut, or coir fabric is used for this purpose the fiber rolls are usually referred to as coir rolls or "fiberschines". These coir rolls are typically manufactured in 35 cm or 40 cm (12 in or 18 in) diameters and lengths of 6 m (20 ft). They are staked or otherwise anchored in place at the toe of a streambank, generally at the stream-forming flow stage within a zone of perennial growth. (USDA, 1996).

5. PURPOSE

Coconut fiber rolls are placed at the bottom of streambanks to help prevent scour and erosion as shown in Figure 2. They are used to provide channel and shoreline stabilization in areas of low to moderate shear stress. They act as a type of "soft" toe armor and encourage the growth of vegetation. A "soft toe" will function and endure on the insides of meanders, and in tangent zones that are beyond the influences of a channel bend. Vegetative establishment in and around the fiber roll increases surface roughness, slows water velocity, and causes sediment to drop out and accrete at the toe. This system is quite flexible and can conform to existing curvature of a streambank.

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:

The system can be considered for erosion protection along the toe of streambanks where water velocities are not excessive and where vegetation establishment and toe accretion are desirable. They protect the base of a streambank without significant site disturbance.

Complexity:

Low. Coir or coconut rolls are pre-manufactured, relatively lightweight and easy to install.

Design Guidelines / Typical Drawings:

Consult individual manufacturer's specifications. Normally a shallow trench wide enough to accommodate the fiber roll is first excavated at the toe of the slope to a depth slightly below channel grade. The fiber roll is placed in the trench. Wooden stakes or poles are driven in on either side of the roll on 60 cm to 120 cm (2 to 4 ft) centers (depending on anticipated velocities) to keep it in place as shown in Figure 3. Once the fiber roll is staked in place, soil can be backfilled behind it. Finally, appropriate vegetation, is planted (plugged) in and around the fiber roll.

Figure 3. Schematic drawing of coconut fiber roll installation

Coconut Fiber Rolls Typical Drawing
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7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Coconut fiber rolls do not intrude visually; they provide a soft, natural boundary to a river's edge. They help establish healthy riparian habitat conditions that will provide cover. Individual stems or clumps of various sizes and types of reed grass (Calamagrostis spp), sedges (Carex spp), and bulrushes (Schoenoplectus spp) can be inserted into and around the fiber roll (Schiechtl and Stern, 1997).

8. HYDRAULIC LOADING

Only limited data has been collected for shear stress or velocity tolerances of coir or coconut fiber rolls. In general, fiber rolls should only be used under relatively low to moderate shear stress and velocity conditions. Available data shown in Table 1 comes largely from empirical information or from vendors' design criteria (Allen and Fischenich, 2000). Failure of coconut fiber rolls has been attributed to several mechanisms, viz., flanking, undercutting, and anchor failure.

TABLE 1: Limiting Shear Stress And Velocity Levels For Fiber Geotextile Rolls
(after Allen and Fischenich, 2000)

9. COMBINATION OPPORTUNITIES

Fiber rolls provide opportunities for conjunctive use with a variety of other bank protection systems. They can be combined with upper- and mid-slope bank armor (see Techniques: Live Staking, Live Fascines, Brushlayers, Erosion Control Blankets (ECBs), Turf Reinforcement Mats (TRMs)). They could also be used in combination with discontinuous, redirective river training methods (see Techniques: Spur Dikes, Vanes. and Bendway Weirs).

10. ADVANTAGES

Fiber rolls help to protect slopes from undermining resulting from toe erosion while trapping sediment that encourages plant growth within and around the structure. Coir (coconut husk) fiber used in these rolls has a high tensile strength (Gray, 1989), is biodegradable, and has excellent moisture retention and sediment trapping properties to encourage plant growth. The rolls are flexible and can conform to existing bank curvature. They are easily installed with wooden stakes and over time blend naturally into the aquatic environment.

11. LIMITATIONS

Prefabricated materials and products can be expensive. Coir fabric, while quite durable compared to other natural fiber products, has an effective life of only 6 to 10 years in many applications. Natural fiber rolls should be avoided in channels which are actively incising and in reaches with large debris loads and/or significant ice build up.

12. MATERIALS AND EQUIPMENT

The main material is the coconut fiber roll itself, which is typically manufactured in 35 cm or 40 cm (12 in or 18 in) diameters and 6 m (20 ft) lengths. A backhoe with narrow bucket is useful for excavating a shallow trench (in which the fiber roll is placed) and for lifting the rolls into place, especially when wet. Wooden construction stakes (5 cm x 5 cm x 90 cm (2 in x 2 in x 36 in)) are also needed to keep the roll in place. Suitable rooted plantings, e.g., individual culms and clumps of reed grass, bulrush, and sedge etc. should be chosen according to site specific conditions and objectives by a plant specialist.

13. CONSTRUCTION / INSTALLATION

A recommended construction procedure (Maryland, 2000) for natural fiber roll installation consists of the following steps:

•  Install fiber rolls so that they rest against the bottom of the waterway in ponds or lakes. A shallow trench wide enough to accommodate the fiber roll is first excavated at the toe of the slope to a depth slightly below channel grade. In streams and rivers, place the fiber roll above any necessary toe stabilization measures.

•  Use wooden construction stakes or earth anchors with cable assembly systems to anchor the fiber rolls in place. The stakes should be notched approximately 13 cm (5 in) from their tops and driven partially into the ground on either side of the bundle at a spacing of 0.9 to 1.2 m (3 to 4 ft) as shown in the Typical Drawing. Use twine to tie from the notch in one stake to the notch in stake directly opposite. Then drive the stakes so that the twine is secured against the top of the roll. The top of the stake should be flush with the top of the roll after driving.

•  Lace together the ends of adjacent rolls with twine by making a number of passes in the end netting between the rolls and pulling the twine taut. In places where a fiber roll does not abut another, bend the end inward and bury it in the bank to prevent water from washing in behind and dislodging it.

•  Insert or plug plants in an alternating pattern along the top of the fiber rolls in gaps between the coir fiber netting. Appropriate species and a spacing ranging from 0.15 to 0.3 m (6 to 12 in) should be selected by a plant specialist according to such site characteristics as soil properties, anticipated post-construction bank slope, water chemistry, amount of available sunlight, and expected duration of inundation during high stream flows. If water levels are too low for the fiber rolls to be submerged 1/2 to 2/3 of their diameter, plants should be plugged inside the soil/log interface to receive adequate moisture.

•  Soil backfilled behind the fiber roll should be seeded and/or plugged with appropriate vegetative species viz., reed grasses, bulrushes, and/or sedges, and covered with an erosion control blanket or coir fabric mat.

Helpful information concerning construction with coir geotextile rolls, specifically guidelines dealing with bank preparation, physical handling and placement, and securing them in place is provided by Allen and Fischenich (2000). The very characteristic that gives coconut husk fibers good moisture retention properties also makes coir rolls difficult to handle when wet. A roll weighs about 10 to 30 kg/m (7 to 20 lb/ft) when dry, depending on their diameter, and can be handled by two to four people. When allowed to absorb water, it can increase in weight seven fold making it difficult to maneuver by hand. Accordingly, these fiber rolls should be positioned in their final resting places in a relatively dry condition if possible and/or when water levels in a stream are low.

14. COST

The cost of coir geotextile rolls (or logs) themselves (Washington, 2003) ranges between $20 and $40 per linear m ($6 and $12 per linear ft), depending upon their diameter and density. Installed toe treatments with plantings cost about $86 per linear m ($26 per linear ft). A similar treatment with the addition of erosion-control fabric and willow bundles behind the coir-roll at the toe runs about $100 per lineal m ($30 per lineal ft). These costs are for material and construction only for fiber rolls installed primarily in Washington between 1995 and 2000, and do not include design or post construction components of the project. Based on available data (Maryland, 2000), costs for coir geotextile rolls installed at the toe (in 1999) are $201 per running m or $61 per running ft.

15. MAINTENANCE / MONITORING

Coir is a relatively durable material, but its finite effective life (about 6 to 10 years) requires that the long-term integrity of this bank-stabilization technique relies on the roots and shoots of vegetation. Installed plantings may require initial irrigation, weed control and/or protection from browsing animals. The anchoring system may also need periodic maintenance or replacement with special attention paid to the twine (or wire) laced between the stakes over the coir rolls.

Monitoring should focus on potential weak points in the design and construction of this technique and on elements such as the stakes and binding twine (or wire) used to hold coir rolls down. Potential weak points include junctions between rolls, transitions between undisturbed and treated banks and between different bank treatments. Monitoring should consist of periodic field inspections and photo documentation. Additional information with regard to a plan to monitor both coir fiber roll performance and to evaluate vegetation establishment success can be found in the integrated streambank protection guidelines (Washington, 2003).

16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE

Reasons for failure include excessive shear stress leading to scour and undermining, inadequate anchoring conditions, a poor vegetative establishment environment (e.g., shade, toxic soil/water chemistry, etc), and poor construction methods. If the substrate beneath the fiber roll is non-cohesive material, such as sand or silt, anchoring may be problematic because of the lack of sufficient skin friction to hold the anchors (stakes) in place (Allen and Fischenich, 2000). Conversely, a substrate laden with interspersed rocks may make it difficult to drive the stakes in place.

An example of a Coconut Fiber Roll failure can be found in Racin and Hoover (2001), pages 88 through 93.

17. CASE STUDIES AND EXAMPLES

Coconut fiber rolls have been used extensively in practice. Schiechtl and Stern (1997) describe the use of fiber rolls to protect streambanks and small rivers with fairly constant flow rates and fine grained bed load. They also cite the use of fiber rolls for the protection of newly established reed plantings on lake shores prone to erosion. Figure 4 shows a fiber roll soon after construction along a lakeshore in California; whereas Figures 5-7 show views of fiber rolls used along stream channels.

Please visit the Photo Gallery for more pictures.

18. RESEARCH OPPORTUNITIES

The combined use of Coconut Fiber Rolls with discontinuous, redirective techniques, e.g., Bank Barbs, Vanes, and Bendway Weirs, merits additional investigation. The potential value of fiber rolls for improving near shore aquatic habitat also deserves some attention.

19. REFERENCES

Allen, H.H. & Fischenich, J.C. (2000). Coir Geotextile roll and wetland plants for streambank erosion control. EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-04), U.S. Army Engineer Research Development Center, Vicksburg, MS. (pdf)

Gray, D.H. (1989). Engineering and Physical Properties of Coir Geotextiles. Report prepared for Belton Corp., June 1989

Maryland Department of the Environment, Water Management Administration (Follweiler, J eds.) (2000). Maryland’s Waterway Construction Guidelines, Section 3 Channel Stabilization and Rehabilitation Techniques, Baltimore, MD. (pdf)

Schiechtl, H. M. & Stern, R. (1997). Water Bioengineering Techniques for Watercourse Bank and Shoreline Protection. Blackwell Science, Inc. 224 pp.

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)

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