1. CATEGORY

2.0 – Bank Armor and Protection

2. DESIGN STATUS

Level I

3. ALSO KNOWN AS

Live brush bundles, willow wattles.

4. DESCRIPTION

Live fascines are bundles of live branch cuttings placed in long rows in shallow trenches across the slope on contour or at an angle. Fascines are used for biotechnical stabilization of slopes and streambanks.

5. PURPOSE

Fascines are utilized as a resistive measure to protect the toe and face of an eroding streambank; they are also very effective for erosion control on long bank slopes above annual high water. Fascines reduce effective slope length, dissipating the speed and energy of runoff moving down slope, and simultaneously trapping sediment. The branch cuttings, rope ties, and wooden stakes combine to provide structural elements that resist hydraulic forces acting on the slope. The terraces behind the fascines provide a stable platform for workers or for access to steeper areas. Additionally, the terraces formed by the rows of fascines will trap sediment and detritus, promoting vegetative establishment. The partially buried bundles can root and grow, providing strong, long-term protection. Fascines provide excellent protection against surficial erosion; detaining runoff and increasing infiltration if aligned on contour, and directing drainage away from a slope area installed at an angle.

6. PLANNING

Design Guidelines / Typical Drawings:

Fascine spacing and configuration vary depending upon slope, exposure and purpose.

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Live fascines may be used for erosion control and vegetation establishment on long slopes, road fills, road cuts, gullies or slumped areas, eroded slopes, eroding streambanks or lake shores. Frequently, they are used to repair small earth slips and slumps or to protect slopes from shallow slides 0.3-0.6 m (1-2 ft) deep. This technique is useful on slopes requiring other planting materials such as woody vegetation, transplants, seeded grasses, and forbs. Live Fascines provide soil stability while promoting vegetation and cover establishment.

Vegetation establishment is enhanced because the bundles provide a suitable microsite for plants by reducing surface erosion, increasing infiltration rates, and by forming a series of terraces with shallower slope angles. As the riparian vegetation roots and grows, soil decompaction, shading, and development of nest sites, food resources and shelter occur.

8. HYDRAULIC LOADING

Escarameia (1998) suggests that all bioengineering treatments be limited to situations where mean flow velocity is less than 1 m/s (3.3 ft/s). Fischenich (2001) presents data from a variety of published sources that suggest live fascines will withstand shear stresses up to 60 N/m² (and velocities of 1.8 to 2.4 m/s (5.9 to 7.9 ft/s)). Fripp (personal communication, 2002) presents data from Gerstgrasser (1999) that show fascines may withstand up to 100 N/m² and 3 m/s (9.8 ft/s), data from Schiechtl and Stern (1996) for live fascines of 60 N/m² (right after construction) and 80 N/m² (after 3 to 4 seasons of growth), and data from Schoklitsch (1937) indicating fascines may be used in situations with shear stresses ranging from 10 to 50 N/m².

Dr. Gerstgrasser reported that while the fascines themselves could withstand high velocities, the unprotected, horizontal soil areas between the fascines showed accelerated erosion (personal communication, 2000).

9. COMBINATION OPPORTUNITIES

Live stakes can be used in addition to the construction stakes to increase slope stability and revegetation. Fascines can be used in combination with Erosion Control Blankets and Mats, and other planting and erosion control treatments such as seeding and live rooted transplants.

10. ADVANTAGES

11. LIMITATIONS

This technique should only be installed during the dormant season of the plant material used. Live fascines are only appropriate on slopes that are not undergoing mass movement and on streambanks above the annual high water (AHW) level. Because the reinforcement and does not penetrate as deeply into the slope as other techniques, such as brushlayering, they are not immediately as effective and in arid or semi-arid regions the actual rooting and growth may be limited.

12. MATERIALS AND EQUIPMENT

Fascines are made of long straight brushy branches 1 to 5 m (3 to15 ft) long up to 40 mm (1 ½ in) in diameter. The branches comprising fascines are harvested from tree and shrub species capable of propagating from cuttings, typically willow (Salix spp) species (McCullah, 2002). Coyotebrush (Baccharis pilularis), dogwood (Cornus spp.), cottonwood (Poplar spp.), and Elderberry (Sambucus spp.). If adequate quantities of suitable material is unavailable, the fascines may be constructed from 50% of species unlikely to root, e.g., alder, maple, birch, or aspen. See Special Topic: Harvesting and Handling for collection, transportation and storage techniques.

13. CONSTRUCTION / INSTALLATION

Tie cuttings together to form bundles, tapered at each end, 2-10 m (6-30 ft) in length, depending on site conditions or limitations in handling. The completed bundles should be 15-30 cm (6-12 in) in diameter, with the growing tips all oriented in the same direction. Stagger the cuttings in the bundles so that the tips are evenly distributed throughout the length of the bundle. Compress and tightly tie the bundle every 30 cm (1 ft) with rope or twine of sufficient strength and durability. Hemp, jute, cotton, or other biodegradable rope may be used (McCullah, 2002).

Installation should progress from the bottom to the top of the slope. Install bundles into trenches dug into the slope on contour. Spacing of contour trenches (fascines) is determined by soil type, potential for erosion, and slope steepness. See Table 1 below for general spacing guidelines.

TABLE 1: General Installation Guidelines
(Sotir & Fischenich (2001))

Trenches: The trench should be shallow, about ½ the diameter of the fascine. The trench width should be 30-45 cm (12-18 in), depending on the slope angle, but should be at least 2.5 cm (1 in) wider than the bundle. In non-cohesive soils, the trench should be lined with a coir erosion control blanket or netting prior to installation of the fascine (Sotir & Fischenich, 2001).

Staking: Stake fascines firmly in place with one row of construction stakes on the downhill side of the bundle, not more than 1m (3 ft) apart. Place a second row of stakes through the fascines, near the ties, at not more than 1.5 m (5 ft) apart. Overlap the tapered ends of adjacent bundles at least 45 cm (18 in), so the overall thickness of the fascine is uniform. Use two stakes at each bundle overlap, such that a stake is driven between the last two ties of each bundle.

Live stakes, if specified, are generally installed on the downslope side of the bundle. Drive the live stakes below and against the bundle between the previously installed construction stakes. Repeat the preceding steps to the top of the slope, placing moist soil along the sides of the live bundles. When finished, all live stakes should be trimmed, such that a maximum 7.5 cm (3 in) of stake protrudes above the bundle (Sotir & Fischenich, 2001).

Keys: Fascines should be keyed into the bank at least 1 m (3 ft) on both upstream and downstream ends (Sotir & Fischenich, 2001).

Backfilling: Proper backfilling is essential to the successful rooting of the fascine. Backfill bundles with soil from the slope or trench above. Work the backfill into the fascine interstices and compact behind and below the bundle by walking on it and working from its terrace.

Seed and mulch: Shallow slopes, generally 3:1 or flatter may be seeded and mulched by hand. Steeper slopes may be hydraulically seeded, and the mulch anchored with tackifier or other approved methods.