Description
Live fascines (LF) and inert fascines (IF) are long tubular bundle structures made of cuttings of living woody plant material, placed in trenches across the slope of a bank and fastened with wooden stakes (Figure 1 & 2). LF are are expected to sprout and develop a root mass for soil reinforcement and a top growth for surface protection (Gerstgraser, 1998). The IF is not intended to grow, but can be used to protect the toe of the streambank while other vegetation becomes established (Figure 2) (Sotir & Fischenich, 2001).The plant-filled trenches break up the length of the bank face, shortening each slope segment and reducing the energy available for erosion. Once established, the live rooting material will act like a living fence for sediment trapping. In one growing season the roots and shoots can grow along the entire length of the slope thus ensuring a good stabilization of the slope. The lines of vegetation placed parallel to the contour of the shore can break up the erosive force of small waves since the plants grow in lines perpendicular to the source of energy. Fascines also help as drainage system for wet zones, by soaking up ground water seepage which can de-stabilize a slope.
Straw wattles
Straw wattles technique is very similar to live fascines and it differs only for the material used. Manufactured cylinders of compressed, weed-free straw (wheat or rice) are installed in a shallow trenches forming a continuous barrier along the contour (across the slope) to intercept water running down a slope. The cylinders of recycled, compressed, 100% agricultural straw, are wrapped in tubular wrapped in biodegradable tubular plastic or similar encasing material. They are considered an economical alternative to other erosion control measures for a temporary sediment control (the measure is effective in a period of 1-2 years).
Advantages:
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Fast and simple construction;
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Useful for wet slopes;
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Short implementation time;
Disadvantages:
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Intensive labour: construction of fascine bundles;
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Flexible branches necessary;
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Susceptible to rockfall;
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Scarce reinforcement of deeper soil layer;
(Georgi & Stathakopoulos, 2006)
Figure 2. On the left: installing a live fascine structure; on the rigth: Installing an inert fascine structure (Sotir & Fischenich, 2001).
Design methods
Live cuttings are usually bound together with untreated/un-dyed bailing twine (30-45 cm spaced) in bundles of 15-25 cm diameter and 1-6 m length. The bundles themselves need to contain at least five cuttings, each being a minimum of 3-5 cm in diameter (Figure 10). However, the exact dimensions will be governed by the available plant material (Morgan & Rickson, 1995). The fascines bundles are placed in trenches deep enough (25-40 cm) to accommodate them, with spacing up the bank face depending on the soil type. If it is a loose erosive soil, the spacing between two trenches should be 1-1.5 meters. If the soil is cohesive and less subject to erosion, the spacing can be 1.5 – 2 m between two trench rows. Moreover a row should be placed at any ground water seepage line or spring. Fascine bundles are secured every meter with wedge-like dead stakes, sometimes in alternance with live stakes, of 60-90 cm length. The trenches are backfilled with loose soil, without entirely covering the live fascines, to encourage rooting. Some techniques contemplate also the use of a mulch to retain moisture and reduce surface erosion (https://www.ernstseed.com/products/bioengineering-materials/).
Period of installation: during plant dormant season.
Materials: Willow/red-osier dogwood cuttings or chestnut peggs; baling twine; dead wedge stakes made of hardwood; live stakes.
Figure 3. Scheme of Live Fascines with a particular of a tubular bundle. (USDA - NRCS EFH Chapter-16, 1996).
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Mostly applicable on slopes where runoff flows can produce erosion processes. They can be also used as drainage in very wet soils. |
| Topple | 0 | |
| Slide | 7 | |
| Spread | 2 | |
| Flow | 7 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Most suitable for slopes where differing substrates (e.g. topsoil onto raw soil) are not sufficiently interlocked. Suitable for entrapping debris sediments along the slope. Not suitable for rock slopes, because the stakes and the fascines cannot be fixed properly, and vegetation cannot be established. Not suitable for rock slopes, because the stakes and the fascines cannot be fixed properly, and vegetation cannot be established. |
| Debris | 6 | |
| Rock | 1 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Highly effective in protecting surficial layers. With a scarce reinforcement of deeper soil layer. |
| Shallow (0.5 to 3 m) | 7 | |
| Medium (3 to 8 m) | 1 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 1 | Adequate for contrasting small volumes of extremely slow or very slow-moving soil. Less suitable for contrasting higher volumes of fast moving soil. |
| Slow | 4 | |
| Very slow | 7 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 4 | Applicable irrespective of groundwater conditions. Indirect effects on groundwater levels due to root-water uptake from plants during evapotranspiration. Particularly suitable where ground water level is high. Only for live fascine. |
| High | 6 | |
| Low | 6 | |
| Absent | 2 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Typically used for reducing the rainsplash erosion, it can be suitable for contrasting the back erosion along a streambank. This technique is suitable when placed in riverbanks. |
| Snowmelt | 7 | |
| Localized | 7 | |
| Stream | 7 | |
| Torrent | 4 | |
| River | 6 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | A well implemented measure can be reliable permanently. |
| Feasibility and Manageability | 10 | Labour demanding but after installation maintenance is not needed. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Short implementation time during the dormant season, but at least one vegetative season is required for the vegetation to be established. |
| Environmental suitability | 10 | It involves only live or woody materials (indigenous plants are preferred), suitable with the surrounding environment. |
| Economic suitability (cost) | 8 | The cost can be low if the material is provided directly on site. However, for long slopes the demand of material could be high. |
References
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Georgi, J., & Stathakopoulos, I. (2006). Bioengineering techniques for soil erosion protection and slope stabilization.
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Gerstgraser, C. (1998). “Bioengineering methods of bank stabilization,” GARTEN & LANDSCHAFT, Vol. 9, September 1998, 35- 37.
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Morgan R.P.C., Rickson R.J. (1995). “Slope Stabilization and Erosion Control: A Bioengineering Approach”. E & F Spon, London, England
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Ohio Department of Natural Resources. Ohio Stream Management Guide No. 14: Live Fascines. http://www2.ohiodnr.com/portals/soilwater/pdf/stream/stfs14.pdf.
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Sotir R. B., Fischenich C. (2001). “Live and Inert Fascine Streambank Erosion Control”, ERDC TN-EMRRP-SR-31, http://el.erdc.usace.army.mil/emrrp/pdf/sr31.pdf
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USDA NRCS Engineering Field Handbook, Part 650, Chapter 16, Streambank and Shoreline Protection. December 1996