| ROOTWAD REVETMENTS |
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1. CATEGORY
2.0 – Bank Armor and Protection
2. DESIGN STATUS
Level II
3. ALSO KNOWN AS
Rootwads, Rootwad Composites, Pervious Tree Revetments, Cedar Tree Revetments
4. DESCRIPTION
Rootwad and tree revetments are structures constructed from interlocking tree materials. These structures are continuous and resistive type methods, distinguishable from discontinuous and redirective methods such as large woody debris (LWD) structures or rootwad deflectors. Rootwad revetments and tree revetments are primarily intended to resist erosive flows and are usually used on the outer bends of streams. While all of these structures are made from tree parts, they differ in their fundamental purpose.
Rootwad revetments use root mass or a root ball, commonly called a rootwad, of a tree plus a portion of the trunk (boll) in combination with rocks and biotechnical methods to construct continuous bank protection. Rootwad revetments are designed such that the rootwads are oriented upstream into the stream flow and are frequently placed against each other for continuous armoring along the entire length of the meander bend. In addition to providing continuous, resistive bank protection, rootwad revetments also function secondarily as a redirective method that moves the current line away from the streambank so the bank is less susceptible to erosion through hydraulic forces.
Tree revetments are comprised of whole trees, commonly cedar, where the tree butts are cabled to the bank and the crowns are overlapped to form continuous armor along the outer bank of a meander bend (Edminster et al. 1949). Structures made with numerous small branches (for example those made from cedar or juniper trees) are more efficient in trapping and retaining sediment than those with larger, fewer diameter branches. Felled trees may be used to deflect erosive flows and promote sediment deposition at the base of eroding banks.
Rootwad revetments and tree revetments require a thorough and immediate revegetation plan for complete and long-term project success, and if not constructed properly, fish habitat may be less than desired (Sylte, Fischenich, 2000).
5. PURPOSE
The uses of naturally-occurring materials are intended to mimic natural systems and increase aquatic habitat. Rootwad revetments and tree revetments are primarily intended to resist erosive flows and armor are usually used on the outer bends of streams. The woody material provides resistance and energy dissipation against stream flow and wave action. These pervious structures increase hydraulic roughness and can promote deposition. Structures made with numerous small branches (for example those made from cedar or juniper trees) are more efficient in trapping and retaining sediment than those with larger, fewer diameter branches. Rootwad composites, due to the rootwad fan protruding out into the stream, tend to move the current line away from the streambank so the bank is less susceptible to erosion through hydraulic forces. (Sylte, Fischenich, 2000).
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:
Rootwad and tree revetments are typically are most frequently used for stabilization and erosion resistance along the outer bank of meander bends. Rootwad revetments have been used successfully on many types of stream systems. As a general rule, these methods are considered if the natural materials are found at or near the site, if large woody debris is part of the stream system and contributes to the "proper functioning condition", if the riparian zone will be allowed to function naturally as the material decomposes, and if a thorough revegetation plan can be implemented immediately. The revegetation plan is best if it combines biotechnical practices.
A variety of architectures are available: a "shingled" configuration (cedar tree revetments) where tree butts are cabled to the bank and crowns overlap along the water line (Edminster et al., 1949), a shingled arrangement where trimmed crowns are buried in the bank and butts protrude into the channel (Biedenharn et al., 1998) and large, single trees cabled to the bank (Section 32 Main Report).
Rootwad revetments are best used:
On meandering streams with occasional out of bank flow conditions (McCullah, 2004).
To protect eroding streambanks, and create scour pools and other instream fish rearing and spawning habitat (McCullah, 2004).
Where these types of structures are naturally-occurring in adjacent stream reaches or in similar stream types (McCullah, 2004).
Where natural materials are desired to provide structural diversity, velocity differentials, scour, undercut banks, and substrate sorting (Sylte and Fischenich, 2000).
Where sediment deposition is undesirable (Sylte and Fischenich, 2000).
Where the rootwad fan reaches from base scour elevation to the annual high water elevation (Sylte and Fischenich, 2000).
Where there is at least one stable meander above and below the project reach, ensuring consistent incoming flow direction (Sylte and Fischenich, 2000).
Bed elevation is either naturally stable or maintained at a stable elevation (Sylte and Fischenich, 2000).
Where bed and banks have greater than 15 % silt or clay composition, or if less than 15 %, only on very small, meandering streams (Sylte and Fischenich, 2000).
Where failure of the revetment would not endanger lives or structures (Sylte and Fischenich, 2000).
Where temporary stabilization is needed for riparian establishment, and riparian vegetation will thrive (Sylte and Fischenich, 2000).
Complexity:
Moderate.
Design Guidelines / Typical Drawings:
Design of rootwad revetments should address the following areas: rootwad dimensions, orientation and spacing, habitat requirements and revegetation (design guidelines taken from Sylte and Fischenich, 2000).
Rootwad dimensions: The fan (root mass) of the rootwad should be of sufficient diameter to reach from the depth of maximum scour to the annual high water elevation. If one fan is not sufficient, rootwads may be stacked, providing this does not jeopardize the structural integrity of the installation. The length of the rootwad trunk should be 4 times the projected scour behind the rootwad; this length can range from 3 m (10 ft) on small streams (less than 4.5 m (15 ft) wide) to over 6 m (20 ft) on larger streams. The trunk should be firmly attached to the fan.
Footer dimensions: The footer should have a diameter at least three-quarters that of the rootwad trunk to provide the necessary support to the rootwad.
Orientation: The rootwad fan should be facing directly into oncoming flow (at a 90° angle to incoming velocity vectors); if the angle varies from this, it may be up to 15° towards the stream. Directing the rootwad fan towards the adjacent bank will likely result in accelerated erosion of the bank, and potential revetment failure. The rootwad should be placed at an elevation such that the fan reaches the maximum scour depth.
Footer orientation: The footer log should be placed roughly parallel to the streambank, and extend past the rootwad fan for a length sufficient to protect the bank against any eddying downstream of the rootwad. The footer should be placed at an elevation appropriate to support the rootwad at the elevation specified above; this will typically be slightly above the maximum scour depth. In some cases, footer logs may not be necessary.
Spacing: Spacing between the rootwads should be such that velocity vectors are directed away from the streambank for the full distance between rootwads. This will typically be between 3 and 4 times the projecting length of the structure.
Habitat Requirements: For maximum habitat benefits, the fan of the rootwad should be placed within 1 m (3 ft) of the bank. Placing the fan this distance from the bank allows for consistent water circulation on all sides of the fan, habitat niches throughout the structure, and increases velocity diversity and scour and textural diversity of the bed and bank.
Vegetation: Vegetation of the bank is crucial to the long-term success of this installation. Vegetative methods are addressed more thoroughly in Vegetation Alone, Willow Posts and Poles, Live Staking, and Live Brushlayering. Vegetative efforts should be focused most intensely on those areas just upstream of the rootwad fan, and the area between the rootwad trunk and the footer log, where eddying may occur.
Rootwad Revetment Cross-Section
Typical Drawing
Rootwad Revetment Plan View
Typical Drawing
7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS
By combining boulders, logs and live plant materials to armor a streambank, fish habitat is enhanced, while creating a natural looking stabilization structure. Rootwad revetments can be used for a wide range of fishery enhancement structures for various stream types. Vegetated rootwads combined with vegetated riprap can protect the streambank, while providing in-stream and overhead cover for fish, resting areas, shade, detritus, terrestrial insect habitat, and habitat diversity.
8. HYDRAULIC LOADING
Allen and Leach (1997) report allowable velocities for rootwad revetments of 2.7 m/sec (8.9 ft/sec).
9. COMBINATION OPPORTUNITIES
Rootwad revetments may be used in combination with Large Woody Debris Structures, Live Siltation, Live Brushlayering, Willow Posts and Poles, Vegetation Alone, Live Staking, Live Fascines, Live Brush Mattresses, Vegetated Riprap, and Cobble and Gravel Armor.
10. ADVANTAGES
Rootwad revetments are usually less expensive than other revetments, as they utilize natural materials that can usually be found on or near the site (Sylte and Fischenich, 2000).
The rootwads and footer logs eventually decompose, allowing the riparian vegetation to take over, and restore the bank to a naturally-functioning condition.
Rootwads create habitat complexity and hydraulic diversity, and assist in substrate sorting.
Rootwad revetments induce less local sediment deposition than other flow deflection structures (Sylte and Fischenich, 2000).
11. LIMITATIONS
Rootwads are susceptible to flanking, and their performance is highly dependent on flow direction; therefore, they should only be installed for erosion control where upstream and downstream ends are stable, and there is at least one stable meander upstream of the installation (Sylte and Fischenich, 2000). If the technique is being installed for habitat enhancement or riparian restoration, these limitations do not apply.
Successful incorporation of vegetation into the revetment is important for long-term erosion control, as the rootwad will decompose within about 15 years.
Rootwad revetments should not be installed on streams where the bed and banks are uniform sand, except on very small, meandering streams (Sylte and Fischenich, 2000).
12. MATERIALS AND EQUIPMENT
Rootwads with appropriately-sized fan and trunk, footer logs, rock (if specified), and materials, such as willow poles and branches for vegetating the structure. An excavator with a thumb will facilitate precision placement of rootwads and other materials.
13. CONSTRUCTION / INSTALLATION
Use a backhoe or excavator to set a "footer" log in a trench excavated below the thalweg (lowest point in the channel), running roughly parallel with the bank. A second log with the rootwad attached should be set on top of the footer log diagonally, forming an "X". The root wad fan should be pointing upstream, directly into the flow, and the butt end should be set in a trench excavated into the bank (McCullah, 2004).
Large boulders and willow poles should be used to secure the root wad, especially close to the fan. A filter layer, either graded aggregate or filter fabric, placed under the rock will prevent the washout (piping) of fines through the armor layer (McCullah, 2004).
Further incorporation of live woody cuttings (see Live Brushlayering and Willow Posts and Poles for cutting specifications) will enhance the structure. The length of the cuttings will depend on the depth through the rock and rootwad to the native soil. If necessary, a terrace may be cut into the bank, behind the structures to provide more planting area close to the water table (Sylte and Fischenich, 2000).
14. COST
Rootwad revetments cost between $40 and $200 per linear m ($12 to $60 per linear ft) of bank treated, with an average cost of $80 per linear m ($25 per linear ft) (Sylte and Fischenich, 2000).
15. MAINTENANCE / MONITORING
The rootwad revetment should be inspected after the first several floods, and at least once a year, preferably following the flood season, after that. Examine the structure for evidence of undercutting or flanking, and inspect vegetation for survival, growth, and evidence of herbivory or disease. If animal damage is observed, exclosures may be necessary until plants are established (Sylte and Fischenich, 2000).
16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE
The most frequent reasons for failure of rootwad revetments are flanking and undercutting of the structure.
Flanking may occur if upstream or downstream meanders are unstable, causing changing flow directions, the footer log does not extend far enough to protect the entire structure, or the trunk is not keyed in to a sufficiently stable substrate. Undercutting may result if the rootwad is not set at an appropriate elevation, or the trunk is not embedded far enough.
17. CASE STUDIES AND EXAMPLES
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| Rootwad Revetment prior
to vegetation establishment. Clear Creek, Redding, CA. January,
2004. |
Rootwad Revetment following vegetation establishment. Clear Creek, Redding,
CA. May, 2004. |
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| Rootwad Revetment with VMSE. Anchorage, AK. |
Rootwad Revetment with brushlayering.
Green River, WA. Photo courtesy of Alan Johnson. |
Please visit the Photo Gallery for more pictures.
18. RESEARCH OPPORTUNITIES
Thresholds for allowable shear stress have not been developed, and would greatly assist in specification of these structures (Sylte and Fischenich, 2000).
19. REFERENCES
Allen, H. & Leech, J. R. (1997). Bioengineering for Streambank Erosion Control; Report 1, Guidelines. TR EL-97-8. 90 pp.
Biedenharn, D. S., Elliott, C. M., & Watson, C. C. (1998). Streambank stabilization handbook. Veri-Tech, Inc., Vicksburg, MS, CD-ROM.
Edminster, F. C., Atkinson, W. S., & McIntyre, A. C. (1949). Streambank Erosion Control on the Winooski River, Vermont., USDA Circular No. 837, 54 pp.
McCullah, J. A. (2004). Bio Draw 3.0. Salix Applied Earthcare, Redding, CA
Sylte, T. L., & Fishenich, J. C. (2000). Rootwad composites for streambank stabilization and habitat enhancement, EMRRP Technical Notes Collection (ERDC TN-EMRRP-SR-21). U.S. Army Engineer Research and Development Center, Vicksburg, MS. (pdf)
