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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Vegetation is the most natural method for protecting streambanks because it is relatively easy to establish and maintain and is visually attractive. However, vegetation alone should not be seriously considered as a countermeasure against severe bank erosion where a highway facility is at risk. At such locations, vegetation can best serve to supplement other countermeasures.
Vegetation can effectively protect a bank below the design water line in two ways. First, the root system helps to hold the soil together and increases overall bank stability by forming a binding network. Second, the exposed stalks, stems, branches and foliage provide resistance to flow, causing the flow to lose energy by deforming the plants rather than by removing soil particles. Above the water line, vegetation prevents surface erosion by absorbing the impact of falling raindrops and reducing the velocity of overbank flow and rainfall runoff.
Vegetation is generally divided into two broad categories: (1) grasses, and (2) woody plants (trees and shrubs). A major factor affecting species selection is the length of time required for the plant to become established on the slope. Grasses are less costly to plant on an eroding bank and require a shorter period of time to become established. Woody plants offer greater protection against erosion because of more extensive root systems; however, under some conditions the weight of the plant will offset the advantage of the root system. On high banks, tree root systems may not penetrate to the toe of the bank. If the toe becomes eroded, the weight of the tree and its root mass may cause a bank failure.
There are several synonymous terms that describe the field of vegetative streambank stabilization and countermeasures. Terms for the use of 'soft' revetments (consisting solely of living plant materials or plant products) include bioengineering, soil bioengineering, ground bioengineering, and ecological bioengineering. Terms describing the techniques that combine the use of vegetation with structural (hard) elements include biotechnical engineering, biotechnical slope protection, bioengineered slope stabilization, and biotechnical revetment. The terms soil bioengineering and biotechnical engineering are most commonly used to describe stream bank erosion countermeasures and bank stabilization methods that incorporate vegetation. Where riprap constitutes the "hard" component of biotechnical slope protection, the term vegetated riprap is also used.
Due to a lack of technical training and experience, there is a reluctance on the part of many engineers to resort to soil bioengineering and biotechnical engineering techniques and stability methods. In addition, bank stabilization systems using vegetation have not been standardized for general application under particular flow conditions. There is a lack of knowledge about the properties of the materials being used in relation to force and stress generated by flowing water and there may be difficulties in obtaining consistent performance from countermeasures that rely on living materials. Nonetheless, stabilization of eroding stream banks using vegetative countermeasures has proven effective in many documented cases in Europe and the United States.
Most hydraulic engineers in Europe would not recommend the reliance on bioengineering countermeasures as the only countermeasure technique when there is a risk of damage to property or a structure, or where there is potential for loss of life if the countermeasure fails (TRB 1999). Soil bioengineering is not suitable where flow velocities exceed the strength of the bank material or where pore water pressure causes failures in the lower bank. In contrast, biotechnical engineering is particularly suitable where some sort of engineered structural solution is required because the risk associated with using just vegetation is considered too high. However, the use of soil bioengineering and biotechnical engineering with respect to scour and stream instability at highway bridges is a relatively new field. Research has been conducted, but these techniques have generally not been tested specifically as a countermeasure to protect bridges in the river environment.
Design of biotechnically engineered countermeasures to minimize rates of stream bank erosion requires accounting for hydrologic, hydraulic, geomorphic, geotechnical, vegetative, and construction factors. Although most of the literature dealing with biotechnical engineering on rivers is associated with stream bank stabilization relative to channel restoration and rehabilitation projects, it is also generally applicable to bank stabilization associated with bridge crossings. Bentrup and Hoag (1998), Johnson and Stypula (1998), U.S. Army Engineer Waterways Experiment Station (1998), and the Federal Interagency Stream Restoration Working Group (1998) provide detailed guidelines, techniques, and methods of biotechnical engineering for bank stabilization in the United States. Guidelines, methodology, and design of biotechnically engineered streambank stabilization in Europe and the United Kingdom are discussed in Schiechtl and Stern (1997), Morgan et al. (1997), and Escarameia (1998).
The following discussion is drawn primarily from concepts presented in NCHRP Report 544 entitled, " Environmentally Sensitive Channel- and Bank-Protection Measures," (McCullah and Gray 2005) where biotechnical techniques are referred to as "vegetated riprap."
Continuous and resistive bank protection measures, such as riprap and longitudinal rock toes are primarily used to armor outer bends or areas with impinging flows. These continuous and concentrated high velocity areas will generally result in reduced aquatic habitat. It has been widely documented that resistive techniques in general and riprap in particular, provide minimal aquatic habitat benefits (Shields et al. 1995). Recently the concerns over the poor aquatic-habitat value of riprap, both locally and cumulatively, have made the use of riprap alone controversial (Washington Department of Fish and Wildlife 2003).
Since streambank protection designs that consist of riprap, concrete, or other inert structures alone may be unacceptable for lack of environmental and aesthetic benefits, there is increasing interest in designs that combine vegetation with inert materials into living systems that can reduce erosion while providing environmental and aesthetic benefits (Sotir and Nunnally 1995).
The negative environmental consequences of riprap can be reduced by minimizing the height of the rock revetment up the bank and/or including biotechnical methods, such as vegetated riprap with brush layering and pole planting, vegetated riprap with soil, grass and ground cover, vegetated riprap with willow (Salix spp.) bundles, and vegetated riprap with bent poles.
Combining riprap with deep vegetative planting (e.g., brush layering and pole planting) is also appropriate for banks with geotechnical problems, because additional tensile strength is often contributed by roots, stems, and branches. In contrast, trees and riparian vegetation planted only on top of the bank can sometimes have a negative impact (Simon and Collison 2002).
Correctly designed and installed, vegetated riprap offers an opportunity for the designer to attain the immediate and long-term protection afforded by riprap with the habitat benefits inherent with the establishment of a healthy riparian buffer. The riprap will resist the hydraulic forces, while roots and branches increase geotechnical stability, prevent soil loss (or piping) from behind the structures, and increase pullout resistance.
Above ground components of the plants will create habitat for both aquatic and terrestrial wildlife, provide shade (reducing thermal pollution), and improve aesthetic and recreational opportunities. The roots, stems, and shoots will help anchor the rocks and resist 'plucking' and gouging by ice and debris.
Specific ways vegetation can protect stream banks as part of a biotechnical engineering approach include:
In addition, biotechnical engineering is often less expensive than most methods that are entirely structural and it is often less expensive to construct and maintain when considered over the long-term.
The critical threats to the successful performance of biotechnical engineering projects are improper site assessment, design or installation, and lack of monitoring and maintenance (especially following floods and during droughts).
Some of the specific limitations to the use of vegetation for streambank erosion control include:
More importantly, the type of plants that can survive at various submersions during the normal cycle of low, medium, and high stream flows is critical to the design, implementation, and success of biotechnical engineering techniques. In addition, the combination of riprap and vegetation may be inappropriate if flow capacity is an issue, since bank vegetation can reduce flow capacity, especially when in full leaf along a narrow channel.
In an unstable watershed, careful study should be made of the causes of instability before biotechnical countermeasures are contemplated (see HEC-20) (Lagasse et al. 2001a). Since bank erosion is tied to channel stability, a stable channel bed must be achieved before the banks are addressed. Scour and erosion of the bank toe produce the dominant failure modes (see HEC-20), consequently, most biotechnical engineering projects documented in the literature contain some form of structural (hard) toe stabilization, such as rock riprap (Figure 6.1), rock gabions, cribs, cable anchored logs, or logs with root wads anchored by boulders (Figure 6.2). Note the use of a fascine bundle in Figure 6.1 as part of the rock toe protection. Toe protection should be keyed into the channel bed sufficiently deep to withstand significant scour, and the biotechnically engineered revetment should be keyed into the bank at both the upstream and downstream ends (called refusals) to prevent flanking. Deflectors such as fences, dikes, and pilings may also be utilized to deflect flow away from the bankline.
Other factors that need to be considered when selecting a design option include climate and hydrology, soils, cross-sectional dimensions (is there sufficient room for the countermeasure), flow depth, flow velocity (both magnitude and direction), and slope of the bankline being protected. Most methods of biotechnical engineering will require some amount of bank regrading. Because structure design is based on flood velocities and depths, one or more design flows will need to be analyzed. Of particular interest is the bankfull or overtopping event, since this event generates the greatest velocities and tractive forces. Local (at or near the project site) flow velocities should be used for the design, especially along the outside of bends. The erosion protection should extend far enough downstream, particularly on the outer banks of bends. The highest velocities generally occur at the downstream arc of a bend and on the outer bank of the exit reach immediately downstream. As noted, the countermeasures should be tied into the bank at both ends to prevent flanking.
Five methods for constructing vegetated riprap have proven effectiveness. Typical design concept sketches of the five methods are provided as Figures 6.3 through 6.7. These sketches are reproduced from NCHRP Report 544 (McCullah and Gray 2005). It should be noted that the key hydraulic design variable "design high water" is not defined in these sketches, and "average high water" (AHW) and "average low water" are only qualitatively described.
There are many environmental benefits offered by vegetated riprap, most of which are derived from the planting of willows or other woody species in the installation. Willow provides canopy cover to the stream, which gives fish and other aquatic fauna cool places to hide. The vegetation also supplies the river with carbon-based debris, which is integral to many aquatic food webs, and birds that catch fish or aquatic insects will be attracted by the increased perching space next to the stream (Gray and Sotir 1996).
The exclusive placement of predator-perching type habitat may not be appropriate where fish-rearing habitat is desired. In that situation, large rocks and logs located above the average high water line (AHW) might be replaced with shrubby-type protective vegetation. An additional environmental benefit is derived from the use of rock, as the surface area of the rocks is substrate that is available for colonization by invertebrates (Freeman and Fischenich 2000). The small spaces between the rocks also provide benthic habitat and hiding places for small fish and fry.
As indicated by U.S. Army Engineers Waterways Experiment Station (WES 1998), plants should be positioned in various elevational zones of the bank based on their ability to tolerate certain frequencies and durations of flooding, and their attributes of dissipating current- and wave-energies. The stream bank is generally broken into three or four zones to facilitate prescription of the biotechnical erosion control treatment. Because of daily and seasonal variations in flow, the zones are not precise and distinct. The zones are based on their bank position and are defined as the toe, splash, bank and overbank zones (Figure 6.8).
The toe zone is the area between the bed and the average normal stage. This zone is often under water more than six months of the year. It is a zone of high stress and is susceptible to undercutting and scour resulting in bank failure.
The splash zone is located between the normal high-water and normal low-water stages and is inundated throughout much of the year (at least six months). Water depths fluctuate daily, seasonally, and by location within the zone. This zone is also an area of high stress, being exposed frequently to wave-wash, erosive currents, ice and debris movement, wet-dry cycles, and freeze-thaw cycles.
Because the toe and splash zones are the zones of highest stress, these zones are treated as one zone with a structural revetment, such as rock, stone, logs, cribs, gabions, or some other 'hard' treatment. Within the splash zone, flood-resistant herbaceous emergent aquatic plants like reeds, rushes, and sedges may be planted in the structural element of the bank protection.
The bank zone is usually located above the normal high-water level, but is exposed periodically to wave-wash, erosive flows, ice and debris movement, and traffic by animals or man. This zone is inundated for at least a 60-day duration once every two to three years and is influenced by a shallow water table. Herbaceous (i.e., grasses, clovers, some sedges, and other herbs) and woody plants (i.e., willows, alder, and dogwood) that are flood tolerant and able to withstand partial to complete submergence for up to several weeks are used in this zone. Whitlow and Harris (1979) provide a listing of very flood-tolerant woody species and a few herbaceous species by geographic area within the United States.
The overbank zone includes the top bank area and the area inland from the bank zone, and is usually not subjected to erosive forces except during occasional flooding. Vegetation in this zone is extremely important for intercepting overbank floodwater, binding the soil in the upper bank together through its root system, helping reduce super-saturation of the bank, and decreasing the weight of unstable banks through evapotranspiration processes. This zone can contain grasses, herbs, shrubs, and trees that are less flood-tolerant than those in the bank zone. The rooting depth of trees can be an extremely important part of bank stability. Besides erosion control, wildlife habitat diversity, aesthetics, and access for project construction and long-term maintenance are important considerations in this zone.
Descriptions and guidelines for biotechnical engineering treatments or combinations of treatments, and plant species that can be used in the treatments are described in detail by WES (1998), Bentrup and Hoag (1998), and Schiechtl and Stern (1997). The following is a brief summary of some of the major types of biotechnical engineering treatments that can be used separately or in some combination.
Toe Zone. Structural revetments such as riprap, gabions, cribs, logs, or root wads in a biotechnical engineering application are used at the toe in the zone below normal water levels and up to where normal water levels occur. There are no definitive guidelines for how far up the bank to extend the structural revetment. Instead, it is common practice to extend the revetment from below the predicted contraction and local scour depth up to at least where the water flows the majority of the year. Vegetative treatments are placed above or behind this structural toe protection (see Figures 6.1, 6.2, and 6.7).
Splash Zone. Several treatments may be used individually or in combination with other treatments in the splash zone above or behind the structural toe protection. These include coir rolls and mats, brush mattresses, wattles or fascines, brush layering, vegetative geogrid, dormant posts, dormant cuttings, and root pads.
Coir is a biodegradable geotextile fabric made of woven fibers of coconut husks and is formed into either rolls (coir roll) or mats (coir fiber mats). Coir rolls are often placed above the structural toe protection parallel to the bank with wetland vegetation planted or grown in the roll. Coir fiber mats are made in various thicknesses and are often prevegetated at a nursery with emergent aquatic plants or sometimes sprigged on-site with emergent aquatic plants harvested from local sources.
Brush mattresses, sometimes called brush matting or brush barriers, are a combination of a thick layer of long, interlaced live willow switches or branches and wattling. Wattling, also known as fascine, is a cigar-shaped bundle of live, shrubby material made from species that root rapidly from the stem. The branches in the mattress are placed perpendicular to the bank with their basal ends inserted into a trench at the bottom of the slope in the splash zone, just above the structural toe protection. The fascines are laid over the basal ends of the brush mattress in the ditch and staked. The mattress and fascines are kept in place by either woven wire or tie wire that is held in place by wedge-shaped construction stakes.
Both are covered with soil and tamped. Figures 6.1, 6.3, and 6.7 show examples of this type of treatment.
Brush layering, also called branch layering or branch packing, is used in the splash zone as well as in the bank zone. This treatment consists of live branches or brush that quickly sprout, such as willow or dogwood species, placed in trenches dug into the slope, on contour, with their basal ends pointed inward and the tips extending beyond the fill face. Branches should be arranged in a criss-cross fashion and covered with firmly compacted soil. This treatment can also be used in combination with live fascines and live pegs.
Vegetative geogrid is also used in the splash zone and can extend farther up into the bank zone and possibly the overbank zone. This system is also referred to as "fabric encapsulated soil" and consists of successive walls of several lifts of fabric reinforcement with intervening long, live willow whips. The fabric consists of two layers of coir fabric which provide both structural strength and resistance to piping of fine sediments.
Dormant post treatment consists of placing dormant, but living stems of woody species that sprout stems and roots from the stem, such as willow or cottonwood, in the splash zone and the lower part of the bank zone. Post holes are formed in the bank so that the end of the post is below the maximum predicted scour depth. Posts can also be planted in riprap revetments.
Dormant cuttings, also known as live stakes, consists of inserting and tamping live, single stem, rootable cuttings into the ground or sometimes geotextile substrates. In the splash zone of high velocity streams, this method is used in combination with other treatments, such as brush mattresses and root wads. Dormant cuttings can be used as live stakes in the brush mattress and fascines in the place of or in combination with the wedge-shaped construction stakes (Figures 6.1, 6.5, 6.6, and 6.7).
Root pads are clumps of shrubbery composed of woody species that are often placed in the splash zone between root wads (Figure 6.2). Root pads can also be used in the bank and overbank zones, but should be secured with stakes on slopes greater than 1V:6H.
Bank Zone. This zone can be stabilized with the treatments previously described as well as with sodding, mulching, or a combination of treatments. Sodding of flood-tolerant grasses can be used to provide rapid bank stabilization where only mild currents and wave action are expected. The sod usually must be held in place with some sort of wire mesh, geotextile mesh such as a coir fabric, or stakes. Coir mats may extend into this zone. Shrub-like woody transplants or rooted cuttings are also effective in this zone and are often placed in combination with tied-down and staked mulch that is used to temporarily reduce surface erosion. For areas where severe erosion or high currents are expected, methods such as brush mattress should be carried into the bank zone.
Contour wattling consists of fascines, often used independent of the brush mattress, placed along contours, and buried across the slope, parallel or nearly parallel to the stream course. The bundles can be living or constructed from wood and are staked to the bank. Contour wattles are often installed in combination with a coir fiber blanket over seed and a straw mulch to prevent the development of rills or gullies (WES 1998) (Figure 6.5).
Brush layering with some modifications can be used in the bank zone. Geotextile fabrics should be used between the brush layers and keyed into each branch layer trench to prevent unraveling of the bank between the layers (WES 1998).
Overbank Zone. Bioengineered treatments are generally not used in this zone except to control gullying or where slopes are greater than 1V:3H. In these cases, brush layering or contour wattling may be employed across the gully or on the contour of the slope.
Deep-rooting plants, such as larger flood-tolerant trees, are required in this zone in order to hold the bank together. Care should be taken in the placement of trees that may grow to be fairly large since their shade can kill out vegetation in the splash and bank zones. Trees planted in the overbank zone are planted either as container-grown or bare-root plants.
Depending on their shade tolerance, grasses, herbs, and shrubs can be planted between the trees. Hydroseeding and hydromulching are useful and effective means of direct seeding in the overbank zone.
Biotechnical engineering can be a useful and cost-effective tool in controlling bank or channel erosion, while increasing the aesthetics and habitat diversity of the site. However, where failure of the countermeasure could lead to failure of a bridge or highway structure, the only acceptable solution in the immediate vicinity of a structure is a traditional, "hard" engineering approach. Biotechnical countermeasures need to be applied in a prudent manner, in conjunction with channel planform and bed stability-analysis, and rigorous engineering design. Designs must account for a multitude of factors associated with the geotechnical characteristics of the site, the local and watershed geomorphology, local soils, plant biology, hydrology, and site hydraulics. Finally, programs for monitoring and maintenance, which are essential to the success and effectiveness of any biotechnical engineering project, must be included in the project and strictly adhered to.