Chapter 2

Multiple Objective Soil Bioengineering For Riverbank Restoration (Alton Simms and Robbin Sotir, Robbin B. Sotir & Associates, Inc.)

Introduction

Today, stream and riverbank protection efforts are expected to address issues such as habitat, aesthetics, and water quality in addition to such needs as flood control and erosion protection. It is common knowledge that integrated streambank protection designs that include vegetation are likely to satisfy these multiple objectives. Soil bioengineering systems utilize vegetation as a principal component and can provide sound streambank protection while maximizing ecological and water quality benefits. Streambank protection designs that consist of riprap, concrete, or other inert structures alone are being accepted less frequently because of their lack of environmental and aesthetic benefits. Consequently, there is greater interest in designs that combine vegetation and inert materials into living systems that can reduce erosion while providing environmental and aesthetic benefits. This integratable technology is therefore responsive to these increasing concerns.

This case study describes soil bioengineering systems that have been used to meet specific aquatic and riparian habitat objectives, such as providing overhanging cover for fish and riparian habitat. Examples are presented to illustrate the use of these systems.

Information has been prepared in tabular form that may be a useful guide for evaluating alternative soil bioengineering streambank protection measures and selecting those that best achieve the desired project objectives (Table 3). This procedure has been developed and used on several projects where environmental objectives were major concerns, including a major sport fishing stream in Alaska and the Ottawa River in Canada, which divides the Provinces of Ontario and Quebec.

Table 3. A matrix to help compare the benefits of different streambank protection measures.

Environmental Benefits of Soil Bioengineering Systems

Soil bioengineering systems for stream and riverbank protection consist of structural engineering components and integrated ecological systems that provide protection for the entire riverbank over a reach or an entire system. There may be several soil bioengineering components capable of providing erosion protection for a given site, depending on the type of erosion or failure problem that exists. The specific design chosen may depend on several factors, including the level of risk that is acceptable, cost, and/or environmental and aesthetic objectives.

Table 3 summarizes the flood conveyance, habitat, water quality, recreation, and aesthetic benefits. Table 4 summarizes the major environmental benefits of the most common soil bioengineering methods employed in streambank protection that utilize woody vegetation. Such tables can be useful in helping to select specific soil bioengineering methods that can be incorporated into streambank protection designs to maximize specific environmental requirements. For example, the branches that overhang the water along the riverbank provide shade and protection from predators making it an excellent choice as part of a bank protection system on streams where such habitat is scarce. There may be other constraints that affect the choice however. Some methods might cause too much flow constriction or might cause erosion of the opposing bank if used on very small systems. However, this is not typical in the case of rivers. All of the soil bioengineering methods have a common geotechnical benefit of providing root reinforcement in the soil mantle. The more deeply installed methods, such as brushlayer, positively affect the direction of seepage. Hydrologically, these methods serve as horizontal drains converting parallel flow to vertical flow. Hydraulically, vegetation reduces velocities and redirect flows. Soil bioengineering projects are typically considered aesthetically pleasing and become more so over time.

The species of woody vegetation selected for inclusion in soil bioengineering systems can have a significant effect on the habitat benefits. Various species of willow are the most common woody plants used in soil bioengineering because of their excellent rooting ability. While willow can provide good overhanging cover and shade for streams, good nesting habitat for some species of birds, and some cover for mammals, it is not noted as an excellent food source for land animals. There are other plants that may be better choices for accomplishing specific habitat objectives. Such plants can be added to soil bioengineering designs to provide specific habitat benefits for target species. Chapter 18 of the Natural Resources Conservation Service Engineering Field Handbook (Soil Bioengineering for Upland Slope Protection and Erosion Reduction) provides information about growth habits, habitat value, and rooting characteristics for a variety of plants adapted in the United States.

Table 4. Environmental benefits of soil bioengineering for streambank protection.

Ottawa River

In 1996, Public Works and Government Services Canada undertook the stabilizing and remediation of a biomedical disposal site along the banks of the Ottawa River in Hull, Quebec, Canada. This site is situated adjacent to a highly visible, popular, and passive recreational green space known as Jacques Cartier Park. This area offers spectacular views of the River and Parliament Hill (Ottawa) on the opposite bank.

As part of the remediation of the site, excavation of the contaminated soils and materials was replaced with a sand/clay subsoil mix (Figure 10). The resulting embankment was then topped off with an approved topsoil blend. Due to the steepness of the constructed slope and the river below, surface stability was of vital importance. This was accomplished via the use of live fascines on the contour with erosion control fabric known as coir (Figure 11).

Other project objectives included: preparing a foundation, where over time a natural community of indigenous plant materials for upland and riverine habitat would evolve; improve aesthetics; and establish a long-term, maintenance-free natural slope along the Ottawa River within its highly urbanized context. The success of this project to meet the desired goals enabled Public Works to designate the area as an extension of Jacques Cartier Park (Figure 12).

Kenai River

The Kenai River in Alaska is a world class sport fishing stream noted for its trophy Chinook salmon fishing. In heavily used public access areas, such as Soldotna Creek Park and Centennial Park, bank vegetation had been destroyed by foot traffic and the streambank was eroding rapidly (Figure 13). Because of the potential impacts on rearing habitat and movement of young Chinook, Alaska Fish and Game would not permit dikes of any kind or hard structures such as bulkheads. They also discouraged the use of riprap above the elevation of the ordinary high watermark.

A 650 foot section of streambank at Soldotna Creek Park was stabilized using soil bioengineering methods. Overhanging cover was provided by live siltation constructions and live cribwalls (Figure 14). In wet areas, native sod rolls and live fascines were used to stabilize the bank line and reestablish vegetation. Large rocks, placed randomly in the shallow water in front of the live cribwalls, and small rootwads, anchored further out, were used to create additional fish cover (Figure 15). The soil bioengineering installations survived the 1995 flood, the largest on record, with minimal damage. This same flood ravaged banks protected with riprap and other hard structures.

Summary

Water resource projects, by their very nature, involve multiple objectives, and streambank protection is no exception. In addition to controlling erosion, we must meet water quality, habitat, aesthetics, and other environmental objectives. Integrated soil bioengineering designs that employ woody vegetation meet these environmental objectives better than other types of streambank protection alone. Maximum benefits are derived by choosing soil bioengineering methods and selecting the vegetation to achieve specific environmental objectives. The success of soil bioengineering on the Ottawa River and the Kenai River indicates that this approach to riverbank protection and restoration is applicable to address multiple objective goals.

References

Robbin B. Sotir & Associates, Inc. unpublished files.

Sotir, R.B. 1997. Presentation on Management of Landscapes Disturbed by Channel Incision. Conference sponsored by U.S. Department of Agriculture, US Army Corps of Engineers, and the University of Mississippi, Oxford, Mississippi.

US Department of Agriculture - Natural Resource Conservation Service (USDA-NRCS). 1992. Engineering Field Handbook: Chapter 18 - Soil Bioengineering for Upland Slope Protection and Erosion Reduction.

Contact Person

Alton P. Simms
Robbin B. Sotir & Associates, Inc.
434 Villa Road Marietta, GA 30064-2732
sotir@mindspring.com