Chapter 3

Comparison of Soil Bioengineering and Hard Structures for Shore Erosion Control: Costs and Effectiveness (Tim Patterson, Environment Canada)

Introduction

The objective of this case study is to compare the costs and effectiveness associated with both soil bioengineering and hard structures. Through research, the common principles and materials used in bioengineering were discovered. An investigation of how these principles were applied to various sites across Southern Ontario was then conducted. This included visitation of the sites, as well as interviews with people involved.

Manufactured "hard" structures designed to prevent erosion will often become cracked and damaged as they age. Since they are "dead" materials, they cannot maintain or repair themselves as plant materials can. Hard structures do, however, provide immediate effective erosion control against severe elements that would wash away newly placed plant materials. This is especially true for lake front lands. For this reason, some constructed bioengineered sites incorporate hard structures.

The main thrust of bioengineering involves the harvesting and planting of dormant cuttings or branches from tree and plant species in order to provide a natural basis for erosion control. These cuttings are arranged into individual stakes (live stakes) and/or put into bundles. These bundles can be arranged in a variety of ways. The most common arrangements are called fascines, brushlayers, and brushmattresses. Cuttings are usually taken from dogwoods and/or willows. This is mainly because the cut branches of these species are able to take root and grow on their own. As the cuttings grow and extend their root structure, the soil becomes more stable.

Bioengineering is used as a natural, long-term solution to erosion control. A bioengineered site is considered successful when there is little or no evidence of human intervention, usually several years after planting. The site should become better protected with time. For example, cuttings planted into a bank beside a watercourse can go through additional stages of erosion control for the bank. The cuttings add a bit of protection against soil sliding down the bank, immediately after being planted. Roots from the cuttings begin to hold the soil together more as they grow and interconnect with each other. Eventually, some of these roots will likely extend out into the watercourse, slowing down flows and creating fish habitat. As the trees mature more, the thicker roots in the watercourse are able to partially deflect flows away from the bank, decreasing erosion. During flooding conditions, these roots not only help protect against erosion, but can trap soil, sand, and small stones which add to the bank material.

For the majority of applications in bioengineering, the only types of tree cuttings that may be successfully used are those that can grow on their own after being cut (when dormant). There are three common types of trees in Ontario that can provide such cuttings: willows, dogwoods, and poplars. Willow and dogwood cuttings are the most commonly used for bioengineering projects. Although these species are used to establish a firm root structure in the soil, native plants tend to invade a bioengineered site over time, mixing in with the willows and dogwoods. These invading species usually do not harm the integrity of the bioengineered site and are often beneficial in aiding to the root structure.

The technique of bioengineering is becoming more popular among municipalities and Conservation Authorities (CAs) in Ontario. Where most municipalities and CAs did not incorporate bioengineering techniques only 10 years ago, most do today for at least some of their erosion control projects.

There are two main reasons municipalities and CAs choose bioengineering over concrete and steel. The first reason is that it is considered to be more environmentally friendly. The second reason is financial. For most sites, it is actually cheaper to implement bioengineering than it is to create a hard structure, especially for the long term. Material, transportation, and labor costs are generally more expensive for hard structures.

The main reason that some local governments do not choose to utilize bioengineering is because they are unsure of its effectiveness. Bioengineering principles are relatively unknown (although not unproven), and thus an uncertain solution to erosion in the minds of many. The limits of theability of bioengineered sites to resist erosion are even less certain and very site specific.

Because municipalities typically have larger budgets than CAs, some of the best combinations of bioengineering techniques can be found at projects paid for by cities. Often, these techniques are in combination with hard structures.

Costs

Costs vary for different types of bioengineering techniques. There is often no cost for labor and/or materials. Labor is often done by volunteers. Materials, such as cuttings for live stakes, fascines, and brushlayers are sometimes found either on or off site, or are donated.

Hard structures have a specific design life to them, but bioengineering designs typically do not. This may be partly because bioengineering was little used in North America 10 years ago compared to its use now, so there are few projects older than 10 years to compare with (except for sites in Europe). While this may be true, the theory behind bioengineered designs is that they are living and self-repairing. Once established with a good design, they increase in strength, and after a period of 2 to 3 years they should be capable of resisting high stream flows. They should also be capable of self-repair. Branches or roots that become broken or die are gradually replaced with more growth. Since hard structures cannot repair themselves, they require long-term maintenance. This means that the gap in costs between a hard structure and a bioengineered site will continually grow.

Most of the case studies detailed have successfully achieved stable erosion control using bioengineering. Proper planning and adaptation to site conditions played a big role in these successes. This included knowing the limitations to each type of planting or soft structure and deciding if and where they should be used. Recognizing where rip-rap or rocks should be used instead of, or in conjunction with, a soft structure was also very important.

Although careful planning went into most of the case studies, unforeseen or unanticipated problems have occurred at some sites, resulting in partial or complete failures in the bioengineering designs. There are many problems that can occur due to the combined complexities of factors such as the characteristics of tree species used, soil conditions, local climate, random storm events, immediate and surrounding land use, area wildlife, pedestrian traffic, skill of the laborers, and the project design, among other things.

The success of a bioengineered site can only be conclusively determined after the first 2 or 3 years. Live stakes, fascines, brushlayers, and brushmattresses are very vulnerable to poor site conditions, erosion, and vandalism during this time, while their root structures are growing. It is essential that the required amount of sunlight and soil moisture, necessary for the species of cuttings used, be a part of the site conditions, as this was the main reason for failed areas of sites in the case studies.

Natural channel design goes well with bioengineering. Because this involves the removal or relocation of soil, this adds to the cost considerably.

Bioengineering could still use more public and municipal support. Although it is becoming a popular alternative to hard structures, there are still some municipalities that seldom or never use bioengineering designs. This support should come gradually, as the overall effectiveness of bioengineering projects become better known and understood.

Bioengineering is much more widely used in riverine environments over lake shores. This accounts for the fact that few case studies involve lake shore sites. Where bioengineering is used at such sites, it is often in combination with hard structures such as armorstone or boulders. This is because the erosive force of waves along a shoreline is frequent and usually too overpowering to allow tree cuttings to grow, even if aided by geotextiles and cribwalls. For adequate erosion control in many low flow creeks, however, hard structures may be limited or avoided altogether in favor of bioengineering.

Comparing costs taken from the case studies, live stakes, fascines, brushlayers, brushmattresses, root wads, and log jams are the lowest costing components of bioengineering, followed by geotextiles, rip-rap, and live cribwalls (Table 5). Natural channel design is above these costs. Hard structures cost even more (Table 6).

Bioengineering in a riverine environment is usually significantly less expensive than hard structures on a per meter basis (Figure 16). Comparing natural channel design case studies with large concrete channels, the difference is about threefold. Comparing case studies using basic bioengineering designs with those using large concrete channels, the difference is even more, depending on site conditions.

Table 5. Unit costs of selected materials or components of bioengineering (Sources: City of Mississauga, Cooksville Creek Tender Contract; Environment Network; Grillmayer, 1995; Belton Industries, Inc.; Brad Glasman, personal communication).

Table 6. Sample costs per meter of hard structures (Sources: Ken Cullis & Jake Vander Wal, personal communication; Joe Hollick, personal communication; Otonabee Region Conservation Authority, Scott's Plains Park Tender Contract; R.V. Anderson Associates Ltd., 1992).

Environmental Benefits

In addition to the cost benefits of bioengineering, the environmental benefits, which are not as easily measured, are an important factor. Wildlife habitat, green space, and aesthetic qualities are in high demand. This is apparent by the number of citizens' and special interest groups that have made contributions to several case studies.

Lack of information and understanding is a big obstacle to soft engineering practices. Authoritative guidelines for soft engineering are not nearly as abundant, or as clear, as specifications that exist for hard structures. Municipalities and other government bodies will be more likely to approve soft engineering designs if specifications are known and carry the same weight as those for hard structures.

Figure 16. The cost for bioengineering compared to hard structures (cost/m).

References

Belton Industries Inc. Anti-Wash/Geojute: Natural Erosion Control Fabric. Pamphlet. Atlanta, Georgia.

City of Mississauga. 4 Nov. 1997. Tender Contract # 17 111 97-134. Cooksville Creek Channel Stabilization Works.

Cullis, K. and Vander Wal. North Shore of Lake Superior Remedial Action Plans. Personal communication.

Environment Network. 1998. 1998 Bioengineering Material & Naturalized Products Price List. Collingwood, Ontario, Canada.

Glasman, B. Upper Thames River Conservation Authority. Personal communication.

Great Lakes 2000 Cleanup Fund web site. 1998.

Grillmayer, R. 1995. Soil Bioengineering Technical Report: Black Ash Creek Rehabilitation Project, 1992-1994. Collingwood Harbour Remedial Action Plan. Report # ISBN 0-7778-4080-4, Ontario Ministry of Environment, Toronto, Ontario, Canada.

Grillmayer, R.G. 1998. Soil Bioengineering. Subsection in: A Stream Rehabilitation Manual for Ontario - Draft. Ontario Streams web site.

Hollick, J. City of Burlington. Personal communication.

Otonabee Region Conservation Authority. 6 Nov. 1997. Tender Contract No. OTR97-02. Scott's Plains Park Erosion Control Works.

Patterson, T.S. 1999. Comparison of Soil Bioengineering and Hard Structures for Riverine and Shoreline Erosion Control in Ontario: Costs and Effectiveness. Final Report for Environment Canada, Burlington, Ontario, Canada.

RV Anderson Associates Limited. 1992. Halton Channel Lining Maintenance Study. RVA 3450.10. Report for the Halton Region Conservation Authority.

Steering Committee Report. 1994. Assessment of Benefits of Subwatershed Planning and Naturalizing Stream Systems, March 1994. Environment Canada, Credit Valley Conservation Authority, Ministry of Natural Resources.

Contact Person

Tim Patterson, Environment Canada
National Water Research Institute
Aquatic Ecosystem Restoration Branch
867 Lakeshore Road
P.O. Box 5050
Burlington, Ontario L7R 4A6
tim.patterson@cciw.ca

John Shaw, Environment Canada
Great Lakes Cleanup Fund 2000
867 Lakeshore Road
PO Box 5050
Burlington, Ontario L7R 4A6
john.shaw@ec.gc.ca

Tom Muir, Environment Canada
Atmospheric Environment Branch
Water Issues Division
867 Lakeshore Road
PO Box 5050
Burlington, Ontario L7R 4A6
tom.muir@ec.gc.ca