Robbin Sotir: Stabilizing Streambanks with Native Vegetation

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Robbin B. Sotir is President of Robbin B. Sotir & Associates, Inc., a soil bioengineering consulting firm in Marietta, Georgia. Ms. Sotir is an accomplished practitioner in the field of soil bioengineering. She has authored over 125 publications and serves as the Chair of ASTM subcommittee D18.25.10 “Soil Bioengineering”. She wrote the standards for soil bioengineering in the USDA/NRCS Engineering Field Manual, as well as having co-authored the book, Biotechnical and Soil Bioengineering Slope Stabilization, published by John Wiley & Sons, Inc.

WaterLaws: Ms. Sotir, to what does the term soil bioengineering refer?

Robbin Sotir: Soil bioengineering is an integrated technology that uses sound engineering practices in conjunction with integrated ecological principles. It is a means to assess, design, construct and maintain living vegetation systems, to repair damage caused by erosion and land failures and to protect and enhance functioning systems.

WaterLaws: How is soil bioengineering used to stabilize a streambank or shoreline?

Ms. Sotir: Soil bioengineering is one component in an overall approach. Its application is always based on sound engineering practice, first and foremost. Soil bioengineering includes methods that simply support and stabilize the top few inches of the soil surface as well as those that reinforce to a depth of 10 or 15 feet with the use of synthetic geo-grid or geo-composite materials. In some cases soil bioengineering offers a complete alternative to a “hard” method such as a retaining wall or riprap, but in many cases it is a very useful adjunct to a conventional method. In former times, unless crib systems were used, the natural angle of repose was a limitation for a bioengineered system.

In many situations, especially urban areas, space does not allow for that. Now, using vegetated reinforced soil slopes, or VRSS, we can build and establish vegetation to a slope angle of 4V:1H or perhaps 6V:1H, which is a 70% grade. Cost-wise, it is competitive with wall systems in part because you can utilize soil strength and reuse existing on-site soils. Sometimes I hear it said that you can use soil bioengineering in any circumstance. I would not say that. It is not effective in all circumstances. But if properly selected and designed, soil bioengineering has substantial structural qualities. It is not just a vegetative treatment. From the first moment of installation, soil bioengineering offers immediate erosion control, whether it is on a slope, a stream or a shoreline. And over time, this support grows stronger as root and top systems develop.

WaterLaws: How does the cost of soil bioengineering compare with traditional methods?

Ms. Sotir: A soil bioengineered approach such as a VRSS offers more benefits than, for example, a conventional retaining wall, including improvements to water and air quality, noise absorption and stormwater runoff reduction. With all of this they are typically competitive with conventional methods. A concrete wall, for example, will increase noise, heat and runoff  substantially.  The planning stages of soil bioengineering, however, do cost more than conventional approaches.  More comes into play. In a conventional approach, you would require a geo-technical engineer, maybe a civil engineer and a surveyor. With soil bioengineering, you need a surveyor, a civil engineer, geotechnical expertise, perhaps a geomorphologist, biologist or botanist, a soil bioengineer. You may need ecologists, horticultural people, ornamental or perhaps native plants specialists. The team is larger, the result is a more complete kind of solution with more function. We have enhanced wildlife habitat, ecological diversity. We have improved aesthetic quality. But the system must function the same as a conventional system from a structural, hydraulic and hydrologic perspective.

WaterLaws: Is there uncertainty about the cost and availability of plant materials?

Ms. Sotir: That can occur, although we haven’t run into it. We work all over the United States and in Canada. We specify either cuttings from nature or rooted stock in the way of bareroot plants, tublings or container stock, depending on the time of year, the particular site requirements and future planned use. There are companies that produce cuttings and also some of the structures such as live fascias or live stakes.

WaterLaws: How do soil bioengineered approaches compare with traditional methods with respect to durability and long-term cost?

Ms. Sotir: Because they do change and they are living and somewhat flexible, soil bioengineering systems tend to grow stronger with age, whereas concrete structures are strong initially but tend to break down with age. From that perspective, soil bioengineering very well may have a longer lifetime. Again, it is going to depend on where it is installed and for what purpose. But the function will typically be longer and broader. There will be a greater range of function for a longer period of time. 

WaterLaws: Is soil bioengineering work seasonal?

Ms. Sotir: Depending on the part of the country, the best time for the work is from the fall, when you are entering the plant dormant season, until the buds break in the spring. The season can be expanded by using refrigeration vans to keep branches in an environment with high humidity and above freezing.  In this way you probably can extend the season for several months. The other option is to use rooted plant materials. Once you’re getting into summer it can be a lot drier, so a higher proportion of plant materials may not survive and deep irrigation would be necessary. Not every branch or plant needs to survive, but you need to have a survival rate sufficient for both immediate and long-term function.

It is important, again, to emphasize that this is not soil bioengineering versus conventional engineering. It’s more how we can integrate these pieces together and create a better whole. In any situation, the goal will be to select the best combination of the two methods, keeping in mind that soil bioengineering is built on sound engineering practice and ecological principles.

WaterLaws:  Is soil bioengineering done with the same degree of “hard engineering” as conventional methods, or does it rely more on experience and intuition?

Ms. Sotir: I assume when you use the words “hard engineering” you mean conventional. The engineering is just as rigorous, especially if we are using a reinforced system. Absolutely. We have to understand what’s going on with the geo-technical soil components and water velocities and fracture forces. In stream and river setup, it is important to know what the morphological and fluvial characteristics are. Is the system stable? In a river situation if the application is a straight section, an outside bend, an inside bend, we must know where the normal high water elevations are, what the flow velocities are going to be, what the tractive forces will be, and the orientation. If the water is coming directly into a bank, there must be a hard system. Soil bioengineering is not going to do it. We need to select the means of toe protection and how high the toe protection will need to be. If we vegetate right down to the bottom area and there is toe scour, that system is going to be gone. Slopes may be dry or wet, be composed of clay or sandy soils. Putting all of the pieces together is critical in making the final design decisions.

WaterLaws: So in soil bioengineered systems, the soil regime itself plays more of a structural role than it does when it is simply underneath a more structured system?

Ms. Sotir: Yes. Underneath or behind a wall or riprap system, the structure needs to support the weight of the soil. Conversely, a root-reinforced soil mantle allows for natural release of water and, at the same time, consolidates the soil particles. The roots are holding those soil particles together so that it is functioning as a unitary mass.

WaterLaws: Are you then more particular about the type of soil that you have? Will you need to bring soils in if the soils on site are sandy or not cohesive?

Ms. Sotir: Typically, we use the soils on site. We might bring in additional clays or something like that if required by the geo-technical. But soil bioengineering works very well in sandy soils with low cohesion because it develops that root system. Soils must also be tested for fertility and may need amelioration for that purpose.

WaterLaws: Is the choice of vegetation based on a structural assessment, or do other things come into play?

Ms. Sotir: Certainly some of it comes from experience. We have developed information applicable to different types of problems, for example, a point of stream scour. We also consider the soils and slopes. Are we looking at sandy soils with very low cohesive capabilities? That would make a difference as to what type of surficial system we use and how it is installed. For example, if it were a wet site and only surface protection is needed, we would use a configuration that would help to drain the water, such as live fascias on angle. For a dry site, we might put those live fascias on contour. With just some moisture in a streambank, we might be able to use a brush-mattressing system. Sites steeper than its natural angle of repose or those with deep failure planes are somewhat more difficult and would require further investigation. Remember, from the moment of installation it needs to be stable. As an example, you need to be able to put a particular flood event through, such as the 50- or 100-year. And if it’s done right, if you’ve selected the right method and the configuration is appropriate, it should perform well throughout and after the event.

WaterLaws: You have been in the Twin Cities recently to work with the Minnehaha Creek Watershed District and the Minneapolis Park and Recreation Board on the stabilization of Minnehaha Creek. [Editor's Note: Minnehaha Creek is the outlet for Lake Minnetonka west of the Twin Cities. The Creek meanders easterly through the cities of Minnetonka, Hopkins, St. Louis Park, Edina and Minneapolis before flowing over Minnehaha Falls (featured in Longfellow's "Song of Hiawatha"), and into the Mississippi River.] The creek lies within a parkland corridor, but that doesn't prevent it from receiving accelerated stormwater runoff from an entirely urbanized area. Can you talk a bit about streams in the urban context?

Ms. Sotir: Under natural circumstances, streams are able to move and change and alter themselves and continue to develop equilibria. Maybe a tree falls and that changes the stream flow pattern; over time, the stream reestablishes an equilibrium. There are many different kinds of streams, and they respond in different ways according to the cause. Some streams are highly meandering, others are somewhat straighter. There are branched and braided stream systems. The nature of the stream depends, among other things, on the soils and the climatic conditions. When we go from a natural stream system to an urban one, land use changes in the watershed. Stream change is one reflection of the anthropogenic conditions in the watershed. If you have a healthy watershed, a watershed that is containing storm events, then you are not going to be increasing the amount, rate or duration of storm flows into a stream. Urbanization removes natural stormwater holding areas, replaces natural infiltration with hard surface, involves loss of soil and vegetation root and top growth systems, and accelerates drainage through engineered stormwater systems.

WaterLaws: How does urbanization affect stream evolution?

Ms. Sotir: These all increase the amount, number of events and duration of flow of  stormwater running off and they decrease natural infiltration. This also lowers the water table that is the base flow in many areas of the country. Erosion is natural.  However, the effects of urbanization dramatically enhance those negative impacts. Plus we attempt to confine the stream. Streambanks and beds have been hardened and have been put into pipes and culverts, dramatically reducing aquatic and riparian habitat. Open sections have been shortened, increasing the bed slope. So many interventions have been done that have greatly enhanced negative impacts and change in urban stream systems. These are not natural changes.  Many stream systems may take 20 to 50 years to readjust from the effects of urbanization.

While storm flows are  increasing, groundwater recharge is diminishing. In the Twin Cities area, stream flow is from groundwater. If you don’t have infiltration, pretty soon your groundwater table will drop and your baseflow will go down, or there won’t be any base flow at all in the summer. This happened to Minnehaha Creek for much of the last year. Some influence is also due to the lake levels above. Not only did we have a rain deficiency, but we weren’t getting rainwater infiltrating into the ground. Most of it becomes just direct runoff into the stream.

WaterLaws: Is a groundwater-fed stream system more vulnerable to urbanizing impacts?

Ms. Sotir: It would depend on the event circumstance and the specific stream system. Any natural stream system is fundamentally affected by urbanization because so much may have changed. A system that relies on groundwater for its baseflow could become critical in terms of its aquatic life, for example. Water quality and stability too could be affected. Streams are drainage systems, but now they may have lower groundwater tables and need to accept more frequent and longer-duration flow events. All of that can create tremendous havoc within a stream system. The bed typically degrades first. It deepens in response to scouring by increased higher velocity flows. Then, with deepening, the banks become unstable, and so the channel begins to widen. At the same time, higher velocity flows will apply new forces to the streambank at bends and will cause the channel to straighten over time. That is the normal process. Retaining walls and other engineered means of controlling the stream channel frequently accelerate flows and impacts down stream. Eventually you have a wide, straight drainage conduit.

WaterLaws: You have raised these broader issues with the City of Minneapolis and the Minnehaha Creek Watershed District. This has led us to recognize fundamental questions: Do we seek to restore the creek system toward a more natural equilibrium, or does it continue to evolve toward a pure storm sewer? And are “quick fixes” to address specific locations of erosion or scour in a stream futile?

Ms. Sotir: Minnehaha Creek is a gorgeous creek, just gorgeous. It is extraordinary to find such a long stretch of stream with excellent buffer areas right through a city. It will take time but I believe the right thing is going to happen. That area is really worth it. A balance needs to be found that will allow the creek to develop full function within the new regime.

Repairs and point stabilizations are still important to hold off further degradation. Most importantly, you need to determine whether or not the bed is lowering. If the bed is going down and if the channel is widening, that needs to be stopped. Otherwise, if you install soil bioengineering, conventional rock treatments, they will all eventually fail. Normally, holding the bed elevation, or “grade control,” is done by setting rock in the bed at the channel elevation at sensitive points of scour. If the channel is widening, these grade control installations will need to be set into the bank perhaps 10 feet. But while you can do some of these sorts of spot repairs, you need to be aware that this a system watershed problem. It is not a local problem, such as where a tree has fallen into the stream and the stream has moved over.

WaterLaws: So, is it fair to say that simply coming in and stabilizing a streambank at certain locations is not all that you would advocate as part of a broader stream system protection effort?

Ms. Sotir: That’s fair. Apart from immediate stabilization, the kind of actions that need to be taken are watershed-wide actions. We need to enhance surface water management by installing stormwater detention within the watershed. System-wide actions include riparian corridors, buffers and so forth, but the problem can’t be solved without important efforts to correct the problems upstream. These have to be done together with water quality practices, in light of the conditions and concerns in each particular watershed. You need to begin with physical and biological assessments of your system to identify what is affecting the particular system. But in all cases, you have to address the root causes and not just the symptoms. You have to look at the watershed and not just the stream.  Too frequently, stream repairs are undertaken but the watershed is ignored.

WaterLaws: Does stream stabilization provide any direct economic benefits?

Ms. Sotir: Absolutely. If streams are downcutting and widening, eventually infrastructure and road facilities near those streams are going to be affected. Think about bridges, pipeline crossings, water crossings or sewerline crossings. If a pipe is buried several feet below a streambed, and the stream is downcut five, six, or seven feet, the pipe is now exposed above the bed and the stream is still widening. Similarly, bridge abutments in the banks can become exposed and will need to be replaced. This type of infrastructure is extremely expensive to repair or replace.

WaterLaws: Are communities becoming any more aware of the importance of stream system protection?

Ms. Sotir:  In some ways, yes. We completed a study for Big Creek, a stream system in a rapidly developing area of suburban Atlanta. A number of communities together commissioned the study. The area already is in development, but if they begin to implement some of this work, stream degradation can be greatly reduced and over the years the communities will save hundreds of thousands or millions of dollars in repairs to these stream systems, anticipated infrastructure, recreational benefits, and water quality, aquatic and riparian habitat. 

WaterLaws: Are there other cases in which you have been able to participate at the beginning of a watershed-wide development planning process, where all of these pieces can be put together?

Ms. Sotir: I have to admit, most of it is repair. This is not always true. For example, we were called in to assist in a 12,000-acre development in Albuquerque, where there was nothing at the beginning. But generally speaking, it is rare that a community has the opportunity and chooses to act comprehensively at the front end.

WaterLaws: In your experience, how have regulatory agencies responded to soil bioengineering proposals?

Ms. Sotir: Very well, excellent. We rarely have problems. We almost always can get nationwide permits from the [U.S. Army] Corps of Engineers, assuming the criteria, such as no fill, are met. The Corps is really going in that direction environmentally.  Each of the districts is a little bit different, but in my experience there is a real interest, provided that a proposal is backed by sound engineering.

In the past few years, we have written and revised soil bioengineering standards for the engineering field handbook published by the U.S. Natural Resource Conservation Service. These include chapters for upland slopes and for streambanks and shorelines. The ASTM [Association for Standards and Testing Materials] has created a soil bioengineering subcommittee, which is a pretty amazing development. The engineering principles are becoming quite well established. Many people are becoming very involved and very interested in this natural way of stabilization and restoration.

WaterLaws: Is it easier to get permits for soil bioengineering?

Ms. Sotir: Not in a broad sense. But there will be cases, for example, where certain methods such as sheet piling or a retaining wall could not be permitted and where soil bioengineering can, especially if you are using native plant materials. An advantage lies in the ability to avoid filling in a channel or changing the cross-sectional area of the channel, where space may be a limiting factor. 

Also, the use of vegetative stabilization on banks enhances the filtering of sediments and pollutants from runoff. Hard methods of stabilization don’t do that. This may bring advantages in some areas, such as the EPA Total Maximum Daily Load program, where parties are responsible for reducing pollutant loading along an impaired waterway from both point and non-point sources.

The water quality effects of soil bioengineering have not been well quantified yet.  But I believe that we’re going in that direction. We need good base information, so that we can monitor to compare before and after conditions. Of course, larger reaches of vegetation are needed for water quality improvement. Very small reaches wouldn’t be significant, but we’ve lost our wetlands incrementally, we’re losing our streams incrementally, and I suppose we gain them back incrementally. Everything we do when we touch a stream, if we are moving toward better and more complete function, then I would say we’re going in the right direction.  Reversing the degradation in our watershed’s streams, rivers, wetlands and lakes is a long-term process. Obviously it is far better for environmental function and cost effectiveness to protect and/or enhance systems versus having to do major “restoration.”

WaterLaws: What are the important areas for further research or development to advance the use of soil bioengineering methods?

Ms. Sotir:  We already have a huge body of anecdotal and qualitative information.  However, more information needs to be developed regarding selection of plant types and planting techniques, which method would function most effectively short-term and long-term. More information needs to be developed as to cost effectiveness compared with conventional systems. Of course, interpretation of technology is most important. More environmental baseline information is needed to allow for after-development comparisons. Further research is required as to velocity and tractive force thresholds for different systems.  We also need to better inform and educate communities as to how restoration of natural systems really offers economic benefits. If we can begin to make that connection, people will become extremely interested and that will be the way they go. There are communities now that are developing policies or guidelines directing that  soil bioengineering approaches be considered first before turning to a hard method. Being able to quantify and compare these methods  is very, very important to continuing to establish bioengineering as a method of first resort. I am very excited about where we are going.

WaterLaws: Ms. Sotir, thank you very much for speaking with us today.

Ms. Sotir: It has been my pleasure.

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