Interview: Streambank Solutions

Philosophies on streambank stabilization are radically changing. Where hard-armoring techniques such as concrete and riprap used to be widely accepted, new attitudes and policies are calling for a mix of hard and soft solutions as a more environmentally friendly alternative. But do the new methods do what’s required of them?

Erosion Control spoke with well-known expert Edward Perry, Ph.D., a research civil engineer in Vicksburg, MS. Perry has worked with the US Army Corps of Engineers Waterways Experiment Station in Vicksburg–the principal research, test, and development arm of the corps.–and is now a consulting engineer. He became involved with streambank protection through participation in the Section 32 Program Streambank Erosion Control Evaluation and Demonstration Act of 1974 and served as chair of a committee to disseminate knowledge gained from that program through such means as manuals and a training course. The Final Report to Congress for the Section 32 Program, completed in 1981, represented the state of the art for streambank and streambed protection in the United States at that time.

Perry shares his experience with different types of streambank stabilization and points out that some of the “new” solutions really aren’t.

EC: How have perceptions changed since you’ve been involved in the business regarding hard armoring versus softer solutions such as bioengineering?

Edward Perry: The Greek philosopher Heraclitus said, “You can’t step in the same river twice.” Ralph Peck, an eminent geotechnical engineer, said “Nature ignores specialties.” Taken together, these two sayings give one an appreciation of the problems involved with river stabilization. Rivers are dynamic, highly sophisticated systems, and river stabilization involving hard structures requires a knowledge of fluvial geomorphology, including river mechanics, channel stability, and river-basin management; environmental engineering, including aquatic and terrestrial wildlife; hydraulic engineering; and geotechnical engineering. When bioengineering is included, additional knowledge in horticulture and soil science is necessary.

For the Streambank Erosion Control Evaluation and Demonstration Act, a wide range of materials, such as rock, soil cement, and used auto tires, were used in various configurations, including revetments, retards, bulkheads, dikes, hard points, and breakwaters. Vegetation in the form of grass, woody plants, trees, and anchored trees was also investigated.

Within the past decade increased demands have been placed on the Army Corps of Engineers and the Natural Resources Conservation Service by environmental agencies and conservative organizations to incorporate vegetation into their streambank protection projects rather than to rely completely on traditional methods. Hard armor such as rock revetment and concrete bulkheads are considered by many to have little value for fisheries, wildlife, water quality, and aesthetic appeal. Softer solutions, such as bioengineering, which utilize living and nonliving plants, sometimes in combination with other construction materials, are being given preference (and sometimes required) as streambank protection methods. Regardless of the type of streambank protection used, there are general requirements that will always apply. Toe protection and control of streambed degradation are a necessary prerequisite to streambank protection, and the upstream and downstream ends of the revetment must be protected to prevent flanking.

EC: What are the most prevalent – and acceptable – uses of hard armoring today? How do they differ from past uses?

Perry: Soft armoring preceded hard armoring. As far back as the Middle Ages, streambanks were protected by the use of plants and plant materials such as logs. Many of the methods used were forgotten, only to be rediscovered during the last century. In today’s way of thinking, hard armoring is the option of last resort and is to be used when soft armoring is either inadequate or inappropriate, as discussed in the handbook by Schiechtl and Stern, Water Bioengineering Techniques: For Watercourse, Bank and Shoreline Protection. (Author’s note: The sidebar contains a list of books and reports mentioned in this article.)

Detailed information on hard armoring is contained in a recently completed Corps of Engineers manual, The WES Stream Investigation and Streambank Stabilization Handbook, which covers fluvial geomorphology; general principles of erosion protection; design, construction, and maintenance of hard streambank protection; river training structures; grade-control structures; and bioengineering for streambank protection.

EC: What are the most effective alternatives to hard armoring?

Perry: Soft armoring is the alternative to hard armoring, discounting relocation of the endangered structure or relocation of the channel. When combined with hard armoring, soft armoring provides environmental attractions often at lower cost. Bioengineering streambank protection is covered in the previously mentioned book by Schiechtl and Stern and in several Corps of Engineers and US Department of Agriculture publications.

With bioengineering, the entire streambank is treated to furnish an array of plants to provide ground cover and root penetration for erosion protection, wildlife habitat, and water-quality improvement by providing shade and cover for fish. Bioengineering is accomplished by creating zones to place plant material at various elevations on the bank based on the plants’ ability to tolerate certain frequencies and durations of flooding and their attributes of dissipating current velocities and wave energies.

EC: Realistically speaking, what are your options under extreme conditions when maximum performance is required?

Perry: Plain grass and reinforced grass (grass within geogrid or concrete block) have been used for erosion control ditches and waterways, and limiting velocity versus flow-duration information is available. Grass is not recommended for use if the flow duration is longer than about two days. While the maximum sustainable design velocity for more conventional hard materials, such as rock revetment, concrete bulkheads, and gabions, is generally known from hydraulic model studies, failure thresholds and performance behavior for bioengineering materials, such as woody species, has only recently been addressed. Maximum flow velocities should not exceed 3 fps for herbaceous plantings, 3-5 fps for woody and herbaceous mixed plantings, and 5-8 fps for woody plantings alone. Maximum flows above 8 fps require a more substantial bioengineering treatment such as coir fiber or geotextile roll, brush mattress, and wattling. The larger the stream or the stronger the flow, the more probable that hard materials will be incorporated into the bioengineering design. This is also true when an expensive or critical facility is being threatened. The “hard material” may be a log crib or a structure incorporating rock.

EC: How effectively can techniques be combined?

Perry: Combining hard and soft armoring is the key to their success. This is particularly useful in the toe and splash zones of the streambank. Hard structures are used to protect the toe of the streambank and the upstream and downstream ends of the armoring to prevent flanking. Toe protection can also be achieved by deflector dikes (hard points, groins, bendway weirs, and stream barbs) that protrude into the water, deflect the current away from the eroded bank, and induce sedimentation. These dikes may be composed of rock, vegetation, or a combination of both.

Some special consideration is necessary during the design phase of a project combining hard and soft armoring. Some lead time is required because plants for bioengineering are acquired by purchasing, collecting from the wild, or propagating and growing. Timing of planting is critical for bioengineering. Planting during hot summer months should be avoided. Planting during the dormant season prior to flood events to permit establishment of roots, especially for herbaceous plants, is preferred.

Monitoring and aftercare of a bioengineering treatment up to and after the first one or two flood events is essential. Following repair of weak spots, the bioengineering treatment will gain strength with time as the vegetation becomes established.

EC: How do the costs of hard-armor solutions compare to those of “softer” solutions in terms of planning, materials, and labor?

Perry: Bioengineering has a steeper learning curve than conventional hard-armor methods because it has recently come to the forefront as the method of choice and there are a limited number of people trained in the discipline. The success of bioengineering in streambank protection in the US is due in a large part to pioneering efforts of Robbin Sotir (www.sotir.com). Given the design guidance now available, the planning costs of bioengineering and conventional hard armor should be about the same.

Material and labor costs for bioengineering and conventional hard armoring vary tremendously depending on availability of materials, hauling distances, labor rates for the geographic area, and other factors. A common denominator for arriving at costs is labor in terms of person hours required to install the particular treatment. Then material costs, equipment rental, and so on are added onto this. In general, since bioengineering streambank protection is labor-intensive, the cost advantage will be greatest in regions where labor is inexpensive, skilled in agriculture, and conscientious.

EC: Describe a specific streambank stabilization project. What options were considered, and how did they compare?

Perry: This project is discussed in Allen and Leech’s Bioengineering for Streambank Erosion Control. It compares the actual costs of bioengineering treatments with the estimated costs of traditional erosion control, in this case, riprap revetment, under similar conditions in the same areas. The table shows comparisons for two cases, Court Creek in Illinois and the Upper Truckee River in California.

What is not shown in the Upper Truckee River example is that the site is next to a golf course and the sponsor is trying to provide shaded riverine aquatic (SRA) habitat for native brown trout. The vegetative geogrid will provide SRA by providing willows that overhang the bank. The riprap revetment requires a flatter slope and uses more valuable golf course land. Thus, the project objectives and potential benefits and impacts must be considered when comparing various streambank-protection options. And this doesn’t even take into consideration the damage Tiger Woods could do to his Titleist Titanium 975 driver in the unlikely event that his golf ball ever landed in the riprap revetment!