Combining Techniques

INTRODUCTION

Streambank protection guidance documents often present protection measures as isolated techniques; however, they can and often are, employed in combinations of two or more. Vegetation, in particular, is often combined with structures and nonliving materials. The term "biotechnical slope protection" has been coined to describe this approach. Biotechnical methods are defined as the combined or integrated use of plants and structures to protect slopes against erosion and shallow mass wasting (Gray & Sotir, 1996; Gray & Leiser, 1982). This integrated approach entails the use of plant-structure associations such as plantings on slopes (or fills) above conventional revetments or retaining structures placed at the toe of slopes, or alternatively, the incorporation of vegetation directly into the structure itself (e.g., vegetated gabion, crib, and rock walls, vegetated revetments, etc.). Vegetated "soft armor" (viz., planted ECBs, TRMs, and GCSs) and "hard armor" (e.g., vegetated ACBs and gabion mattresses) are yet another example of a combined or integrated approach to slope and bank protection.

OTHER COMBINED APPROACHES

Structural elements can also be combined with soil bioengineering treatments, e.g., the use of live staking in combination with rock riprap, a technique known as "joint planting". Soil bioengineering refers to, and is strictly defined as, the arrangement and embedment of vegetation (primarily live cuttings) in the ground where they act as soil reinforcements, barriers to earth movement, wicks, and drains. Examples of this approach include Live Staking, Live Fascines, Live Brushlayering, Live Brush Mattresses, Live Gully Fill Repair, etc., techniques which are described in detail elsewhere (Gray and Sotir, 1996; Schiechtl and Stern, 1998; USDA, 1996). Combined or integrated treatments are highly versatile not only in regard to the erosion processes they address, but also with regard to location on a slope where they can be employed. Schematic diagrams with illustrations (DiPietro, 2000) of these combined approaches are shown in Figures 2 - 4.

Figure 1. Mid bank with vegetated gabions and vegetated riprap along toe, Branciforte Creek, California, February 2002

Nonliving or "hard" structures and materials are used to assist vegetative protection in at least three ways.

TOE PROTECTION

One of the main conclusions drawn from extensive research on streambank erosion and protection in the 1970s was that simply grading the bank to a stable slope and planting vegetation without toe protection is ineffective (USACE. 1981); similar conclusions have been reached by others (Shields et al. 1995). In most stream channels, shear stress reaches a maximum at the toe of concave banks, and this region is unsuited for terrestrial plants because it is either permanently or frequently inundated. Emergent aquatic vegetation has been used in conjunction with geotextiles or structure in this zone along smaller streams (Biedenharn et al. 1999).

PROTECTION OF FLANKS

Many streambank protection projects fail at the upstream or downstream end. Local flow acceleration at the downstream end of the protection project often leads to local scour that progressively undercuts the protection and eats its way upstream. At the upstream end, erosion associated with impinging flow sometimes allows flow above or behind the protective measures and eventual failure. Stone, timber, logs, gabions, coir rolls, and other types of structures are used to build deflectors and refusals at the upstream or downstream endpoints of other types of protective schemes. Deflectors are spurs or barbs. Refusals (also called tiebacks or roots) consist of rock riprap or other heavy granular material buried in deep trenches dug perpendicular to the channel.

Figure 2. Vegetated (live staked) rock mattress at toe and erosion control blanket (ECB) at mid bank supplemented by brushlayering and live staking (from DiPietro, 2000).

Figure 3. Vegetated rock gabions and gabion mattress apron at toe supplemented by erosion control blanket (ECB) and live fascines at mid bank & upper bank (from DiPietro, 2000)

Figure 4. Rock apron and vegetated MSE and rock toe supplemented by erosion control blanket (ECB) with live stakes on upper bank
(from DiPietro, 2000)

PROTECTION DURING THE PERIOD OF ESTABLISHMENT

Plants are often combined with biodegradable products or plant materials that shield the banks from erosive flow during the period of establishment, but disappear later.

POTENTIAL ENVIRONMENTAL BENEFITS

Combining treatments allows maximum flexibility in meeting the objectives of bank stabilization and habitat development. Selection of an appropriate toe protection structure, e.g., longitudinal stone toe with spurs, can create ideal water/shore interface conditions and scour holes that may provide stable pool habitats. A well vegetated (or re-vegetated) bank can improve aquatic and riparian habitat in addition to providing important functional benefits (Coppin and Richards, 1990). Streambank vegetation provides cover, shade, and insect food sources for fish and other aquatic organisms near the water's edge. Upper and mid bank vegetation yields cover and habitat opportunities for small mammals and other riparian wildlife.

IMPLEMENTATION AND INSTALLATION

The mix of bank treatments best-suited for a given location depends on a number of site and soil parameters, such as bank inclination and height, bank material, channel substrate, stream discharge and velocity, erosion processes to be mitigated, and desired environmental benefits. While no general specifications can be written for combined treatments, some general guidelines can be offered for treatment selection based on the concept of "streambank zonation" (shown schematically in Figure 5), originally introduced by Logan et al. (1979), and subsequently adopted by others (e.g., Henderson and Shields, 1984, Biedenharn et al. 1999). The Logan zonation pattern based on elevation mirrors plant community zonation on undisturbed, stable natural banks. This zonation was initially developed for the Missouri River, a large river with relatively mild stage fluctuations due to the size of the watershed and reservoir controls. Thus the adjective “normal”, applied below to river stages, is more meaningful than for a small, flashy stream.

The toe zone is the portion of the bank below the average annual minimum water surface elevation. Along the Missouri River, this zone is typically flooded all year. Protection in the toe zone is usually intended to prevent undercutting by either armoring the bank deflecting high currents. Surficial erosion by current is typically the dominant process in this zone, and this region should be protected using structures (e.g., stone or rock revetment, stone toe, lunkers, large woody debris structures, geotextile rolls, etc.) or vegetation integrated into such structures (e.g., emergent aquatic vegetation incorporated into coir roll). Application of vegetation within this zone is inversely proportional to the duration of inundation. Larger rivers offer less opportunity for use of vegetation in the toe zone than small, flashy streams. When the toe zone is permanently inundated, and thus not subject to cycles of wetting and drying, appropriately sized low-cost materials such as chalk, limestone, soft sandstone, shale, or soil-cement may often be used for toe protection (Henderson and Shields, 1984).


Figure 5. Streambank zonation concept

The splash zone is the zone of normal water surface fluctuation, or the region higher than average annual minimum water stage and lower than the annual average maximum stage. Along the Missouri River, this zone is flooded at least six months per year, but water depths fluctuate a fair amount. This zone experiences high levels of erosive stress, wet-dry and freeze-thaw cycles, ice and debris gouging, and wave wash along wide streams. The splash zone may be quite steep; however, stresses due to currents are usually lower for the splash zone than for the toe zone. Woody plants and structures involving woody plants that root from cuttings (e.g., brush mattresses) are often applied to the splash zone, especially if much of the period of inundation occurs during the dormant season for plants. Herbaceous wetland plants may be used in the splash zone if energy levels and bank slopes are not too great (Biedenharn et al., 1999).

The freeboard zone (or bank zone) is the region between the splash zone and the top bank (i.e., above annual average maximum stage). Along the Missouri River, this zone is flooded for at least 60 days once every two to three years. This region is subjected to weathering, current erosion during high flows, wave wash, ice and debris gouging, and sometimes human and animal traffic. Many types of flood-tolerant plants may be used in this zone if soil and slope conditions permit. Nonliving materials suitable for use here include gravel and stone (but smaller than that required in lower zones), geotextiles, and compacted clay layers.

The terrace or floodplain zone lies above the top of bank. This zone, which is rarely flooded, can be readily eroded when it is flooded if it is not well-vegetated. Plants in this zone influence slope stability both positively and negatively. Plants contribute to stability by removing moisture from the soil profile via transpiration and reinforce the soil with roots; however, they detract from stable conditions by facilitating rapid infiltration during rainfall events, and large trees add weight that loads the top of steep banks (Simon and Collison, 2002; Collison and Simon, 2001). When rainy seasons coincide with plant dormancy, transpiration is near zero while banks are at the greatest risk for slope instability. Design of protection within the floodplain zone should address movement of water from the floodplain to the channel via overbank runoff or subsurface flow. The former can trigger gully development, while the latter can cause piping and geotechnical instability. A number of conventional drainage and soil bioengineering drainage techniques can be used to address these problems.

These important levels of flow in stream systems are addressed in this manual. In general, Annual Low Water (ALW) refers to the toe zone and Annual High Water (AHW) or Bankfull Discharge is comprised of the toe and splash zones. The bank and top of bank zone is generally included in Design High Water (DHW).

DELINEATION/SELECTION CRITERIA

Criteria for delineating bank zones at a given site are poorly developed, as are guidelines for selecting compatible treatments for each zone. The key principle is that the designer should select measures that address the dominant erosion mechanisms active in each zone. It is also desirable for treatments to reproduce or mimic habitat features and functions of naturally stable banks. Ease of construction may also be an issue, since some regions of the bank may be difficult to access by machines or laborers.

RELATIVE COST

Cost will depend upon the particular combination of techniques employed along a given reach of stream. Conventional structural techniques, viz., hard armor and structural toe protection, tend to be more expensive than conventional planting used alone. Combination methods are likely to be intermediate in terms of cost alone but may also be highly cost effective because of the flexibility they provide for adding environmental benefits and meeting stabilization objectives.

REFERENCES

Biedenharn, D. S., Elliott, C. M., & Watson, C. C. (1998). Streambank stabilization handbook. Veri-Tech, Inc., Vicksburg, MS, CD-ROM.

Collison, J. C. & Simon, A. (2001). Beyond root reinforcement: The hydrologic effects of riparian vegetation on riverbank stability. In Proceedings of the Conference Wetlands Engineering and River Restoration 2001, American Society of Civil Engineers, Reston, VA, published on CD-ROM. (pdf)

Coppin, N. J., & Richards, I. (1990). Use of Vegetation in Civil Engineering. Butterworths: Sevenoaks, Kent (England).

DiPietro, P. (2000). Soil bioengineering and ecological systems. Geotechnical Fabrics Report, Vol. 18, No. 7, pp. 30-35.

Gray, D. H. & Leiser, A. (1982). Biotechnical Slope Protection and Erosion Control. Van Nostrand Reinhold, New York, N. Y.

Gray, D. H. & Sotir, R. (1996). Biotechnical and Soil Bioengineering Slope Stabilization. John Wiley and Sons, New York, N. Y.

Henderson, J. E., & Shields, F. D., Jr. (1984). Environmental Features for Streambank Protection Projects. (Technical Report E-84-11), US Army Engineer Waterways Experiment Station, Vicksburg, MS.

Logan, L. D., et al. (1979). Vegetation and mechanical systems for streambank erosion control along the banks of the Missouri River from Garrison Dam downstream to Bismarck, North Dakota. U.S. Forest Service, Missoula, Montana. Prepared for the U. S. Army Engineer District, Omaha, U. S. Forest Service, Northern Region, and North Dakota State Forest Service.

Schiechtl, H.M & Stern, R. (1998). Water Bioengineering Techniques for Riverbank and Shore Stabilization. Blackwell Science, London, England

Simon, A. and Collison, A. J. C. (2002). Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability. Earth Surface Processes and Landforms 27(5):527-546.

USACE. (1981). Final Report to Congress, The Streambank Erosion Control Evaluation and Demonstration Act of 1974, Section 32, Public Law 93-251. Main Report and Summary and Conclusions. Washington, D. C.

USDA Soil Conservation Service. (1996). Chapter 16: Streambank and Shoreline Protection. Part 650, 210-EFH, Engineering Field Handbook, 88 pp. (pdf)