Levee Armoring: Woody Biotechnical Considerations for Strengthening Midwest Levee Systems
Douglas Wallace, Clifford Baumer, John Dwyer and Frank Hershey
- Forester, USDA Soil Conservation Service, Columbia, Missouri; Civil
Engineer, USDA Soil Conservation Service, Columbia, Missouri; Associate
Professor of Forestry, University of Missouri - Columbia, Columbia, Missouri;
Watershed Specialist, USDA Forest Service, Columbia, Missouri.
Presented at the Restoration of Aquatic Ecosystems symposium, The Association of State Wetland Managers, St. Paul, Minnesota, June 20-23, 1994.
- The floods that occurred throughout the Midwest during the summer
of 1993 were of historic proportions (Ilanhack, 1994). Because of the duration,
velocity and extent of floodwaters, levee failures resulted in extensive damage
to agricultural land, rural and urban infrastructures, and human resources
(Scott, 1993).
In Missouri, agricultural floodplain areas along the Mississippi, Missouri, Grand, Chariton, Locust, Thompson and Osage rivers were particularly impacted. According to the State Emergency Management Agency, about 3.1 million acres of Missouri land flooded. The Food and Agricultural Policy Research Institute estimated crop losses on this land at $247 million. The USDA Soil Conservation Service (SCS) predicts that reclaiming cropland damaged by flooding will exceed $600 million (Soil Conservation Service, 1993).
Secondary levees associated with major rivers and upstream tributary levee systems in Missouri experienced an estimated 2019 levee breaks (Soil Conservation Service, 1993) during the 1993 floods. A number of non-scientific surveys conducted by Missouri Department of Conservation and SCS personnel after the flooding indicate that levees with forested buffer zones or with woody vegetation cover on the levees may have saved many levees from damage and reduced the severity of damage to others.
The beneficial effects of woody vegetation is not surprising considering the increasing use and successful applications of woody vegetative materials in stabilizing slopes and reinforcing soils (Gray and Leiser, 19g2; Soil Conservation Service, 1992). In light of these recent observations and uses, this paper examines the potential woody interactions with levee systems and presents some levee armoring designs.
- Woody corridor development and woody levee cover appear to be
critical elements in increasing levee integrity (Shields and Gray, 1993).
An informal aerial review of a segment of the Missouri main-stem levee showed a dramatic increase in levee failure (Table 1) as woody corridor width decreased. Observations from Missouri Department of Conservation personnel (Young, 1994) contend that tree screens are credited with saving levees as well as floodplain fields from flood scour. Field reviews completed in February 1994 by the authors (unpublished) on a portion of Shoal Creek in Caldwell county revealed only a single levee break (internal tile line failure) with woody cover and woody corridors of 20 to 100 meters in width. Levees, with woody corridors completely absent and in grass sod, upstream from the wooded corridor site experienced multiple breaks in a similar length of levee.
O.S. Scheifele, an early advocate of woody vegetation for protection of streambanks and levees, made similar observations following the flood of June, 1927 near Memphis, Tennessee (Scheifele, 1928).
- "
- It was interesting to inspect various sections of the levees after the big flood. Wherever a heavy stand of native willows or other forest trees were growing in the burrow (sic) pits and on the land between the river the erosion from wave action and current was very slight and on miles of levee where tree growth existed no injury was caused whatever. On the contrary, where land was cleared and there were no obstructions to break the waves, injury and destruction were evident along the entire distance."
The relationship of woody plants and levee integrity may be the result of woody plant effects on altered river hydraulic conditions and biomechanical interactions of woody root systems and earthen levee material.
- For many years engineers have used Manning's equation (Chow, 1959)
to describe steady uniform flow in an open channel where:
V = 1.49 RA(2/3) SA(1/2)/ n and
- V = the mean velocity of flow in feet per second
R = the hydraulic radius of the channel in feet
S = the slope of the energy grade line in feet per foot
n = the coefficient of roughness
The hydraulic radius is dependent on the geometry of the channel cross section; the slope of the energy line is essentially the average slope of the channel bottom; and the coefficient of roughness is a measure of the roughness of the channel boundary. With all other variables held constant, the greater the roughness, the slower the velocity of flow. The roughness coefficient is dependent on many factors but the two most important influences are surface roughness and vegetation.
In natural streams, this coefficient ranges from approximately 0.025 for clean straight channels to 0.150 loy floodways with heavy stands of timber and underbrush (Chow, 1959). This range represents a potential six-fold change in velocity as a result of roughness alone. During a flood event, the vegetation in a wooded stream corridor creates drag forces opposing the flow which dissipate energy, and reduce flow velocity (Henderson and Shields, 1984). Flood waters are less likely to cause erosion and scour as energy is dissipated by vegetation. In addition, sediment carried by flood waters will drop out of suspension as velocity is slowed.
- V = the mean velocity of flow in feet per second
- Naturally vegetated stream corridors exhibit a level of channel
stability that is lost when vegetation is removed. In addition to slowing the
velocity of flow, vegetation helps protect streambanks from surface erosion and
slope failure. Tall grass and brushy vegetation tend to lay down during a flood
event, dissipating energy and providing resistance to scour (Henderson and
Shields, 1984). Root fibers, especially woody root fibers, increase the shear
strength of the soil (Schiechtl, 1980). Roots also create a fibrous mat that
resists scour of the surrounding soil matrix (Henderson and Shields, 1984).
Vegetation further enhances slope stability by transpiring moisture from the
root zone to the atmosphere (Gray and Leiser, 1982).
Woody material associated with levee systems raise a number of engineering concerns (Shields and Gray, 1993). Hynson et al. (1985) list three potential problems posed by trees on levees. First, they suspect that seepage could occur if tree roots penetrate into embankment areas that have high pore pressures during flooding. Second, windthrow could lead to slope stability or seepage failure. Third, trees can hinder inspection and flood fighting if lower limbs are not periodically pruned. Hynson et al. (1985), however, did not document any failures that actually occurred by these mechanisms. Shields and Gray (1993) note that root-induced piping has not been documented scientifically. Windthrow is a valid concern for isolated or widely scattered trees but is less likely for dense uniform stands and not a concern for small trees and shrub growth forms. Access for inspection and flood fighting may be safeguarded by designs that permit travel access on levees.
- Based on hydraulic and biomechanical considerations, we believe
maximum levee protection should incorporate both woody corridors and levee
woody plantings. Four designs using combinations of woody materials are
suggested for trial use and study on Midwest levee systems. All the following
designs would be incorporated with a river side woody corridor of appropriate
width.
It is important to note that the use of woody material on levees is not an accepted policy by any federal agency at this time.
- Design
The first design uses woody shrub material on the river side of the levee from the toe to the levee crest (Figure 1). A number of native shrub species are suitable for establishment and use (Table 2) with this system. To reduce the chance of insect and disease problems and to increase site diversity, a minimum of three species is recommended. Growth forms and site requirements of the selected shrubs should be compatible. The crest and field side levee slopes would be vegetated with grass sod.Management
A periodic cycle of cutting (5-7 years) would be the only needed woody maintenance. This will allow the shrub regrowth to maintain a vigorous and healthy vegetative condition and allow improved inspection. For the sod areas on the levee, the use of current U.S. Army Corps of Engineers maintenance guidelines (1982) would be continued.Advantages and Disadvantages
This design scenario would aid river side slope stability and protect the levee against wave wash. Habitat diversity would be increased by the addition of a shrub component. Free access to the top and field side of the levee would be maintained for any necessary high water inspections and operations.
Levee Shrub System
- Design
The second design uses woody shrub material on both sides of the levee (Figure 2). Choice and selection of shrub species would be similar to design 1. The top of the levee would be maintained in a grass sod.Management
Woody shrub management would be similar to design 1. To maintain adequate protection for the levee, cutting shrubs for rejuvenation should be avoided on both sides of the levee within the same segment. Annual mowing of the top of the levee would be needed to maintain sod cover and prevent woody encroachment.Advantages and Disadvantages
The advantages and disadvantages would be similar to design 1 with increased acreages of diversified habitat and added field side slope protection. Visual inspection capabilities and access would be reduced.
- Design
The third design uses tree and shrub material on both sides of the levee (Figure 3). Tree establishment would be continuous from the woody corridor to the midslope on the river side of the levee. Shrub material (Table 2) would complete the front slope to the levee crest and cover the top half of the field side of the levee. Tree species would cover the lower half and extend into the former field area. The crest would be maintained in a grass sod. Native riparian tree species (Table 3) should be used.
Management
Shrub management of this system would be similar to design 2. An added management consideration would be required for the levee tree component. Optimum tree growth occurs generally between 30 - 60 years of age. Management schemes, such as periodic tree harvests, would be needed to maintain the levee stand in this age range.Advantages and Disadvantages
Advantages of this system would include increased lower levee slope protection, periodic saleable wood products, increased habitat diversity, debris and sand filtering, and continued access along the top of the levee. Disadvantages include reduced access to both the levee front and back slopes.
Levee Tree System
- Design
Design four would establish and maintain a forested community over the entire levee structure and river side corridor. Adequate tree densities would be needed to keep stands fully stocked to reduce the possibility of wind-throw hazards. Species selection (Table 3) should be guided by site conditions, landowner objectives, and maximization of site diversity and income.Management
Optimum tree growth occurs generally between 30-60 years of age. Management would be based on keeping the stand in pole and small sawtimber size-classes which would reflect this age range. Vigorous tree growth and stand health would allow maximum tree densities and strong root systems. Periodic tree harvests would be needed to maintain this condition and allow capture of saleable wood products.Advantages and Disadvantages
Advantages of this system would include increased total levee slope protection and stability, periodic saleable wood products, debris and sand filtering capabilities, and reconstruction of a natural riparian ecosystem system. Disadvantages include very limited access to entire levee system.
- Historically, flood protection measures in the Midwest have ignored
and actively excluded woody vegetation from levee systems. In light of recent
reviews of levees in Missouri, the attitude that woody vegetation is
undesirable and negatively affects the maintenance and stability of the levee
structures may be unjustified and an unwarranted position.
Instead of excluding woody vegetation, levee designs should actively incorporate woody materials as corridor plantings between the levee and river and as protective cover on the structure itself as long as inspection and flood fighting capabilities are maintained.
Levee armoring with properly designed woody material will slow floodwater velocities, dissipate energy, reduce scouring potential, and increase soil shear strengths. These hydraulic changes and biomechanical attributes would increase levee protection and stability, improve wildlife habitat diversity, establish natural riparian ecosystems, reduce maintenance costs, reduce flood damage to floodplain fields by trapping sediment and debris, and provide an opportunity for secondary wood products to be harvested.
Field studies and research are needed to evaluate the full potential of levee armoring designs and reduce the negative aspects such as reduced access and inspection capabilities.
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Henderson, J.E., and F.D. Shields, Jr., 1984. "Environmental Features for Streambank Protection Projects", Technical Report E-84-1 1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi.
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Schiechtl, Hugo M., 1980. Bioengineering for Land Reclamation and Conservation. University of Alberta Press, Edmonton, Alberta, Canada.
Scott, J., 1993. Relentless Rivers. Missouri Department of Natural Resources Missouri Resource Review 10(3): 8-13.
Shields, Jr., F. D. and D. H. Gray, 1993. Effects of Woody Vegetation on Sandy Levee Integrity. Water Resources Bulletin 28(5): 917-931.
Soil Conservation Service, 1992. Chapter 18: Soil Bioengineering for Upland Slope Protection and Erosion Reduction. In: U.S. Department of Agriculture Soil Conservation Service Engineering Field Manual. Washington D.C., 52 pp.
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