The approach of soil bioengineering is to design and construct self-maintaining systems that dissipate the energies that cause erosion. Soil bioengineering primarily involves plant-based systems but also includes other natural materials such as stone, wood, and plant fibers. In fact, materials are very important in this field, and they are a critical component in designs. The materials, both living and nonliving, must be able to resist and absorb the impact of energies that cause erosion. Design in soil bioengineering involves both the choice of materials and their placement in relation to erosive energies. Grading — the creation of the slope of the land through earth-moving — is the first step in a soil bioengineering design. Shallower slopes are more effective than steep slopes because they increase the width of the zone of energy dissipation and therefore decrease the unit value of physical energy impact.
Soil bioengineering designs are becoming more widely implemented because (1) they can be less expensive than conventional alternatives and (2) they have many by-product values. Soil bioengineering designs have been shown to be up to four times less expensive than conventional alternatives for both stream (NRC, 1992) and coastal (Stevenson et al., 1999) environments. In addition, the by-product values of soil bioengineering designs include aesthetics, creation of wildlife habitat, and water quality improvement through nutrient uptake and filtering. The wildlife habitat values are often significant and may even dominate the design as in the restoration of streams for trout populations (Hunt, 1993; Hunter, 1991) or the reclamation of strip-mined land. Although soil bioengineering systems are multipurpose, in this chapter the focus is on erosion control. Chapter 5 covers the creation of ecosystems whose primary goal is wildlife habitat or other ecological function. As an example, Figure 3.9 depicts a possible design for stream restoration that would serve dual functions.
In some situations soil bioengineering is truly an alternative for conventional approaches to erosion control from civil or geotechnical engineering. However, other situations with very high energies require conventional approaches or hybrid solutions. Conventional approaches to erosion control involve the design and construction of fixed engineering structures. These include bulkheads, seawalls, breakwaters, and revetments which are made of concrete, stone, steel, timber, or gabions (stone-filled wire baskets). Such structures are capable of resisting higher energy intensities than vegetation. The most common and effective type of structure for bank protection along shorelines or in stream channels is a carefully placed layer of stones or boulders known as riprap (Figure 3.10). The rock provides an armor which absorbs the erosive energies and thereby reduces soil loss. Rock fragments which make up a riprap revetment must meet certain requirements of size, shape, and specific gravity. A
sample design equation for the weight of rock fragments to be used in coastline protection, known as Hudson's formula (Komar, 1998), is given below:
W = weight of the individual armor unit d = density of the armor-unit material g = acceleration of gravity
H = height of the largest wave expected to impact the structure k = a stability coefficient
S = specific gravity of the armor material relative to water
A = angle of the structure slope measured from the horizontal
Gray and Leiser (1982) have given a related design relationship for riprap stone weight for a stream channel situation in regard to current velocity.
In addition to the structures described above, conventional approaches to erosion control employ various geosynthetics, which are engineered materials usually made of plastics. These take the form of mats used to stabilize soils, and they include geotextiles, geogrids, geomembranes, and geocomposites (Koerner, 1986).
The heart of soil bioengineering is new uses of vegetation for erosion control that can replace or augment the conventional approaches. Soil bioengineering designs are covered in several important texts (Gray and Leiser, 1982; Morgan and Rickson, 1995; Schiechtl and Stern, 1997) and in trade journals such as Erosion Control and Land and Water. A few designs are reviewed below as an introduction, but detailed case studies are covered in subsequent sections of the chapter for urban, agricultural, stream, and coastal environments. This is a very creative field with many sensitive designs that have been derived through trial and error and through observation and logical deduction about physical energetics at the landscape scale. Various kinds of vegetation are employed to control erosion, depending on the environment. Woody plants such as willows (Saliaceae) are used in stream environments and mangroves on tropical coastlines; herbaceous wetland plants such as cattails (Typha sp.) are used in freshwater and cordgrass (Spartina sp.) in saltwater environments. Direct mechanisms of erosion control by living plants include (1) intercepting raindrops and absorption of rainfall energy, (2) reducing water flow velocity through increased roughness, and (3) mechanical reinforcement of the soil with roots. Living plants also indirectly affect erosion through control of hydrology in terms of increased infiltraton and evapotranspiration. Plants are used in soil bioengineering designs in many ways. Individual plants are planted either as rooted stems or as dormant cuttings that later develop roots. Groups of cuttings are also planted as fascines (sausage-like bundles of long stems buried in trenches), brush-mattresses (mat-like layers of stems woven together with wire and placed on the soil surface), or wattles (groups of upright stems formed into live fences). Willows in particular are preadapted for use in soil bioengineering along streams because of their fast growth and their ability to produce a thick layer of adventitious roots (i.e., roots that develop from the trunk or from branches), and also because their stems and branches are elastic and can withstand flood events (Watson et al., 1997). Schiechtl and Stern (1997) show many line drawings of how these and other kinds of plantings are used in slope protection. Often plantings are used in hybrid designs along with conventional approaches as shown in Figure 3.11. Protection of the "toe" or lower portion of a slope with resistant materials is especially important because this location receives the highest erosive energy. Thus, a typical hybrid design would include rock armor at the toe of the slope with plantings on the upper portion of the slope.
Other natural, nonliving materials besides stone are often included in soil bioengineering designs. For example, tree trunks are used in several ways. Log deflectors have a long history of use in streams to divert flow away from banks. Owens (1994) describes a similar though more elaborate kind of structure using trunks with branches which he terms porcupines. Root wads — tree trunks with their attached root masses (Figure 3.12) — also have been used as a kind of organic riprap in streams to absorb current energy (Oertel, 2001). All of these uses are made even more effective when the trees to be used are salvaged from local construction sites rather than harvested from intact forests. Other examples of natural nonliving materials used in soil bioengineering designs include hay bales, burlap, and coir, which is coconut fiber. Coir is an especially interesting natural material used as a geotextile to stabilize soil and provide a growing media for plants. Its special properties include high tensile strength, slow decomposition rate due to high concentrations of lignin and cellulose, and high moisture retention capability. Uses of coir are described by Anonymous (1995b) and by Goldsmith and Bestmarn (1992) whose company has patented several fabrication methods for coir geotextiles.
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