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Biotechnical groundcovers, soil bioengineering and landform grading have greatly increased our ability to control erosion and sedimentation. Erosion control practices in the USA are driven mainly by concern over the impact of accelerated erosion and sediment on the quality of the nation's waterways and reservoirs. The impact on water quality is very significant-sediment is the number one pollutant in our waterways. Suspended solids and turbidity drive up water treatment costs; sedimentation degrades aquatic habitat, reduces reservoir capacity, and decreases hydraulic conveyance in rivers and streams. Pesticide and fertilizer residues-- attached to eroded soil particles in urban and agricultural runoff-- pollute lakes and streams. According to a recent National Water Quality Inventory conducted by the US Environmental Protection Agency, siltation and nutrients (nitrogen and phosphorous) from erosion impair more miles of rivers and streams than any other pollutant. Erosion control measures basically fall into two major categories, namely measures to decrease erosive forces versus measures to increase erosion resistance. Measures to decrease erosive forces mainly entail decreasing the velocity of water (or wind) flowing over a bed of soil and/or dissipating the energy of the water in a defended area; a grade stabilization structure is a good example of this approach. Measures to increase erosion resistance, on the other hand, basically consist of stabilizing the soil and/or covering its surface with a protective layer; grass, straw mulch, rock riprap, and biotechnical ground covers-- erosion control blankets and turf reinforcement mats-- are good examples of this approach. Dense covers of grass and herbaceous plans probably provide the best long term protection against surficial erosion. Attractive vegetation supplies numerous environmental and landscaping benefits as well. Plant leaves and foliage intercept raindrops, the stems and roots filter sediment out of the runoff, and the roots reinforce the soil and bind the particles together; furthermore, the stems and foliage increase surface roughness and slow the velocity of runoff. Vegetation by itself also has certain drawbacks: it takes time to establish and is initially vulnerable to washout and drought. Also limited to the velocity of flow it can withstand when used in temporary channels or grassed waterways, vegetation is also difficult to establish on very steep slopes. A variety of techniques and products have been developed to overcome these limitations. "Reinforced grass" refers to agrass surface which has been artificially augmented with an open structural coverage (mats, meshes, or interlocking concrete blocks) to increase its resistance to erosion above that of grass alone. These products can be manufactured with known specifications, are easily transported and delivered to a site, and can be installed relatively rapidly. Rolled erosion control products in two-dimensional, open-weave fabrics, meshes, and nets or three-dimensional geosynthetics are simply unrolled on a slope, then fastened securely "to the ground. These biotechnical groundcovers are designed to take advantage of the protective benefits provided by both a vegetative and structural coverage in a complementary manner. Biotechnical composites which provide this reinforcement typically are composed of non-degradabie elements which furnish temporary erosion protection, enhance vegetative establishment, and ultimately become intimately entangled with living plant tissue (roots) to extend the performance limits of vegetation. A variety of biotechnical composites or groundcover systems are now available to suit different purposes and slope conditions, including such products as erosion control blankets, turf reinforcement mats, and geocellular containment systems. By using living plant materials, i.e. stems and branches, as the main structural components or reinforcements in the soil , mantle, soil bioengineering can also be employed to enhance the performance of vegetation. Live cut stems are purposely arranged and imbedded in the ground in various configurations, also functioning as drains which remove excess soil moisture. Immediate reinforcement is provided by the imbedded live cut stems and branches; secondary stabilization occurs as a result of adventitious rooting which develops along the length of the buried stems. Techniques such as live staking, brushlayering, and live fascines work on this principle. The USDA Natural Resources Conservation Service has added a chapter on the use of soil bioengineering for upland slope protection and erosion control to its Engineering Fieldbook. Guidelines for the use, construction and selection of soil bioengineering measures are also described in a recent book by Gray and Sotir titled Biotechnical and Soil Bioengineering Slope Stabilization (See LASN Bookstore, November 1996, page 35). Both transportation corridors and residential developments in steep terrain require that some excavation and regrading be carried out to accommodate roadways or building sites. The manner in which this grading is planned and executed, and the nature of the resulting topography or landforms that are created affect not only the visual or aesthetic impact of the development but also the mass and surficial stability of the stability of the slopes and effectiveness of landscaping and revegetation efforts. A linear or planar slope will exhibit higher soil loss than a slope with a concave or decreasing gradient near the toe. Likewise, drainage channels brought down and across a slope in a curvilinear manner, which lengthens flow path and reduces gradient, are less susceptible to erosion than exposed channels brought directly down the face of a slope. Landform grading essentially attempts to mimic nature's hills. Very few hillsides are found in nature with linear, planar faces. Instead, natural slopes consist of complex landforms covered by vegetation that grows in patterns adjusted to hillside hydrogeology. Shrubs and other woody vegetation growing on natural slopes tend to cluster in valleys and swales where moisture is more abundant. Landform graded slopes are characterized by a variety of shapes including convex and concave forms. Downslope drain devices either follow natural drip lines in the slope or are tucked away and hidden from view in special concave swale and convex berm. Vegetation patterns that are found in nature are also mimicked. Random patterns or uniform coverage should be avoided; instead the vegetation is placed where has a better chance of surviving and where it does a better job of holding soil. Trees and shrubs require more moisture, and they also do a better job of stabilizing a soil mantle against shallow mass wasting. Accordingly, it makes sense to cluster them in swales and valleys in a slope where runoff tends to go. Shrubs should also be heavily concentrated along the drainage flow of each swale; by purposely controlling the drainage pattern on a slope, runoff can be concentrated in concave areas where needed or where it can best be handled by woody slope vegetation. Conversely, runoff and seepage will be diverted away from convex-shaped areas. These areas should be planted with grasses or more drought tolerant herbaceous vegetation. Irrigation needs are thus reduced by careful control of drainage pattern on a slope and selection of appropriate plantings for different areas. A judicious selection and application of all three of these technologies- biotechnical groundcovers, soil bioengineering, and landform grading- greatly enhances our ability to control both surficial erosion and shallow mass wasting in a cost-effective, environmentally compatible, and visually attractive manner. We should not lose sight, however, of fundamental principles that must underlie and guide any successful erosion control program: restricting grading or disturbance in sensitive areas, minimizing the size of disturbed areas, limiting the duration of exposure, retaining natural vegetation whenever possible, constructing hydraulic conveyance facilities to handle increased runoff, and installing erosion control measures early on in the life of a project. LASN FOX GLOVES, POPPIES AND GRASSES PROTECTING THIS CUT SLOPE ALONG HIGHWAY 1 IN CALIFORNIA IS A FINE EXAMPLE OF HERBACEOUS PLANTINGS CONTROLLING EROSION ALONG A HIGHWAY RIGHT-OF-WAY. BIOTECHNICAL GROUNDCOVERS, SOIL BIOENGINEERING AND LANDFORM GRADING HAVE GREATLY INCREASED OUR ABILITY TO CONTROL EROSION AND SEDIMENTATION. "REINFORCED GRASS" WATERWAY. A TURF REINFORCEMENT (TRM) HAS BEEN USED AS A LINER. PHOTO TAKEN PRIOR TO FILLING WITH SOIL AND SEEDING. Photo provided courtesy of Synthetic Industries, Inc. Rows of fascines have been placed on contour on the regraded crest of this highway (above) Two years later, the fascines have rooted and leafed out, thereby protecting the slope against erosion and shallow sliding (right) Imbedded live cut stems can provide immediate slope reinforcement; secondary stabilization occurs as a result of adventitious rooting that develops along the length of the buried stems. STREAM CHANNEL PROTECTED BY A COMBINATION OF BOARD CHECK DAMS AND LIVE WILLOW STAKES. THE STAKES HAVE ROOTED AND LEAFED OUT. VEGETATION HELPS PREVENT CHANNEL EROSION AND ENHANCES THE PERFORMANCE OF THE CHECK DAM SYSTEM. Cost-effective and environmentally attractive, soil bioengineering methods are finding increasing application as methods of combating both streambank and hillside erosion problems. These techniques have surficial and mass erosion on cut and fill slopes along highways, for gully stabilization, and for watershed rehabilitation. During construction, vegetated geogrids are pulled over earth lifts (above); after hydroseeding with a grass seed mixture, the slope has a natural appearance. (left) The grass protects the slope face against erosion and shades it from radiation. CONVENTIONALLY GRADED AND LANDSCAPED HILL SLOPE WITH FLANAR FACE, RECTILINEAR DRAINAGE DITCH, AND UNIFORMLY SPACED PLANTINGS.

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June 17, 2019, 8:39 am PDT

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