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How to Maximize the Benefits of Slope Plantings

Inappropriate placement (location) of a tall tree on a stream bank. Large, rigid stem on bank tends to cause local scour. The tree is also susceptible to wind throwing and toppling.

Vegetation affects both the surficial and mass stability of slopes in significant and important ways. A number of different hydro-mechanical mechanisms can be identified which explain the protective role of vegetation. These range from mechanical reinforcement and restraint by the roots to modification of slope hydrology as a result of rain drop interception by the foliage and soil moisture extraction by the process of evapo-transpiration.

A tight, dense cover of grass or herbaceous vegetation provides one of the best protections against surficial rainfall and wind erosion. Conversely, deep rooted, woody vegetation is more effective for mitigating or preventing shallow, mass stability failures. The loss or removal of slope vegetation can result in either increased rates of erosion or higher frequencies of slope failure. This cause and effect relationship can be demonstrated convincingly as a result of many field and laboratory studies that have been reported in the technical literature.

Surficial (Rainfall) Erosion

Banyan tree growing next to near vertical cut slope. Roots have spread over and grown into the slope. The stem and roots of the tree help to buttress and support the slope.

Vegetation plays an extremely important role in controlling rainfall erosion. Soil losses due to rainfall erosion can be decreased a hundred fold by maintaining a dense cover of sod, grasses or herbaceous vegetation. The beneficial effects of herbaceous vegetation and grasses in preventing rainfall erosion are tabulated below:

• INTERCEPTION: Foliage and plant residues absorb rainfall energy and

prevent soil detachment by raindrop splash.

• RESTRAINT: Root systems physically bind or restrain soil particles

while above-ground portions filter sediment out of runoff.

• RETARDATION: Stems and foliage increase surface roughness and slow

velocity of runoff.

• INFILTRATION: Plants and their residues help to maintain soil

porosity and permeability thereby delaying onset of runoff.

Streambank Erosion

Effective and attractive use of herbaceous plantings (including poppies and fox-gloves) to protect a cut slope against surficial erosion along a highway in northern California.

Streambanks are subjected to erosion and scour by flowing water. The erosive power of flowing water increases with velocity. Slope vegetation can help to reduce this type of erosion in the following manner: above ground shoots bend over and cover the surface and/or reduce flow velocity adjacent to the soil/water interface, while below ground roots physically restrain or hold soil particles in place. Dense, grass swards and low shrubby species which extend numerous, non-rigid branches and leaves into the flow (e.g., sandbar willows) are the most effective.

Some controversy exists about the wisdom of allowing woody vegetation to grow on levees and streambanks…..particularly on armored sections. Objections that have been raised include loss of conveyance from increased roughness, difficulty of inspection, hindrance to flood fighting operations, and alleged threats to structural integrity as a result of root penetration and subsequent piping. In response to these objections it should be noted that in large rivers, additional channel roughness will have a negligible effect on the stage of the design flood. Furthermore, field studies have shown that woody vegetation does not adversely affect the structural integrity of a levee. No open voids or conduits clearly attributable to plant roots were observed in levees that have been studied. On the contrary, the presence of plant roots reinforces the soil and increases the resistance to shallow bank failures and sloughing in a measurable manner.

Mass Stability

In general vegetation has a positive influence of mass stability although in some instances vegetation can have a destabilizing effect. But even in the latter case, there are many steps and actions that can be taken to minimize or mitigate these negative influences. The influence of vegetation (both beneficial and adverse) are summarized in Table 1.

A. Beneficial Effects

Coppicing or cutting of tree close to the ground can produce multiple shoots or “whips” and thereby eliminate the danger of wind throw/toppling, preserve the benefits of roots and foliage.

As noted in Table 1 the benefits of vegetation greatly outweigh detrimental effects. The most obvious way in which woody vegetation enhances mass stability is via root reinforcement. Extensive laboratory studies on fiber reinforced sands indicate that small amounts of fiber can provide substantial increases in shear strength. These findings have been corroborated by field tests on root permeated soils.Soil buttressing and arching action associated with roots and the stems/ trunks of woody vegetation are also important components of slope stabilization (see Figures 1 and 2). In addition, evapo-transpiration by vegetation extracts soil moisture and increases storage capacity. This means that vegetated slopes can tolerate greater inputs of water before exceeding field capacity and developing positive pore water pressures within the soil mantle. Lower pore water pressures increase stability and resistance to slope failures.

B. Detrimental Effects

Deeply rooted Ponderosa pine tree buttressing and supporting a steep slope mass behind it. Unbuttressed portion of slope to left of tree has failed.

The primary detrimental influence on mass stability associated with woody vegetation appears to be the concern about external loading and the danger of overturning or uprooting in high winds or currents. If a significantly sized root ball is unearthed during uprooting it could reduce the stability of a cross section depending upon a tree's position on the slope. This problem is likely to be more critical for large trees growing on relatively small dams or levees (see Figure 3). With regard to external loading, levee embankment slopes are generally shallow enough that the main component of the overburden weight may act perpendicular to, rather than parallel to, the failure surface, thereby increasing stability. However, the location of trees on the embankment must be considered in any slope stability analysis in order to ascertain the extent to which their weight might affect the balance of forces.

Maximizing Benefits of Vegetation

Woody shrubs (e.g., ceanothus and baccharis) are deeper rooted and provide better protection against shallow mass movement than grasses and herbaceous vegetation.

As noted previously, vegetation mostly has a beneficial influence on mass stability of slopes, but it can also have detrimental or adverse effects as well (see Table 1). Fortunately, several strategies and procedures can be adopted to maximize the benefits of vegetation while minimizing its liabilities. These strategies include: (1) selection of the appropriate species for particular site conditions and stabilization objectives, (2) proper placement or location of vegetation, (3) grading and site preparation, and (4) management of the vegetation to mitigate undesirable characteristics or problems. These strategies and procedures are summarized in Table 2.

A. Selection Strategies

Selection and placement of vegetation on levee (or embankment) side slopes to maintain viewing access from levee crest and to facilitate inspection of levee.

Vegetation should be selected for desired stabilization objectives and be compatible with soil and site conditions. The latter includes consideration of soil type, water availability, nutrient status, and soil pH, climate, possible browsing pressure, regulations governing the use of exotic or non-native species, etc. Certain types of plants are intrinsically better suited than others for specific stabilization objectives. Woody vegetation is stronger and deeper rooted than herbaceous plants and grasses and provides greater mechanical reinforcement and buttressing action at depth. Accordingly, woody plants are superior for mass stability. Grasses and herbaceous vegetation, on the other hand, grow close to the surface and provide a tight, dense ground cover (see Figure 4). They tend to be superior, therefore, in intercepting rainfall and preventing surficial erosion. Shrubs are not as deep rooted as trees nor can they be expected to provide as much buttressing restraint as trees. On the other hand, shrubs are more flexible, have less above ground biomass, and exert less surcharge on a slope (see Figure 5). They may preferable, accordingly in riverbank and levee stabilization where these attributes are advantageous. The characteristics and relative advantages/ disadvantages of different plant types for various applications are summarized in Table 3.

B. Placement Strategies

Conventionally graded slope with uniform placement of slope plantings. Slope face is planar with mid slope bench and linear drain running straight up and down slope.

Several different placement or locational strategies can be invoked to maximize the utility of slope plantings and minimize possible problems. One of the main objections raised to vegetation on slopes is that it obstructs views and hinders access. These objections have been raised by both homeowners living on hillsides and inspectors examining river levees. These problems can be addressed by pruning and coppicing techniques which are described in the next section. They can also be addressed by placement of vegetation on a slope according to its height and shape or density of the foliage crown. Smaller shrubs should be grown near the top of slope and larger trees placed near the bottom. This simple procedure will not only improve views from the top, but also eliminate weight from the top of the slope, and put maximum buttressing restraint and reinforcement near the base where it is most needed. In the case of river levees plants can be located in such a way to meet both stabilization objectives and to create relatively clear fields of view for inspection and access purposes (see Figure 6).

C. Grading Strategies

Slope face is contoured and sculpted with swales and berms. Slope runoff is removed via a drain that curves its way down slope.

Another approach is to locate vegetation in conformance with "landform grading" practices. Landform grading replicates irregular shapes of natural, stable slopes. Landform graded slopes are characterized by a continuous series of concave and convex forms interspersed with swales and berms that grade into the profiles. Re-vegetation in conjunction with landform grading entails planting vegetation in patterns that occur in nature as opposed to specifying either uniform or random coverage (see Figures 7 and 8). Trees and large shrubs tend to require more moisture, and they are also better at stabilizing against shallow slope failures than herbaceous vegetation. Accordingly, trees should be clustered in swales and valleys in a slope where runoff tends to concentrate and evaporation is minimized. Shrubs and trees should also be heavily concentrated along drainage flow lines of each swale. Conversely, seepage and runoff tend to be diverted away from convex shaped areas. These areas should be planted with more drought tolerant herbaceous vegetation. Irrigation needs are thus reduced by careful control of drainage pattern on a slope and selection and placement of appropriate plantings for different areas.

D. Management Strategies

American beach grass (dune grass) traps drifting, blowing sand and helps to build protective dunes.

Several different planting and/or management strategies can be employed to enhance desired characteristics of vegetation at a particular site. More vigorous and deeper rooting can be accomplished in a variety of ways, namely by:

•Watering for longer times at less frequent intervals

•De-compacting or ripping a soil before planting

•Weeding to minimize competition from unwanted plant

Fire is often used as management tool....sometimes with unanticipated consequences. Levees are frequently fired to rid them of woody vegetation. Burning, however, promotes explosive growth of fire adapted species which may not be the vegetation of choice for soil erosion control.

An interesting approach to mitigate the adverse effects of vegetation on slope stability is the practice of coppicing. Coppicing is a timber harvesting or pruning method that involves the production of new trees from the old stumps (see Figure 9). This procedure leaves the root system intact while generating smaller, multiple stems near the cut area. Many tree species have the ability to regenerate or sprout from dormant buds along their stems, and lend themselves to coppicing.... especially northern hardwoods that have dormant buds on the lowest parts of their trunks. Examples include willows and most maples and locust trees. Some species such as aspen also produce new sprouts from their roots, which are referred to as root suckers.

Use of dune walkover to protect beach grass and other dune vegetation against foot traffic and trampling. Slope plantings in general and dune vegetation in particular are excellent for protecting slopes against erosion but are vulnerable to traffic.

Best results with coppicing are obtained if the stumps are cut after leaf drop in the late fall or winter. Red maples, silver maples, and black locust sprouts can grow more than 6 feet the first season. As the stump sprouts grow, they can be thinned and pruned to the desired height and number of trees per stump. Coppicing mitigates two main adverse effects from the list in Table 1, namely, surcharge (#7) and wind throwing (#8) whilst retaining beneficial effects. There may be some initial loss of beneficial influence interception (#1), but this is temporary and greatly outweighed by the attendant benefits. Coppicing allows one to retain views (a reason for tree removal on slopes), enjoy smaller trees, and retain the hydromechanical benefits provided by a tree's living root system.

Another simple yet effective management techniques is to control pedestrian and vehicular traffic and in critical areas that are protected by vegetation. Coastal dunes are a good case in point. Foredunes play a critical role in a shoreline defense system. Dune vegetation is very effective at trapping drifting sand and helping to build and accrete dunes (see Figure 10), but this same vegetation is very vulnerable to trampling and traffic. The use of boardwalks and walkover structures (see Figure 11) is an effective way of preserving the protective role of vegetation.

Browsing damage to new plantings can be controlled by the use wire mesh cages (see Figure 12) or plastic sleeves placed around stems of saplings.

Vegetation improves the resistance of slopes to both erosion and mass wasting. Conversely, the removal of slope vegetation tends to accelerate or increase slope failures. Herbaceous plants are the most effective for improving the resistance to surficial erosion, whereas woody vegetation is more effective for preventing shallow mass wasting.

Wire mesh cages used to protect newly planted slope vegetation against browsing animals. Other types of protective devices, e.g., plastic sleeves, can be employed as well.

The mechanical or reinforcing effect of plant roots on the stability or factor of safety of slopes can be described and accounted for in a systematic manner. Likewise, beneficial hydrologic influences can be identified as well. Vegetation modifies the hydrologic regime in a slope by intercepting rainfall in the foliage and by extracting and transpiring soil moisture via the roots.

Fortunately, several strategies and procedures can be adopted to maximize the benefits of vegetation while minimizing its liabilities. These strategies include: (1) selection of the appropriate species for particular site conditions and stabilization objectives, (2) proper placement or location of vegetation, (3) grading and site preparation, and (4) management of the vegetation to mitigate undesirable characteristics or problems.


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June 18, 2019, 8:47 am PDT

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