A vegetated surface serves several roles in the control of stormwater.
    The surfaces of the vegetation (leaves, twigs, branches, rough bark) intercept and hold a portion of the rain,which then evaporates back into the atmosphere before reaching the ground surface. The more layers of vegetation (canopy, understory, shrub layer, ground cover, leaf litter), the more interception that occurs. Precipitation that is not intercepted, reaches the groundby drip through,either by dripping or by running downthe trunk. Consequently the vegetative cover discipates much of the energy of the rain drops and thus decreases the occurrence of rainsplash erosion, which in turn initiates the process of surface or sheet erosion during a storm. The vegetation reduces the occurrence of sheet erosion by holding the soil particles in place with its roots as the stormwater runs off along the surface.
    Some of the precipitation enters the soil by infiltration. Of this, some evaporates back into the atmosphere, some is drawn down below the root zone by gravity where it may enter the water table, and some is held in the root zone by the soil particles. The amount held in the root zone is dependent upon the soil texture and structure. Some of the water in the root zone enters the roots of the plants and is released back into the atmosphere from the leaves of the plants by transpiration. The deeper the roots go, the more water returned to the atmosphere, and the less that remains in the soil.
    Water in the soil occupies the spaces between the soil particles, that would otherwise be occupied by air. As the volume of water in the soil increases, it exerts a pressure on the soil particles that pushes them apart. Water may also enter fractures in bedrock and the water pressure may separate the rock faces from each other. On slopes this may lead to slope failure or mass wasting, such as rock falls or slides. Slides are brought about at planar surfaces, such as at the interface of soil and the weathering surface of bedrock. As the soil particles are pushed apart, they separate from the rock surface, thus the friction that held them in place against gravity is reduced and a slide may occur. 

    The first effect is the increase in soil surface erosion due to the loss of vegetation and its functions - interception, holding the soil in place, and evapotranspiration.
    The second is the increased chance of slope failures due to the added downward weight of structures (buildings, roads, vehicles) placed on the soil surface, and to increased entrance of water into the soil. Thus a volume of ground water that was too small to cause a slump under natural conditions, can now cause a slump as the added weight of the structures overcomes the friction that held the soil in place. Or a storm that would not have caused a problem now causes slope failure because a greater percentage of the precipitation is entering the soil.
    As a consequence of the above, human activities on slopes should be controlled, and in some cases prevented. Although homes can be built on slopes if their weight is transmitted directly to bedrock, the fracturing of the bedrock must also be taken into account. "In urban areas, the overloading of slopes is occurring more frequently as buildings are sited on artificial fills on steep hillsides that were formerly avoided" (Dunne and Leopold, 1978, p. 563). Human activities that alter the hydrology of the hillslope, such as clearing of vegetation, diversion of stormwater from roads or roofs, leakage of water from septic tanks, water pipes and sewerage lines, or the use of dry wells, are frequent causes of damaging landslides.
 

    Sewage from the house enters the septic tank where the organic compounds are partially broken down by bacteria. Effluent from the tank contains many solid and soluble organic compounds, inorganic solutes and bacteria. The effluent flows slowly through pipes in the filter field and drains into the soil over a large area. "Bacterial action in the soil further breaks down the solid and soluble organic compounds. Some of the inorganic solutes become bound to soil particles, and the remaining solids, including bacteria, are filtered out by the soil. The effluent then drains to groundwater and streams with a greatly reduced potential for contamination" (Dunne and Leopold, 1978, p. 184).
"For efficient operation the filter field should be located in a large volume of soil into and through which the effluent can move easily. The drain pipe should be buried below the topsoil, usually at a depth of at least 60 cm. Below it there should be at least 1.5 m of soil above the bedrock so that the effluent will be adequately filtered before reaching the bedrock. ... Filter fields should terminate at least 15 m from a lake or water course so that there is adequate distance for filtering of the effluent within the soil" (Dunne and Leopold, 1978, p. 184).
    "Filter fields ideally should not be located on hillslopes with gradients greater than about 0.10 (10 percent), and on such slopes should be laid along the contour and in a zig-zag pattern. In this way, flow velocities within the drain pipes will be slowed and the effluent will seep into the soil over the whole field. If the soil is shallower than is ideal, the effluent may return to the ground surface downslope of the drains on a steep hillslope" (Dunne and Leopold, 1978, p.184).
    "Some state health agencies allow the absorption area per bedroom to be estimated from the texture of the soil" (Table 1), butthis does not take into account the"hazard from seasonal waterlogging or steep slopes" (Dunne and Leopold, 1978, p. 187).
 

    Hillslopes can easily be destabilized by human activities and by the occurrence of infrequent meteorological, as well as seismic, events (Sidle et al., 1985, p. 1). The economic impact of the mass movement of soil can be measured in terms of direct costs related to damage to homes and other buildings; to personal property; to water and gas pipes, storm and sanitary sewers, roads and sidewalks; and to street and traffic lights, and other electric cables (Sidle et al., 1985, p. 5). There are also indirect costs, such as implementing preventative measures to reduce future impacts, loss of real estate values in the area purchase of title to landslide prone properties by local government, loss of tax
 

Table 1:Minimum absorption areas for tile fields in various soils(after Dunne and Leopold, 1978, from U.S. Public Health serice Publication #526)

Soil Texture	Absorption Area, ft3 bedroom
Gravel or coarse sand	70
Fine sand	90
Sandy loam	115
Clay loam	150
Sandy clay	175
Clay with a small amount of sand and gravel	250
Heavy clay	unsuitable

 

revenues on conversion of private property to public property, damage to personal property, lost time from work, legal fees, moving expenses, and social costs; and emergency services (medical, housing and subsistence (Sidle et al., 1985, p. 5). There are also impacts related to flooding and sedimentation downstream. 

    There are five major types of soil mass movement: falls, topples, slides, lateral spreads, flows, and combinations thereof. These movements can occur in bedrock,in debris and in fine earth sediments (Sidle et al., 1985, p.11). Debris slides, avalanches, and flows, or combinations thereof, are the most common types of soil mass movement on managed and unmanaged steep (> 25o) slopes; these movements occur with high antecedent soil moisture levels and intense high volume rainfall (Sidle et al., 1985, p. 12). In addition to the above, soil erosion also occurs as "dry ravel," which consists of gravity initiated downslope movement of individual soil grains, aggregates and coarser fragments (Sidle et al., 1985, p. 18).

    Several factors affect slope stability: pore water pressure, soil strength parameters, slope gradient, and depth of soil to potential fracture plane, as well as micro-fissure structure in clay regoliths, pore-water chemistry, macropore networks in soil, and clay sensitivity (Sidle et al., 1985, p. 26). 

Geomorphic factors

a) Geologic and tectonic settings

    Certain types of terrain are predisposed to mass movements because of geologic factors: structure, lithology and tectonic setting (Sidle et al., 1985, p. 31). Rock structures, such as bedding planes, folds, joints, and faults, affect the stability of natural hillslopes. Rocks of different competence may be separated by downslope dipping planes, which may, along with joints and fractures oriented in the same direction, impede vertical infiltration and root penetration, and may in turn act as failure planes. On the other hand, horizontal or cross-dip bedding may result in increased slope stability. "Fault zones often contain fractured, crushed, or partly metamorphosed rocks. The inherent geologic weakness of these zones can be increased by deep percolation of water into the bedrock and subsequent chemical weathering of crushed rocks into clay-rich soil, which is susceptible to slump-earthflow type failures" (Sidle et al., 1985, p. 32). Shallow soil layers lying on hard, massive or impermeable rocks on steep slopes or resistant flow rocks lying on clay-rich rocks are susceptible to mass movement of soil (Sidle et al., 1985, p. 34). Such soils may have little attachment to underlying crystalline igneous and metamorphic rocks and thus are subject to debris slides and avalanches.

b) Slope gradient

    It is difficult to specify a lower limit of slope gradient for soil mass movements (Sidle et al., 1985, p. 36). Soil creep occurs on slopes that range in gradient from 1.3o to 25.0o. Rotational slumps occur on slopes of 7 to 18o. Rapid soil mass movement occurs on slopes greater than 25o, particularly when the soil layer is weakly attached to underlying rock. Soil lying on rock with slopes greater than 35o is subject to mass movement.

c) Slope shape

    Slope shape has a major affect on the distribution of water content and thus on soil mass movement (Sidle et al., 1985, p. 38). Concave areas with ridges concentrate subsurface recharge water in small depressions and gullies on the slope, which raises pore pressures rapidly and makes the areas susceptible to debris-slides. Depressions and gullies may have soil layers twice as thick as on the upper slopes, which makes them even more susceptible to mass movements (Sidle et al., 1985, p. 38).

Hydrologic properties of soils

    The rate of water movement within and water holding capacity of the soil are very important relative to slope stability (Sidle et al., 1985, p. 42-45). These soil properties are affected by the size distributions of particles and the spaces betweenthem, water input, slope gradient and shape, distance to water table, evapotranspiration and land use. Large spaces between soil particles, or macropores, are formed by decayed roots, earthworms, animal burrows, cracks from freeze-thaw or drying or subsurface erosion,and spaces between soil aggregates. Presence of macropores greatly increases the rate of water flow through the soil. Macropore size decreases with depth in the soil, which in turn influences the depth at which lateral subsurface flow occurs in the soil. Such lateral flow may occur in the upper permeable organic layers or at the interface with bedrock or other impermeable surface where shear stress is the greatest, and result in reduced shear strength and a shallow landslide. A high water holding capacity may result in retention of water from previous storms, antecedent moisture levels, and thus lessen the amount of precipitation in a following storm needed to saturate the soil. Thus thin soils on slopes recharge quickly and may lead to debris slides.

Engineering properties of soils

    Shear strength of the soil is dependent on the normal stress on the slip surface, cohesion of soil particles, and the internal angle of friction (Sidle et al., 1985, p. 45-46). The normal stress is affected by the weight or density of the soil, which increases with increased moisture content, the soil depth and slope gradient. Water infiltrating into the soil increases the weight of the soil and the water pressure within the soil pores and brings about unstable conditions. Soil cohesion is brought about by electromagnetic attraction between soil particles, cementing by organic and inorganic materials, and surface tension in unsaturated clay particles. Cohesion is limited by the point at which water content deforms the soil enough to cause rupture, the amount of water needed to cause the soil to flow. The internal angle of friction or degree of interlocking of soil particles and aggregates is dependent on the shape, size, and packing arrangement of the soil particles. Physical and chemical weathering of primary minerals, such as biotite and pyroxenes, results in production of clay particles (Sidle et al., 1985, p. 46). 

    Stability of hillslopes is dependent on hydrologic processes such as rainfall, rate of infiltration into the soil, subsurface flow rate, and evapotranspiration (Sidle et al., 1985, p. 48-51). Overland flow tends to be minimal in vegetated, undisturbed areas, and thus the major downslope flow is subsurface. If the infiltration rate is greater than the downslope transmission rate, the excess is pulled by gravity into the deeper layers of the soil to form or add to the groundwater; shear strength in this deeper layer of the soil drops. Tension cracks, soil pipes and macropores in the soil may also divert subsurface flow downward into the deeper soil layers. Increasing water flow into deeper layers brings water into potenial failure zones and decreases slope stability. 

    Transpiration by vegetation reduces soil water content (Sidle et al., 1985, p. 54). Trees transpire more than do understory shrubs or short ground vegetation due to their higher surface roughness (Sidle et al., 1985, p. 54-55). Vegetation types with deeper roots are able to transpire more water and dry the soil to greater depths. Evapotranspiration may thus reduce slow soil movement, or earthflow, rates.

Groundwater-slope stability relationships

    Increases in pore water pressure increase the likelihood of shallow rapid slope failures (Sidle et al., 1985, p. 57). Vegetation removal decreases transpired soil water loss and thus increases soil moisture levels; this can result in increased rates of soil slumps and creeps.

    Clearcutting of forests is followed over a 10 year period by increasing number and size of shallow landslides (Sidle et al., 1985, p. 58-59, 63, 65, 66). The length of the period is related to the time involved in decay of tree roots, with 50% or more of the tensile strength lost on the average in 2 years. The actual occurrence of landslides is dependent upon root deterioration, occurrence of a storm at or above the failure threshold and specific site factors, such as slopes over 30%. Roots are particularly important in increasing soil cohesion and shear strength when they cross shear zones. While alive the roots are able to stretch and readjust their positions when under peak shear stress.

Influence of roots on slope stability

Roots increase slope stability in several ways (Sidle et al., 1985, p. 60-62).

1. Roots can penetrate through failure prone surface layers into more stable deeper layers within the root zone and thus increase slope stability.

2. Roots can form a dense interwoven network in the upper 50cm of forest soil, which holds the soil in place.

3. Roots of a tree can form a localized center for soil reinforcement. With appropriate spacing of trees, the roots can provide soil arching restraint and increase shear strength of the soil.

Root deterioration and its effect on slope stability

    With the forest intact, the roots stabilize the soil in two ways: they anchor the soil in place by passing through loose material at the surface to more compact layers beneath, and they provide channels along the roots for rapid subsurface drainage which reduces the risk of surface soil saturation during heavy storms (Sidle et al., 1985, p. 66).

Failure of rock masses, as well as of soil material, is frequently triggered by seismic events (Sidle et al., 1985, p. 66, 68).
 

The effect of forest cover removal is dependent on the density of remaining trees and shrubs, the rate and type of regenerating vegetation, site characteristics, and water input (Sidle et al., 1985, p. 73-74, 76). Partial removal of trees exposes the remaining forest stand to wind action and blowdowns, which can further increase landsliding. Conversion of hillside vegetation from trees to grasses increases the rates of mass movement. 

    In the early 1970s the US Federal Highway Administration estimated it spent $50 million a year to repair landslide damage (Sidle et al., 1985, p. 79). However, low traffic volume roads are built without the extensive engineering and preventative control measures which are used in federally funded highway systems. Low volume roads are frequently constructed on steep slopes and decrease slope stability in four ways: 1) embankment fill adds weight to slope, 2) slope is increased on both cut and fill surfaces, 3) support is removed on the cutslope, and 4) concentration and rerouting of stormwater from road surface. Blockage of, lack of, or too few culverts, can cause soil saturation from storm water runoff in specific areas on the fill slope, which brings about slope failure. Routing of runoff into slope depressions can bring about shallow, rapid failures. 

    Recent urban development on hillsides has increased landslide activity, particularly on fill and cut slopes (Sidle et al., 1985, p. 84-86). Slope stability is decreased in several ways: "(1) removal of support by excavation, (2) mechanical overloading by fill placement, (3) concentration of water on the site or introduction of additional water, and (4) extensive removal or conversion of vegetation" or a combination of the above. Introduction of additional water involves water from septic tanks, swimming pools, artificial ponds, dry wells and other concentrated storm runoff, leakage from sewer lines, and from irrigation or watering of vegetation. Where slopes are more stable, soil creep may be accelerated, which in turn may cause damage to residences and public services. Cutslopes can cause debris avalanches on highly weathered, jointed bedrock, due to loss of lateral slope support. "Fill material and structures placed on hillslopes load the existing terrain and must be carefully engineered. Compaction of fill material is critically important to increase fill strength and minimize subsurface water inputs from upslope. Organic material and live vegetation should be excluded from fills to prevent eventual settlement and slope failure. Natural instability factors, such as compressible soils and extensive subsurface seepage, should be avoided in fill sites." Revegetation can reduce shallow slides and surface erosion; however, grasses and herbs are limited to reducing only surface erosion. On slopes over 45o strong rooted woody shrubs are preferable to trees, as trees add weight to the slope and transmit vibrations to the soil when subject to heavy winds.

    Detailed information and references are given in Sidle et al., (1985, Chapter 6) in relation to engineering and control methods to reduce slope destabiliztion. Specifically cited are the Uniform Building Code of the International Conference of Building Officials, the grading code for Los Angeles, and papers by Erley and Kockelman (1981), Leighton (1976), Neilsen et al. (1979), and Slosson (1969). The Los Angeles code restricts construction on slopes over 26o, has mandatory setbacks from cut and fill slopes, and sets specific requirements in relation to drainage around buildings, grading during the rainy season, thickness of fill, moving of soil, and as to size and type of grading equipment; it also sets up a schedule of seven mandatory inspections during construction. In some locations regulations have been set on lot sizes, such as one residence per 1.6 ha on a 24% slope, or on the percentage of the site that must remain in a natural state.

Serpentine is a hydrous magnesium silicate with a hardness ranging from 2 to 5 (Leet and Judson, p. 480-481). Talc or soapstone is also a hydrous magnesium silicate with a hardness of 1, which makes it the softest of all rocks. Serpentine rock can be converted into a talc-magnesite on exposure to carbon dioxide in the process of weathering (see below). Once formed, rocks may undergo metamorphic changes as they are subjected to different conditions of temperature (up to a maximum of 800oC at which point the rock melts), pressure and chemical environment, and thus may vary depending on the pH and mineral content of the surrounding material (Ehlers and Blatt, 1982, p. 511). These changes in environmental conditions bring about extensive changes in the structure and chemical composition of the rock (while the rock remains solid), so that it is now considered to be metamorphic rock. Mafic minerals are rich in Mg and Fe. The igneous rocks which fall into this group are pyroxene, amphibole, and olivine (Ehlers and Blatt, 1982, p. 101). The so called ultramafic rocks are composed of more than 90% mafic material; for instance, dunite is 90 to 100% olivine and the remainder is mostly pyroxine (Ehlers and Blatt, 1982, p. 106). The main chemical constituents of ultramafic rocks are MgO and SiO₂, with small amounts of FeO and CaO. Metamorphism of these minerals usually involves reactions with water and to a lesser extend with carbon dioxide (Ehlers and Blatt, 1982, p. 637-638). Thus prior to metamorphic changes, ultramafic rocks can be thought to range in composition from mostly MgO to mostly SiO₂. During metamorphism the water and/or carbon dioxide content of the rock increases as a result of the processes of hydration and carbonation. Formation of serpentine, or serpentinization, is one of the most common of these metamorphic reactions when igneous ultrabasic rock (with a chemical composition intermediate between Mg₂SiO4 and MgSiO₃, such as found in olivine and pyroxine) is hydrated on exposure to water at temperatures around 400° C. The chemical composition of serpentine can be generalized as Mg₃Si₂O5(OH)₄. Hydration of an ultramafic igneous rock with a slightly higher content of SiO₂ leads to the formation of talc, Mg₃Si₄O₁₀(OH)₂. In fact serpentinite, which is derived by hydration of primarily olivine and pyroxene, contains accessory minerals such as talc, chlorite and carbonates (Ehlers and Blatt, 1982, p. 516).
    Serpentinization involves induction of water into a rock and subsequent hydration under high temperatures, which in turn results in either an increase in volume, or a decrease in density of the solid rock and increased porosity. This involves loss of Mg and Si and/or increased internal fracturing (Ehlers and Blatt, 1982, p. 639).
 

    Igneous and metamorphic rocks are mechanically unstable when exposed to the atmosphere (Ehlers and Blatt, 1982, p. 268-282). They were formed under high temperatures (above 200o C) and pressures with exposures to relatively low levels of oxygen and water as compared to levels in the atmosphere. It is this instability that leads to breakdown of the rock to form sediments or soil. The stability of minerals varies with iron-oxides being the most stable, amphibole and pyroxene low in stability, and olivine the least stable (Press and Siever, 1978, p. 97); pyroxine and olivine are the minerals that give rise to serpentine on metamorphosis, as indicated above). Thus the mafic minerals from which serpentine is formed are highly susceptible to weathering when exposed at the surface due to the low ironoxide content. Once formed, serpentinite is susceptible to alteration on exposure to carbon dioxide, a normal component of rainwater which has been absorbed during passage through the atmosphere. If the water in contact with the rock contains more than 5% carbon dioxide (carbon dixide dissolved in water forms carbonic acid), chrysolite and lizardite are eliminated from that portion of the rock to leave a talc-magnesite zone. Thus the fractured nature of serpentine leads to penetration of water throughout the rock and creation of talc-magnesite zones within the serpentine formation. "Various degrees of alteration may be found, from a thin rim of talc-magnesite around the serpentinite body to almost complete replacement" (Ehlers and Blatt, 1982, p. 640).
    Serpentine rock is composed of three minerals which differ in their fracturing or way in which they break up. The fracturing type can only be detected by x-ray diffraction techniques (Ehlers and Blatt, 1982, p. 639). Lizardite tends to be platy, chrysotile fibrous, and antigorite either fibrous or platy. Fibrous minerals differ greatly in chemical composition, but can be used to make heat and fire resistant fabrics, insulating boards and shingles. Because of its fibrous structure, chrysotile serpentine has been used to produce 95% of the asbestos produced in the United States (Leet and Judson, 1958, p.438).
    Thus in the process of metamorphism and chemical weathering, ultramafic rocks are reduced in hardness from 5-7 to a hardness of 2-5 for serpentine and then to a hardness of 1 for talc. Hardness is related to the strength of bonding between atoms and ions that make up the rock (Press and Siever, 1978, p. 69-70). Covalent bonding is stronger than ionic bonding and van der Waals bonding is the weakest. The weakness of talc is because its silicate layers are weakly held to each other by van der Waals bonding, and thus the layers slip and break apart easily.

 
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